Jeff;
There are a number of possibilities that could cause this problem, one of
which is contamination.  The problem you are describing sounds as though the
LPI is not completely processed, which could be another area of concern. Not
knowing the conditions of processing your particular LPI mask, it appears
that the wave temperature is relaxing the the polymer above its Tg (plastic
flow region) in order to obtain additional adhesion. There could still be
traces of moisture in the mask, some solvent retention, incomplete UV
crosslinking and/or improper final processing, which could include a final
bake or a final UV cure or both. The fact that the adhesion improves
indicates the latter as the possible cause. This can be an intermittent
problem depending again on the processing conditions and how tightly the
procedure is adhered to. This does not necessarily mean the LPI mask is
questionable nor does it mean the board manufacturer's procedure is
questionable. It is only indicative of the tight process control required
for the mask using the manufacturer's equipment.  Another possibility is
absorption of moisture on storage and I'm sure you will hear of others.
If you have questionable boards I would suggest baking them prior to
populating.  Bring the boards to at least 100 Deg. C. for 30 min. to drive
off any moisture and 150 Deg. C. if there is entrapped solvent. Then test
for flaking. You can also use the SM-840 C procedure for testing for
adhesion, if it is still applicable.
I hope I have given you necessary information to help you in determining the
cause of the problem. If you do determine what the actual problem is, I
would appreciate knowing how you resolved this issue.

Fred Shubert
-----Original Message-----
From: Jeff Hempton <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Wednesday, September 30, 1998 1:44 PM
Subject: [TN] solder mask (LPI) flaking


>     Fellow Technetters,
>       On some of our bare boards before build, with normal board handling,
>     (before ANY processing has been done) we are noticing that the solder
>     mask is flaking off. This condition is no longer there after wave
>     solder. Our impression is that the mask is not fully cured. Are there
>     other things that could cause this (contamination)?
>       Is there a test that we can perform on the bare boards to prove/test
>     this?
>     Any thoughts would be appreciated!!
>      Thanx in advance, group!
>
>     Jeff Hempton
>     Sr. Mfg. Eng'r
>     United Technologies Electronic Controls
>     email: [log in to unmask]
>
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