Ioan,
When I look to the phase diagram of Sn-Cu I see a steep increase of
the melting point when the copper/tin balance increases. I don't have
a large Sn-Cu phase diagram, only a small one.
But, if  0.7% copper in tin gives an eutecticum of minus 5dC with
respect to the melting point of tin, I expect for 1.75% copper in tin
a delta T of at least about + 20dC, so about minimum 250dC. With a
reflow temperature of 246dC, the BGA sphere will neither melt nor
dissolute the SAC on the solder pad.

Regards,
Peer Langeveld
Consultant Soft Soldering Processes



2006/11/15, Tempea, Ioan <[log in to unmask]>:
> Hi TNLF-ers,
>
> please have a look at http://stevezeva.homestead.com/files/Ioan_Ball.jpg , many thanks to Steve again.
> What can I tell you: no-clean SAC305 at the bottom, the balls read a sort of SAC with 3%Ag and 1.75%Cu
> The reflow profile seems to be OK, I measure 246C under the periphery of the BGA, with a thermocouple that is squeezed there. I know it is not rigorous, I did not drill, I am not measuring inside the joint. But even if I am 20C off, the materials should be molten and coalesce, isn't it?
> The intermetallics solder paste - PCB pad are OK, the wetting is good, the BGA balls seem to have changed shape, they are almost collapsed, there is physical contact between the balls and the paste, but no joint.
> The question is: why do I see the separation line? What can cause that? Should I bother investigating the process, or do I have a sort of contamination? Or the BGA ball alloy has a higher melting point?
> Thanks,
> Ioan
>
>
>
>
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