Controlled erosion encompasses abrasively drilling, grooving or milling a part. Micro-abrasive blasting is powerful enough to drill holes in ceramic substrates, yet gentle enough to cut slots in fragile silicon and glass wafers. MicroBlasting is “shockless,” generating neither heat nor vibration.
- Ring bearings – milling channels to control fluid transfer and heat buildup
- Ring laser gyros – applying serial numbers to lenses without micro-cracks
- Cutting composites
- Wafers – cutting slots, holes and apertures in thin, fragile substrates; contouring or beveling edges of silicon power devices to expose junctions
- Inkjet printer cartridges – cutting slots in silicon wafers to create ink reservoirs near the nozzle
- Wafer carriers and end effectors
- IC epoxy removal
- MEMS – machining vias in glass and silicon
- Ceramic rings – creating a dimpled pattern on the surface
- Glass engraving
- Egg lacing
- Thermocouples – removing MgO to expose contacts on heat sensing harnesses
- Cutting strengthened glass
- Nitinol tube thinning
- Mechanical heart valves – graphite removal
- Devesting crowns and bridges
MicroBlasting is controlled erosion at its best, as it provides very tight control over the media delivery process to accurately mill or etch surfaces. Controlled erosion can be performed on everything from hard ceramics to soft epoxies. Here are some key variables, requirements and notes to consider:
The blaster is not usually considered a variable in micro-abrasive blasting applications. It is taken for granted that every blaster supplies a consistent flow of abrasives. However, in order to control erosive depth to 70 nanometers, a precise abrasive flow is required. Even small fluctuations in the abrasive flow cause variation of particle velocities at the nozzle, imperceptible to the human eye, and resulting in uneven erosion of a surface.
Comco recognizes the importance of a consistent flow of abrasive and developed a patented modulator system to feed a precise amount of media into the air stream at 60 times per second.
Optimizing Media Flow
Removal rate is tied directly to the number of particles that strike the surface and their speed.
- Quantity: More abrasive improves process speed as long as each particle strikes the surface of the part, rather than the particle in front of it.
- Velocity: Increasing media density decreases velocity. Every additional gram per minute decreases velocity by one meter per second.
- Type: Aluminum oxide is the most common abrasive used on hard, brittle materials. Sodium bicarbonate is a better choice for soft material, because it cuts through the fibrous structure without “burning” the surface.
- Quality: Variation in the size, distribution and quality of the abrasive impacts removal rate. Even subtle variations significantly impact the end product. A bottle of carefully-sized sodium bicarbonate is not the same as your supermarket baking soda or the industrial abrasive supply equivalent.
Controlled erosion applications typically require automation. The human operator becomes a major source of variation when MicroBlasting to remove nanometers of material. Automation greatly extends the capabilities of MicroBlasting.
What Affects Surface Finish?
- Substrate properties: The most significant variable is the composition of the substrate itself. Crystalline surfaces erode evenly and fracture along crystal interfaces. On a sintered material, the ceramic and the binder may erode at different rates. When this happens, it is common to see the ceramic “pluck” from the binder, creating a large crater. Over time this plucking will result in a wavy surface finish.
- Particle velocity: Higher pressure yields faster removal rates, but creates a rougher surface.
The vast majority of controlled erosion applications are done with automation. By automating the parts handling component of the system, much tighter tolerances can be achieved, in some applications, as tight as 0.5 microns. Automation is almost always required to etch pockets into a substrate.
There are two approaches that can be used with automation: direct machined and masked.
A small nozzle and accurate parts-handling capabilities cut a pattern into the substrate. The focused abrasive stream avoids overspray, eliminating the need for a mask and reducing abrasive consumption. This makes direct machining ideally suited where large, simple sections of material need to be removed.
A metal or polymer photoresist mask exposes only the areas that require blasting. The nozzle is swept back and forth over the entire mask to erode fine layers of the exposed surface. For this reason, masking is common in applications with many small features.
The above approaches, direct machined and masked, can be applied in manual applications when tolerances are not critical or volume is not sufficient for automation.
Bearing surfaces must run cooler to meet the new efficiency requirements in jet engines. Etching precision channels into the surface of the ring creates an air bearing. Because these bearings are made from hard, brittle materials, etching a flow path with a tolerance of 0.5 µ presents a significant challenge.
MicroBlasting, however, is surprisingly well-suited for this requirement, as it provides tight control over particle velocity to accurately control etching depth. Aluminum oxide, the ideal abrasive for this application, is only slightly softer than diamond and able to cut through these hard surfaces easily.
Comco’s distinctive automated systems are able to monitor the abrasive stream for both quantity and consistency. These measurements are fed back into the system to keep the rate of etching constant.
MEMS and Microfluidics
As technology advances, more silicon wafers incorporate both semiconductor and mechanical features. The manufacture of ink jet printer cartridges is a good example of this convergence. The via, or hole, that connects the reservoir of ink to the individual nozzles is now built into the same silicon wafer that manages the signal. To integrate this, a via must be drilled into the middle of each wafer segment. The shape of the via is critical and must ensure a smooth flow of ink to each nozzle.
MicroBlasting cuts an organic shape, improving the fluid dynamics of the ink flow. The blaster drills into the wafer without generating heat or vibration, limiting the formation of micro-cracks.