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

Estimated reading time: 23 minutes

Camden: Absolutely. Really that's what it comes down to. Working where I work, doing the work that we're doing, we really are a reliability lab. Because whether we look at your product upfront or after a failure analysis, we're either instilling the reliability upfront before the initial product release, or we're looking at your failures and determining how to optimize the process to build the reliability back into that process before it goes back out the door.

Goldman: Of course, you always hope that you get that upfront opportunity to prevent.

Camden: Sure. And like I said earlier it's a lot cheaper to do it up front.

Goldman: What else can you tell me about your talk this morning?

Camden: It was on two very important issues. Apparently, it was a good turnout here for rainy Wednesday morning in West Penn. There were quite a few people in the audience with a lot of good questions. People that are working with no-clean fluxes and they're cleaning them at the direction of either their customer or some of their internal engineers think that it needs to be cleaned. One of the main problems with cleaning a no-clean flux is what you leave behind. No-clean flux based on the amount of rosin or resin in your material can be extremely difficult to clean. Especially under QFNs, micro-BGAs, the low standoff components, because you've created this barrier that's very hard to penetrate. QFNs are complex under the best conditions and even when you've got a solid ground plane you can't get the gasses to vent out properly and you certainly will struggle to clean those properly.

Any time you've got a no-clean flux, that outer shell is meant to remain in place and become basically a poor man's conformal coating. If you partially clean a no-clean flux, what you're doing is you're ripping off the band-aid. You're exposing the wound. And if you leave any of that material behind that was meant to be bound within that outer resin shell, now it's very hydroscopic, it's very conductive. It can be as bad as water-soluble flux residues.

In the first talk this morning we discussed some of the failure modes of no-clean. It all comes out to electrochemical migration, electrochemical leakage. These issues are very prevalent in the industry for people who are trying to clean no-clean. And just because you can't see a residue doesn't mean it's not there. We touched on making sure you have the right equipment parameters and making sure you're using things like belt speed, temperature, impingement angles, and saponifier concentration. A lot of these little things play together in concert to make an effective wash process. And then being able to do localized extractions and ion chromatography after you've cleaned, to improve the effectiveness.

Historically, the ROSE tester has been used, but that is going by the wayside for several reasons, and rightfully so in 2018. It was never meant to be used as acceptance criteria. It was supposed to be process monitoring to see if something was completely out of whack somewhere in your process. You would catch that. And the materials that were around when the ROSE tester was implemented are no longer being used at all. These materials that that tester was meant to test have gone by the wayside, but the tester was still in place and the criteria had never changed. But that is coming to an end to a certain degree. And a lot of engineers are really seeing the benefit of not using the ROSE tester for anything more than just the occasional process monitoring tool.

When you start looking at a process is, you really need to focus on parts like QFNs because they're the hardest to process whether using no-clean or water-soluble flux. If you focus on parts like that and you end up doing localized extractions, obviously I'm going to promote the C3 because it's an automated localized cleanliness tester, but it does a 0.1 square inch area extraction so you're able to look at very specific areas of the board. And what you don't do versus ROSE or even standard extractions for ion chromatography is you don't normalize out these pockets of contamination. For instance, if you've got a rework area and it's not very clean when you're done and then you take that whole board and you try to do the cleanliness measurement by dropping that in five gallons of IPA deionized water, you've got one square inch area that is dirty and you've got 148 that are clean. Then when you put in your dilution factors for anything like that you're taking length time, width time, two times 10% for population factor. You've normalized that out over the entire surface of the board.

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