"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:

"Dave Plowman (News)" wrote:

Jeff Liebermann wrote:

Dry joints? I've never had to lubricate my solder connections.

It's what describes a failed solder joint as well as anything - they
can look sort of dried out.

That is called a 'cold solder joint' in the US, because it is
typically caused by too low of a soldering temperature to make a good
joint.

Don't you talk about 'wetting' when solder flows? Turns from a solid to a
liquid? But whatever - either description works for me.


Wetting is when the liquid solder flows completely over the oxide
free surface. It is one step of the soldering process, and the hardest
to control in automated soldering. A profile has to be created, and
carefully followed for constant quality. Air temperature, humidity and
barometric pressure changes can cause problems, as well as moisture that
has ben adsorbed into the blank PC board and components. Any of these
can cause cool spots, and in extreme cases cause the PC board to
delaminate as the moisture is boiled away.


FWIW I'm not sure all failed joints are caused by too low a temp - you see
them on factory soldered boards too. Famously like round a LOPT - where
the vibration and heat causes the problem.


Solder passes from a liquid to a solid state, not a dry state. Some
alloys also pass through a plastic state that allows movement as it
cools into the solid state. Any movement during the plastic state is
what gives the dull gray, rough surface of a cold solder joint. Use of
eutectic solder, which has a very narrow temperature range with a
plastic state, it reduces cold solder joints. 63/37 was the preferred
alloy, before the EU screwed the pooch with their ROHS bull****.

As far as bad solder on heavy pins, it takes more to bring them to
the proper soldering temperature, and causes more problems. You can
wave solder everything else perfectly, and not get the pins on a
transformer anywhere near hot enough for proper wetting. Since it takes
longer to bring them to the proper temperature, the pins need to be as
clean and oxide free as possible.

I was involved in improving our reflow process when the company moved
to higher pin count surface mount ICs. We had a horrible initial
failure rate when we added the newer devices to our process. We went
through a half dozen allows and ball sizes in the paste solder, and
several different flux blends before we found what we needed. At least
we started with a brand new Heller oven.


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