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by bananas
4482 days ago
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Yes exactly. This happens all too often unfortunately. It's why you didn't see anything BGA packaged in the defence industry for a number of years -- they are not mechanically stable. I did have a reference for this but I can't find it now. Also the multi-layer boards tend to bend when you repetitively heat/cool them resulting in the actual metal traces cracking inside. Sometimes there's enough contact after this oven cycle for it to reconnect BGA packages and board traces semi-reliably but like hell I'd rely on this method for long-term stability. I did a spell post-university reworking things that pick and place machines had screwed up and it was pretty much entirely packages like BGAs where there were arrays of solder connections. The production guys were always returning prototype devices due to mechanical problems on the boards as well and they were coming back with socketed LGAs and soldered PGAs. |
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There is also a problem with rework. BGAs are hard to get off the board without destroying the board in the process, especially on multi-layer boards. For cheap commercial boards where the automatic decision is to scrap the board when the chip fails, that's fine. For $10k+ circuit card assemblies on a low volume defense production line, scrapping the board is a last resort. This applies to production defects as well as field returns.
> Also the multi-layer boards tend to bend when you repetitively heat/cool them resulting in the actual metal traces cracking inside.
Another problem is delamination (separation of the board layers). Delamination allows contaminants to get in on the traces and possibly start shorting things out. It seriously degrades the reliability of the board. That was the biggest problem for us when trying to rework CCAs. We had no BGAs, but we did have a card that used a few parts with thermal pads on the bottom. It took heat from both sides of the board to get the chip off, and it was very easy to apply too much heat and delaminate the board in the process.