Wires and such

[tom cloud]

        [ snip ]

>Like David said, soldering don't cut it when you are talking about wires
>that move around. Even wires that attach to a circuit board are (gasp)
>wrapped, not soldered, when used in high vibration environments.

        [ snip ]

Soldered connections are still the standard for the majority of hi-
reliability connections.  Mil-Spec (I mention this, because mil-spec
and NASA specs are some of the toughest in the world) and NASA allow
crimp only under very stringent guidelines (of course, the same could
be said for soldering).  Wire wrap is NOT COOL for anything.  It's only
for temporary connections or for connections which are in a non-hostile
environment (i.e. no humidity or temperature extremes, no vibration, etc.
-- essentially in a building).

The definition of a good electrical connection:
        1. Low resistance.
        2. Mechanically sound.
        3. Gas tight (so no oxidation can occur).

Wire wrap barely meets those criterion.  It's low resistance because
there's a minimum of ten wraps around a square post (40 contact points)
using silver plated wire.  It's hardly mechanically sound -- if for no
other reason than it uses solid wire which easily breaks.  It is gas
tight because the wire is "work hardened" by being pulled under tension
around the post and therefore becomes springy and maintains a constant
pressure trying to unwrap itself (the wire is indented by the post
and those indentions, combined with the springiness of the wire, keep
the wire pulling against the 40 or more contact points.  That is why
the wire usually breaks when unwrapping a wire wrapped connection --
the copper has been work hardened.

Stranded wire is used for most applications because it is less likely
to break (copper, silver, etc. are *work hardened* by being bent,
twisted, hit, compressed, etc. -- that's why copper gaskets must
be annealed with a torch before being re-used).  Solid wire should
only be used where there is no possibility of the wire being moved
(like jumpers on circuit boards) and where vibration won't cause
it to break.  The advantages of solid wire are that it holds its
shape better than stranded and it's slightly cheaper.

Now, for soldered and crimped connections.  Crimping is a technology
like front wheel drive:  it's to save money for the manufacturer.
Soldering is very difficult to inspect.  It's very hard to train
a worker to produce consistently high quality solder joints and it's
easy to have a joint that looks good on the surface but which does not
join to the metal underneath, giving a *cold* solder joint.  (Recommend
a book on soldering from an electronics store or Rat-Shack).  Crimping,
on the other hand, is fast and, if the right tools and connectors are
used, can give very consistent results.

What's this about not soldering wires that move around?  Well, solder
*wicks* (by capillary action) up the strands causing the wire to
effectively become solid.  This increases its likelihood of breaking.
So, the rules specify how far the solder is allowed to wick (about 1/8").
There is no evidence that says properly crimped connections are any
better than properly soldered ones.  The reason I suggest soldering
is that most of the problems encountered in electrical / electronic
circuits are due to faulty connections.  You are not likely to find
good crimp connectors at your local parts house (nor will you find
a good soldering iron or solder there, for that matter).  It is just
my experience (and I have a bunch installing industrial electronics in
vehicles and ships doing instrumentation and communications) that
soldered connections, done by someone who cares, are far less likely
to give problems than crimped ones -- and wire wrap must NEVER be used
outside of a controlled environment like a home or garage.  NEVER
use wire wrap in a car, etc.

[A quick history of wire wrap.  It was patented by Gardner-Denver
company and popularized by the telephone company.  They used to have
to solder wires to terminal blocks in the switch station to hook up
new subscribers or to change 'phone numbers.  This was real messy,
and the terminal blocks were damaged after being soldered on several
times.  Wire wrap was much faster (they use punch blocks now).
The interesting thing to note here is that they originally thought
wire wrap would last indefinitely in a controlled environment.  Turns
out, however, that the connections began to fail after about fifteen
years even in the closed switch stations.  There's enough vibration
just from seismic activity, apparently, to affect the connections, plus
the wire's pull seems to relax.  Then the silver begins to oxidize
under the contact point and, eventually, the resistance of the connection
reaches unacceptable levels.]

Now, don't misunderstand, I crimp connections in my car.  And I'll put a
wire-wrapped doo-dad in it if I want to.  Just like I sometimes buy
parts from Rat-Shack when I know they're poor quality.  But, I won't
crimp or wire-wrap anything that can cause me to walk home!!!  I just
want the list members to know what's been found to work reliably --
and why.

Tom Cloud

Tom Cloud <cloud at peaches.ph.utexas>
I have no idea where any of the preceeding stuff came from ...