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Acrylic Coating De-Wetting?

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Acrylic Coating De-Wetting? | 25 July, 2017

I'm currently working on bringing a manual coating process/workcenter online and my QA team is giving me some grief.

For background, we're using Humiseal 1B31 thinned with thinner 521, mixed at 1:1 by volume. Spraying through a Binks HVLP with 10 PSI inlet pressure at the gun. Below is a rough process outline:

- Wash (Aquastorm with chemistry, can't remember what we're using off the top of my head) - Bake (about 100c for a couple hours) - Plasma Clean (confirmed that adhesion is greatly increased, waiting for dyne pens to confirm, contact angles on DI water drops go from near 60 degrees, to almost nothing at all, so surface energy shouldn't be a problem) - Mask (ATS-667 tape for masking, known compatible with AR coatings, no silicone adhesives) - Plasma Clean again if too much time has passed during mask stage - Coat, 3 coats per side, resulting in 2-2.5 mil finished thickness as measured on an aluminum coupon

On some smaller chip resistors (0201/0402) the coating almost appears to be missing (images attached), but from the images alone and looking at the assembly I'm not seeing any signs of de-wetting on the resistors, just a darker part due to the resistors already being black. I also used a plastic probe to scrape coating off the top of one of these resistors and could verify that the thickness is about the same as on chip caps.

Mostly looking for opinions based on the pics (the top of some larger parts are intentionally mask, ignore those).


Thanks! -Phil

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Acrylic Coating De-Wetting? | 26 July, 2017

Ah...the joys of subjective analysis!

At a glance, these pictures look fine to me. Other than the IC's, which you've indicated were masked, and, maybe R12 in the last picture and CC1384 in the first image, coating looks uniform and well distributed.

The darker parts on the top of the resistors looks to be a question of contrast between the black of the component, and the fluoresence of the die. If you've had to scrape off coating to measure the thickness, and your QC is calling it a failure, then, it seems more like a false call than anything else.

Cheers, ..rob

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Acrylic Coating De-Wetting? | 28 July, 2017

I'm pretty sure that in the job description of each QA related position it lists "Ensure Process Engineering is always being hassled because you inspect to target and not acceptable."

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Acrylic Coating De-Wetting? | 31 July, 2017

Ain't that the truth!!

In a previous life, we did a (somewhat brief) cost analysis of class three rework being performed on class two boards. When we realized how much time/money we were losing due to unnecessary rework, we decided to start taking action.

You should have seen the looks we got from people when we told them to stop rejecting class 2 acceptable assemblies. Their pride was deeply wounded. But, the fact remained that we were regularly (as in, nearly every board on every job) reworking acceptable assemblies because of pride in workmanship issues.

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