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Measuring Conformal Coating Adhesion
Measuring Conformal Coating Adhesion
Coating adhesion has been a difficult property to measure. This paper reviews a new tape peel test that can be applied to the PCB or component surface.
Materials Tech

Authored By:
Christopher Hunt and Ling Zou
National Physical Laboratory, Teddington, UK
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Summary
Coating adhesion has been a difficult property to measure, and the industry has made do with a scratch test that is only capable of qualitative tests. NPL with industrial partners, have developed a tape peel test that can be applied to the PCB or component surface.

The choice of tape is critical in achieving complete wetting of the fabric, and good adhesion to the coating. The tape is applied with liquid coating to the substrate, and then cured, leaving a flying unbonded section for clamping on during the pull test. The method shows clear differences in adhesion between different coating types: acrylics, polyurethanes and silicones.

The effect on coating adhesion of surface cleanliness and the cure state of the resist were investigated. Coatings were generally observed to perform well with these problems. However, coating adhesion to components and some resists proved much more variable, with some coatings failing to adhere to problematic components. Surface energy measurements using a wetting angle technique were also used and compared with the peel data.
Conclusions
We have demonstrated a adhesion peel test for conformal coatings that can detect when the adhesion becomes less than optimal for silicone, acrylic and polyurethane coatings.
  • Selection of the tape material is critical to achieve the desired coating to substrate failure during peel. A high surface area woven fabric with a loose weave was found to be ideal, and a 65% polyester and 35% cotton mix cloth with 15% open area was selected.

  • A limitation of the test is that the tape has to be applied with the coating, by fully immersing the tape in the coating and then curing.

  • The peel test is not particularly sensitive to the variables of peel speed, peel length and tape width. It is envisaged that a wide range of peel test equipment could be utilised.
    As originally published in the IPC Printed Circuit Expo, APEX & Designer Summit Proceedings

  • Application of the test to a range of material variables was proved successful, and included the cure state of the coating, contamination of the substrate, surface energy state of resists and components. This test was extremely effective at revealing the low adhesion of coatings to plastic encapsulated IC packages. The test could be readily applied to coating process optimisation.

  • Separation in the peel test was not always at the coating substrate interface, and both failure surfaces should be checked to identify the separation path. The silicone revealed low adhesion forces for removing the tape from the coating, a problem for the test, but on contaminated surfaces, the weaker coating substrate bond could be investigated. For the acrylic and polyurethane coatings, the force to remove the tape from the coating was significantly higher than for the silicone. The silicone had the least dependence on substrate surface condition.

  • Where low adhesion occurs, the test peeled the coating from the substrate. Where the tape was pulled from the coating, it is indicative that the coating has sufficient adhesion for its functional purpose.
Initially Published in the IPC Proceedings
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