Indium Corporation - Mailchimp

Test Methods for ECM Assessment & Process Control: Sources of Contamination

Category:
  • Flux
  • Wave Solder
  • Videos

  • Phil Zarrow: Brook, with regard to contamination, there's a number of different sources and the first one that comes to my mind is handling—the human factor, if you will. And I'm remembering a very iconic industry story: Our good friend Terry Munson, in a previous employer—large automotive manufacturer, did a test. And what he did was he measured cleanliness of work in progress in the morning at various stages, and then he came back after lunch and measured the same people—
    using the same people, same equipment, same work in progress, obviously different circuit boards. He measured the contamination levels of what was present. And he found that after lunch it went way up.
     
    Brook Sandy-Smith: Sky-rocketed.
     
    Phil Zarrow: Absolutely. People had eaten, there's food, they put lotion on their hands, they've-
     
    Brook Sandy-Smith: Touched their hair.
     
    Phil Zarrow: Touched their hair.
     
    Brook Sandy-Smith: Changed their clothes.
     
    Phil Zarrow: Things we don't want to talk about.
     
    Brook Sandy-Smith: Went out for a smoke.
     
    Phil Zarrow: Yeah, we'll leave it to people's imagination. Exactly, but let’s just say they transferred said contamination to the circuit boards. And interestingly enough, as a result of that, of Terry's findings if you will, today I don't know of an automotive facility that I've been into, whether it's a CEM, or an OEM building automotive, that their operators do not wear gloves, either prior to final soldering or until after conformal coating. So Terry's definitely left his legacy in the industry.
     
    Brook Sandy-Smith: Well definitely. And that's a huge uncontrolled source…
     
    Phil Zarrow: Human factor.
     
    Brook Sandy-Smith: …for contamination. I mean that kind of trumps anything that you would get from your board fabrication or your incoming components or your incoming materials. You can control for those things. You can clean all of your materials when they come into your facility, or clean them after assembly. But the handling factor, people touching the assemblies and putting them away, is totally uncontrolled.
     
    Phil Zarrow: So you did mention there, incoming components boards, cleanliness levels, how well they were handled and processed previously. But what are some of the other sources of contamination? For example, flux entrapment and it's two connotations.
     
    Brook Sandy-Smith: Flux entrapment.
     
    Phil Zarrow: Two connotations.
     
    Brook Sandy-Smith: Well, definitely. I mean, flux is always seen as a source of ionic contamination that could be present at the end of the assembly. Any material that you put in contact with the assembly could contribute to electrochemical migration, because sometimes these materials have things in them that you wouldn't expect to be there, or they weren't processed properly, or didn't see the same thermal excursion that they were designed to, which could also compromise that level or reliability that you would expect.
     
    So two of the critical ways that flux can become entrapped and become more of a problem then was initially realized is: 
     
    Under low-standoff components, or under RF shields, or in a pocket design where there just isn't space for the volatiles to escape. Or, it's an area where just not as much heat is present because the high thermal mass is wicking away the heat that you would be expecting the flux to see.
     
    The other factor there, is a lot of companies will take assemblies and palletize them or put them in fixtures to make some areas available ‘til selective soldering or localized processes and other areas shielded.
     
    Phil Zarrow: Right, or on a wave solder machine.
     
    Brook Sandy-Smith: For instance, on a selective wave soldering machine. So then you have fluxes that are being sprayed onto areas where they're supposed to be areas that are keep-out areas. But often time the flux will wick under those pallets and then not see the thermal excursion that's needed to make them or render them benign. And then you once again are imparting a source of ionic contamination, which is not processed in the way that you were expecting.
     
    Phil Zarrow: Now having done process failure analysis and trying to track down where this contamination is coming from, sometimes it defies a global cleaning methodology, that basically a local testing methodology is required.
     
    Brook Sandy-Smith: Absolutely. There are emerging test methods such as, C3 testing, which was introduced to the industry by Terry Munson, who we mentioned earlier. This kind of a test method has a small nozzle that uses cool steam to extract residues on a very small area. So if you have one component that's a problem, or you're always handling boards on a certain corner, you can test one area instead of extracting the entire contaminants on the whole board. So you can see on specific areas how the cleanliness is being affected.
     
    Phil Zarrow: And you've authored papers on this particular subject.
     
    Brook Sandy-Smith: Absolutely.
     
    Phil Zarrow: Where can we find them?
     
    Brook Sandy-Smith: You can find them at www.indium.com, or if you come up with some specific questions you could email me directly at bsandy@indium.com.
     
    Phil Zarrow: Excellent, Brook. Thank you very much.