silver finishes on pcbs

Hi Jack,

I once tried this finish ( without changing any process parameters ) and encountered wetting problems even in the first reflow process. It was thought as cheaper alternative to Ni/Au we use for planarity reasons. Heard of the same problems others had. It might be that things are different today than 4 years ago. At least your supplier should proof that whatever you want to do with the PCB will work or tell you which process adjustments are necessary.

What I do with new material is always a testrun to see if and how it works. BTW, the supplier doesn�t offer this finish anymore here.

Others may have different oppinions about that.

Wolfgang

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Silver finish???? What kind ??? electroplated or immersion??? [anyone giving hints??]

ELECTROPLATED

Silver should never be electroplated onto copper, because it diffuses into the copper at a blistering rate and then solderability goes into the dumper. [Silver migrates into copper at a rate 1,000 times greater than tin according to Tautscher.] You can watch a 1 um silver plating on copper disappear in less than 24 hours. Seriously consider using a nickel buffer against this diffusion.

Some people talk about plating silver to a 5um thickness to overcome this diffusion of silver into the copper. This thickness of silver does remain silver that�s nice and silvery, but it�s: * Expensive * Possible that this is a poor approach for a solderable surface stand point. Silver-tin intermetallics are long and brittle. When the silver content approaches 5% lead-tin solder connections become very unreliable.

Fresh silver is very solderable. Sulfur in the air makes silver less solderable and reacts with silver to form silver sulfide that could creep and cause shorts to adjacent leads. * Proper protective packaging could prevent this [I have no experience in trying] deterioration. I do know that cardboard contains beaucoup sulfur. * OSP on the silver may help.[Again, no experience.]

IMMERSION

Did someone say OSP? On OSP soaked silver things ... No comments beyond those in => Immersion Silver PCB Surface Plating - Dennis VanBuren 14:21:24 02/01/2000 [Wudja mean "not much luck" searching the archives? What is this haggis???][Is that like saying "what is this chopped liver??"]

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Jacqueline,

Asuming you mean immersion silver:

Advantages - Very flat finish compared to HASL, more easily solderable than bare copper with OSP and more process tolerant than this in our experience, reasonable shelf life, cheaper than immersion gold and doesn't require nasty chamicals (immersion gold)or high temperature excursions (HASL)in the PCB.

Disadvantages - Not very widely available, requires careful handling to avoid damaging OSP coating, mind you, handling PCB's by their solderable surfaces is not good practice anyway!!

We have been using this finish on some of our finer pitch assemblies for about a year now (Alpha Level). The majority of these are reflow and wave solder or multiple reflow processes. We are using the same process setups as for equivalent HASL boards and have not seen any real difference in wetting performance and it is better than immersion gold.

Regarding the process time limitations as mentioned by Wolfgang, a good production control regime means that partly processed PCB's should not hang around on the shop floor for too long anyway. Typically ours have no more than a max. of 3 days between all the various processes.

Storage time for immersion silver - we have a max. shelf life limitation for stock of 6 months, our supplier guarantees solderability for 12 months minimum.

All in all we rather like it over the alternatives.

regards,

Pete B.

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