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PCB007-July2019

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58 PCB007 MAGAZINE I JULY 2019 Feature Interview by Andy Shaughnessy I-CONNECT007 I recently spoke with Foresite CEO Terry Munson during the IPC High-Reliability Forum and Microvia Summit in Baltimore. Terry dis- cussed his presentation on the causes of con- ductive anodic filament (CAF), the dangers of resin starvation, and what advice he'd give to PCB designers to avoid those types of failures. Andy Shaughnessy: Can you give us a rundown of your presentation? Terry Munson: Thanks for the opportunity to share. I talked today about CAF and what's causing those types of failures. We're seeing an increase in those type of failures in auto- motive, medical, and even some of the server board construction. Class 2 and 3 hardware are both experiencing similar issues. And the dif- ference between CAF and electrochemical mi- gration boils down to this: CAF is inner layer, and the electrochemical migration is external. Shaughnessy: I thought the dendrites and CAF were almost interchangeable. Munson: Yes, dendrites. We've seen a lot of external failures; it's harder to find those in- ternal failures when things are shorted and when there are two vias that are shorted to- gether or to the ground plane. Dendrites grow on the surface due to flux residues, board fab- rication residues, external moisture, and mi- cro-condensation—droplets of water sitting there. Internally, the only thing you have as a contaminant source is the etching materi- als for the prepreg, the desmear process, and then the plating process. In the barrel itself— if the resin flows well and there isn't any resin starvation in the weave—you have no place for that plating chemistry to go, so it just gets flushed out and is relatively clean behind the barrel. When you have resin starvation, and fiber- glass is exposed, that fiberglass bundle be- comes a collector. All that low surface tension plating chemistry works right in. Now, when you plate a big copper barrel right next to that, the chemistry is going to do what chem- istry is designed to do: dissociate the copper Avoiding CAF Failures at the IPC High-reliability Forum Terry Munson

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