Thank you Richard for expanding my short replies :-)
Vladimir
SENTEC
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-----Original Message-----
From: "Stadem, Richard D." <[log in to unmask]>
Date: Fri, 27 Feb 2009 11:11:56
To: <[log in to unmask]>
Subject: Re: [TN] Reflowing CBGA component (with high Sn balls) using Eutectic solder paste
Just to clear up the confusion from the previous responses:
To my knowledge, the IBM PowerPC components are ceramic ball joint
arrays (CBGAs) that utilize a Pb90 ball, not Sn90. The melting
temperature of these primarily lead balls is way up over 250 deg. C.
They are not intended to be brought into liquidus. These lead balls are
attached to the CBGA package with Sn63 solder, and they are intended to
be used with standard Sn63 solder paste to form the SJ to the pads on
the assembly.
Ceramic-body CBGAs typically use Pb90 solder balls because the lead
balls have a higher modulus of elasticity, providing some strain relief
between the very low CTE of the alumina oxide component and the higher
CTE of a standard FR-4 PWB.
Because the Pb90 BGA balls do not go into liquidus, but the Sn63 paste
does, a small amount of the lead ball dissolves into the molten Sn63 to
form the solder joint.
It is important, however, to either increase the standard reflow profile
and/or the MRT slightly to get good wetting of the Sn63 solder over the
Pb90 ball and get a good homogenous mix, but you do NOT need to melt the
Pb90 ball.
I have found that if you attempt to use the standard reflow profile you
will get what Mr. Macko is seeing, good wetting on the board pad, but
not quite perfect on the Pb90 ball. Increasing the TALT or the MRT
slightly (perhaps by 10 deg. C) usually fixes the issue.
If your IBM PPC CBGAs do in fact have Sn90Pb10 solder balls, then you
would use a Pb-free profile with enough TALT and a high enough MRT to
form a good homogenous SJ.
I do not have any experience with soldering Pb90 CBGAs with lead-free
solder, but I assume the principle is the same as soldering with Sn63.
However, with the higher temperature lead-free profile and using a Sn99
solder, I am assuming more of the lead ball will be dissolved into the
solder joint, and the homogenity will be less, which will result
possibly in a solder joint of lesser reliability. But I may be wrong
about that. Others with experience in this care to comment?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joe Macko
Sent: Friday, February 27, 2009 10:04 AM
To: [log in to unmask]
Subject: [TN] Reflowing CBGA component (with high Sn balls) using
Eutectic solder paste
Hi All,
My question is related to the soldering of IBM PPC CBGAs type components
(that have Sn90Pb10 solder balls) using Sn63 paste onto ENIG finished
boards. What are the "determining-factors" to improve the
wetting/diffusion of the eutectic solder paste into the high Sn solder
balls. The eutectic solder appears to wet well to the ENIG pads located
on the board but occasionally I see eutectic solder fillets that do not
always appear to fuse well with the high Sn content balls (if fuse is
the correct word?). thx
Best Regards,
-Joe
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