Thanks to everybody who replied, including the non-metallurgists ;-} I have certainly learned things!

I think the secret lies in that 270C melting point. I use a regular SAC305 soldering recipe (right semantics, not profile; see, I got this one Werner), hence the 240C target peak. Of course this will not move the 1.75%Cu alloy.

One thing is that the component data sheet says the alloy is SAC305. But it also recommends 260C for peak temperature. I find this very weird, since somebody told me we need to solder at MP + 20C, so my 246C should be more than OK. Not to mention that many ICs on the board are speced per J-STD-020C for max peak of 250C.

Question 1: why this 260C recommendation, when 240C should be OK?

Then, I would like to know more about the case of high temp solder balls. Why a joint will be formed around 210C between SnPb paste and 90/10 balls and the solder would not wet at 246C on a 270C solder ball? The flux cushion is there in both cases. And why the SAC ball should melt anyway, if solder wets on 90/10, why shouldn't it wet on other higher temp balls? Is it because the Pb goes into solution with the Sn whereas the high concentration Sn-Sn interfaces absolutely have to mix?

Thank you very much,

Ioan



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