IF reflowing the internal MCM joints is a genuine concern, and you are
going to reflow attach these with other devices rather than separately,
your best bet is probably a "Pb-free" paste. These use SAC type alloys
melting in the 217C range.
More important than alloy though you need minimum superheat on
processing to limbo under the 230C peak so find a decent supplier.
However this implies virtual zero delta T in your process.
Also you need to be aware of the implications for changing your solder
type, it would be a good idea to review the surfaces finishes etc with
your prospective supplier.
If that doesn't work there maybe 1 or 2 other possibilities but these
will cost.
Example: Separately attaching the MCM would allow internal Sn63 as now,
but you could use a lower MP solder [than Sn63] for device attach.
Regards
Mike Fenner
Indium Corporation
T: + 44 1908 580 400
M: + 44 7810 526 317
F: + 44 1908 580 411
E: [log in to unmask]
W: www.indium.com
Pb-free: www.Pb-Free.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Sergey Ermolin
Sent: Tuesday, December 16, 2003 4:04 PM
To: [log in to unmask]
Subject: [TN] high-temperature alloy and process for a multi-chip module
Hi.
I am desinging a multi-chip module. In my desing, two off-the-shelf IC's
in TQFP packages are mounted on an FR-4 PCB. This board has a Land Grid
Array footptint on the bottom and will be soldered to the motheboard
using a standard reflow process with Sn63Pb37 alloy. What alloy and
temperature profile would you recommend for assembling the multi-chip
module itself? The chips on the module were not designed to withstand
more than 230 deg C and are normally soldered using the same Sn63Pb37
alloy...
Thank you for your help...
Regards,
Sergey Ermolin
Dust, inc
2560 9th Street, #218
Berkeley, CA 94710
510/225-2137
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