Look for Eric and Paco at APEX in Las Vegas. They will be participating in Standards Committee meetings and hanging out on the show floor.

Eric Camden will be presenting, "Relationship of Via Size and Cleanliness," at APEX. His presentation will be on April 7, 2010 at 3:00 p.m., session 25.

His presentation will explore multiple via sizes, different approaches to cleaning, and, based on the cleanliness data, what might be the most viable option for producing good performing product.

Selective Coating Removal

It is not uncommon at Foresite for our staff to be challenged with new product issues by our customers. This past year we were approached by a customer whose product had been coated in paralyene. After a short time, less than a year in the field, the paralyene was discoloring and adversely affecting product performance. Complete removal of the paralyene was not an option since specific areas of the product required the coating.

Having been successful in developing methods for stripping conformal coating, we looked at similar ways to remove the paralyene. These methods were quickly ruled out, due to the inability to control which areas were stripped. As we narrowed our options, plasma cleaning became the most viable choice. We began experimenting with recipes for the plasma oven and designing fixtures to protect the areas where paralyene needed to be retained. After fine tuning the process, plasma cleaning has proven to be extremely effective. We have seen 100% removal from specified locations while maintaining the integrity of the remaining, coated areas.

More recently we have been experimenting with plasma cleaning to increase the solderability of surfaces. A customer had product that had been stored and unused for over five years. The assemblies had oxides in vias and on pads, resulting in areas that were not conducive to soldering. Utilizing a short-dwell plasma exposure, the oxides were removed, surfaces were solderable and product was fully recovered.

Cross sectioning allows for in-depth analysis of the inner layers of PCBs, PCBAs, and components, catching weaknesses that are almost impossible to pinpoint. A cross sectioned sample can effectively reveal stack-up structures, PCB plating thickness, solder coverage, and conditions affecting wetting.

Traditionally, an area is cut from the board and encapsulated in a 1" diameter epoxy puck. The specimen is then rough cut and repeatedly polished until the area of interest is exposed and can be inspected, photographed and reported upon. An alternative to the puck is a three-point fixture. Utilizing the fixture, we are able to easily and quickly focus on the area of interest, commonly concentrating on an area as small as two or three solder balls on a BGA. The fixtured sample still requires polishing; however, there is less material to remove allowing for faster processing. Both methods can produce satisfactory results.

There are still instances when the puck should be the method of choice; however the three-point fixture is a more efficient way for us to perform cross section analysis, when applicable.