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Masking of PWBs prior to conformal coat

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BJ

#41064

Masking of PWBs prior to conformal coat | 19 April, 2006

Is there a preferred/efficient method(s)(e.g. tape/masking compounds, boots etc.) for masking off specific areas and/or components on a pwb prior to conformal coating?

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#41067

Masking of PWBs prior to conformal coat | 19 April, 2006

Hi BJ, Everyone's favourite answer...it depends!

If the volume if large, masking boots will save you both, time and money in the long run. Custom boots are relatively expensive to buy, but they pay themselves over time by savings on masking tape. make sure that the boots have a tight fit to the masked location (they expand, so order them reduced in size).some small and/or hard to stick to locations, pink rubber mask is very useful (requires an additional bake out to cure it) make sure they peel off clean and not leave any residue on the board. the masking tape material depends on the coating material. improper tape selection will result in coating wicking under the tape onto the keep out areas. boots are made up of silicone and rubber. avoid silicone boots for AR, UR coatings as silicone acts as a contaminant for these materials.

Finally, depending on what class assemblies you are coating, you can decide how much money to be spent on masking material. We use manual masking and spray process for our class 3 product, and auto coating machine for our class 2 assemblies.

hope this

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#41071

Masking of PWBs prior to conformal coat | 19 April, 2006

I'd also say to be very careful about contamination espacialy with boots. Some companies use oily products on there boots so be careful.

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#41072

Masking of PWBs prior to conformal coat | 19 April, 2006

Here's an other option called seal and peel http://www.interfluxusa.com/misc.html

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#41073

Masking of PWBs prior to conformal coat | 19 April, 2006

Silicone-based coatings "repel" most foreign,non-compatible materials like peelable solder mask, and rubber or vinyl boots. The result is, uncured material around the vicinity of where the boot was. Trust me. Been there and done that.

As a rookie Engineer, my 1st project was a dip CC process. I came up with a masking process that used card-board "boots" and around the perimeter of the mask area, a thicker viscosity silicone coating that would have to be hand applied to gasket and hold the boot down. It was cumersome, but the best solution at the time. We had an unsealed potentiometer where the silicone CC would "wick up" into the pot's innards as soon as it heat cured, and consequently, would ruin the pot's functionality.

I tried to justify a PVA which would've solved all our problems, but then our factory shut down after a hostile takover.

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#41085

Masking of PWBs prior to conformal coat | 20 April, 2006

I am using Silicone based boots with SR coating with good results. I Use Vinyl and Rubber boots with AR and UR coatings.

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#41086

Masking of PWBs prior to conformal coat | 20 April, 2006

Amol, good call there. Silicone mask and silicone coatings are friendly with each other.

This was 1995, and at the time, there were no silicone boots available on the market.

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SD

#41131

Masking of PWBs prior to conformal coat | 21 April, 2006

First of all, do not use kapton tape! It tends to cause a dewetting effect around the tape. A call to Humiseal will explain why.

There's not really a blanket answer to your question....

Removing cured liquid mask can often result in a fillet effect with thicker coatings. You'll end up with a crater around the masked area after mask removal.

Liquid mask generally does not work well in small amounts. In other words, if you are masking an area with liquid mask, make it thick so it separates from the coating and is easy to grab for removal. A thin layer of mask can easily become part of the coating layer.

Humiseal has some tapes available that work fairly well, but a cheaper alternative is to use masking tape used in autobody painting. You may want to test for ESD safety when choosing the tape.

Preformed caps are great for higher volume production. However, they can only be used if the coating is sprayed.

In my experience, when dipping a board for coating, the only way to go is liquid mask. If the tape is not completely sealed to the PCB, then capillary action will pull the coating under the tape. It happens instantly.

Also, in some cases, a stencil works well. We recently had an order for a small # of boards that required a high voltage area to be coated. I cut out a quick stencil out of cardboard and sprayed the area. Worked perfectly and cost virtually nothing.

There is a variety of factors to determine what's best. Unfortunately, part of that involves doing it the wrong way the first time! You'll generally need to experiment a bit for each situation.

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