Explaining ASTM D3359 Adhesion Testing for Conformal Coatings（2）
The blades have a unique angle between 15° and 30° and are spaced at 1 mm or 2 mm.
1 mm spacing for coating thickness up to 50 microns
2 mm spacing for coating thicker than 50 microns
Method A is recommended to be used for thicknesses above 125 microns
Cuts are made at a consistent pressure into a fully cured board. After the first cut, the board is rotated 90° before a second cut is made. Tape is applied and required to rest for 60 seconds before removal. Remove the tape firmly.
The adhesion tape is recommended to be certified as per ASTM D3359 standard for optimum pull strength.
Common Mistakes for Method B Cross-Cut Adhesion Test
Through our many years of practice, we've assembled some helpful tips to ensure you get the most accurate results possible when conducting adhesion tests.
Keep the following in mind:
· Coatings must be fully cured as per the TDS recommendations
· The type of blade used is based on the thickness, be sure you have the right equipment for your coating
· Positioning of the tool and force is critical for achieving clean cuts
· Following the standard instructions on how to place and remove the tape
· Quality of the blade. The blade is recommended to be replaced every 50 cuts.
Additionally, there are two primary reasons to conduct adhesion testing.
For material qualification:
The coating material can be applied on:
1. A flat surface, generally FR4 panel with similar solder mask as the production assemblies
2. Unpopulated PCB panel
3. PCB panel with solder paste but no components
For Quality checks or part of internal process:
1. Coated assembly with components (generally adhesion test is done on an area without components. Minimum flat surface required 50 x 50 mm)
Performing and Analyzing the Adhesion Test
We've talked about many of the considerations and steps already, but as a quick reminder:
1. Coat and cure the board with conformal coating
2. Using the proper tool, make the cuts. With a consistent pressure and angle:
a. Score one line of cuts
b. Rotate the board
c. Score the second line of cuts perpendicular to the first
3. Apply certified tape to the board
4. After letting the tape rest for 60-90 seconds, remove the tape firmly at a 180° angle
5. Analyze results (see below)
Interpretation of the results:
0B – majority of coating is pulled from the cut area
2B – deep scratches and coating pulled from some of the squares
4B – deep scratches with minor delamination along the lines, but no coating is pulled
5B – clear lines with no delamination
4B and 5B ensured good quality of the coating when the same conditions are maintained:
· Surface energy of the substrate
· Level of contamination on the PCB surface
· Coating application, thickness, and curing method
Getting proper adhesion for your conformal coating material is essential.
Application errors that result in delamination or peeling impose a good deal of risk onto the circuit board. We have also seen and assisted with conformal coating bubbling and conformal coating cobwebbing or wicking, two similarly dangerous conformal coating errors.
LINK：https://blog.humiseal.com/ultimate-guide-to-adhesion-testing
