Explaining ASTM D3359 Adhesion Testing for Conformal Coatings（1）
Conformal coatings offer tremendous protection to printed circuit board assemblies, but this assumes they properly adhere to the PCB. Application errors are all too common, causing delamination and otherwise poor adhesion of the material. So before producing at scale, you should routinely conduct adhesion tests to validate the integrity of your production processes.
This blog will cover the hows and whys of adhesion testing. Specifically, we will explore the following:
· Benefits of Adhesion Testing
· Causes of Delamination
· Reviewing ASTM D3359 and Required Tools
· Performing and Interpreting Adhesion Tests
What is PCB Adhesion Testing?
Adhesion testing evaluates conformal coating material's ability to adhere or "stick" onto substrates (such as PCB solder masks and components). This test can predict the long-term performance of the coatings in highly demanding environmental conditions.
Ultimately, the level of adhesion directly impacts the grade of the PCB, the integrity of the manufacturing process, and the quality of the conformal coating application.
ASTM D3359 is the referenced standard for adhesion testing. This standard is also referenced in the IPC-HDBK-830. It's important to note that adhesion testing is not a required part of IPC-CC-830 Rev. C or IPC-A-610.
Two major concerns adhesion testing evaluates are delamination and conformal coating pealing.
So adhesion testing can help predict the likelihood of delamination, but what causes delamination in the first place?
3 Main Causes of Delamination
Delamination is the splitting and subsequent separation of the layer of conformal coating material and PCB. It represents a severe failure in performance and must be avoided. Some common causes of delamination include:
1. Contamination on the components or on the solder mask
2. The surface energy of the PCB assembly is low
3. Weak adhesion of the coating during masking compound removal. This happens when:
(1).The coating application is too thick
(2).Solvent-based coatings cured at too high temperature and for too long
Adhesion Test Standard: ASTM D3359
ASTM D3359 is the recognized standard for testing the adhesive strength of coatings and paints on a substrate. Primarily, the process includes applying coatings and allowing them to cure completely. The coatings are scored with a blade; a certified tape (meeting standardized pressure requirements) is applied to the coating and quickly removed.
The standard included grading criteria to evaluate the adhesion of the coating based on the amount of material remaining after the removal of the tape. This is a visual review process, and we will include images taken directly from the standards below.
The standard identifies two methods of testing:
· Method A (X-Cut Tape Test)
· Method B, (Cross-Cut Tape Test)
While both tests are effective, they are designed for different material thicknesses. Method B covers coatings in our range of PCB applications. As such, we will be reviewing considerations for the Cross-Cut Tape test.
Tools Required for Adhesion Test - Method B Cross-Cut
As the name implies, the Cross-cut test utilizes a specialized cutter and blade to score parallel lines of uniform thickness into the cured coating. The blades create a pattern of 6 or 11 individual cuts with each pass of the tool. For method B, create two perpendicular slices, one in each direction.
LINK：https://blog.humiseal.com/ultimate-guide-to-adhesion-testing
