Palladium Silver surface finish

Indy: How is this situation different than the last time you posted: http://www.smtnet.com/Forums/Index.cfm?CFApp=1&Message_ID=29371

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> Hi, I am trying to assemble a LTCC component > with Pd/Ag surface finish on to Sn/Pb/Ag solder > and Sn/Ag/Cu solder. I am passing it through a > reflow. I have observed poor wettability at the > solder-pad joint. Does anyone have information > regarding the solderability and wettability of > Pd/Ag as against the above mentioned solders. > Any help will be appreciated > > thanks Indy

I hate to say never but, you will never see wetting that is what you are used to seeing with 63/37sn/pb plating on those IC leads. A couple of things to keep in mind is that the solder paste flux that you utilize should have the highest activity that you can get from your vendor whether it be no-clean or water wash, also when using the lead -free paste sticking with the strickly tin, silver 96.5/3.5 sn/ag eutectic helps quite a bit. Most importantly the reflow profile should be a ramp to spike as opposed to a ramp,soak,spike profile, this will allow the flux to remain active right up to actual reflow, also run your PCBs faster by say 30% to 40% to minimize the dewetting that will occur with longer liquidous times, you will need to raise your reflow zone to accomodate the extra speed. Very rarely will you see any wetting at the top of the lead as with 63/37 solder. Wetting will be front back and sides of the component lead at best generally. Those of us that have been running the lead-free process for a while now have come to expect the not so pleasing changes that come along with it. I hope this helps and good luck!

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