On Wed, 03 Aug 2005 06:06:29 GMT "Michael A. Terrell"
wrote:

Jim Adney wrote:

On Tue, 02 Aug 2005 05:35:33 GMT "Michael A. Terrell"
wrote:

if I have trouble pulling all the
solder out of a hole, I add fresh 63/37 solder to the 80/20 that was
used for wave soldering. It now has a lower melting point, and will
come out easier.

I do this too, but I didn't realize that the wave solder was
non-eutectic. Why do they use the higher melting point solder?

80/20 solder is used because it goes to the solid state faster when
it cools, and it reduces cold solder joints form differential expansion
of the PC board, the solder and the component bodies and leads. The
board temperature is slightly below the melting temp of the solder, so
it sinks a little of the heat to help with the cooling as well.

I'm having trouble understanding this part. Since the 80/20 solder
solidifies at a higher temp than the 63/37, this means that the board
with components must cool down over a larger delta T. Wouldn't this
make any differential expansion problems worse?

And why would they heat the board to less than the melting temp? How
do you get the solder to flow down a PTH that is at a temp below where
the solder solidifies?

Thanks again,

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Jim Adney
Madison, WI 53711 USA
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