I used to do a lot of soldering of MEMS LCCs to a ceramic block that was
in turn soldered to the PWB. The process was to pre-bump all of the
sintered pads on the ceramic block with solder, reflow the MEMS on the
block and then later the subassembly would be reflow soldered to the
PWB.
We initially had a problem with the sintered silver traces/pads leaching
away into the 63/37 solder. When I switched to a 66/32/2Ag alloy the
problem disappeared. So I can tell you for sure it works when using that
alloy.
However, when we attempted to use a SAC alloy we had a much higher rate
of the silver traces/pads leaching away. Virtually all of the sintered
silver dissapeared into the solder joint.
With SAC you are soldering with a much higher tin content. I presume
high tin content is what caused the sintered silver traces to dissolve
(leach away) much more than other alloys. For this type of soldering to
ceramic I would not recommend a SAC alloy be used; an exemption should
be sought.
This is the problem with RoHS, how to cover all of these exemptions. I
believe that Pb9010Sn solder balls on CBGAs are exempted from RoHS (BUT
I could be wrong). I do not know if there are any other exemptions
covering issues like this.  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Greg Major
Sent: Monday, June 25, 2007 5:42 PM
To: [log in to unmask]
Subject: [TN] Silver Leach

Hello TN,

When soldering to a ceramic board with silver tracks sintered to it, has
anyone come across a requiremnt to use a higher silver content than SAC
to prevent leaching of the silver?

I'd assumed that the (almost) 4% silver in SAC would have been more than
enough to make the solder and substarte virtually electro-chemically
equal and prevent leach/dissolution.

Pre pb-free, I remember reading about folks using 96S alloy (Tin & 3.7%
Ag) to solder to such boards.

Given the liquidus point of 96S, I wouldn't have thought there would be
much differnce with SAC.

Any ideas anyone?

cheers

Greg

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