You've got to make very sure that the solder wire's core-flux or the liquid
assist flux plays well with the conformal coating and there are definately
combinations of fluxes and coatings that don't get along.  This requires
lots of testing.  The coating must adhere well to the circuit board with
the flux residues present, so adhesion tests must be done.  this is a good
starting point:

http://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2.4.1.6.pdf

Then one must test the adhesion, etc. after exposures to adverse
environmental conditions, such as high temperature and humidy and
temperature cycling.  Additionally SIR or MIR measurements should be made
for the flux residue and  conformal coating combination.

http://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2-6_2-6-3-4A.pdf

http://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2.6.3.3b.pdf

http://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2-6-3-7.pdf

The surface insulation of the flux covered with coating should be as good
or better than the SIR of the flux by itself.  Similarly the insulation
resistance of the flux and the coating together should be only. moderately
worse than the coating by itself.   After testing, coating should adhere
well to the board with minimal indications of vesication (moisture
bubbles).

I've tested combinations of flux and coatings where  the individual
materials have high SIR by themselves, but have very low SIR together.
There are also combinations that work just fine.

Karen Tellefsen - Electrical Testing
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908-791-3069



                                                                           
             Ken Bloomquist                                                
             <Kennyb@TECHNICAL                                             
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                                                                   Subject 
             03/04/2010 12:47          [TN] Conformal Coat Over No-Clean   
             PM                        Flux                                
                                                                           
                                                                           
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              Ken Bloomquist                                               
             <Kennyb@TECHNICAL                                             
               SERVICES.NET>                                               
                                                                           
                                                                           




We have a twelve leaded, through-hole component that has to be hand
soldered
on the circuit board. The assembly gets conformal coated with urethane. We
would like to hand solder these twelve leads using no-clean and then
conformal coat the board. The component would be added as a final step
after
the assembly has first been cleaned.

Since this is not a "total no-clean" process do you see any issues applying
conformal coat over these twelve no-clean solder joints?

Best regards,

KennyB


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