On Mon, 15 Sep 2014, Noel Chiappa wrote:
From: Tothwolf
The conductivity of tin/lead solder is simply
higher than that of the
wire and terminal alone
I think you meant 'resistance', not "conductivity", right?
Right. The resistance of tin/lead solder is higher than the copper wire or
brass terminal (copper+zinc).
Can you tell
what size the wire is?
Sorry, I'm not good at telling those larger sizes apart. I looked on the
insulation to see if it said, and all it gave was some manufacturer
data, and a 600V rating.
Well, the 600V rating does tell you that it has a really heavy insulation.
Maybe they
used too large of terminal for the wire and the crimped
connection is bad?
I can verify that the opening in the terminal was 'full' of wire - i.e.
wire and terminal were size-matched.
So it was either 12 or 10 AWG.
I honestly don't think that it was the
wire-terminal connection that
caused the problem. For one, the insulation on the wire right up at the
terminal shows no damage (it's still nice and flexible, I can put
thumbnail prints in it - unlike the insulation on the terminal, which
was extremely brittle (I wound up cutting the insulation off the
terminal with a Dremel to check out the crimp - replaced with
heat-shrink), and if the wire-terminal connection had been bad, I'd have
expected the wire there to get pretty hot. Also, when I pulled the
terminal off the lug, melted solder from the terminal-PCB connection had
run down onto the lug, and you can see the shape of the terminal in it.
So either i) they soldered the lug onto the PCB after the terminal had
been placed on the lug (very unlikely), or the solder melted from the
overheat. (Some overheat!) Would that much heat from a poor
wire-terminal joint have made it to the PCB? Anyway, given both of
these, I suspect it was either the terminal-tab connection, or the
tab-PCB connection.
It sounds like the connection between the tab and terminal is where things
went south then. It still could have been caused by either an
overload/overcurrent condition or a bad connection between the two. I have
no doubt that terminal would have been glowing orange when it got hot
enough to melt the solder.
Is that terminal a dual crimped type with a separate
insulation crimp?
If you look into the end of the connector, there would be a
secondary
crimp grabbing the wire's insulation.
I _think_ it's double crimped, but it's not 100.00% definite. Most of
the other terminals, all the smaller ones, are very definitely double
crimped. The handful of other large ones (from the full-wave bridge to
the heat-sink/bus-bar) look a lot like this one - strong indications of
a double crimp, but not absolutely positive.
If the other terminals have an insulation support, it is probably safe to
assume that one did as well. I would have expected them to use high
quality terminals with an insulation crimp anyway.
At the very
least, the pc board mount tab should be resoldered.
Yep, did that - although I'm a tiny bit worried that I got a cold solder
joint on the pins, the solder didn't really flow up them the way it
should. There may be burned gunk (probably from the old rosin) on them;
I may remove the existing solder, and take corrective action.
The original solder will likely have to be removed first. As hot as it got
(hot enough to burn the tin plating on the terminal), it would have cooked
the solder too. The black stuff isn't burned rosin flux, but rather burned
tin. If you don't have one on hand, a RMA flux pen will help when you
resolder the terminal.
MG Chemicals 835 10ml flux pen:
http://www.amazon.com/dp/B0080X79HG