George, I am usually 100% with you, but this time I am only 90% with you.
Kevin, I need to answer as follows. The key here is the word "Immersion" - meaning the "I" in both Immersion tin and immersion silver. Immersion plating involves taking one metal ion out of solution to deposit it on some base metal surface (copper). BUT, a bit of the base metal (the copper circuits) must go into solution to provide the reducing electrons for the tin or silver metal to deposit. It is the same electromotive (battery) effect for all reduction/oxidation reactions.
Kevin, yes your supplier could take off the immersion tin (metal stripping). There are many strippers that dissolve tin, without taking off copper. Then, he could apply immersion silver by simply re-plating the boards in an immersion silver bath. NO, he could not simply IMMERSION plate silver over tin. There is not enough "battery power" in the electromotive series to do that. Immersion plating reactions all have an order, just as battery cells give different voltages - based on the pair of metals that make up the battery.
George's example in board fabrication uses electroplated tin to protect the circuit traces in etching. Only once have I seen immersion tin used as an etch resist - where the circuits were formed by laser abrading the very thin immersion tin, and the alkaline etchant was run at a deliberately low pH.
One final warning - your boards will have less copper on them after the second immersion plating with the silver. You ought to check the thickness by cross section. I doubt there is an issue, as immersion plating does not take off too much copper for each plating, but better safe than sorry later in performance or blow holes in assembly.
Denny Fritz
-----Original Message-----
From: Wenger, George M. <[log in to unmask]>
To: TechNet <[log in to unmask]>
Sent: Thu, Jan 27, 2011 2:40 pm
Subject: Re: [TN] Re-finish ImSn to ImAg ?
Kevin,
Yes it is possible to remove Sn and then do ImAg plating. Actually many PCB
abricators use Sn as an etch relief when they fabricate circuitry on PCBs and
hen etch the Sn off before a final surface finish is plated. That final finish
ould be almost anything (e.g., ImAg, ENIG, ImSn, HASL, etc.)
Regards,
eorge
eorge M. Wenger
enior Principal Reliability / FMA Engineer
ndrew Corporation - Wireless Network Solutions
0 Technology Drive, Warren, NJ 07059
908) 546-4531 Office (732) 309-8964 Mobile
-mail: [log in to unmask]
-----Original Message-----
rom: TechNet [mailto:[log in to unmask]] On Behalf Of Glidden, Kevin
ent: Thursday, January 27, 2011 2:19 PM
o: [log in to unmask]
ubject: [TN] Re-finish ImSn to ImAg ?
Question of PCB finishes...
We changed specification on a PCB from ImSn to ImAg. The PCBs were built with
mSn, so we returned them. The PCB supplier is stating the ImSn was removed and
hey were replated with ImAg. Is this possible, or did they just plate Ag over
n? If it IS possible, is it safe for the rest of the PCB?
Thanks,
Kevin
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