Hi Jim -
A mild bake to dehumidify a board isn't unreasonable, but some of the times
and temperatures you are mentioning are a mite to the extreme. Bake cycles
such as these can promote a fair amount of surface oxide formation and worst
of all, considerable intermetallic growth at the copper/tin interface. None
of this is going to improve solderability, and it may have a very negative
effect in the long term. Ideally, find the cause, eliminate it, then reduce
or eliminate the bake and get on with life.
There are several reasons why boards delaminate, water type moisture being one
of them. The others include: solvent type moisture, which often tends to
cross link with the resin; poor surface prep on the internal layer surfaces;
improper storage or preconditioning of the prepreg layers; and incomplete cure
of the laminate structure.
I would expect the board fabricator to have tight control on his prepreg
handling. I'd also expect to see a post laminate bake in his process, just
after final cleaning and prior to solder mask (if used).
May I suggest you invest in some DSC or TMA testing on samples of the boards
in the "as received" condition? If the problem is moisture or resin cure
state, this testing should help isolate it. The laminate manufacturer, via
the board supplier, should be able to provide Tg data and standard DSC or TMA
plots of his materials behavior to correlate with the test results.
Encapsulated microsections of the delaminated area can tell a lot about its
nature as well. At what elevation did the delamination occur (prepreg,
laminate, metal surface, etc.)? Did it delaminate between two plies of glass
mat or did it tear up a glass mat?
The next area to take a close look at is how the boards are being handled and
shipped. Reusable vapor barrier containers, bags, etc., can go a long ways
toward avoiding the pickup of environmental moistures. This is particularly a
problem in coastal areas and the southern states. Got to remember that board
is honeycombed with micro-capillaries that will pick up moisture fairly
rapidly if allowed. There can be a lot of available moisture in trucks,
shipping docks, stock rooms, etc.
Good luck - Kelly
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