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Effect of Reflow Environment on No-Clean Residue and Surface Insulation Resistance (SIR)

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  • Phil Zarrow: Eric, I understand you've been doing some work in experimentation on the effect of reflow environments on residue and, particularly, surface insulation resistance.
     
    Eric Bastow: Right. As has been discussed before, and I think you would probably use the same value or similar value that I would use, if you look at the electronics world, largely dominated by commercial electronics, 80% of it—or there abouts—is no-clean, so meaning that especially in the case of a solder paste that flux residue remains on the assembly for the life of the assembly.
     
    Phil Zarrow: Right.
     
    Eric Bastow: When we talk about SIR, surface installation resistance, that is the means by which we measure the electrical reliability of that residue. If you look at the flux chemistry, if you will, when we do SIR testing, or we measure SIR testing, typically we're capturing that large part of the market which is reflowing in air. Within that market, there's a minority of people that are reflowing in nitrogen.
     
    The flux manufacturer, when they're doing the J-Standard test, they're thinking okay, this is going to be used in an air-reflow environment. When they do those tests they're doing it in air, unless the product is specifically made to be reflowed in nitrogen. There are especially in the case of solder paste, no-clean solder paste, rosin-based no-clean solder paste, where again you have the majority reflowing in air, but a minority reflowing in nitrogen. Does that have an effect on the electrical reliability of that flux chemistry? You have to look at the chemistry of the material, and you have to be knowledgeable on why would you even want to think about that.
     
    If you look at the chemistry, there's a particular group of ingredients called activators. It is their function to remove oxidation—be it on the board itself, component leads, et cetera. If you think about when you're re-flowing in air, you have the potential of, what I think of as. a revolving door, where you're cleaning oxides, but because you have air and heat you're recreating oxides at the same time.
     
    Phil Zarrow: Right.
     
    Eric Bastow: Theoretically, those activators have to work harder or longer or more consumed in an air-reflow environment as opposed to a nitrogen-reflow environment.
     
    Phil Zarrow: Right, because you have a double whammy: You have the oxygen present, and you've got heat which is a catalyst for oxidation.
     
    Eric Bastow: Absolutely. If you look at those situations side-by-side in terms of what is ultimately left over in the residue, at a theoretical level you would have more activators, or the potential for more activators, to be left over when reflow to nitrogen, than as opposed to being reflowed in air.
     
    Then you have to take it the next step further and say, “Why does that matter where SIR is concerned?” That's because if you look at all the many things that can affect SIR performance, board contamination, all those sorts of things. If you're looking at just the flux chemistry itself, the material or the ingredients in the flux chemistry that can have an impact on SIR performance are the activators. If you have more or less leftover in one scenario versus the other, yes, the potential exists for some sort of difference in the SIR performance of that flux residue.
     
    Phil Zarrow: Very good. One question I have for you: With the experimentation you've been doing with nitrogen, one of the areas of controversy, discussion, however you want to put it, is what level of nitrogen to use in terms of measured PPM O2. And, ovens have come a long way as far as containment, and there's also the purity of your source nitrogen that you're using. In terms of the experimentation you've been doing, what level of nitrogen have you been using?
     
    Eric Bastow: Thankfully, for a lot of the reasons you just mentioned, we have very good nitrogen supply, and the ovens are very up to date on that sort of thing. There were times when I watched the meter quite a bit, if you will. It was not uncommon to have values that single digits for oxygen.
     
    Phil Zarrow: Wow, come a long way.
     
    Eric Bastow: Yeah. I'm very convinced that we had a very good nitrogen environment.
     
    Phil Zarrow: Obviously, what you're finding I'm sure is that flux chemistries have come a long, long way as well.
     
    Eric Bastow: Yes. That was one of the things I'm trying to capture in the work that I have going on now, is: First of all, keeping in mind that not all chemistries are equal.
     
    Phil Zarrow: Right.
     
    Eric Bastow: I'm trying to capture three different variations. One is an ROL0, traditional type ROL0, with a traditional type residue. When I say traditional type residue, I'm differentiating between that and a residue that's been optimized for pin probing.
     
    Phil Zarrow: Right.
     
    Eric Bastow: I have one with a traditional residue. I also have an ROL0 with a residue that is optimized for pin probing—a little softer, a little waxier as I like to say. Then I also have an ROL1, or a halogen-containing material with a traditional residue that I'm looking at. 
     
    Back to your specific question, we've gotten very intelligent and very good at selecting the activators, so there's a lot of confidence in how they're going to act in the residue. Also, you can never overlook—however sophisticated we become and as time goes along—can never overlook the simple almost miraculous ability of the rosin to encapsulate and immobilize nasties that could be leftover in the residue, regardless of where they come from, the residual activators or what have you. They just do such a great job of locking everything in place, and ultimately keeping those SIR values high, and ensuring good reliability of the residue.
     
    Phil Zarrow: Right. This is a very intriguing and very important topic. Where can we find this paper?
     
    Eric Bastow: All of our papers, Indium Corporation papers, are on our website, which is www.indium.com, and a lot of the papers specific to SIR performance are written by myself. If somebody has a particular interest in that, they can feel free to contact me directly, they can send me an email, ebastow@indium.com.
     
    Phil Zarrow: Right to the source.
     
    Eric Bastow: Yes, sir.
     
    Phil Zarrow: Eric, thank you very much.
     
    Eric Bastow: You're very welcome.
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