push the "bubling" and it springs back. It does not look like solder mask. We may have to fight with board house as they don't think it's their problem. Do you know any way to find the real reason? Any idea or help will be appreciated.
We had the exact same problem in early stages of lead-free. Very random positions of defect + some boards were 100% O.K. and others were an explosion of what I see in your picture. Turned out to be bad processing/materials from board house. I'm don't remember exactly what the process problem was called but we ended up going to a different material and have not had a problem since.
2) STD FR4 material with HASL finish, 4 layer board. Reflowed under tin-lead temps with another 4 layer board same time(no delamination problem).
3) We uasualy do not pre-bake board unless board is too old.
4) We do not have the board on hand right now so can not check date code. The board is about 2-3 weeks old. After we found delamination problem, the board house brought us a same board from their stock and reflowed it under same profile, got same problem.
1) Was the Pre-preg material that was used on this board within its Shelf life? and ask for the Incoming inspection CofC for the Pre-preg material. 2)Was this order shipped from Stock - this is why I asked about the Date code. FR4 material can suck up moisture as its sits in and uncontrolled environment.
From the looks of the board I see some dry weave in the left edge of the board and more on right side of board, which would lead me to believe that boards had resin starvation issues. Could be caused by old pre-preg material or wrong Press cycle selected. Pic's attached. You should not be able to see the weave of the material this easy.
Was this the first time that you had De-Lam on this Part number? if not you may want to change the stack-up of the board and go with Higher resin pre-preg or go with 2 ply pre-preg. This would remove any resin starvation issues. Just a suggestion.
I would say this issue is a board supplier issue not Assembly.
We have seen this exact same problem. Our vendor agreed it was their problem and also agreed it was a moisture issue within the PCB layers, not the solder mask. We have taken to baking our boards, especially when they are manufactured during the late spring and summer when it is humid.