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July 13, 2008

Effect on Eutectic Melting point for a multiple alloy board

How do multiple lead-free alloys, combined with a tin-lead Solder Paste, lead-free components, and tin-lead components effect the Eutectic melting points for a solder joint?

As an example: Let's say I am starting with SnPb solder paste and add a few SnPb components with an ENIG (Immersion Gold over Electroless Nickel) circuit card and I add a SnAgCu (Tin, Silver, Copper) alloy BGA. Next, I add some SOICs, QFPs, and PLCCs where the specs identify Tin (lead-free) leads.

This brings ENIG, SnPb, SnAgCu, and possibly 100% Sn all together in one spot, with the common denominator being the SnPb solder paste.

R. S.

Experts Comments

The simplest answer is that you get a mess.

Consider this; if you take equal amounts of 10/90 tin-lead solder and eutectic 63/37 solder and melt them you end up with 37.5/63.5 solder. The tin-lead binary phase diagram tells us that the liquidus of this composition is 247C instead of the 183C we would expect with eutectic solder or the 299C for 10/90.

Add some SAC with silver, copper and more tin and the resultant mess makes an indistinguishable porridge that defies classification. Given that the eutectic for tin silver is over 200C and the addition of more tin to SAC raises the liquidus beyond 217C, one could assume that you would need higher temperatures to get this porridge to melt and form a joint. But the actual temperature is almost impossible to predict.

The bottom line is don't mix because you'll never know for sure what is going on and even worse if you get it to make an acceptable looking joint there is no knowledge about reliability.

Fred Dimock
Manager, Process Technology
BTU International
Mr. Dimock is the manager of Process Technology at BTU International. His extensive experience in thermal processing includes positions at Corning, GE, and Sylvania. He has authored numerous articles on lead free processing and process control, taught classes at SMTAI, and participated in the IPC Reflow Oven Process Control Standard committee.
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