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Coating adhesion problems

#30613

Coating adhesion problems | 20 September, 2004

We use Envibar UV 1244 to coat our PCBs after wave soldering. We clean them with alcohol, and then we coate them manually with a brush. Then they are passed through a UV Oven.

We have adhesion problems, and we started by looking at the Envibar, but it seems they have no specs to that. What would be the right direction to find the problem?

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#30623

Coating adhesion problems | 21 September, 2004

We know nothing about Envivar UV 1244 and you weren't real clear about the details of the problem or the materials [eg, ws flux, solder mask, etc], but we'd speculate your adhesion problem goes back to the cleanliness of your board. First by 'alcohol', we assume you mean isopropanol, isopropyl alcohol, IPA. * IPA is a hazardous material. Your people need to use proper protective gear when using this material. * Flash point of IPA (~12�C TCC) is sufficiently low that all electrical equipment within a few meters of the bath must be flameproofed or intrinsically safe, if you don't want a fire. * IPA will not do dip in removing flux residues from your board. 'Cleaning' with IPA just spreads the res uniformly across the board. The proper cleaning method depends on the type of flux that you use in your soldering process.

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#30625

Coating adhesion problems | 21 September, 2004

> Cleaning method will depend on the type of flux > that you use in your soldering process.

Yes it is isopropyl alcohol. We use "no-clean" halide free flux (Multicore X33-06i). We still cleaning and I beleive it is because we roto trim the leads after wave soldering, and to be sure about no residues we do that alcohol process.

We are trying to find the specs and type of solder mask from our PCB supplier in order to look for some incompatibilities with the coating.

What about curing time/temp? Are those critical factors for adhesion?

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#30627

Coating adhesion problems | 21 September, 2004

Several things to check are: * Ask Multicore if their flux res and your conformal coat are compatible. Conformal coats and NC flux res are notorious for having compatibily issues. * Conformal coat a bare board to determine if there is there is enough surface free energy from your solder mask to prevent dewetting. * Bake a board after cleaning with IPA and before coating, because IPA is about 50% water [depending on what IPA you buy and all that]. [Doesn't the IPA make your solder connections turn a milky, white color?]

Different coating materials have varying cure conditions, but we'd speculate that the coating dewets fairly quickly after application, so that cure is not really the problem. The problem is getting the coat to stick the board. [We think.]

Going back to your reason for IPA cleaning, if you are trying to remove filings from your Rototrim, why do you need to use IPA? Is the flux res sticky?

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#30628

Coating adhesion problems | 21 September, 2004

> Conformal coat a bare board to determine if there > is there is enough surface free energy from your > solder mask to prevent dewetting.

Sorry for my ignorance, but what is "enough surface free energy"? How do I determine that?

>* Bake a board > after cleaning with IPA and before coating, > because IPA is about 50% water [depending on what > IPA you buy and all that]. >

The purpose of this test is to elminate any alcohol residues. Right?

> Different coating > materials have varying cure conditions, but we'd > speculate that the coating dewets fairly quickly > after application, so that cure is not really the > problem. The problem is getting the coat to > stick the board. [We think.] >

dewets=dries?

You are saying the curing process is not critical to adhesion? What role plays the solder mask compatibility with the conformal coating?

> Going back to > your reason for IPA cleaning, if you are trying > to remove filings from your Rototrim, why do you > need to use IPA? Is the flux res sticky?

Well, I'm not sure about it, I really beleive that process is there since the regular fluxes were used. I have to find that.

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#30638

Coating adhesion problems | 21 September, 2004

Q1: What is "enough surface free energy"? A1: Look here for background: http://www.azom.com/details.asp?ArticleID=1575

Q2: How do I determine surface free energy? A2: We look at it a relative term. For instance, a semi matte finish on solder mask has more surface free energy than a glossy finish.

Q3: What is the purpose of baking the board after cleaning with IPA and before coating? A3: We�re thinking than maybe the water from the IPA is interfering with proper adhesion of the coating.

Q4: Does dewet = dries? A4: Dewetting. The condition in the conformal coating in which the liquid coating has not adhered intimately with one or more the components. Characterized by an abrupt boundary between the coating and the component lead or conductor. Can be distinguished by a "rolling back" of the coating from the lead or conductor.

Q5: You are saying the curing process is not critical to adhesion? A5: No, we believe that your coating has probably rolled-back from the surface long before curing is intended to occur.

Q6: What role plays the solder mask compatibility with the conformal coating? A6: This goes back to the �surface free energy� of Q1. This is basically saying that is the solder mask surface is real slick or greased-up [with maybe nc flux res], it will more difficult for the conformal coat to stay spread-out and will cause it to "rolling back" on itself [dewet].

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#30704

Coating adhesion problems | 24 September, 2004

OK, We have tested and changed several things in our process, but we still getting the same result.

We are trying another type of coating. We will see later.

My question now is what is adhesion wise acceptable in the industry. I know many of you will point to IPC adhesion tests, but that will give you a rating from 0 to 5, but what is pass/fail criteria.

Thanks.

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#30708

Coating adhesion problems | 25 September, 2004

IPC-HDBK-830, Guidelines for Design, Selection and Application of Conformal Coatings says:

A conformal coating may have several functions depending on the type of application. The most common are: * Inhibit current leakage and short circuit due to humidity and contamination from service environment. * Inhibit corrosion. * Improve fatigue life of solder joints to leadless packages. * Inhibit arcing, corona and St. Elmo's Fire. * Provide mechanical support for small parts that cannot be secured by mechanical means, to prevent damages due to mechanical shock and vibration.

You have decided to conformal coat your board for a particular reason. You have selected a conformal coating material with properties that are different from other materials, most likely. You specific materials [eg, solder mask, nc flux residue, etc] that interact with the coating. You have developed a specific production process.

We do not test for adhesion. We test for: * Hardness * Coating thickness

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