- Conformal coating failures in the field are rarely caused by the coating material itself; they are caused by inspection methods that cannot verify curing or thickness.
- Boards that pass visual or standard AOI inspection can still fail if the coating is applied too thin or has not fully cross-linked.
- Hyperspectral imaging prevents field failures by verifying the chemical composition and curing state of the coating across the entire board.
When a high-reliability PCB fails in the field due to moisture or corrosion, the first instinct is to blame the conformal coating material or the application process. But the coating material is rarely the problem. The real failure happened weeks earlier, when an inspection system passed a board that wasn't actually protected.
Why Visual Inspection Misses the Root Cause
Visual inspection misses the root cause of field failures because it only checks for the presence of a UV tracer, not the integrity of the coating. A human operator or a standard camera looks at the board under a blacklight. If it glows green or blue, it passes.
But a board can glow perfectly and still fail in a humid environment. If the coating was applied too thin, it won't provide an adequate moisture barrier. If the curing oven profile was off, the polymer won't fully cross-link, leaving it vulnerable to chemical attack or thermal cycling. Visual inspection cannot see thickness, and it cannot see curing state. It only sees the tracer.
What Most Plants Get Wrong About Coating Defects
Most plants get wrong the assumption that if the coating is present, the board is protected. They treat conformal coating inspection as a binary check: is the coating there, yes or no?
In reality, conformal coating is a chemical process. The protection comes from the physical properties of the cured polymer. If you are only checking for the presence of a dye, you are not verifying the physical properties of the coating. You are verifying the application process, but you are blind to the curing process. This is why boards that pass inspection on the line still fail in the field.
What Actually Works: Chemical Verification
To stop field failures, you have to verify the chemical state of the coating before the board ships. This is where Headwall hyperspectral imaging changes the equation.
Instead of looking for a glowing tracer, a hyperspectral camera captures the full spectral signature of the board. It detects the actual chemical composition of the conformal coating.
Because it measures the chemical signature, it can verify that the coating is present, that it is applied at the correct thickness, and that the polymer has fully cured. If the curing profile drifts, the hyperspectral system will catch the change in the chemical signature before the board leaves the plant.
Frequently Asked Questions
What causes conformal coating to fail in the field?
Conformal coating fails in the field primarily due to improper application thickness, incomplete curing, or poor adhesion to the substrate. These issues compromise the moisture and chemical barrier, allowing corrosion to attack the components. Check out this article for more on this topic.
Why does standard AOI miss conformal coating defects?
Standard AOI misses conformal coating defects because it relies on traditional RGB cameras to detect a UV tracer dye. It cannot measure the thickness of the clear polymer or verify if the chemical curing process is complete.
How can you verify if conformal coating is fully cured?
You can verify if conformal coating is fully cured by using hyperspectral imaging to analyze the chemical signature of the polymer. This method detects the molecular changes that occur during cross-linking, confirming the curing state without destructive testing.
What is the best way to inspect conformal coating on aerospace PCBs?
The best way to inspect conformal coating on aerospace PCBs is to use automated hyperspectral imaging. This provides a data-driven, chemical-level verification of coverage and curing, eliminating the subjectivity and risks associated with manual UV inspection.
If you are dealing with unexplained field failures on coated boards, the problem is likely your inspection method. We can help you implement a system that actually verifies the coating.
-Nate
Happy to help you with any conformal coating inspection questions your team has.