Ioan,
You have local contamination...
Look at the both the shape of the interface and interfacial granularity
between the solder on the pad and the solder ball...
Smooth curved interfaces typically mean no alloying or no wetting to
between materials... The temperature is hot enough for the solder paste
to melt, so there should have been some localized wetting to the Pb-free
ball but it can't be confirmed in this view...Wetting increases the
interfacial granularity..
Try cleaning the BGAs prior to placement...
Remember Pb-free means more Sn oxide on the bulk surface...
Paul
Paul Edwards
Process/Quality Engineering
[log in to unmask]
Tel: 408-433-4700
FAX: 408-433-9988
Surface Art Engineering
81Bonaventura Dr.
San Jose, CA 95134
DUNS: 944740570
CAGE/NCAGE: 1XZ48
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
Sent: Wednesday, November 15, 2006 6:57 AM
To: [log in to unmask]
Subject: [TN] Separation in BGA joints
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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