As I am unsure how to correctly address you, I hope you will not mind me
referring to you as Lum.

I think that your problem is probably related to coating material that is
under the power module. Despite this being mounted flush to the board, the
coating material generally exhibits a low surface energy and/or high level
of capillarity.

Because the power module probably has a high thermal efficiency, I would
assume that there is uncured material underneath. What you could be seeing
is residual solvent gasses that have pushed through the coating material.

It could be trapped air, but I doubt it.

Solution: Try an extended heat cure on the coating for 24 hours at 80 deg C,
but leave sufficient time for the coating to "rest" before putting in the
oven - yeh! - Its a bit like baking bread!

If you could let me know what the coating material is, I may be able to give
you more info.

Regards, Graham Naisbitt

[log in to unmask]

Concoat Ltd             Phone: +44 1276 691100
Alasan House          Fax:      +44 1276 691227
Albany Park
CAMBERLEY
GU15 2PL UK

Any opinions expressed are those of this employer.

-----Original Message-----
From: Lum Wee Mei <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 01 September 1998 03:29
Subject: [TN] Bubbles from conformally coated board


>The test engineer came back to us to complain that bubbles are found
>along the edge of the power modules after going through the temperature
>cycling tests. This problems are found on both coated boards. The power
>modules are mounted completely flushed onto the board and subject to
>conformal coating using the diping method. The coated board undergo QC
>to ensure all components are completely coated, inlcuding the edges
>around the modules.
>
>What could be the problem?
>
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