Conformal Coatings - Managing Compatibility Challenges

Posted 07/12/2011 by $username

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Conformal coatings are cured or dried in several ways as well.  Hot air, UV, vapor deposition/polymerization are some common techniques.

A key to the successful use of a conformal coating is compatibility with any residues from solder paste or wave soldering flux.  The science behind the interaction of flux residues and conformal coatings is an area of great interest, but due to the huge number of possible combinations of paste, flux and conformal coatings, not every combination is fully characterized.

Of course the easiest way to eliminate an interaction between conformal coatings and soldering flux residue is to use a water soluble paste and wave soldering flux.  Water cleaning the residues is an additional process step that uses water and creates effluent, but in certain mission critical assemblies, this may be the best process choice.  A frequently asked question is how to clean no-clean paste and wave flux residues.  In the past few years, manufacturers of cleaning chemistry have created a large body of data on the cleanability of no-cleans.

The lowest cost process is to use a no-clean paste and/or flux, and conformal coat without cleaning. However, in the case of epoxies, certain ingredients used in solder paste can retard or prevent the curing of the epoxy.  This can be reduced or eliminated in some cases by managing the reflow profile and solder bath dwell time of the assembly.  Again, leaving no-clean residues is the lowest cost option, not always the easiest.

How can you test for no-clean residue/conformal coating compatibility?  One approach is to expose the assembly to the fluids or vapors that are likely to cause damage.  Such tests include a standard 85% humidity / 85 °C temperature exposure and a test for surface insulation.  More exotic tests are used to check for exposure to industrial gasses, acid rain, coffee and anything else that may cause a failure in an electronic assembly.

What strategies have you used to protect your assembled circuits with conformal coatings?