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Immersion Silver Thickness >30microinches

Several immersion silver publication discuss process effects of Ag>30micro-inches. Higher silver deposits contribute to micro-voids @ the intermetallic layer of the solder connection. Another phenomenon refers to flux vehicle in paste reacting with high silver and retarding flux outgassing (resulting in voids). These types of defects impact solder joint strength. According to Dr. Lee (Indium Corporation), another phenomenon can occur with fillet lift from solder pad. Whereas, the component and fillet de-tach from PCB land. This defect is know as "Chocolate Chip Cookie on Wax Paper." Basically, the solder fillet stays intact with component termination and lifts off PCB leaving a smooth finish indicating solder paste was present. I was told if immersion silver does not dilute completely into solution during Reflow a false fillet can form to the immersion silver, not the underlaying copper. I refer to this phenomenon as the pop-corn syndrome when component is subject to post process stresses or additional thermal heating. Has anyone else in the industry encountered this pop-corn phenomenon? If so, has immersion silver thickness >30micro-inches been a lead contributor?

James Hall
TRW Automotive Electronics - Auburn, New York
2006

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Dear Sir,

I am familiar with the problem you call "popcorn effect." My company plates printed circuit boards (rigid and flex). During our prototype of plating printed circuit boards in a continuous plating line, we found that having immersion silver > 0.000015" created poor adhesion and caused solder issues for our customer who sells them to Ford. We recommend that you use an electrolytic silver instead to eliminate this problem. This way you can pulse plate and create interrupted grain boundary structures to help stop problems from occurring during the solder phase.

Hope this helps!

Best Regards,

Stephen Sharretts
- York, Pennsylvania
2006