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Compatibility of Conformal Coatings versus No-Clean Solder Paste Flux Residues: When To Clean?

  • Indium Corporation
  • Conformal coating compatibility with no-clean flux residues has been a major topic for years – becoming even more popular recently with companies looking to cut manufacturing costs and processes.

    Unfortunately, no industry standards definitively determine or define “compatibility” between conformal coating materials and no-clean flux residues. This does not mean that people are always automatically cleaning the flux residues before conformal coating their boards. Nor does it mean that most people are shooting in the dark with their decision to clean or not. I have a number of customers who conformally coat over no-clean flux residues, smart customers who have taken the time to do their due diligence and create their own standards and test methods to determine compatibility. These companies also run the tests and apply their standards on their materials.

    Here at Indium Corporation we look at materials compatibility from three different viewpoints:

    The first is the simplest: visual appearance. Does the coating look like it has adhered to the board, components, and/or flux residue? A lack of adhesion will usually result in bubbles, crazing, and a number of other visual defects or anomalies.

    The next step/test that could be taken to determine compatibility would be to measure actual adhesion. “Tape Tests” are often used to measure adhesion. However, these Tape Tests all have a number of imperfections and variables that accumulate to result in a lack of both accuracy and repeatability. For example, many of these Tape Tests are very operator dependent and rely upon the speed at which the tape is removed, the angle at which it is removed, the force in which it is removed, etc. Different operators can, and do, have very different results. Tape Tests are also dependent on the tape. Variables include: age, shelf life, tack strength, adhesion to certain materials, tape brand, tape width and/or length, even test temperature. Another issue with this method has to do with the conformal coating material. What happens when the conformal coating material is a silicone? There isn’t much that will stick to a silicone, so using the Tape Test with this material is probably worthless. Here are several Tape Test procedures that are present within the industry:

    Note that the ASTM test method neuters itself by stating all the possible flaws that could be present.

    The third viewpoint is electrical reliability using Surface Insulation Resistance (SIR). Even this is not easy to conclusively determine because there are two different test methods for SIR. One each for:

    Another issue with SIR testing involves 3D parts. The National Physical Laboratory (Britain) and the SMART Group (a British trade association) have been trying to conquer these SIR challenges for years. They do have a draft of a standard written, but nobody wants to stick their neck out, without further testing and proof, to release this standard; so it has been in limbo for quite some time. For further reading on this topic, read this paper, authored by my colleague Andy Mackie and me. It provides further insight into the issues that our industry is facing with regard to compatibility. Note the chart, in this paper, that highlights the differences between the two IPC SIR test methods.

    Now that you know Indium Corporation’s position on conformal coating compatibility with no-clean flux residues, it is time to consider the point of view of the companies who manufacture and supply conformal coatings. The majority recommend that, when in doubt, clean the no-clean flux residue before conformally coating. This takes any and all of the guessing out of the equation.

    As stated earlier, there are companies that conformally coat over no-clean flux residues with success. Some of these companies are very successful and well-known aerospace, automotive, and military organizations. I am sure they have created their own standards and test methods to mitigate risk and doubt, while assuring performance and reliability. Unfortunately, they have not yet shared their methods and/or experiences with the rest of the industry, so we cannot yet use their expertise to guide us as we converge on one industry standard. If you are one such organization, I’d love to hear from you.

    We have not done much work with conformal coating “compatibility” trials due to the vast number of conformal coating materials commercially available and the vast number of no-clean flux formulations that we offer. Even if there were a standard that would define what it means to be “compatible”, the sheer number of conformal coating products, when combined with the sheer number of no-clean solder fluxes, creates an extremely large matrix of necessary tests.

    In conclusion: When in doubt, clean.

    If you have any other questions or concerns please feel free to contact me at any time.