The cooling rate of solder directly effects the resulting
metallurgical microstructure of the solder joint.  In solder, a rapid
cooling (not quenching, but of several degrees C per second)
results in a lamellar eutectic microstructure, where there are very
fine, ordered grains of Sn and Pb rich areas.  (The grains look like
fine lines throughout the solder).  A slower cooling rate will cause
the microstructure to have coarse, globular grains of Sn and Pb in
a basically random arrangement.
        The lamellar microstructure (from fast cooling) is much better at
resisting the motion of dislocations, and thus gives a stronger,
more creep resistant structure.  Also, the faster cooled solder will
look shinier - which makes the QA people happy, while it is not
necessarily a great indicator of how fatigue resistant the actual
joints are.  Here's the kicker - at time, especially with elevated
temperatures, the lamellar microstructure will grow fairly quickly.
(They'll even coarsen at room temperature.)  Thus the quicker
cooled solder joints will have superior initial strength, but over time
and temperature will have comparable strength to the slower cooled
solder joints.



Brett Goldstein
EVI, Inc.


> Technet,
>
> I have recently read in an article that the most favourable type of solder
> joint is formed from rapid cooling of the solder.
>
> I am a bit confused with this as I have always been under the impression
> that rapid cooling of metals results in tight compact grain structures
> (very hard, but brittle). This is in contrast to slow cooling that forms
> long grain structures resulting in a stronger and tougher material (which
> I would have thought is what would be ideal for a solder joint - good
> shock resistance).
>
> Regards....
>
>
> Clayton Gardner
> Engineering Manager
>
> A.E.M.S
> 11-13 Fiveways Boulevard
> Keysborough VIC 3173
> AUSTRALIA
> Ph. 61 +3 9701 5499
> Fx. 61 +3 9701 5422
> Mb. 0416044552
> Email:  [log in to unmask]
> WEB:  www.aemspl.com.au
>

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