Indium Corporation - Mailchimp

BGA Red Dye Penetrant Testing

Ok, so it happened again. Another urgent request was brought to me for action. This time it was a customer who had performed a Red Dye Penetration test on a Ball-Grid Array (BGA) that was attached to a board using our paste. A single BGA on a single board was the evaluation (Sample size is a WHOLE other topic of conversation!) The picture is one that I took of the pad that showed no wetting. There are a number of causes of why the paste did not wet only these two pads. Including poor paste transfer, BGA contamination and board contamination. Paste transfer is certainly an issue that is at the forefront of every BGA issue. The first issue that I think of is a contaminated stencil. If their cleaning process were incomplete or sloppy, then this would definitely cause some issues. Especially if the stencil apertures were not completely cleaned out or if the solvent used was not completely removed. Solvent left in the apertures when paste is introduced would wreak havoc on reflow. But, in this case there was sufficient paste printed, because the analysis of the solder joints confirmed that the spheres of the good joints were the same volume as the suspect BGA solder joints. Had the paste had issues with transfer efficiency, those spheres would be comparably undersized. As with Head-in-pillow, the solder spheres on the component may also play a role in the situation, where again, the increased silver content or higher than normal oxide contamination would hinder good wetting. But, would this have shown itself as non-wetting to the pad? Probably not, but rather as head-in-pillow, or a "cold" solder joint. So, where does that lead us? Directly to board manufacturing and storage, or direct pad contamination. Board manufacturing issues would show itself, I believe, more widespread than just two localized pads. Especially since these are regular production boards. This is the same for poor board storage, where an oxide layer had may build up on the pads. But, again, it would probably be more widespread. Therefore, my money is on contamination. We have all been to production facilities, and very rarely do I see any of the operators using gloves. In fact, at one facility, where they were placing solder preforms by hand, we brought up the idea of using gloves. Their defect rate dropped to <1%. As a matter of fact, we do not permit the wearing of silicone bracelets, as the silicone has shown to rub off and contaminate some testing. The silicone actually prevents the wetting of the flux and solder! Even our body's natural skin oil is an inhibitor to wetting. Which is exactly what I believe happened here. More information may be found at Online Help: Indium Knowledge Base.