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PCB immersion silver discolouration

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#56613

PCB immersion silver discolouration | 20 September, 2008

Hello All,

I used to hear that PCB with immersion silver finishing, its non component location pads, tend to be discolour (Yellowish) after went through heat cycle, such as reflow oven. However, if the component solderbility on the PCB is OK, can we accept it even some of non component pad location become yellowish?

BTW, what cause it become yellowish?? Is this sign of PCB oxidation?

Many thanks, Sean

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#56617

PCB immersion silver discolouration | 21 September, 2008

As long as the surface is solderable, you should be fine. Silver tarnish a well0-known thing. Make sure, though, there is no sulfur (S) "around".

Regards,

Vlad

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#56630

PCB immersion silver discolouration | 22 September, 2008

We see this fairly often. It doesn't affect solderability. Our analysis makes us think this coloration is copper. The reason copper appears on the surface may be that heat cycles increase the thinness and porosity of the immersion silver [IAg] coating, which allows copper to diffuse and migrate to the surface during heat cycles. Fresh, well done IAg boards can withstand: * Up to four lead-free reflow cycles with only a small decrease in wetting. * At least two lead-free reflow cycles before wave soldering with only a small decrease in wetting.

On fresh boards, we see sulfide (Metal|sub x|S) and sulfate (Metal|sub x|SO|sub 4|) on fresh IAg boards at less than 0.2% each.

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#56642

PCB immersion silver discolouration | 23 September, 2008

Hello Vlad, Davef,

Your valuable inputs are much appreciated. Many thanks,

Sean.

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