QFN / BGAs with Water-Soluble Fluxes, Bad Practice?

In my experience, before my current employment, I have personally seen damage from the water soluable fluxes left under a BGA. In consideration, I have not seen it from an assembly process where the boards are cleaned thoughly, but I have seen it during the rework process. The flux was not thoughly cleaned from underneath. When the board started failing months later, the part was removed again. There was visable damage to the traces and pads below the BGA. It looked like acid had been spilled on it. It was not the first or the last at that place of employment. I ended up removing all the water soluable from the area as I did not want to be responsible for other persons cleaning under the BGA's. Water soluable is something you must make sure that the cleaning process is very though.

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It is very common to use low residue flux with low-standoff parts.

On the other hand, sure you can clean under low-standoff parts. We've talked about this previously. Search the fine SMTnet Archives to find threads like: http://www.smtnet.com/Forums/Index.cfm?CFApp=1&Message_ID=27716

Taking a different angle, here's a paste of an oldy but goody approach suggested by our old friend Steve Gregory: If you wanna get a "warm n fuzzy" that your cleaner is capable of doing the job. One thing you can do is what I was taught when selecting a cleaner to buy, is get yourself some glass plates (heat resistant tempered glass to be sure!) and make yourself up some glass slugs, or chips if you will, that are the same size as the largest BGA you think you'll do. Glue them to the glass plate at different stand-off heights and squirt some colored flux beneath the slugs. Send it through a reflow oven to bake it some, and then it's bathtime! Then just take a gander at what the plate looks like after it comes out of your cleaner. That'll show you without a doubt, what your cleaner is capable of. [S Gregory]

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