Hi Ingemar,
You write:
>W,
>can't you tell a little about the 'time to maximum wetting' also? It's
>remarkable how little this is mentioned. Tx oh guru.
Should have thought of that, but since I am at my son's house and am
distracted by my 3 granddaughters (6, 4, and 2) who all want the attention of
their 'Opa,' I hope I am forgiven by making up for this oversight now.
Timing is indeed essential and a big part of what profiling tells you.
A widely quoted, but irrelevant, reflow parameter, the time above the Solidus
temperature of 183°C for Sn/Pb solder (the Liquidus is the same as the
Solidus only for the eutectic solder composition), ranges from about 75 to 95
seconds. However, a better and more appropriate guideline is 15 seconds at
or above Liquidus+20°C is just adequate for soldering to copper; for
soldering to Ni I recommend Liquidus+35°C.
Since this has to be at the solder joint of the whole assembly heating up the
slowest, all other SJs will see higher T's for longer times. However, other
than for too thin a HASL-coat, I have never seen a SJ failure due to too
much IMC thickness. A major component manufacturer has reported that the peak
reflow temperature for BGAs with Ni/Au metallization had to be increased
from 235 to 255°C to prevent interfacial separation between the Ni and the
solder.
You may watch for my Reliability column in Global SMT & Packaging magazine.
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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