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January 1998

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From:
Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 29 Jan 1998 09:52:41 EST
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Hi Marcello,
Unfortunately, your solder paste vendor is giving you a rule-of-thumb (30
seconds from the time the solder becomes liquidus, 183 deg. C), which works
well enough in most cases, but is technically insufficient and inaccurate. [By
the way, for eutectic Sn/Pb solder the Liquidus (the temperature above which
the solder is liquid or in a liquidous state) is 183*C, but for most solders
the Liquidus is higher because of non-eutectic compositions or other
contaminations, such as Cu or Au]. To properly reflow solder, which includes
the wetting of the surface to be soldered as well as forming some
intermetallic compound (IMC) layer with a constituent of that surface, e.g.,
Cu, Ni, you not only need to have liquid solder and a solderable surface, but
enough heat input to bring that surface constituent into solution with Sn. At
Liquidus that will take quite a long time. A better rule-of thumb is to have
every solder joint at least 10 seconds at or above Liquidus+20*C for a Cu
surface and Liquidus+35*C for Ni and Alloy 42 surfaces (Ni disolves much more
slowly into Sn than does Cu). That is the reason, why reflow profiles should
be measured at the solder joint that will heat up the slowest, e.g. near the
center of BGAs. In general, the solder joints of larger components (more
thermal mass) will heat up slower than small component or the PCB; differences
of as much as 20*C have been measured with 'Forced Convection' relow ovens,
more with 'IR' reflow ovens. The thickness of the IMC layer is unimportant
(you do not want it too thick, but that is outside of normal processing), as
long as you have some thickness. Your Au coating will let the solder spread
more easily over the surface, but it does nothing to enhance or improve
wetting other than presewrve the solderability of the underlying surface.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]

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