When the joint moves while in the plastic stage it is still electrically
connected -- when the joint moves with a eutectic mixture it cracks
-- the joint is very weak when still close to the liquid stage. With the
non-eutectic mixture the crystals of tin provide strength while the
solid is cooling.

I'm not an expert on the physics of materials, but this flies in the face of
what I think I understand.

How can a eutectic mixture ever be "close to" the liquid stage? To
paraphrase Yoda -- "It either is, or is not."


I might not have explained it very well, but I assure you, with total
honesty, that the reason 60/40 was popular was the better reliability
for point to point soldering and the reason 63/37 is now taking its
place is that point to point has all but disappeared and the eutectic
mixture gives slightly lower thermal stress.

You might not consciously not be telling the truth, but that doesn't mean
what you say is correct.


Either that, or you believe our antecedents were morons, who while
they knew about the eutectic mixture, were too stupid to use it, or
too clumsy to mix it, or they thought a few dollars extra per ton of
solder was too much to pay for the good stuff.

The difference in price is not just "a few dollars extra per ton". A
significantly higher price is almost certainly the explanation.

When I was a kid and assembled kits, the instructions always warned that the
connection /had/ to be mechanically strong before it was soldered. (I
remember the Knight-Kit photographs very well.) This isn't true in
practice -- as I commented in a recent posting, J Gordon Holt fought against
it -- but it virtually guaranteed that the wires wouldn't move during the
soldering process. If the solderer heated the joint sufficiently, a cold
joint was essentially impossible, eutectic solder or not.

This didn't keep customers from making bad joints, or altogether missing
joints to be soldered. These, I believe, were the principal causes of
non-working kits. Eutectic solder would have helped only a little (ie,
there's a limit to how much you can prevent human error).

About a year ago I had to replace a high-current driver in an electronic
crossover. The device was a five-pin TO-style product. Removing the PC board
to unsolder it would have required removing all the input and output jacks.
(Yes!) So I had to clip the device's pins near the body, unsolder and pull
out the pin stubs, then suck out any remaining solder. I was grateful I had
eutectic solder, because it made it easy to re-fill the holes and suck them
clean, then solder in the new chip with virtual certainty the connection
would be good. (It was.)

Confession time... One basic rule of soldering is "Get the joint hot enough
to melt the solder. Do not melt the solder directly with the iron." I've
been ignoring that for nearly half a century, and have never had a bad
joint.


According to the Kester Website...

"Sn60 has a plastic range and puts down a slightly thicker coating of
solder. Sn60 is often preferred for lead tinning and other solder coating
applications. Sn63 is eutectic and as such has no plastic range. Generally
it flows better than Sn60 and is the preferred alloy for wave soldering and
surface mount applications."

This isn't a complete answer -- I assume the cheaper 60/40 makes sense when
coating, because no joint is involved. However, "flows better" seems to be a
desirable characteristic when soldering joints.

I'm willing to be proved wrong about anything. I'd like to see a document --
preferably from a solder manufacturer -- that explains why 60/40 is less
likely to produce a cracked joint than 63/37.