Investigating Analytical Tools for Testing Cleanliness with Foresite's Eric Camden

Estimated reading time: 16 minutes

Your higher standoff, larger-pitch type components are much easier to clean so you focus on a QFN on a micro-BGA, any of the bottom terminator components like that that are very difficult to process. If you focus on those, if you get those clean, set up your wash parameters and your thermal profile right to process those, then you'll be in good shape for the rest of the board. In general, we've found that to be true over the years.

So that was the first talk. And then the second presentation I gave today was on localized cleaning—hand cleaning after hand solder or rework or whatever that operation might be. Just like any other wash process there's a lot of right ways and a lot of wrong ways to do it, and we've seen both. To give you a synopsis on that, basically knowing where your effluent is going, being able to control the amount of solvent you're putting down to do the cleaning and then knowing where that spreads to. Once you bring a solvent into play with a flux residue you've made that a mobile phase containment basically. If you've got neighboring components that have a low standoff, well that surface tension of that solvent and flux residue is so low it's going to easily wick its way underneath SMT type components or whatever it might be. And if you don't do a good job of rinsing that whole area then you're just doing contamination relocation. I've heard it called vacuuming without a bag. The area that you're cleaning looks good, but it's just spreading it everywhere else.

Like any other cleaning process, it's verifying that it is effective using the right analytical techniques. There are a lot of wrong ways to do localized cleaning, but if you know to test not only the area that you cleaned but create a cleanliness map around there, looking at all the neighboring components, you can say, "Okay, I've cleaned this area of interest but I'm also not contaminating the neighboring components."

It's been a good day here at the West Penn show because a lot of people have come up to me since the morning presentations and we've had further conversations on what their companies are doing.

Goldman: That's great. The big thing about the no-clean versus others is that they're notoriously hard to clean. And I'm just curious about this. Why don't people then go back to fluxes that aren't no-clean?

Camden: This is one of my bugaboos about no-clean. I believe the people who process their boards with no-clean flux and then wash them, they are in the mindset that if they partially clean a no-clean flux it doesn't matter because it's a no-clean flux and what's left behind will be non-conductive and non-corrosive. But that simply isn't the case. If I see somebody that's going to do a wash process no matter what, don't make it harder on yourself. Use the water-soluble fluxes.

Goldman: So if you use a flux that needs to be cleaned, aren't you further ahead then?

Camden: There's zero doubt in my mind when I look at a water-soluble board that's been washed that if I see residues, they are conductive. If you're processing with no-clean and you don't fully remove all the residues and you see some white haze here and there, that's mostly likely flux residue left behind. You can't look at that and say, "Yes that's active. Yes, that's corrosive. Yes, that's conductive, it's going to cause a problem." Given the option, process with the water-soluble flux and not a no-clean if you're intention is to wash. I think really at this point like with a lot of changes in the industry it's just education and time. We need to teach these people who are requiring the washing of a no-clean flux that what you're doing may be more detrimental than leaving it in place.

Goldman: Just get the regular flux out there.

Camden: Or leave the no-clean on there. Visual inspection is one of the worst criteria for determining a board's quality and in turn its reliability. I've been doing this for 18 years and I can't look at a board, I can't look at a residue, and say that's good or bad. There's just no way to tell by looking at a board. But there are so many companies out there that say, "I want a clean, shiny solder joint. And that's how I know it's a good quality board." Well, think about when they went to lead-free, they didn't have shiny solder joints anymore. What you thought was the be all end all criteria has now gone away because you were forced to go lead-free. I think it will take some time the same way with the cleaning of no-cleans.

Goldman: Yeah, everybody thought no-cleans meant there was nothing there. That's not true.

Camden: Correct. It's lower risk. There are lower active components to it to begin with, but that doesn't mean that you have no risk of contamination or no residues. It's just a matter of making sure that it's been thermally processed to a level that whatever is left is near benign. But that's just not always the case.

Goldman: Well, this has been interesting. And I thank you for your time.

Camden: Perfect. Thanks Patty.

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