Alright Ioan, you asked for it,
<< I use a regular SAC305 soldering recipe, hence the 240C target peak.
A: The 240C target reflow peak will work IF indeed the target reflow peak 
temperature is measured on the slowest-heating SJ in the assembly.

<< 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.
A: The recommendation is a 'cover-your-behind' recommendation—unfortunately, 
many people do not properly profile, and that recommendations is aimed at 
those people.

<< 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?
A: The "flux cushion" has nothing to do with any of this. For both SnPb paste 
and 90/10 balls the Solidus temperature is 183C [their Liquidus temperatures 
are different , however], so they become at least 'pasty' at 183C and the 
solder paste fully liquid at 183C or just slightly more, facilitating wetting. For 
the SAC-solders the Solidus is 217C and the Liquidus slightly higher; 
however, even fully liquid they have significantly greater surface tensions and 
inferior wetting characteristics [this is likely related] than SnPb solders [this 
is the cause for more icicling, bridging, shorts] thus making proper wetting 
more difficult.

Werner

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