An example of this that I can report: I needed to solder to ceramic
blocks that had sintered silver traces and pads. When using standard
63/37 solder paste, the sintered traces sometimes dissolved into the SJ.
From what I learned about why they use .5%-.7% Cu in Pb-free paste to
better form an IMC with the copper on the board, I reasoned that the
same phenomena may help in soldering to silver traces with a reduction
in leaching. So I switched from 63/37 solder to 62/36/2%Ag. The
dissolving of the silver traces and pads was significantly less. We
never saw that problem again.
Did I have a different solder joint metallurgy when using the 2% silver?
Of course. So I re-qualified the process by cycling to failure the same
way. The mean time to failure dropped from 1200 cycles to about 950 when
using 2%Ag. However, this was still more than enough to satisfy the
customer. More importantly, all 50 samples passed to a minimum of 950,
as opposed to 42 passing to 1200 with 8 failures when using 63/37. When
I went back and looked at the 8 failures soldered with 63/37, it became
apparent that they were due to the pad leaching away such that the
connection opened up, not from brittle fracture.
Which would you rather have? The embrittlement of the solder was most
likely there when I added 2% silver solder to the mix and it did reduce
the overall life to failure, but did not contribute to the opens seen as
outliers in the process that was out of control using 63/37. There were
fewer failures with a more brittle joint to 950 cycles. The 2% silver
process reduced the overall life, but increased the reliability of all
samples in the lot to get to that 950-cycle point.
I told you all of this to make an observation that supports what Werner
is stating. You may be able decrease the leaching of the silver pads by
adding some silver to the solder, but you will pay a price by increasing
the brittleness of the IMF itself. You seldom get anything for nothing.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Tuesday, March 07, 2006 8:10 PM
To: [log in to unmask]
Subject: Re: [TN] soldering to PdAg cap

Hi John,
No metrics on the leaching 'slow down'--only anecdotal info, so I do not
know for sure that even the slow-down is real--physically, I cannot
think of a mechanism for txhe slow-down, but too many people report
having observed it.

Werner

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