Yehuda:

Polyimide boards are very hygroscopic, they absorb moisture very easily and 
quickly. It is the moisture trapped inside the board which rapidly expanded 
during your reflow process which caused your delamination and measle 
problem.  As a matter of normal shop floor practice, we always bake 
polyimide boards prior to any reflow process whether it is reflow for 
surface mount and/or wavesoldering. Ideally, when the board cools to room 
temperature, the board is processed. For surface mount, the board is cooled, 
then screen printed and reflowed immediately. For wavesoldering, we bake the 
board with components in place, removed, allowed to cool approximately 30 
minutes, then wavesoldered.

You must be aware of what your relative humidity is at the time of 
soldering. If the relative humidity is greater than 75%, it may be 
impossible to relow since the board will re-absorb the moisture you have 
just baked out !

The process parameters generally speaking for baking  range from 2.0 hours 
at 250F to 6.0 hours at 150F. (Note this topic has been debated for years, 
some feel that in order to dry properly you must bake over the boiling 
temperature of water. While others believe that board solderability is 
adversely affected when baking at higher temperatures. It is up to you to 
determine what drying profile you want to use. I've had success with all 
different ranges of settings, the important thing is to dry.

Following these steps will eliminate all of your delamination and measle 
problems

Bill Kasprzak
Moog Inc.
716-652-2000 ext 2507
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