Cleaning up after laser processing
The cleanliness of laser-processed parts is of paramount importance, especially when producing medical, semiconductor and aerospace components.
But even with “clean” laser processes, such as those performed at ultraviolet wavelengths and with short pulses, some debris usually remains on the workpiece. That means some post-lasing cleaning likely will be needed.
Laser-etched glass before (left) and after ultrasonic cleaning. Note that the debris is almost entirely removed, but “chip-outs” of the brittle glass remain. It is possible to minimize, but difficult to eliminate, this effect without applying a sacrificial layer or performing a post-laser grinding process. Photos courtesy PhotoMachining.
Following are some common ways to clean lased parts.
Physical scrub. A gentle scrubbing is the simplest, most economical method. This is a contact approach, so extreme care must be taken when choosing a tool and scrubbing solution—if used—and how aggressively and long the part is scrubbed. Cotton swabs, clean-room wipes or soft brushes normally are applied to the small and delicate parts manufactured by laser micromachining. Typical solvents include acetone, methanol, ethanol and water (mixed, perhaps, with a cleaning surfactant).
Common, inexpensive items can sometimes be used with great success. For instance, a pencil eraser is handy for cleaning carbon and debris from the contact pads on printed circuit boards. A piece of Styrofoam is also good. Aside from man-hours, these items cost almost nothing to use.
Ultrasonic. An ultrasonic cleaner works via ultrasonic vibration, usually from 15 to 400 kHz. It’s often employed for delicate items, like optics, precious metals, surgical instruments and electronics.
In operation, the object to be cleaned is placed in a chamber containing a suitable conducting fluid, either an aqueous or organic solvent. An ultrasound-generating transducer, either built into the chamber or lowered into the fluid, is electronically activated to produce ultrasonic waves. The cleaning action is produced by the energy released from the formation and collapse of microscopic cavitation bubbles, which gently break up and remove contaminants from the part surface.
An ultrasonic cleaner can reduce or eliminate dependence on the harsh chemicals often found in industrial cleaning applications. Ultrasonic devices for the home and hobbyist are readily available for as little as $50. Industrial units cost anywhere from a few hundred to a few thousand dollars, and the operating costs are very low, assuming no exotic fluids are used.
Electropolishing. With this method, a metal workpiece immersed in a temperature-controlled electrolyte bath is connected to the positive terminal (anode) of a DC power supply. A negative terminal is attached to an auxiliary electrode (cathode).
When electropolishing a rough metal surface, the protruding sections of the profile dissolve faster than the recesses. This desired behavior effectively smoothes and cleans the surface. The total amount of dimensional change required to obtain the polish effect is very small (about 0.00025").
Plasma. Plasma cleaning eliminates wet processing while reducing costs related to chemical disposal, water usage and water treatment. Labor costs are lower, too, because there are no baths to maintain.
This method is primarily used with organic materials, such as those used to make semiconductor wafers, and to strip resist from printed-circuit-board panels and flex circuits.
The surface to be cleaned is placed in a vacuum chamber, then gas is introduced and converted to a reactive plasma by a power supply. The plasma reacts at the surface, and volatile byproducts are removed by the vacuum pump. The addition of a relatively inert gas, such as nitrogen or argon, stabilizes the plasma and controls the ionization rate. Reactive oxygen species oxidize organic contaminants on the surface, creating volatile species that are pumped away.
Sacrificial coating. Another method is to cover the material being processed with a second material, like a film, that is washed off post-lasing along with any generated debris. Soapy water is the simplest sacrificial coating to apply. The operator dips the parts in soapy water, lets them dry and then, after processing, rinses them.
Another method is to apply a photoresist to the surface and rinse after processing.
Both cleaning methods are capable of removing loose debris generated during the laser process, but care must be taken to ensure that the parts in question are not compromised by the coating or rinsing process.
It should be noted that post-laser cleaning of microparts can be risky because, in some cases, the cleaning technique could cause a much bigger problem than leaving the debris on the part surface. For instance, care must be taken when electropolishing to ensure that via diameters are not enlarged by overexposure.
The risks involved in cleaning usually dissuade a contract manufacturer from cleaning laser-processed parts. If the shop does agree to do it, the cleaning technique used is well-documented and established procedures are rigorously followed to ensure delivery of the proper end product. µ
About the author: Ronald D. Schaeffer, Ph.D., is CEO of PhotoMachining Inc., a high-precision laser job shop and systems integrator in Pelham, N.H. E-mail: [email]email@example.com[/email].