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Bubbles in conformal coat

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Bubbles in conformal coat | 27 August, 2009

I am having trouble with little air bubbles in my conformal coat. It ranges from a few pin sized bubble to looking like spit. We are using Dow 2620 silicone conformal coat applied with a PVA sprayer machine. The conformal coat comes in bladder bags that are placed in the pressure tank. When I bleed the lines the conformal coat looks non-bubbly and then bubbles start forming. One of my operators suggested adding some thinner to the CC in the bladder bag, on the theory that the stuff is so thick that moving it somehow induces bubbles. Any thoughts would be appreciated.

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Bubbles in conformal coat | 27 August, 2009

There are many possibilities: * Conformal coat is incompatible with surface residue on the board * Conformal coat is incompatible with solder mask on the board * Moisture is out-gassing from the board * Conformal coat is too thick and does not allow the gaseous products to get out [your operator's theory] * Thinner is evaporating too quickly to allow the bubbles to escape * Coating is being cured at too high of a temperature that causes the surface of the coating to skin over and trap the volatiles inside of the conformal coating

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Bubbles in conformal coat | 28 August, 2009

Yes, you must thin Dow CC for any spray operations. We use Dow 2577 daily for multiple customers and we always thin it for spray operations and 2577 is considered medium viscosity. Dow recommends using up to 60% solvent (Xylene). Your 2620 is low viscosity so I would imagine the 60% is a good starting point. We only hand spray it, I won't pollute my machine with silicone. It's too difficult to switch it over for UR and AR.

Start with 60%, experiment with that and adjust from there. Don't bother with a heat cure just let it sit at room temp. Dow SR is one of the easiest coating we use every day.

Let me know if you have any other questions.

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Bubbles in conformal coat | 2 September, 2009

Is this an established process that is just now exhibiting these problems, or is this a brand new process you're just setting up? What is the rest of your process like? What is done to the boards before you coat them? Water washed? Solvent washed? Sat in a stock room for 6 months? What is your post spray process like? Air cure? Elevated temp? If so how hot for how long?

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Bubbles in conformal coat | 10 September, 2009

I currently spray DOW 2620 with my PVA650 without any problems. I do not thin it but I do have a distance between the coating machine and IR oven to provide a flash off period. I notice bubbles only when the flash off time is too short and the coating is allowed to cure too rapidly.

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Bubbles in conformal coat | 9 October, 2009

We use Dow 2620 silicone conformal coat applied with PVA1000 sprayer machine. If used on a daily bases, we have no problem. If we let the machine sit idle for over a week, we will get bubbles in the conformal coating. Sometimes, purging the line will not resolve the issue. We have found out that if the coating tank is in full pressure during the idle days, air will get into the bag. To resolve the issue, we will close all valves going in and out of the coating tank and then release all the air pressure to the tank. This has been our process for the past 6 months and we have had no air bubbles in the coating.

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