Difficult to tell for sure, but it looks to me like insufficiently 
cleaned flux residues or attack by them of the substrate. What kind of 
flux and cleaner/process are you using? Polyimide (not polyamide) resins 
are chemically very tender and hygroscopic, compared with FR-4 and they 
are especially sensitive to basic products including saponifiers, 
particularly if their polymerisation is not perfect. OTOH, the low 
pressures of the Parylene process would ensure that there were no 
volatiles present at the time of coating.

In short, there is something that is preventing a good adhesion and my 
best bet is that there are flux residues (perhaps invisible) either on 
or in the polyimide surface.

Brian

Juan T. Marugán wrote:
> Good evening from Spain.
> 
> The problem we have is the following. After soldering, we conformal coating a 
> class 3 polyamide board (blind vias) with Parylene C. Until this moment, all is 
> OK.
> 
> Then, the board pass the ESS -thermal cycling-, and it is at this moment 
> when appears a halo around the solder joints. The test tape performed in that 
> area fails. I�ll send some pictures to Steeve Gregory for reference.
> 
> We have coated at the same time (and processed in the same conditions, 
> that is soldering, cleaning�) the �suspicions� polyamide boards with other type 
> of boards (polyamide & rigid). We only obtain negative results with the first 
> ones.
> 
> We have sent some boards to an external lab for FTIR testing, and we are 
> waiting for the final results.
> 
> Our though is that cause of the problem is the PCB, but we need to 
> demonstrate it. It's like some contaminant appears under the conformal 
> coating caused by the temperature (from the inner layers of the PCB?)
> 
> So, what we need is that somebody turns the �light� on.
> 
> Thank you for your help.
> 
> 
> 
> Juan T. Marugán López
> Supply Chain Management
> Indra Sistemas SA (Spain)
> 
> [log in to unmask]
> www.indra.es
> 
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