To goo or not to goo - Solderi

[Don McGrath <vintner@GENIE.COM>]

Peter VanDerWal writes:


  > Whoa!  Bad idea,  you should never use solder on a crimp connection.
  > Solder breaks down when under compression and you'll end up with a high
  > resistance connection.

PETER B HOWELL writes

  >         Now here's an effect I haven't heard of before.  How does
  > pressure  break down the solder, and what does it break down into.  I'll
  > refrain  from any alchemy jokes until I learn more.  :-)

Peter VanDerWal writes:

  > ... Solder actually melts the metals being soldered and forms an  alloy
  > with them. So instead of having two pieces of copper glued   together by
  > solder (a Lead/Tin alloy) you end up with two pieces of  copper welded
  > together with a Copper/Lead/Tin alloy....
  > ...If you submit this Copper/Lead/Tin alloy to high pressure it
  > eventually  breaks down into it's composite metals.  I don't know why,
  > I didn't  understand the physics of it 10 years ago, but since I read
  > about the  same reaction in 3 different books written by different
  > authors I  accepted it.

It's true that a Cu/Pb/Sn alloy does form at the interface between the
solder and the copper wires.  But it is a very, very thin alloy layer and
contains a very small percentage of copper.  60/40 tin-lead solder melts at
183 C (361 F).  The melting point of copper is 1083 C (1980 F).  At the
temperatures where solder flows, very little copper dissolves into the
solder.  The solid solubility of copper in lead is less than 0.007%. The
solubility of copper in tin is less than 0.006% at the Cu-Sn eutectic
temperature of 227 C (441 F).

I think Mr. VanDerWal is assuming that the joint is soldered first, then hot
crimped.  Yes, this would subject the solder to high pressure.  But even so,
the solder would not "break down".  Metallic phase diagrams change little
with pressure, even thousands of atmospheres of pressure.  Lead-tin solder
is a eutectic system.  When it solidifies, it separates into two phases, one
phase being essentially pure tin and the other phase being lead containing
about 2% dissolved tin.  So Mr. VanDerWal is correct in saying that solder
"breaks down".  But it always does when it freezes, regardless of pressure
effects.  So the high pressures encountered during crimping should have no
effect on the solder.

I think most people who both crimp and solder do the crimping first; then
flow solder into the joint to fill the voids (if it is indeed possible to
fill all the voids).  In my personal opinion, I don't think that adding
solder would do much to improve the conductance of the joint, although it
might enhance the corrosion resistance of the joint by preventing air or
other corrodents from entering between the unbonded wires.  My opinion is
that a good hex crimp (a double crimp!) is all that is needed, and that if
the joint is protected with a section of polyolefin heat shrink tubing (with
meltable adhesive liner), that should be sufficient to prevent all
corrodents from entering the joint (except perhaps a little air that might
diffuse through the tubing).

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 |  Don McGrath                                         \\        |
 |  Villa Helena Winery   <donmcgrath@alum.mit.edu>   O O O O     |
 |  St. Helena, CA, USA   <vintner@pobox.com>          O O O      |
 |  Tel (707)963-4334                                   O O       |
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To goo or not to goo - Solderi