C1.1.6 Ultrasonic Machining 

Technology suitable for small quantity production

Micro ultrasonic machining (MUSM) is a method derived from conventional ultrasonic machining, in which a tool and free abrasives are used. The tool that is vibrated at ultrasonic frequency drives the abrasive to create a brittle breakage on the workpiece surface. The shape and the dimensions of the workpiece depend on those of the tool. Since the material removal is based on brittle breakage, this method is suitable for machining brittle materials such as a glass, ceramics, silicon and graphite [9]. The chip size required for micromachining can be realized when submicron particles are available for the use as an abrasive. Microtools can be supplied in the same way as that in micro-EDM, because the same types of tools are used to specify the corresponding shapes of products, although the microscopic removal phenomenon is completely different. The major problems are the accuracy of the setup and the dynamics of the equipment. Ultrasonic vibration of the machining head makes accurate tool holding difficult. The implementation of a transducer and rotation mechanism is too difficult to maintain high equipment precision. However, recent development has overcome most of these difficulties. In the earliest works, the vibrations were applied to the tool, resulting in problems in tool holding and in machining accuracy. In order to overcome tool holding problems, the on-the ­machine tool preparation was introduced. In this approach the tool was soldered to the machine head prior to its preparation then machined by wire electro discharge grinding (WEDG) to the desired dimensions and the subsequent machining of workpiece took place on the same machine tool. However this method prevents measurement of the size and the shape of the fabricated tool. By means of the on-the-machine fabrication of the tool, holes in silicon with 20 µm in diameter have been produced [10].

The main problem in tool holding accuracy arises from the soldering process that has been applied to solve the problem of loosening caused by ultrasonic vibration. If the tool material is soldered before it is fabricated into a microtool, the completed tool setting is free from the low accuracy of soldering.

A further development is the introduction of vibrations applied to the workpiece instead of to the tool [10]. This enables a better tool holding and the use of a high-precision spindle mechanism. A high- precision rotation/feed mechanism is essential for most machine tools. The introduction of a vibration mechanism such as that in USM is, therefore, an idea contrary to the concept of the micromachining. One of the solutions to this problem is to attach the vibration mechanism to the workpiece side [10]. This will cause little problem in terms of accuracy because a transducer can simply be inserted between the worktable and the workpiece. Such a setup enables the use of a universal, high­precision machine head and its influence on workpiece holding accuracy is negligible. 

With MUSM, micro holes of 5 µm in diameter were machined in quartz glass and silicon, using a tool with diameter of 4 µm and an abrasive with average grain size of 0.2 µm. Furthermore a microhole with diameter 9 µm and an aspect ratio of 4 was realized in quartz glass. A major problem in MUSM is the high tool wear ratio. Tungsten carbide is used for tool fabrication because tools of approximately Ø 5 µm can be machined and it is tough enough to withstand machining load. However a wear ratio always higher than 0.5 leads to limitations of the machining efficiency with the impossibility of machining deep holes or multiple holes with the same tool. The introduction of sintered diamond tools has enabled to overcome the problem, giving a wear ratio of 0.01 when machining soda glass. However, tools in sintered diamond are limited to a minimum diameter of 15 µm since they are fragile and tend to break during machining.

Rotary ultrasonic machining (RUM) is a hybrid machining process that combines the material removal mechanisms of diamond grinding with ultrasonic machining (USM), resulting in higher removal rates than those obtained by either diamond machining or USM alone. As main tool is made of diamond, steel is not machinable, and its main advantage is obtained machining fragile materials (ceramics, glass), so its application to metals is quite marginal. Expected accuracy is in the order of few microns and roughness of 0.2 microns Ra.

In pure ultrasonic machining (USM), the tool, shaped conversely to the desired hole or cavity, oscillates at high frequency, typically 20 kHz, and is fed into the workpiece by a constant force. Abrasive slurry composed of water and small abrasive particles is supplied between the tool tip and the workpiece. Material removal occurs when the abrasive particles impact the workpiece due to the downstroke of the vibrating tool.

In rotary ultrasonic machining (RUM), a rotating core drill with metal bonded diamond abrasives is ultrasonically vibrated in the axial direction while the spindle is fed toward the workpiece at a constant pressure. Coolant pumped through the core of the drill washes away the swarf, prevents jamming of the drill and keeps it cool. By using abrasives bonded directly on the tools and combining simultaneous rotation and vibration, RUM provides a fast, high-quality machining method for a variety of materials.

In [A_4], micro USM is widely described. Material removal in micro USM is by the mechanical action of abrasives as well as by the cavitation erosion due to rapid pressure changes caused by the ultrasonic vibration of fluid in working zone [A_5] [A_6]. This non-thermal, nonchemical and non-electrical process is especially suitable for the micro machining of hard brittle and inert insulators such as glass, ceramics, composites, quartz, precious

stones and for the machining of fragile and porous materials such as graphite. Irregular shaped hard abrasive particles are dispersed in a liquid medium (called abrasive slurry) and fed into the gap between tool and workpiece. The tool is vibrating with an ultrasonic frequency (usually 20~40 kHz) with an amplitude of several to tens micrometers. When static load is applied between tool and workpiece, abrasive particles impact and chip away material from both workpiece and to a lesser extent from the tool [A_7].

Condition of the abrasive and its grain size affect the machining rate [A_8]. A continuous flow of abrasive slurry flushes away the debris from the working zone. Since actual machining is carried out by abrasive particles, the tool can be softer than the workpiece.

A vibration of the workpiece improves the machining accuracy [A_9]. It not only simplifies the structural design of the tool system, but also stirs the abrasive slurry during the machining to improve renewing particles in the working zone and facilitates debris removal [A_10]. Precise measurement of the vibration amplitude at micron level is a challenging task. An online measurement method proposed in [A_10] drives the tool tip to touch the workpiece surface and captures two vertical positions of the surface with respect to turning on and turning off of the vibration. The difference between two positions is treated as the vibration amplitude. The accuracy of this measurement method is highly affected by the precision and responding time of driving components and force sensor. A force sensor with short responding time and high resolution is required for monitoring and controlling the static load to avoid tool breakage during machining. In some studies, the static loads are recorded under constant tool feed rate [A_11]. The static load under constant tool feed rate usually shows cyclic fluctuate pattern. A closed loop control can be

employed for the better evaluation of the effect of static load as a parameter on machining characteristics. In this case, the tool feed rate is adjusted to obtain a stable static load [A_10].

The mechanism and modeling in macro USM are not yet fully understood. Several models have been proposed to predict the material removal rate, and most of them are with rough accuracy in prediction. Material removal mechanism in micro USM is believed to be similar to conventional USM. However theoretical work in micro USM has rarely been reported and there is a lack of knowledge about the process behaviours of micro USM under various conditions. Theoretical models of macro USM may not

exactly be applicable to micro USM due to effects such as difficulty of refreshing of abrasive particle and debris removal caused by the downscaling of tool and abrasive particle [A_10]. Existing knowledge is far from sufficient to provide a complete understanding and instructive rules for industrial users [A_12]. A tentative mechanistic modeling of material removal in micro USM was proposed in [A_13]. The basic assumptions in this modeling are similar to those in Rotary Ultrasonic Machining (RUSM).

All materials tending to a brittle fracture behavior can be machined by micro USM. Examples include highperformance ceramics, glass, graphite and a part of the fiber-reinforced plastics [47] [A_14]. Geometrical capabilities of micro USM have been testified by drilling, slot machining and 3D machining. Micro holes with a diameter less than 10 μm were successfully drilled on silicon, quartz glass and alumina [A_9] [A_15] [A_16]. Slotting on low melting glass has been reported in [A_11]. The tool wear compensation strategy “Uniform Wear Method” originally developed for micro EDM has been applied in micro USM to successfully generate 3D micro cavities as shown in Figure 17 [A_17].

Micro USM has not yet been commercialized with a functional machine tool similar to micro EDM. However, it is believed that this process could provide solutions to easily and quickly achieve the larger MEMS structures as well as packaging for both prototype and production in silicon, glass and ceramic [A_7].

Proper selection of micro USM process parameters at present is not well understood due to lack of related experimental results [A_17]. Abrasive particle size, vibration amplitude, static load and tool rotation are the main parameters influencing the micro USM machining speed for the given workpiece material [A_10] [A_13] [A_16]. It was found that a slight tool rotation drastically improves the drilling speed. However, there was no significant improvement for speed higher than 50 rpm [159]. The debris accumulation affects the machining speed in micro USM due to the poor fluidic circulation around the machining zone [A_13]. The dependence of dimensional accuracy on tool diameter, vibration amplitude, and abrasive size needs further research [A_12]. By means of calculating the maximum impact force, combined with microcrack models obtained from the research on indentation, it is possible to correlate the depth of microcracks with process parameters. A predictive model for the microcrack depth can be employed to optimally select the process parameters [A_12].

High tool wear is an intrinsic drawback of micro USM. It is difficult to get a constant depth of cut due to longitudinal tool wear. Tool wear is affected by parameters such as vibration amplitude, static load and tends to increase when harder and coarser abrasives are used. As a consequence, harder abrasives, like diamond, cause higher tool wear than softer abrasives such as silicon carbide [A_16]. Therefore, it is necessary to account for and to compensate the tool wear during machining. The feasibility of applying the “Uniform Wear Method” for generating accurate 3D microcavities by micro USM has been tested and found that the tool shape remains unchanged and the tool wear has been compensated [A_17]. Due to inadequate research done on tool wear in micro USM, the selection of tool materials is not well supported by the experimental data and related analysis. Also, the tool wear mechanisms, the wear rate

dependence upon tool hardness, toughness, abrasive type and size, abrasive hardness and material toughness need to be studied to reduce and control the tool wear in micro USM [A_12].

The micro tools used in micro USM can be prepared by a WEDG unit [A_9]. A micro tool with multi tips was made using batch mode micro EDM. Tool material with enough abrasive wear resistance and small deflection under mechanical load is preferable in micro USM. A PCD tool is helpful in reducing tool wear [A_9].

Some of the important micro USM issues requiring systematical research include the study of material removal mechanism, innovative tooling, tool wear mechanism and reduction, on-line sensing, subsurface

damage control, and surface roughness improvement. In addition, in-process monitoring and model-based selftuning strategies are needed for improving the process stability and performance.

[A_4]      Micro and Nano Machining by Electro-Psysical and Chemical Processes, K.P. Rajurkar, G. Levy , A. Malshe, M.M. Sundaram, J. McGeough, X. Hu, R. Resnick, A. De Silva, Annals of the CIRP Vol. 55/2/2006, 643-666.

[A_10]     Hu, X., Yu, Z., Rajurkar , K. P., 2005, Experimental Study of Micro Ultrasonic Vibration Machining. Fourteenth International Symposium on Processing and Fabrication of Advanced Materials, Pittsburgh, Pennsylvania, USA, 197-210.

[47]         IWF, IPT, 2002, Investigation of the International State of the Art of Micro Production Technology MickroPRO.

[A_5]      Kremer, D., Saleh, S. M., Ghabrial, S. R., Moisan, A., 1981, State of the Art of Ultrasonic Machining., Annals of the CIRP, 30 /1: 107-110.

[A_6]      McGeough, J. A., 2002, Micromachining of Engineering Materials, Marcel Dekker Inc., New York.

[A_12]     Zhang, C., Rentsch, R., Brinksmeier, E., 2005, Advances in Micro Ultrasonic Assisted Lapping of Microstructures in Hard-Brittle Materials: A Brief Review and Outlook, International Journal of Machine Tools and Manufacture, 45 /7-8: 881-890.

[A_7]      Medis, P. S., Henderson, H. T., 2005, Micromachining Using Ultrasonic Impact Grinding, Journal of Micromechanics and Microengineering, 15/8: 1556-1559.

[A_8]      Adithan, M., Venkatesh, V. C., 1978, Appraisal of Wear Mechanisms in Ultrasonic Drilling, Annals of the CIRP, 27 /1: 119-121.

[A_9]      Egashira, K., Masuzawa, T., 1999, Microultrasonic Machining by the Application of Workpiece Vibration, Annals of the CIRP, 48 /1: 131-134.

[A_11]     Moronuki, M., Saito, Y., Kaneko, A., Miura, A., Aikawa, C., 2004, Vibration Micromachining of Low-Melting-Temperature Glass. 7th International Symposium on Advances in Abrasive Technology, Bursa, Turkey, 489-494.

[A_13]     Yu, Z., Hu, X., Rajurkar , K. P., 2005, Study of Micro Ultrasonic Machining of Silicon. Proceedings of 2005 ASME International Mechanical Engineering Congress and Exposition, Orlando, Florida USA, 1-8.

[A_14]     Ghahramani, B., Wang, Z. Y., 2001, Precision Ultrasonic Machining Process: A Case Study of Stress Analysis of Ceramic (Al2O3), International Journal of Machine Tools and Manufacture, 41 /8:1189-1208.

[A_15]     Egashira, K., Masuzawa, T., Fujino, M., Sun, X. Q.,1997, Application of USM to Micromachining by On-the-Machine Tool Fabrication, International Journal of Electrical Machining, 2: 31-36.

[A_16]     Choi, H.-Z., Lee, S.-W., Lee, B.-G., 2003, Micro-Hole Machining Using Ultrasonic Vibration, Key Engineering Materials, 238-239: 29-34.

C1.1.7 ECF 

Technology suitable for small quantity production

ECF is a new technique to machine hard materials and other electrochemically active materials. The tool movements are similar to conventional milling machines. Nevertheless ECF is an electrochemical process, hence both workpiece and tool are submerged in an electrolyte and held at a constant potential to prevent corrosion. Short voltage pulses in the range of 10ns to 100ns are applied between tool and workpiece. With these pulses the electrochemical doublelayer capacitance on the surface of the workpiece is charged over the resistance of the electrolyte. Because the resistance increases with respect to the distance between tool and workpiece the time constant of the charging also changes. Since charging takes place only during the time of the pulse, the double-layer is sufficiently charged to generate an anodic dissolution of the workpiece only in a defined working distance around the tool. Surface areas further away from the tool are not affected by the ECF process. This leads to a confined milling with a high spatial resolution. Furthermore the working distance is proportional to the applied pulse width. Thus the working distance and hence the spatial resolution of the process can be varied between 10µm to 5µm and below just by adjusting the pulse width.

ECF is a pure electrochemical process, i.e. ion transport from the workpiece into the electrolyte takes place, and due to this, no remelting of the surface of the workpiece or other conversion occurs. Furthermore virtually no forces act on the tool. Thus tools with diameter down to 5µm and below and with arbitrary shapes - like grooving cutters, rounded cutters or ball cutters - can be used. The milling strategies are similar to that of conventional milling, although the tool in the ECF process does not rotate. This allows production of 3D structures and freeform surfaces. Cutting speed is achieved of about 1µm/s in stainless steel 1.4301.

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C1.3.1 Rolling

Technology suitable for both serial and small quantity production

Rolling is a fabrication process in which the metal is passed through a pair of rolls. There are two types of rolling process, flat and profile rolling. In flat rolling the final shapes of the product are foil (thickness less than 1 mm), sheet (thickness less than 3 mm) or plate (thickness more than 3 mm). In profile rolling, the final product may be a round rod or other shaped bar. This process is gaining an increasing interest if the foil or sheet metal used for subsequent production processes like bending, punching, etc. is having a defined shape of the surface (tailored tape) in order to enable more complex products to be manufactured. 

Rolling is also classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallisation temperature then the process is termed as hot rolling, If the temperature of metal is below its recrystallisation temperature the process is termed as cold rolling. If metallic foils have to be produced, the first process is not suitable due to the large surface to volume ratio yielding high oxidation. 

Another approach is the cold forming of sections by rolling technology where continuously metal sheet or foil is formed by several rolls. As a result of the used bending process, at microscale problems like springback depending on the foil thickness have to be considered.

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C1.3.3 Deep drawing 
Technology suitable for both serial and small quantity production

Deep drawing is a process frequently used at conventional length scale. In order to produce e.g. cups with a diameter of a few millimeters or below, some specifics have to be regarded. As it has been shown, there is a relation between the deep drawing clearance and the punch force. If the ratio between clearance and sheet thickness is increased from 1.05 to 1.75, the increase of the force is about 10%. Furthermore, the shape building is also affected by the clearance, yielding a rough cup wall surface in case of a small clearance. Besides the influence of the geometrical design of the tools, the specimen size also affects the forming limits. When scaling down the cup diameter down to 1 mm, experimental investigations have shown that the maximum drawing ratio is reduced. As it is similar to the conventional scale, friction has an influence on the deep drawing process. To quantify the influence, experiments using solid lubricant, extrusion oil and without lubricant have been performed. As the solid lubricant has only minor influence on the deep drawing results, liquid lubricant is leading to lower deep drawing forces and an increase in the maximum drawing ratio.

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C1.3.4 Bending

Technology suitable for both serial and small quantity production

In industrial application, micro bending processes are frequently used in case of spring, clamp or lead frame production. Considering the characteristics of this process, namely part dimension being close to sheet thickness, conventional FE-simulation programs, which assume plain strain condition in the deformed area, are not applicable. In order to overcome this, a first model has been developed that enables the calculation under plane stress conditions [11] or in the improved version considering the anisotropy of the material [12].Basic investigations on the relationship between miniaturization and bending process have firstly been done in [13]. The analyses of the experimental results have shown that the process forces relative to the size decrease with miniaturization in case of small grains. In case of large grains (only few grains over the sheet thickness), the force is increasing again. This effect has been confirmed by investigations in [14] performing scaled bending experiments on metallic foils. It has been shown, that depending from the thickness of the sheet (scaled from 200 microns down to 25 microns) and from the material structure, bending moment and thus the spring back angle increases when scaling down. This confirms the previously described theory of strain gradient plasticity [15, 16, 17, 18]. When scaling down the foil thickness, two contrary effects appear: the effect of a reduction of the flow stress due to an increasing fraction of surface grains on the overall volume and the effect of an increasing flow stress caused by the increasing density of geometrically necessary dislocations. When the foil thickness is getting smaller, the latter effect gains superior influence resulting in both, higher normalized bending moment and higher spring back angle.

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C1.3.5 Blanking

Technology suitable for both serial and small quantity production

Basic investigations on the effects of miniaturization on the blanking process have been first performed by Kals [13]. It has been shown that the normalized forming force is constant while scaling down sheet thickness due to a lack of free surface. When sheet thickness is below a certain value, the forming force and the ultimate shear strength is increasing. Other investigations have shown that there is a dependency between tool and process parameters and the accuracy of the produced leadframes. The deflection of the leads in the plane of the sheet increases with decreasing width of the lead. The deflection is also influenced by different clearances between tool and die in a progressive tool. They also showed that increasing the strip holder pressure has a positive effect on the accuracy in most cases. Also the dynamic behaviour of the tool is affecting the accuracy [19], e.g. increasing blanking speed is resulting in a decreasing accuracy. Performed experiments have additionally shown that increasing the strip holder force is clearly improving the product quality. Other investigations on the deflection of a punch during the blanking process [20] have shown an increase of deflection when the punch is eccentric relative to the die. A particular blanking process, so-called dam-bar cutting has been investigated by [21, 22]. This is a mechanical trimming process removing the dam-bar between the leads after the IC package is encapsulated. Due to the special shape of the specimen rectangular around the IC, investigations have to be performed considering the anisotropic behaviour of the material in the shearing line. Further investigations on different materials have shown that an increase of the angle α reduces the maximum cutting forces but is leading to an increasing burr height. An important aspect for industrial production is tool life. Therefore, investigations in [22] have been performed to show the dependency of the punch forces and thus the tool stresses from the clearance between punch and die and the used sheet material hardness. While tools made of tungsten carbide (WC) have the longest life time, tools made of bare HSS show the highest wear. An improvement can be reached by using coatings like PVD-TiN or plasma-nitriding. In this case, tools made of steel and WC have been compared according to their tool life in an industrial production process of blades for shavers. While the first only reaches a quantity of 50.000 parts, the latter exceeds 1.15 billion parts. Thus, it is obvious that the usage of the more expensive WC as tool material (factor 1.8 compared to steel) is more economical than the usage of steel. In general it can be said that blanking even at microscale using thin foils is an industrially well established technology.

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C1.3.6 Embossing

Technology suitable for both serial and small quantity production

Studies concerning coining aluminium sheet metal have been performed by Ike and Plancak [23]. Using dies with hole diameters from 0.05 to 1.6 mm, half hard commercially-pure aluminium of 2 mm in thickness and 30 mm in diameter, coining results were evaluated in terms of aspect ratio, the radial position of the hole and the die and the emerging forming load. As it has been shown, the beginning of plastic flow is independent from the radial position of the hole, but the height of the pins is clearly bound to the position of the holes and its diameter. 

At LWP Saarbrücken, Germany [24], also coining of smallest cavities with a thickness of about 400 microns has been investigated. The results show, that in case of micro-coining the influence of the tool geometry and the tool deflection on the forming results must be considered. This is due to rather high nominal stresses (minimum three times the flow stress of the coined material) leading to a large impact on the tool system: tool deformation, tool deflection and tool damage. 

The aim of the investigations on coining technologies at IWU Chemnitz was to establish this technology for the production of geometrically defined micro structures for applications in the fields of micro-fluidics, micro-optics and information technology [25, 26]. In a first set of experiments, tools were made out of single crystal silicon with an almost smooth surface. Thus, it was possible to create structures of 100 microns with radii of some 100 nm in aluminium, copper, brass and steel. 

The analysis of the coined material shows that the shape of the rim strongly depends on the material. While aluminium shows a bulging of max. 30 % of the coining depth, steel shows only max. 5 %. Using fine grained ZnAl alloy in a superplastic state at 250 °C, high precision and surface quality are achieved at low compressive stresses of 25 MPa.

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C1.3.7 Cold Forging

Technology suitable for both serial and small quantity production

Since the most relevant parameters in forming processes describing the material behaviour are the flow stress and the flow curve, it is necessary to perform scaled standard tests to obtain these parameters valid at microscale. Carrying out tensile tests using CuZn15, CuNi18Zn20 [27], copper

[28] and aluminium [29] as well as upsetting tests using copper, CuZn15 and CuSn6 [30, 31] two significant effects have been shown: a reduction of the flow stress when increasing the surface to volume ratio as well as an increasing process scatter. 

A first approach to describe the decreasing stresses has been done by [32] introducing the so-called surface layer model. Based on the assumption, that grains positioned on a free surface have fewer constraints to fulfil than grains within the material, the local forming behaviour of the surface grains must be different. Dislocations induced by a deformation process are able to pile up at grain boundaries but not at a free surface. Thus, lower hardening occurs in the region of the free surface. Decreasing the specimen size leads to an increasing fraction of surface grains and thus a lower integral flow curve.

In case of a full forward extrusion process, scaled down from an initial specimen diameter of 4 mm down to 0.5 mm, an increase in the relative punch force was detected. A possible explanation for this effect can be the increasing friction with decreasing specimen dimension. Further tests to study the effects of miniaturization on friction were made by Messner, using the ring compression test [33].The increase of friction when decreasing the specimen size was analyzed in a more detailed way by [34] using the double cup extrusion (DCE) test which was first proposed by [35] and applied by [36].

In this test – due to the large plastic deformation being well suited to represent metal forming processes – a cylindrical billet is positioned between a stationary and a moving punch. Theoretically, in case of zero friction (m = 0) both cups are supposed to have the same height, as the friction gets higher, the height of the upper cup is increasing. Thus, the change in the ratio between the upper and the lower cup is able to characterise the change in the friction conditions. If absolute values of the friction factors are requested, the method of numerical identification can be used.

Experimental investigations on the frictional size-effects have been performed by [34] scaling down specimens geometrically similar from a diameter of 4 to 0.5 mm with a ratio of diameter to height D0/H0 = 1. The friction increases with decreasing specimen size from a friction factor about m = 0.02 for the largest specimen up to m = 0.4 for the smallest specimen. 

An attempt to describe the frictional behavior on a topographical level is given by a mechanical­rheological model [34] considering the theory of open and closed lubricant pockets. If a forming load is applied to a specimen surface, the roughness peaks start to deform plastically. From this point on, lubricant is either trapped and pressurized within closed areas αCL or squeezed out if a connection to the edge of the surface exists. The forming load can be transmitted into the specimen either by the pressurized lubricant or the flattened asperities.

Due to the scale-invariant production process of a specimen and thus an assumed scale-independent surface topography, the area width where open lubricant pockets appear is constant when scaling down geometry. Additionally, the area of closed lubricant pockets is reduced and thus the real contact area αRC is increased. This leads to an increase in the friction factor. Further independent investigations have confirmed these results [37]. 

Based on the mechanical-rheological model further investigations have been performed in order to describe the size-dependent friction factor analytically [38]. Using Wanheim/Bay’s [39] friction law and the geometrical boundary conditions, it can be shown that in dependency of the surface topography the friction factor changes as it is in a good agreement with experimentally obtained results.

Investigations on micro extrusion processes with high aspect ratios and large strains have shown a significant dependency of the forming results from the material structure [40]. In case of backward can extrusion process, the cup geometry was chosen with a cup wall thickness of about 8 microns. SEM analysis of the shape building reveals a strong influence of the material structure on the shape building, e.g. by an uneven cup height. 

Further investigations using micro hardness measurements to evaluate local material flow also confirmed the above described results. This effect is less distinct in case of fine grained material than in case of coarse grained material. In case of grains being larger than the feature size they are forced to flow into the smaller features and thus in dependency of the size and orientation causes the uneven cup height.

As it was expected from DCE-test results, the increase of the friction factor when scaling down leads to an increase of the ratio between cup height and shaft length for both cases: coarse grained and fine grained material. The minor increase in case of coarse grain, can be explained by the fact that the grain size is in the same range as the feature size. Thus, it is easier for the material to flow into the shaft than into the cup.

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C1.4.1 Ultrasonic Welding 

Technology suitable for both serial and small quantity production

Ultrasonic welding is a solid-state welding process in which consolidation of joining partners is achieved by pressure and friction energy. The components to be welded are clamped between the support bases and tip (sonotrode). The sonotrode is vibrated at a frequency in the ultrasonic range and induce oscillatory stress in the interface between components that disrupt the surface oxides and supply energy for bonding. Ultrasonic welding of wires is called also ultrasonic wire bonding.

Ultrasonic welding can join a wide range of metals and some metal/non-metal combination, including some metallurgical incompatible combinations. Against resistance welding, also high-conductive metals as well as metal with large match of melting temperature could be joined by means of ultrasonic energy. The joint configuration and part geometry for ultrasonic welding show some limitations because they must be customized on the sonotrode geometry.  The smallest wire that could be bonded is in the range of a few microns.

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C2.1.1 EDM

Technology suitable for both serial and small quantity production

Electro Discharge Machining (EDM) is basically a thermal process in which the material is removed by the action of electric sparks between two electrodes made of conductive materials: part-electrode and tool-electrode. The separation space between both electrodes (gap) where the spark takes place is a fundamental process characteristic. Therefore the gap establishes the necessary conditions to establish the electrical field prior to the spark. The electric spark is the material removing effect and presents different stages that only last for some milli- or micro-seconds. The complete sparking cycle is repeated thousands of times per second, generating small craters that conform the final part geometry. EDM is used to machine hard, brittle metals.

In [A_4], micro EDM is widely illustrated. Micro EDM is the successful adaptation of EDM for micromachining from simple holes to complex moulds [A_19]. Here the discharge energy is reduced to the order of 10-6 to 10-7 Joules in order to minimize the unit material removal. Based on the electrode being used, Micro EDM can be classified in to drilling, die-sinking, milling, wire EDM (WEDM) and wire electro-discharge grinding (WEDG) [A_20] [A_21]. An overview of the capabilities of micro EDM is provided in the following table.

Because of the stochastic nature of the sparking, it is difficult to fully explain the mechanism of transient material removal in EDM. It has been known that the mechanism of material removal in EDM is based on electrical energy transfer and thermal process. However, this energy transfer is still not well understood, particularly in micro-EDM using small energy (< 100 μJ) [A_22]. The description of the physical phenomena in the discharge gap has not been established in a common accepted theory [A_23]. The gap phenomena include plasma formation in the dielectric, interaction between electrons and ions, heat transfer and material ejection.

Micro EDM power supply is provided by a relaxation type as well as a transistor type circuits. In a relaxation type circuit, discharge pulse duration is dominated by the capacitance of the capacitor and the inductance of the wire connecting the capacitor to the workpiece and the tool [A_24]. Discharge energy is determined by the used capacitance and by the stray capacitance that exists i) between the electric feeders, ii) between tool electrode holder and work table iii) between the tool electrode and workpiece [A_18]. The transistor type circuit has better controllability and improved capability to handle large currents with high response. Commercial transistor pulse generators can vary the pulse duration, duty factor, and also change the waveform of discharge pulse to reduce the tool wear and improve MRR. However, it is difficult to keep the constant discharge duration shorter than several tens of ns using the transistor type pulse generator [A_18]. By integrating the transistor type isopulse generator with the servo feed control system, about 24 times higher material removal rate has been obtained than that of the conventional RC

pulse generator with a constant feed rate in both semifinishing and finishing conditions [A_25]. Nevertheless, the relaxation type pulse generators are still the better choice for finishing in micro EDM because it is difficult to obtain significantly short pulse duration with constant pulse energy using the transistor type pulse generator.

Gap monitoring and control systems are necessary to avoid or minimize the open circuit, arcing and short circuits during machining. A stable gap control system enables better dimensional accuracy of micro machined features by predicting the gap distance and offsetting tool position.

Ignition delay time (td) is an important indicator of the isolation condition of the discharge gap. Larger gap width causes longer ignition delays, resulting in a higher average voltage. Tool feed speed increases when the measured average gap voltage is higher than the preset servo reference voltage and vice versa [A_18]. Other than the average gap voltage, the average delay time can also be used to monitor the gap width [A_26]. In other attempts, gap monitoring circuits were developed to identify the states and ratios of gap open, normal discharge, transient arcing, harmful arcing and short circuit [A_27] [A_28]. These ratios were used as input parameters for online EDM control based on various control strategies.

Instead of complex shaped expensive tools, simple cylindrical tools can be used to machine complex features by profiling [A_29]. It is possible to use hollow electrodes for drilling above 100 μm diameter holes. For lower dimensions solid tools from sintered carbide that have a diameter of around 50 μm are used [47]. Currently WEDG is the widely accepted and commercialized method to fabricate micro tools [A_30] [A_31]. Using single pulse discharge is an innovative technique to produce 20~40 μm diameter tungsten electrodes in hundreds of microseconds. Repeatability of tool shape after fabrication is hard-tocontrol

in this rapid process [A_32] [A_33] [A_34]. Another method prepares micro rod by self-drilled holes [A_35]. This method does not need initial positioning of the rod with respect to the plate electrode, and the operation is easy and with good repeatability.

Higher heat conductivity, higher melting point and boiling point are desired properties of the tool material. Tungsten which has a high melting point and tensile strength is the predominant tool material in micro EDM [A_36] [A_37]. Tungsten carbide [A_38] and copper have also been used as tool material [A_39]. Electrically conductive CVD diamond is a new entrant. It has shown almost zero electrode wear, even at short pulse duration of 3 μs [A_40]. Mechanical properties and cutting performance of thin wires are the special concern in micro wire EDM [A_41]. In micro wire EDM, apart from tungsten, micro wires made of copper, brass and molybdenum are also commonly used. The tool wear is mainly influenced by polarity and thermal properties of electrode materials [A_18]. The energy dissipation distributed into the anode during discharging is always greater than that into the cathode for both single discharge [A_42] and continuous pulse discharges [A_43]. The carbon layer deposited on the anode surface due to thermal dissociation of the hydrocarbon oil protects the anode surface from wear [A_44]. Thicker carbon layer leads to smaller electrode wear ratio in macro EDM, where the tool is the anode. However, in micro EDM, the tool is typically cathode and hence, deposition of carbon layer is scarce. The effect of thermal properties on electrode wear was investigated in [A_22]. It was found that the boiling point in addition to the melting point of the electrode material plays an important role in the wear of micro EDM tools. It was found that the tool wear ratio reduces with the increase of the tool area [A_37]. Other factors affecting the tool wear ratio, like poor flushing conditions in a deep hole, are difficult to assess and control. This could easily result in wrong estimation of the wear ratio and produced depth [A_45]. Discharge current waveform is yet another factor affecting tool wear [A_18].

Two tool wear compensation methods namely, the linear compensation [A_46] [A_47] and uniform wear method [A_45] have been used in micro EDM. The linear compensation is to feed the tool towards the workpiece and compensate tool wear length after it moves along a certain distance. It is suitable to generate 3D cavities with straight side walls. Uniform wear method includes tool path design rules and tool wear compensation. Tool paths designed based on the uniform wear method can keep the tool wear uniform at the tool tip. This method has been verified by generating 3D micro cavities with inclined side surfaces and spherical surfaces successfully

Micro Wire EDM uses a fine metallic wire to apply the cut. The wire itself is eroded in the process, but it is continuously fed from a spool through the part, keeping a constant diameter. Nevertheless, if the spark energy is too high, it may break. Wire breakage is the most important technical and economic challenge. µ-WEDM uses the same physical principles with just a few differences; the machines provide 
higher resolution, the process parameters are controlled in a finer way to reduce the spark energy and applied wires are ∅0.02-0.03 mm. The most widely used dielectric fluid in µ-WEDM is oil, that permits cutting with a smaller gap, and the wire is often made of tungsten. WEDM is widely applied in the manufacturing industry for machining tools (punch and dies) due to its capability to generate ruled geometries in hard materials.

Not involving abrasive agents, nor any contact between part and tool, the machining forces are very low, but not negligible in the case of micromachining. The forces acting on the wire cause it to bend (static component) and vibrate (dynamic component). As a consequence, the real wire position differs from the nominal position programmed for the guides. Thus, the machined part presents some deviations, especially in wall straightness and also in the corners where the directions of the guides’ movements have been changed. In thin wires, these deviations are important errors especially when machining acute angles.

Micro EDM-milling process, in which a 3D path is commanded to a simple shape electrode. Using simple shape electrode in micro EDM allows achieving tolerances of 5 µm, holes with an aspect ratio of 100, which can be used for nozzles for ink-jet printers, flow channel orifices and pinholes for X-ray measurements.

[A_4]      Micro and Nano Machining by Electro-Psysical and Chemical Processes, K.P. Rajurkar, G. Levy , A. Malshe, M.M. Sundaram, J. McGeough, X. Hu, R. Resnick, A. De Silva, Annals of the CIRP Vol. 55/2/2006, 643-666.

[A_18]     Kunieda, M., Lauwers, B., Rajurkar , K. P., Schumacher, B. M., 2005, Advancing EDM through Fundamental Insight into the Process, Annals of the CIRP, 54 /2: 599-622.

[A_19]     Uhlmann, E., Piltz, S., Doll, U., 2005, Machining of Micro/Miniature Dies and Moulds by Electrical Discharge Machining--Recent Development, Journal of Materials Processing Technology, 167 /2-3: 488-493.

[A_20]     Pham, D. T., Dimov, S. S., Bigot, S., Ivanov, A., Popov, K., 2004, Micro-EDM - Recent Developments and Research Issues, Journal of Materials Processing Technology, 149 /1-3: 50-57.

[A_21]     Masuzawa, T., 2001, Micro-EDM, Proceedings of the 13th International Symposium for Electromachining, Bilbao, Spain, 1-19.

[A_22]     Tsai, Y.-Y., Masuzawa, T., 2004, An Index to Evaluate the Wear Resistance of the Electrode in Micro-EDM, Journal of Materials Processing Technology, 149 /1-3: 304-309.

[A_23]     Schumacher, B. M., 2004, After 60 Years of EDM the Discharge Process Remains Still Disputed, Journal of Materials Processing Technology, 149 /1-3: 376-381.

[A_24]     Lim, H. S., Wong, Y. S., Rahman, M., Edwin Lee, M. K., 2003, A Study on the Machining of High-Aspect Ratio Micro-Structures Using Micro-EDM, Journal of Materials Processing Technology, 140 /1-3: 318-325.

[A_25]     Han, F., Wachi, S., Kunieda, M., 2004, Improvement of Machining Characteristics of Micro-EDM Using Transistor Type Isopulse Generator and Servo Feed Control, Precision Engineering, 28 /4: 378-385.

[A_26]     Altpeter, F., Perez, R., 2004, Relevant Topics in Wire Electrical Discharge Machining Control, Journal of Materials Processing Technology, 149 /1-3: 147-151.

[A_27]     Rajurkar, K. P., Wang, W. M., 1991, On-Line Monitor and Control for Wire Breakage in WEDM, Annals of the CIRP, 40 /1: 219-222.

[A_28]     Snoeys, R., Dauw, D., Kruth, J. P., 1980, Improved Adaptive Control System for EDM Processes, Annals of the CIRP, 29 /1: 97-101

[A_29]     Yu, Z. Y., Kozak, J., Rajurkar, K. P., 2003, Modelling and Simulation of Micro EDM Process, Annals of the CIRP, 52 /1: 143-146.

[A_30]     Masuzawa, T., Fujino, M., Kobayashi, K., Suzuki, T., Kinoshita, N., 1985, Wire Electro-Discharge Grinding for Micro-Machining., Annals of the CIRP, 34 /1: 431- 434.

[A_31]     Masuzawa, T., Yamaguchi, M., Fujino, M., 2005, Surface Finishing of Micropins Produced by WEDG Annals of the CIRP, 54 /1: 171-174.

[A_32]     Takezawa, H., Hamamatsu, H., Mohri, N., Saito, N., 2004, Development of Micro-EDM-Center with Rapidly Sharpened electrode, Journal of Materials Processing Technology, 149 /1-3: 112-116.

[A_33]     Takezawa, H., Itoh, N., Mohri, N., 2001, The Behaviour of Thin Electrode Wear in Electrical Discharge Machining. Proceedings of the 13^th International Symposium for Electromachining, Bilbao, Spain, 727-735.

[A_34]     Takezawa, H., Mohri, N., Furutani, K., 2001, Rapid Production of a Thin Electrode by a Single Discharge Machining. I. Machining Phenomena and Application of Formed Electrode, Journal of the Japan Society of Precision Engineering, 67 /8: 1299-1303.

[A_35]     Yamazaki, M., Suzuki, T., Mori, N., Kunieda, M., 2004, EDM of Micro-Rods by Self-Drilled Holes, Journal of Materials Processing Technology , 149 /1-3: 134-138.

[A_36]     Song, X., Reynaerts, D., Meeusen, W., Van Brussel, H., 1999, Investigation of Micro-EDM for Silicon Microstructure Fabrication, Proceedings of SPIE The International Society for Optical Engineering, 3680 /II: 792-799.

[A_37]     Yu, Z., Rajurkar, K. P., Shen, H., 2002, Drilling of Noncircular Blind Micro Holes by Micro EDM, Transactions of the NAMRI/SME 30: 263-270.

[A_38]     Yu, Z. Y., Masuzawa, T., Fujino, M., 1998, 3D Micro- EDM with Simple Shape Electrode Part1, International Journal of Electrical Machining, 3: 7-12.

[A_39]     Sharma, A., Iwai, M., Kawanaka, K., Suzuki, K., Uematsu, T., 2004, Attempt at EDM of Electrically Conductive Diamond and its Application to Miniature Mold Processing. 7th International Symposium on Advances in Abrasive Technology, Bursa, Turkey, 565-568.

[A_40]     Sharma, A., Iwai, M., Suzuki, K., Uematsu, T., 2004, Low-Wear Diamond Electrode for Micro-EDM of Die-Steel. 7th International Symposium on Advances in Abrasive Technology, Bursa, Turkey, 559-560.

[A_41]     Dauw, D. F., 1994, High-Precision Wire-EDM by Online Wire Positioning Control, Annals of the CIRP, 43 /1: 193-197.

[A_42]     Xia, H., Kunieda, M., Nishiwaki, N., 1996, Removal Amount Difference Between Anode and Cathode In EDM Process, International Journal of Electrical Machining, 1: 42-52.

[A_43]     Xia, H., Hashimoto, H., Kunieda, M., Nishiwaki, N., 1996, Measurement of Energy Distribution in Continuous EDM Process, Seimitsu Kogaku Kaishi/Journal of the Japan Society for Precision Engineering, 62 /8: 1141-1145.

[A_44]     Natsu, W., Kunieda, M., Nishiwaki, N., 2004, Study On Influence of Inter-Electrode Atmosphere on Carbon Adhesion and Removal Amount, International Journal of Electrical Machining, 9: 43-50.

[A_45]     Yu, Z. Y., Masuzawa, T., Fujino, M., 1998, Micro-EDM for Three-Dimensional Cavities – Development of Uniform Wear Method, Annals of the CIRP, 47 /1: 169-172.

[A_46]     Yuzawa, T., Magara, T., Imai, Y., Sato, T., 1997, Micro Electric Discharge Scanning Using a Mini-Size Cylindrical Electrode, Kata Gijutsu, 12 /8: 104-105.

[A_47]     Bleys, P., Kruth, J.-P., Lauwers, B., Zryd, A., Delpretti, R., Tricarico, C., 2002, Real-Time Tool Wear Compensation in Milling EDM, Annals of the CIRP, 51 /1: 157-160.

C2.1.2 ELID

Technology suitable for both serial and small quantity production

C2.1.3 Laser Micromachining

Technology suitable for small quantity production

Laser micromachining removes material in a layer-by-layer fashion. It is an ablation operation causing vaporisation of material as a result of interaction between a laser beam and the workpiece being machined. The removal of material during laser ablation is affected by the characteristics of the laser beam and the workpiece but is mainly determined by the way the two interact. The most important laser radiation features are the pulse length (duration) and repetition rate (frequency). This allows the accumulated energy to be released in very short time intervals, which is what generates the extremely high power. Additionally, the laser beam can be focused on a very small spot. Thus, extremely high intensities (10¹³-10¹⁸ W/cm²) are achievable. The most important substrate material features are the absorptivity in the wavelength of the transmitted radiation, and the thermal conductivity of the substrate. Thus, material transition energies such as the latent heat of melting and the latent heat of vaporisation are most significant. This is demonstrated by the fact that metals melt more easily than ceramics but are considerably more difficult to vaporise and consequently respond less well to laser ablation.

As previously mentioned, when laser radiation impacts on the substrate, electrons in the latter are excited by the laser photons. This absorbs the energy of the photons and generates considerable heat which is transferred to the material lattice in picoseconds resulting in very high temperatures that can create local melting or vaporisation. Energy loss through electron heat transport to the bulk of the substrate is undesirable as it can raise the temperature of the surrounding material and create heat-affected zones. However, for very high intensities, which can be achieved by laser processing, non-linear effects take place and become a factor for stronger energy absorption. In the case of extreme intensities, as in ultrashort pulse ablation, the bond electrons of the material can be directly dislocated. The laser ablation regimes depend on the laser pulse length. In the case of femtosecond pulses, resolutions of 1-2 µm are achievable without a heat affected zone.

Laser micromachining provides a new method of producing parts in a wide range of materials directly from CAD data. Laser micromachining [A_49] is a relatively new process that removes material in a layer-by-layer fashion. It is an ablation operation causing vaporisation. of material as a result of interaction between a laser beam and the workpiece being machined. The removal of material during laser ablation is affected by the characteristics of the laser beam and the workpiece but is mainly determined by the way the two interact. The most important laser radiation features are the pulse length (duration) and repetition rate (frequency). This allows the accumulated energy to be released in very short time intervals, which is what generates the extremely high power. Additionally, the laser beam can be focused on a very small spot. Thus, extremely high intensities (1013-1018 W/cm2) are achievable.

The most important substrate material features are the absorptivity in the wavelength of the transmitted radiation, and the thermal conductivity of the substrate. Thus, material transition energies such as the latent heat of melting and the latent heat of vaporisation are most significant. This is demonstrated by the fact that metals melt more easily than ceramics but are considerably more difficult to vaporise and consequently respond less well to laser ablation. As previously mentioned, when laser radiation impacts on the substrate, electrons in the latter are excited by the laser photons. This absorbs the energy of the photons and generates considerable heat which is transferred to the material lattice in picoseconds resulting in very high temperatures that can create local melting or vaporisation. Energy loss through electron heat transport to the bulk of the substrate is undesirable as it can raise the temperature of the surrounding material and create heat-affected zones. However, for very high intensities, which can be achieved by laser processing, non-linear effects take place and become a factor for stronger energy absorption. In the case of extreme intensities, as in ultrashort pulse ablation, the bond electrons of the material can be directly dislocated. The laser ablation regimes depend on the laser pulse length. In the case of femtosecond pulses, resolutions of 1-2 μm are achievable without a heat affected zone.

Laser ablation is a cost-effective process for manufacturing small batches of parts. It allows parts with complex shapes to be produced without the need for expensive tooling. Laser milling is most suitable for

machining parts with one-sided geometry or for partial machining of components from one side only. Complete laser milling of parts is also possible but difficulties in accurately re-positioning for additional set-ups have to be addressed. The influence of the process parameters is complex and must be optimised to obtain the highest part quality. [A_48]

[A_48]     A Comparison between Microfabrication Technologies for Metal Tooling, L. Uriarte, A. Ivanov, H. Oosterling, L. Staemmler, P. T. Tang  and D. Allen. Multi-material Micro Manufacture, W. Menz & S. Dimov 200X, pp. 1-7

[A_49]     Pham D.T., e.a. Laser milling as a rapid micro manufacturing process. Journal of Engineering Manufacture, vol. 218, January 2004.

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C2.2.1 CVD

Technology suitable small quantity production

Chemical vapor deposition (CVD) is a chemical process for depositing thin films of various materials. In a typical CVD process the substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile byproducts are also produced, which are removed by gas flow through the reaction chamber.

CVD is widely used in the semiconductor industry, as part of the semiconductor device fabrication process, to deposit various films including: polycrystalline, amorphous, and epitaxial silicon, SiO2, silicon germanium, tungsten, silicon nitride, silicon oxynitride, titanium nitride, and various high-k dielectrics. The CVD process is also used to produce synthetic diamonds.

A number of forms of CVD are in wide use and are frequently referenced in the literature.

·         Atmospheric pressure CVD (APCVD) - CVD processes at atmospheric pressure.

·      Atomic layer CVD (ALCVD) - A CVD process in which two complementary precursors (eg. Al(CH3)3 and H2O) are alternatively introduced into the reaction chamber. Typically, one of the precursors will adsorb onto the substrate surface, but cannot completely decompose without the second precursor. The precursor adsorbs until it saturates the surface and further growth cannot occur until the second precursor is introduced. Thus the film thickness is controlled by the number of precursor cycles rather than the deposition time as is the case for conventional CVD processes. In theory ALCVD allows for extremely precise control of film thickness and uniformity.

·         Low-pressure CVD (LPCVD) - CVD processes at subatmospheric pressures. Reduced pressures tend to reduce unwanted gas phase reactions and improve film uniformity across the wafer. Most modern CVD process are either LPCVD or UHVCVD.

·         Metal-organic CVD (MOCVD) - CVD processes based on metal-organic precursors, such as Tantalum Ethoxide, Ta(OC2H5)5, to create TaO, Tetra Dimethyl amino Titanium (or TDMAT) to create TiN. MOCVD is also called as MOMBE when it is under ultra-high vacuum.

·         Microwave plasma-assisted CVD (MPCVD)

·         Plasma-enhanced CVD (PECVD) - CVD processes that utilize a plasma to enhance chemical reaction rates of the precursors. PECVD processing allows deposition at lower temperatures, which is often critical in the manufacture of semiconductors. See also Plasma processing.

·         Rapid thermal CVD (RTCVD) - CVD processes that use heating lamps or other methods to rapidly heat the wafer substrate. Heating only the substrate rather than the gas or chamber walls helps reduce unwanted gas phase reactions that can lead to article formation.

·         Remote plasma-enhanced CVD (RPECVD) - Similar to PECVD except that the wafer substrate is not directly in the plasma discharge region. Removing the wafer from the plasma region allows processing temperatures down to room temperature.

C2.2.2 PVD

Technology suitable small quantity production

Physical Vapour Deposition (PVD) for micromachining of metal is mainly done by sputtering. It consists on the application of metal layers of thickness between 25 nm to 5 microns. Metals like Cu, Al, Ti, Cr, and oxides Al2O3 are the most common ones. It is used to produce diffraction gratings, pressure sensors, and micro moulds.

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C2.3.1 Laser bending

Technology suitable for small quantity production

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C2.4.1 Resistance Welding/Soldering

Technology suitable for both serial and small quantity production

Resistance welding (together with related resistance soldering processes) utilizes an applied forging pressure and heat generated by an electric current thought the workpiece. Resistance welding combines a wide variety of techniques (spot, seam, butt, projection weling), but the most used is spot welding, in which overlapping sheets are welded by local fusion caused by current between cylindrical electrodes that clams also the components together.

Micro resistance welding is typically used in electrical and electronic industry for joining of sheet metals, e.g. for sealing of packages, wire attachment, component assembly and sheet fabrication. Resistance welding is most suitable for welding of metals with low thermal conductivity – in other case a rapid toll wear can take place. Also some limitations in the joint configuration as well as certain contamination of welding zone should be taken into account when resistant welding.

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C2.4.2 Laser Welding/Soldering

Technology suitable for both serial and small quantity production

Laser micro welding is a process to realize a precise weld joints with small distortion on small and tiny metal or plastic parts in the range of micrometers and millimeters size. The principle of laser welding is based on the heating and melting of the material caused by absorption of laser high-power radiation. For laser microwelding two types of laser sources are used: cw laser for seam welding and pulsed laser with pulse duration about 1-10 ms for single spot or spot seam welding. As laser source for welding solid-state laser (as a rule Nd:YAG laser) usually are used.

As laser welding is non-contact, no welding tolls are required and it is possible to weld versatile (and also delicate) joint configuration without contamination of the components. Very good energy focusability and controllability of the laser beam enables welds in the sub-micro range with very small heat-affected zone and heat distortion. Because of the very short welding time (few milliseconds for pulsed spot welding) laser welding is very attractive for series production. An essential requirement for good a weld quality in laser welding, as well as in other micro welding processes, is a small or no gap between joining partners.

Laser soldering, as well as other soldering processes, use filler material (solder), which in liquid state wet the materials to be joined and provide mechanically and electrically stable connections if it is solidificated. The energy for melting of solder is applied by mean of laser beam. There are three different laser soldering processes: single spot, laser line and laser spot line (simultaneous and quasi-simultanious) soldering. The typical laser source for soldering are solid state lasers and particularly diode lasers.

Because the laser energy can be focused and portioned out very exactly, the laser is especially used for soldering of the temperature-sensitive assemblies and boards. Also it is used for joining of assemblies with high thermal capacity that could be not conventionally soldered in the reflow process. Other large application field for laser soldering is the selective rework: soldering out of defective assemblies and soldering in of new components.

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C2.4.3 Bonding

Technology suitable small quantity production

Term “bonding” include two different processes: diffusion bonding and electrostatic bonding. Diffusion bonding can be performed in solid and liquid state and is based on the diffusion of materials in each other that are assisted by uniform application of heat and pressure on the parts to be joined. The most widespread use of diffusion bonding in micro technology has been for die bonding of silicon chips on the gold plated substrates, using the gold-silicon eutectic. It can be used also for plane joining of ceramics to metals, e.g. copper to Al2O3 ceramic.

C3.1.1 ECM

Technology suitable small quantity production

Electro Chemical Machining (or ECM) is a method of working extremely hard materials or materials that are difficult to machine cleanly using conventional methods. It is limited, however, to electrically conductive materials. ECM can cut small or odd-shaped angles, intricate contours or cavities in extremely hard steel and exotic metals such as titanium, hastelloy, kovar, inconel and carbide.

ECM is similar in concept to Electrical discharge machining as a high current is passed between an electrode and the part, through an electrolyte. The ECM cutting tool is guided along the desired path very close to the workpiece but it does not touch it. Unlike EDM however, no sparks are created. The workpiece is eroded away in the reverse process to electroplating. Very high metal removal rates are possible with ECM, along with no thermal or mechanical stresses being transferred to the part, and mirror surface finishes are possible.

C3.1.2 Lithography & Etching 

Technology suitable for small quantity production

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C4.3.1/C4.3.2 Microcasting/Injection Molding

Technology suitable for both serial and small quantity production

In Injection molding the polymer material is heated and melted and then forced into the tool cavity using high pressure. Usually the tool temperature is relatively low compared to the material. Metal injection molding is a recently developed method [41], where a mixture of metal powder and binder is injected into the mold.

The material solidifies under a maintained pressure before it is ejected out of the tool. In micro injection molding very low shot weights (0.01 g) and even smaller part weights have to be realized. Because of the small parts size many cavities can be implemented into one tool in this way making the process cost effective. The tool manufacture is totally relying on manufacturing technologies that can create the necessary microstructures (milling, laser machining, electroforming). An important parameter is holding pressure that influences the product quality since increased pressure improves mould filling. The most important requirement is that the mold must be evacuated before molding, because proper venting during molding is not possible from such microcavities. Although the dimensions of the products are precisely specified during the process, some deformation usually occurs after demolding. The causes include release of compression stress, temperature change, chemical post-reaction and shrinkage by sintering. However, the deformation rate does not change much in repeated operations. After molding, the demolded product is debinded and sintered, and a metal product is finally obtained. As an extension of conventional investment casting, microcasting is also possible [42].

Micro structures with wall thicknesses of 20 µm, structural details in the range of 0,2 µm and surface roughness Rz < 0,5 µm are obtainable [43]. It is possible to produce 2D, 3D and 2,5D micro products by injection molding. With this technology structural details in the order of 10-50 µm and aspect ratios of 10-15 can be obtained.

In microMIM (Metal Injection Moulding) the feeedstock material (metal plus binder) is heated and melted and then forced into the tool cavity using high pressure. Usually the tool temperature is relatively low compared to the material. The material solidifies under a maintained pressure before it is ejected out of the tool. In microMIM very low shot weights (0.01g) and even smaller part weights have to be realised.

C5.2.1 Electroforming 

Technology suitable for small quantity production

In this case it is necessary to machine the inverted features in a material that is easy to dissolve (Al7075 or Al6063), and then produce the mould by depositing nickel or copper on the surface until a suitable thickness has been reached. Since the metals or alloys grown by electroforming are deposited at relatively low temperatures, they tend to be very fine grained. The very fine-grained materials (good sulphamate nickel has a grain size between 70-80 nm) will re-crystallize when exposed to higher temperatures during hot-embossing or injection moulding. Recrystallisation will lead to an increase in grain size and a corresponding decrease in hardness. The influence of recrystallisation on the side-wall roughness in a microfluidic system is in most cases insignificant, but for replication of optical components the increase in roughness can be damaging to the performance.

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C5.2.2 Electroplating

Technology suitable for small quantity production

The process used in electroplating is called electrodeposition. The item to be coated is placed into a container containing a solution of one or more metal salts. The item is connected to an electrical circuit, forming the anode or the cathode of the circuit. When an electrical current is passed through the circuit, metal ions in the solution are attracted to the item. The result is an evenly-coated layer of metal around the item. This process is analogous to a galvanic cell acting in reverse.

C7. LIGA

Technology suitable for small quantity production

The LIGA technology comprises the processes of X-ray lithography, electroforming and moulding. The process combines photoetching and electrical plating. In the first step, a deep etching pattern is made in a photoresist after exposure to synchrotron radiation (SR) through a patterned mask. In the etched grooves, a metal is electrically plated. Finally, the plated part is demolded as a metal product.

LIGA enables the manufacture of micro-components made of non-silicon materials like plastics, metals and ceramics with almost any kind of lateral geometry and very high aspect ratios. This technique can basically produce patterns with straight walls. Since the horizontal section of the product does not change in relation to the vertical position, these kinds of shapes are often called 2.5­dimensional shapes.

Owing to the straightness and high resolution of SR which is alight in the X-ray range, a high-aspect­ratio product with high horizontal resolution of submicron range is obtainable (The LIGA Technique, Catalogue of MicroParts Gesellschaft fur Mikrostrukturtechnik mbH).

For LIGA, in most cases, PMMA is used as resist material. In X-ray-lithography almost parallel high energy synchrotron rays enable the manufacture of very deep structures (up to 1000 µm depth, lateral dimensions down to 0.2 µm, surface quality Ra 0.3 nm) with almost vertical and very smooth side walls (aspect ratios from 50 to 500) [44]. If UV light or lasers are used instead of X-rays, less impressive resolutions and aspect ratios are obtained at relatively low cost. When these structures are produced in polymers, the exposed structured areas can be filled by electroplating with different metals like nickel, gold, copper or certain alloys. Once the PMMA is dissolved, metallic micro structures are left. The metal structures produced can be the final product; however it is common to produce a metal mould. This mould can then be used for injection moulding [45].

Although the materials that can be electroplated are limited to metals, the wide freedom in patterning leads to a wide range of applications.

This technology is also used for fabrication of tools for embossing or coining of polymers, although the development of new plateable alloys with improved thermal and mechanical properties are necessary if temperatures increase above 400 °C.

LIGA competes with WEDM because they target similar types of products. Compared to WEDM, the main advantages of LIGA are:

1)         Narrower space in the pattern is possible

2)         Surface of vertical Wall is smoother

3)         Batch process can cope better with mass production

The main disadvantages are:

1)         Formation of inclined walls or tapered holes is more difficult

2)         Choice of materials is narrow, while WEDM can machine most metals and alloys