Parylene Coating Blog by Diamond-MT

What Is the Proper Adhesion Test for Conformal Coating?

Posted by Sean Horn on Fri, Jan 29, 2016 @ 08:09 AM

You've done your research, chosen a conformal coating provider, and coated your device. Now you want to know if the coating properly adhered.

If you search Google for "conformal coating adhesion test," the results will show 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).

It can be deceiving. In a list of search results, it can appear as if these tests were designed for conformal coating.

That is not the case.

At present, there is no widely-accepted test for conformal coating adhesion.

Adhesion Needs Should Be Determined on a Case-by-Case Basis

Adhesion testing for conformal coating is best determined on a case-by-case basis by the parties involved in the process.

For example, one of our recent clients decided to use the ASTM D3359 scratch test as their standard, even though the test wasn't specifically designed for conformal coating.

ASTM D3359 is sometimes called, "The Tape Test." There are two variations: the X-Cut Tape Test and the Cross Hatch Tape Test.

For the X-Cut Tape Test, a technician will use a knife and a straightedge to make two cuts in the coating to form an “X.” Tape is then placed on the intersection of the two lines and then is 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 in a laboratory environment. Instead of cutting an “X” into the coating, the technician cuts several parallel lines using either a cutting guide or a custom cross-hatch cutter.adhestion_test.jpg

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 then applied and then rapidly removed, followed by a close inspection of the coating to see if any squares pulled away with the tape.

In general, the ASTM D3359 test is a very high standard for conformal coating. But for this company, it provided an important reference point for their certification needs.

One mistake we sometimes see is choosing a standard that’s simply too rigorous for the conformal coating being used. When a test that’s too rigorous becomes part of the certified manufacturing process, a company might find itself forced to throw away or recoat entire batches of its devices later on.

We strongly recommend working with an experienced conformal coating expert to determine adhesion standards for your project.

Coating Tests Require a Test Coupon

All coating tests are destructive. They are not appropriate on a "live" product. Instead, you'll need to create a "test coupon" to use for any adhesion test you'd like to use.

The coupon will be coated at the same time as your devices, using the same coating materials and methods as you do with your actual products. Once the coating has dried, you'll be able to run the destructive adhesion test on the coupon, instead of your products.

Create a Representative Coupon

If you're going to run any kind of adhesion test, you need to create a test coupon that accurately represents the device that's being coated.

If you're coating a printed circuit board, for example, using a plain piece of FR-4 glass epoxy is not a representative coupon.

(FR-4 glass epoxy is the green “card” material on which most circuit boards are printed.)

The connections and the elements on the board are where adhesion problems are most likely to occur. Using a plain coupon without connections or electronic elements might produce a good test result, but it won’t tell you much about how well the coating adhered at the connection points for the board.

Adhesion Testing for Process Monitoring

Adhesion testing is an effective way to monitor your manufacturing process over time.

Some devices require rigorous process monitoring. The manufacturing process for medical and aerospace devices, for example, must often be certified and monitored.

The goal of process monitoring is to identify changes in performance between batches.

A company using the ASTMD3359 “cross-hatch” version of the test might normally see zero to one square removed by the tape. But if the tape suddenly pulls up seven squares, it’s a signal that you might have a problem with that batch of devices.

Every device has different requirements for adhesion performance, and in some situations, formal adhesion testing is not necessary.

If you have questions about process monitoring for your device, be sure to work with your coating provider to establish an adhesion test that’s suitable for your situation.

Adhesion Testing Is a Common Source of Confusion

We understand it is difficult to find information about adhesion testing for conformal coating. It’s not a topic that’s widely discussed online or in trade publications, and a Google search often returns results that describe formal adhesion tests that were not specifically designed for conformal coating.

The best advice we can give is to partner with a conformal coating provider that has a deep knowledge of all types of conformal coating. That’s the best way to ensure you get the right coating for your device, along with the best standards for adhesion and testing in your specific situation.

To learn more about the basics of conformal coating, download our guide:

Basics of Conformal Coatings  Whitepaper

Tags: conformal coating, parylene adhesion, conformal coating adhesion, adhesion testing