Hi Phil

In your message dated Sunday 29, September 1996 you wrote :
> In another case of a ml, measles that were attributed to moisture absorption
> was not eliminated by a 4 hour bake at the supplier and a 4 hour bake at the
> user at 125 deg C  just prior to loading.  However, a 1 hour bake at 10 deg C
> above the Tg of the material solved the problem, the Tg of the material 145
> deg C. (Stress relief, not moisture removal, was perhaps the function of the
> bake).
> 
This would seem to indicate that the resin was not quite fully cured and 
resulted in the free water molecules being left over from the partial 
polymerisation. The full Tg + 10C bake completed the cure and eliminated the 
problem. 

The temperature at the center of the stack will reach the critical point some 
time after the platen, and the best way to calibrate this is with a panel 
fabricated with a thermocouple at the centre and each corner to measure the 
temperature profile placed at the centre of a stack.

We still bake all multi-layer panels after pressing to ensure full 
polymerisation and to relieve stress. This may be a belt and braces approach but 
we do not suffer any problems with measling on 0.125" 6-layer backplanes. 
For HASL, these are typically immersed in solder at 250C for 6 seconds with no 
pre-heat and can withstand several cycles without delamination or measling.

The inner layers should be baked before pressing at low temperature to ensure 
they are dry. Care is needed here as the bake will re-oxidise the reduced oxide 
layer so it must not be too hot or too long.

Another point is to store the pre-preg in as dry an atmosphere as possible since 
it really soaks up water. If the pre-preg is high in moisture you will see 
excessive resin flow and possibly foaming around the edges.

If the resin is fully cured and there is no moisture in the material when 
pressed, I do not think moisture absorption in the finished panel is significant 
enough to cause measling or delamination, although it could cause outgassing in 
less than perfect holes.

Hope this helps,
Regards 
Paul Gould
Teknacron Circuits Ltd
EMail [log in to unmask]

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