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June 2001

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 4 Jun 2001 18:04:08 EDT
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Hi Ryan,
Solder becomes liquid [not liquidous] when it exceeds its Liquidus
temperature. If it is on a non-solderable surface, no metallurgical bond will
form no matter how long it sits there in liquid form. On a solderable
surface, it takes thermal energy, the amount will depend on the metals
involved, to bring the base metal into solution with at least one constituent
of the solder to form an intermetallic compound layer. The longer the time
and the higher the temperature above Liquidus, the thicker the IMC layer.
Also note, the IMC layer will also grow when the solder is not in liquid form
by diffusion, but much slower. If the solder in liquid form is in contact
with the base metal too short a time and/or the temperature of the
solder/soldering surfaces is too low, the metallurgical bond [i.e. the
formation of the IMC] may be inadequate for the requirements we impose on the
solder.
If you use vapor phase soldering, you can have the assembly at the vapor
temperature for the time required for good soldering. In reflow and wave
soldering, because longer dwell times equal ever higher temperatures, we have
to worry on the one hand, about being in the peak reflow zone/solder wave too
long and doing damage to the rest of the assembly, and on the other hand
being there too shortand not get proper metallurgical bonds on all the solder
joints in the assembly. A very fine balancing act, indeed!

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], Website: www.engelmaier.com

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