What Is the Proper Adhesion Test for Conformal Coating?

Essential steps in researching conformal coatings include confirmed testing of coating properties as well as adhesion. An online search for “conformal coating adhesion test” results in a number of documented tests and standards. These include tests from the International Standards Board (ISO) and the American Society for Testing and Materials (ASTM). One must exercise caution, however, since it can appear as if these tests were all designed for conformal coatings. While there is no universally accepted test for the adhesion of conformal coatings, there are methods that can be used and are best determined on a case-by-case basis by the parties involved.

As an example, the ASTM D3359 scratch test has been used as an adhesion standard even though the test was not specifically designed for conformal coating. ASTM D3359 is sometimes called “The Tape Test” and is the most commonly used test to characterize the relative adhesion of conformal coatings to substrates. There are two variations: the X-Cut Tape Test (Method A) and the Cross Hatch Tape Test (Method B).

For the X-Cut Tape Test, a technician will use a knife and a straight edge to make two cuts in the coating to form an “X.” Tape is then placed, by a standardized method, on the intersection of the two lines and is then rapidly removed. The center of the “X” is then inspected to see if tape caused any of the coating to pull away from the device.

The Cross-Hatch Tape Test  is typically used on witness samples of the same material as the substrate being coated. Instead of cutting an “X” into the coating, the technician cuts several parallel lines in perpendicular fashion using either a cutting guide or a custom cross-hatch cutter. The matrix of cut lines then makes up individual squares delineated on the test surface. Cross-hatch cutters will make cuts of exactly the same depth and separation. This creates a series of squares resembling a grid on top of the material. Tape is applied in a fashion similar to that completed in Method A and then rapidly removed. Subsequently, a close inspection of the coating is completed. Rankings of adhesion are then made by the degree to which corners are lifted and/or entire squares are removed.

Coating Tests Require a Test Coupon

All coating tests are destructive and are not appropriate on “live” product. Therefore, a “test coupon” is used for adhesion testing. The coupon will be coated at the same time as the devices, using the same coating materials and methods as the coated products. Once the entire coating process is finished, the destructive adhesion test on the coupon can be completed. To generate a representative adhesion test, the test coupon must accurately represent the device that’s being coated.

Adhesion Testing Is a Common Source of Confusion

It can be difficult to find information about adhesion testing for conformal coating. It is not a topic widely discussed online or in trade publications, and online searches often return results that describe formal adhesion tests that were not specifically designed for conformal coating.

The best advice offered is to partner with a conformal coating provider that has a deep knowledge of all types of conformal coating. This will ensure the appropriate coating is selected for your device and adhesion can be appropriately verified.