MICRO ASSEMBLY/HANDLING
05/14/2012: The preparation done to micro components at the design stage carries through all the way to automated assembly. Because the tolerances are critical and diminishing to microns and sub-microns, the stack-up tolerances of the micro components and their design criteria are scrutinized and their method of manufacturing are also considered for long-term production. The mechanics of micro assembly and the testing, inspection, application knowledge are required for fully functioning specks of dust working collaboratively and repeatedly. Handling these components is a challenge requiring its own article alone but some methods to handling are tiny vacuum suction cups, static-free gloves, and sometimes with tiny tweezers (See Figure 1.6). Below are some of the key joining methods for assembling micro components into micro assemblies.
- AVOIDANCE OF HANDLING: Because these parts are microscopic, the best way to handle them is to not handle them at all. Combining geometry into the least number of parts in a micro assembly is design work worth the effort because picking them up, registering them in nests, joining them to other parts of like or unlike materials can be much more costly than spending the time in the design stage.TWO-SHOT
- MICRO MOLDING/OVER-MOLDING: Two-shot molding (2 injection barrels of two different materials) injecting into a mold at two different locations or in the same location with a rotating mold enables one “set up” of combination geometry and materials. For example, if a pump component requires a seal (or silicone gasket), it is easier to 2-shot mold a gasket in place in an o-ring groove into the same mold as the pump piston than it is to bowl feed an o-ring onto s precision tracking mechanism, grip the o-ring with some scissor grippers and place it over the piston. Firstly, the o-ring is subjected to stretching and shape distortion with the scissors, bowl feeding, and riding on a track of an assembly system.
- LASER WELDING: If 3d geometry isn’t possible to combine, and material strength allows for it, laser welding is a good method for joining micro components. Careful control of laser energy and power densities can be used to selectively clean and strip materials such as wires quickly and non-destructively. Laser beam sizes in the sub-micron and below size range and therefore pinpoint accuracy of multiple laser welds is possible using a nano-positioning table or a multi-axis robot station mounted on the automation rotary or shuttle system. Laser welding can be done using existing wall thickness or an extra “tab” designed into the singular component being welded OR a small amount of similar material can be introduced using wires, etc as is done with soldering in electrical components.
- ULTRASONIC WELDING: There is a time and a place for all joining methods and ultrasonic welding is a good method to join thermoplastics as well as material compatible metals. Because of the size of the parts, spin welding is usually not a good option due to the amount of wall thickness in micro sized parts, however a very tiny (100-200 um) weld bead is sometimes possible and plausible. Specialized and low energy boosters are required for micro components as the amount of energy required for a strong weld is extremely low. Customized ultrasonic horns are also used to dissipate and distribute the correct “tuning fork” focused energy.
- SOLVENT BONDING: Solvent bonding is often used as a quick, low capital investment method for joining micro components in an assembly. Quick and dirty fixtures can easily test the feasibility of multiple and dissimilar materials in a micro assembly using micro and nano pipettes to distribute the tiny amounts of solvents. Appropriate solvents must be used that are compatible with the materials being solvent bonded, especially if the assembly is to be used as an implantable. Although quick and capital-friendly, ramping up a very high volume component/assembly using solvent bonding can be headache-ridden as the method is not easily automated and repeatable, is messy, and difficult to validate on a high volume scale as a result.
- STAKING: Micro staking is a very inexpensive method to join polymer and metal components. In battery cans, for example, crimping or staking is a very common practice to generate good seals for preventing very caustic fluids from escaping the battery vessel. Inexpensive progressive stamping dies enable a moderately fast method for staking both polymers and metals to one another using a pressurized “folding” of one material into another. Cons of this method are that material lot variations ramping up to high volume
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