Hi Rob!
Just to add a quick note to the other response's on immersion tin and
tin whiskers - electrolytic tin is not the only tin finish type that
can exhibit whiskers. Take a look at Solder in Electronics by Klein
Wassink, ISBN 0-901150-14-2, pages 197-202 for other causes of tin
whisker formation. Many folks have solved the tin whisker problem by
using a small percentage of another element (i.e. lead) to "poison"
the tin and prevent the formation of the whiskers. The bigger issue is
the poor solderability of the tin finish itself. Two other sources of
info which may be helpful are:
"Influence of Temperature, and Humidity on the Wettability of
Immersion Tin Coated Printed Wiring Boards", by Urmi Ray, Iris Artaki,
Paul Vianco, IEEE Transactions on Components, Packagaing,and
Manufacturing Technology, Part A, Vol. 18, No.1, March 1995.
AMP's web page - Whiskerless tin:
http://www.amp.com/product/articles/dd50.html
Have fun!
Dave Hillman
Rockwell Collins
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______________________________ Reply Separator _________________________________
Subject: immersion tin
Author: [log in to unmask] at ccmgw1
Date: 1/16/97 11:13 PM
Hello to all,
I thought " tin whiskers" only occurred with electrolytic
plated tin. Is it not the stresses induced during plating that
provide the energy to produce the whiskers? In any case I
have used immersion tins on pcbs for many years, both
as a short term finish in captive shops and as a rework for
outdated HASL boards. The new 1 part liquids make the
process much easier. You need only about 20 mins at
100 F to put down about 20 millionths. thats millionths with
six zeroes, certainly not enough to sustain whisker growth.
As with all plating processes the base metal must be
"meticuously"? well very clean. Copper will require a
microetch , tin/lead may require a solder brightener step
if highly oxidized. If the copper is not clean immersion
tin will not take thus making inspection easy. As always
the solderability time limit will depend storage conditions.
I've seen them last for 6-8 weeks on the floor during a brutal
South Carolina summer.
Remember, all the tin is doing is protecting that perfectly
clean copper surface from the elements with something that
will help your solderpot ratios. Contact your supplier for the
processing details. But try it you'll like it. And if the project
gets hung up for some reason don't fret, just wrap them up like
Cristmas leftovers and keepem in the fridge.
Robert E. Welch
Process Specialist
Email: [log in to unmask]
Modem: 804-239-9120
Fax: 804-237-3048
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