Hi,
When discussing solders and soldering, we should get our definitions and
terminology straight, so we do not talk two differtent 'languages' and miss
any points made.
Tin-lead solder does NOT reflow at 179-183oC. 183oC is the Solidus for these
solders; that is the temperature below which the solder is completely solid.
For the pure eutectic solder 183oC is also the Liquidus (not Liquidous); that
is the temperature above which all the solder is in liqud form. Any other
combination of tin and lead (as well as contaminations from disolved metals,
e.g. copper) will have a Liquidus higher than the Solidus, which remains
constant (except for the mentioned contaminations). The more the combination
differs from the eutectic, the higher the Liquidus. At temperatures between
the Solidus and the Liquidus the solder is pasty.
Reflow does not take place at practical rates just above Liquidus. It takes
thermal energy to affect the eflow process. The reflow process consists of
(1) the liquification of the solder, (2) the flowing of the solder along the
surfaces to be wetted, and (3) the wetting of the surfaces, which involves
the disolution of some of the base metal into,tin to form intermetallic
compounds (IMCs). No IMCs equals no wetting. Therefore, for practical reflow
processes there is a minimum reflow temperature that is about 20oC above the
Liquidus for the solder used when soldering to copper (soldering to nickel or
alloy 42 requires more thermal energy and thus a minimum reflow temperture of
25 to 30oC above Liquidus; or a longer dwell at peak reflow temperature).
The peak reflow temperatures reached will vary across the assembly; solder
joints for small chip components will reach the highest peak reflow
temperatures, whereas solder joints for massive components, acting as heat
sinks, or shadowed components (BGAs) will reach much lower peak reflow
temperatures. The reflow profile will have to be set for these solder joints
to reach minimum reflow temperature.
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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