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Post Reflow changes To Silver Immersion Copper Traces On PWB

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SMTA-Joe

#82001

Post Reflow changes To Silver Immersion Copper Traces On PWB | 16 February, 2019

I am experiencing a "tarnishing" issue on PWB traces that may be causing problems with component reflow.

The PWB traces are made to the following spec:

SOLDER MASK OVER BARE COPPPER WITH SILVER IMMERSION PLATING PER IPC4553, PLATING TO BE 8-12 MICRO INCHES THICK

The traces exhibit what appears to be a "tarnishing" after the PWB is soldered into a housing using SAC305. After the reflow process the traces change color from dull silver to a dull "yellow". I believe this presents issues for further solderability.

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#82003

Post Reflow changes To Silver Immersion Copper Traces On PWB | 16 February, 2019

Your silver immersion layer is dissolving in the solder. I presume this is what is meant to happen. But with copper tracks underneath that should be fine. Does your SAC305 cover all of the pad. If so no problem. If not then you will need to flux your joint to do any further soldering or repair.

sarason

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#82004

Post Reflow changes To Silver Immersion Copper Traces On PWB | 17 February, 2019

Each time that you cook imm surfaces, you give-up coating thickness. Spec IAg can take maybe 4 thermal cycles ... [that's what I remember but those could be PbSn, not SAC cycles]. Anyhow, the more that you decrease coating thickness, the more that Cu begins to show through. People write papers on this kind of stuff.

Two major suppliers of immersion silver technology are:

* MacDermid

* Electrochemicals, Inc

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SMTA-Joe

#82011

Post Reflow changes To Silver Immersion Copper Traces On PWB | 18 February, 2019

the tarnishing is happening across all the traces without solder even being applied. The PCB is soldered into the housing first. Once this is complete the traces have changed, and when it is time to solder the components to the traces this tarnishing is having an effect on wetting.

I did find that adding some liquid flux helps. The flux in the paste for the components must not be enough. But just a little extra drop of flux helps tremendously.

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SMTA-Joe

#82012

Post Reflow changes To Silver Immersion Copper Traces On PWB | 18 February, 2019

Would increasing the thickness to compensate be a reasonable soulluction?

This tarnishing is happening after the first time heat is applied. Could it be a factor of too long in ramp and/or soak?

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#82014

Post Reflow changes To Silver Immersion Copper Traces On PWB | 18 February, 2019

It doesn't matter if the traces have solder or not. We're talking thermal cycles.

RE IAg thickness: I believe the IAg is a self limiting process, similar to IAu. Your fab may be able to help a little, but I don't expect a substantial change. And if you push the fab to running in upper control limits, think upcharges.

If you're getting killed on this, rethink IAg.

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SMTA-Joe

#82018

Post Reflow changes To Silver Immersion Copper Traces On PWB | 18 February, 2019

Thanks. Something to think about!

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#82019

Post Reflow changes To Silver Immersion Copper Traces On PWB | 18 February, 2019

Whose process is your fab using? Contact [MacDermid, Electrochemicals, In, whoever]

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