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June 1999

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Subject:
From:
David D Hillman <[log in to unmask]>
Reply To:
"TechNet E-Mail Forum." <[log in to unmask]>, David D Hillman" <[log in to unmask]>
Date:
Tue, 15 Jun 1999 18:20:13 -0500
Content-Type:
text/plain
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text/plain (91 lines)
Hi Paul - And you thought solderability was a simple subject!  The
solderability comparison data you were looking at was developed using a
wetting balance. During the wetting balance test the fluxed test sample is
lowered into the solder pot (or some equipment uses a solder globule). As
solder "wets" the surface in the case of a tin/lead finish or the finish
diffuses in the case of gold (silver, etc.) an force measurement device in
the wetting balance test head records the force values - basically Young's
equation. The buoyancy of the test sample (the amount of molten solder the
test sample bulk displaces) as it enters the solder bath does give the
instrumentation a negative force but that can be calculated out of the
measurement leaving you with a the wetting force. If a test sample is not
solderable the end result would be a negative force as describe in the
paper you read - no wetting occurred thus no force pulling down on the
specimen to be recorded. An analogy would be dipping your finger coated
with oil into soapy water - you wouldn't get the water to wet your finger
until the soap dissolves the oil allowing the water to wet up your finger
creating a positive wetting force. Take a look at chapter 7.2.1 Wetting
Balance Method in Klein Wassink's book (ISBN 0-901150-14-2) for a better
description and more information. In the gold case you mention if once the
gold diffuses into the solder pot and the underlying substrate is not
wettable then a negative force would result. Hope this helps.

Dave Hillman
Rockwell Collins
[log in to unmask]





PAUL BROWN <[log in to unmask]> on 06/15/99 02:42:17 PM

Please respond to [log in to unmask]

To:   [log in to unmask]
cc:
Subject:  [TN] negative force




Hi all,
I was just reading a paper which was comparing the solderability of
different surface finishes. In it the show electroless nickel/ immersion
gold to have a wetting force of 0 uN/mm at "2 sec" just after
fabrication. this value becomes negative i.e -0.1 to -0.2 after reflow
ect.... What does this mean? Does this mean that solder is actually
repelled by Ni/Au  finish? Is this only due to the short duration
(2sec.) that the dip occurred? Does the value become positive; at say 5
seconds ? How does this affect a surface mount assembly, will parts be
displaced prior to soldering? I was under the impression that gold was
highly solderable......................I need help, but will settle for
answers...............Paul Brown

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