Is it showing up after wave or reflow soldering, or after hand soldering?
If the delamination or blister bridges between traces you have the potential for a short circuit between those traces. A delamination might be repairable, but in most cases it is cheaper to salvage any high cost parts and scrap the board.
Too much heat from a soldering iron can cause a delam or blister around the spot where the iron was touching the PWB. If it showed up after wave or reflow soldering that indicates moisture was absorbed into the board somewhere along the way, at the PWB manufacturer, while in your stockroom, or while the PWB was out on your production floor waiting to be soldered.
A short term fix is to bake the bare PWB to dry it out before you start to work on it. Check the archives for bake profiles, DaveF has some good comments.