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Tombstone/ Poor welting

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#53679

Tombstone/ Poor welting | 18 February, 2008

Hi everybody, I found a defect varistor 0603 Tombstone and poor welting when we use component that terminal is Silver palladium (Ag/ Pd) with Leaded solder paste (63/37). Then we change alternative component that terminal is Ni/ Sn the result not found defect.

We would like to know 1. Silver palladium concern with leaded solder paste? 2. PCB has via on pad (defective location) when we use component that terminal is Ag/ Pd we found Tombstone. But after try to use alternative component that terminal is Ni/ Sn not found Tombstone defect. Why? 3. Do you have any suggestion for reduce or fix this issue?

Thank

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#53680

Tombstone/ Poor welting | 18 February, 2008

Silver palladium is intended for adhesive applications. You need components with a nickel barrier. While you're waiting for others to reply, search the fine SMTnet Archives [ar and pd] to find postings like http://www.smtnet.com/forums/Index.cfm?CFApp=1&Message_ID=49101

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#53730

Tombstone/ Poor welting | 20 February, 2008

I've done some pretty scary cross-sections of chip sized coils with AgPd terminations with Pb free solder. The AgPd went into solution and there was essentially nothing left to solder to. We also had convex fillet shapes that clearly didn't meet workmanship standards. In my opinion, AgPd doesn't belong in a solder joint.

I didn't look at any SnPb joints. I'd expect the same phenomena, but maybe at a slightly reduced level.

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#53745

Tombstone/ Poor welting | 20 February, 2008

Pd/Ag doesn't usually dissolve (leach) nearly as bad as Ag only. In my hybrid days, we would use 62/36/2 solder. The 2% silver helps saturate the solder joint and thus keep the Ag from being leached away by the Sn.

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#53750

Tombstone/ Poor welting | 21 February, 2008

I have seen smt chip varistors with Pd/Ag teminations that had poor wetting in terms of good fillet in LF (sac305) soldering. We found that the solder tended to be attracted to the plating on the part and off the actual pad (silver Immersion). This created SAC solder to bead on each end of the chip and virtually no solder holding component onto the pad. All other parts on PCB where perfectly soldered, only this pd/Ag part gave issues. The customer changed to matt tin plated version of part and got perfect soldering, good fillets, same profile etc.

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#53754

Tombstone/ Poor welting | 21 February, 2008

I agree, the level of Pd plays a role here. Knowing that some versions were probably OK we tried to develop a soldering test for PdAg components with pass/fail criteria, but decided it wasn't worth the engineering effort for a finish that's so rarely used. For the few PdAg parts we were dealing with it was easier to get the suppliers to use a different finish or find other sources.

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