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Silver Immersion Finish - Moisture bake out guideline?

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#61680

Silver Immersion Finish - Moisture bake out guideline? | 23 April, 2010

I am concerned about a 'tarnishing' effect after moisture bake out that affects solderability on a batch of PCBs.

This finish is new to us,and I am wondering if anyone has ideas about how to drive the moisture out of the boards without compromising the solderability of the pads?

Is this a case where I'll have to bake for (sic)9 years at 45C, or is there a more reasonable method for this finish? :-)

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#61681

Silver Immersion Finish - Moisture bake out guideline? | 23 April, 2010

Hegeman: So you receive these boards in seal moisture barrier bags, store them in a controlled environment, and use them as needed; what is your goal in baking these poor babies?

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#61682

Silver Immersion Finish - Moisture bake out guideline? | 23 April, 2010

Therein lies the issue. Unknown storage conditions before we received the PCBs, date codes just expired (2 weeks), first article is de-lam city!

Had a bad feeling about it. Have read some studies showing that there is no linear tie in of visual change(tarnishing after baking) vs. overall solderability, with a threshold around 700-800 angstroms, which is visually looks like a reject.

Also I have seen a proposed IPC1601 doc suggesting that a moisture bake out of 105C for 4-6 hours would be OK for this finish.

Customer has replaced the PCBs with fresh date codes (1 month old), so the issue may be done with, but again, unknown storage conditions before we receive. Customer states(of course) has had the PCBs stored correctly at all times. (That little statement I realize is questionable since I have now located a PCB in the first group with a date code from early 2008. Never return to vendor for strip, bake and re-plate at the 6 months interval suggested by the IPC. Arrgh)

"Hi there Mr.CM, would you please take this bag of parts and PCBs with questionable date codes and build me 50 class 3 devices...TODAY?"

What do you think? Your thoughts are appreciated.

Thanks

'hege

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#61684

Silver Immersion Finish - Moisture bake out guideline? | 24 April, 2010

Hege: Sounds like this customer is a putz. Ask the putz to sign a release, acknowledging responsibility.

Here's a couple more links on imm silver: * http://www.pcbdesignschool.com/wp-content/uploads/2009/08/the-study-measurement-and-prevention-of-tarnish-on-immersion-silver-board-finishes_july-31.pdf [The Study, Measurement, and Prevention of Tarnish on Immersion Silver Board Finishes, L Toscano & D Cullen, MacDermid] * Effects of Storage Procedures and Bake Out on Solderability of Immersion Silver-Coated PCBs | P Vianco, E Lopez, et al, Sandia National Laboratories, January 19, 2010 http://www.flex007.com/pages/zone.cgi?a=55937&artpg=1&_pf_=1

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#61696

Silver Immersion Finish - Moisture bake out guideline? | 26 April, 2010

We had one product that used immersion silver plating. There were a couple of instances where we saw tarnishing, and elemental analysis confirmed the presence of sulfur. These boards did have the anti-tarnish sheets between boards, but somehow they got exposed. We were told that immersion silver boards should never be shipped in cardboard, since it contains sulfur. Not saying this is your problem, but something to keep an eye out for.

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#61701

Silver Immersion Finish - Moisture bake out guideline? | 26 April, 2010

Obviously knowledge in this area is still in a bit of a 'flux'.

Dave, you had the right idea, the "putz"s signed off on the responsibility, and sent a new batch o'boards. FAST. This new batch were at least date code in 2010, and we had no issues soldering.

Caveat. We did perform a quick moisture bake out anyhow (due to the un verifiable storage before we received) 4 hours at 105C. There was no discoloration, and the soldering went just fine.

So how much more do I know today? A little bit more, and that is enough. Thanks to the group for your considerations and postings.

'hege

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