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Why use higher temperatures for copper foil using
60/40 than lead came using 50/50 or 40/60?
Melting temperatures
Part of this is the physical characteristics of the solder
The
graph shows that all compositions of tin/lead alloy solder (above 20% tin)
solidify at the same temperature - 183°C.
Pure lead melts at 327.5°C and pure tin at 232°C. The various proportions of the two metals
melt at different temperatures until at approximately 62% tin, the melting
and solidification temperatures are the same.
This is ideal for running a bead in copper foiling, because there is a
minimum amount of time for the liquid solder to change shape before it
solidifies.
Melting temperatures of some solders
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At 40% tin and 60% lead (40/60) the melting temperature is 238°C.
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At 50/50 the
melting temperature is 212°C.
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At 60/40 the
melting temperature is 188°C, just 5°C above the solidification temperature.
These
figures show the 60/40 solder requires a lower temperature to melt than 50/50
does (24°C difference).
Why should
I run the iron at a hotter temperature for 60/40 then?
There are two separate elements at work here – the
mass of solder being melted and the effects of the pasty range of solder
compositions.
In soldering lead came you are melting small
masses of solder with short pauses between each melting that allow the iron
to partially recover. This means running the iron at 370°C is sufficient to maintain a melting temperature above
238°C for 40/60 solder and 212°C for 50/50.
In copper foil you are melting much greater amounts
of solder, which takes heat out of the iron more quickly than in leaded
glass. The fact is that running a bead
requires melting a much greater volume of solder. The iron needs to run hot to be able to
consistently melt the solder without significant periods when the iron is too
cool to melt the solder quickly. This
is the reason that irons are run hotter in copper foil.
It still does not explain why it is recommended to
run the iron hotter for 60/40 than for 50/50 as their melting temperatures
are so close.
The explanation lies in the pasty range illustrated
in the graph shown above.
You can run an iron hotter than needed to melt the solder, because the
60/40 requires fewer degrees to cool and solidify than 50/50. This allows you to work quickly and still
have a good rounded bead.
The greater pasty range of 50/50 means that you must
be careful about the amount of heat you put into the solder, because the
solder will continue to move for a longer time than the 60/40. The 27°C difference between melting and
solidification shows solidification is not instantaneous. This pasty range allows
flow while the solder cools. This means that the bead will be less rounded,
and it will show minor temperature differences in the wrinkled surface. If you put even more heat than the 410°C that is normally used for 60/40, it will take even
longer for the solder to solidify. The
surfaces effects will then be even more obvious with greater heat.
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The short answer
The explanations for running
hotter with 60/40 than those solders with more lead centre around the pasty
range of solder. When the pasty range is
small, you can put more heat into the solder bead and so work more quickly and
still get a good bead. With wider pasty
ranges you need to reduce the temperature of the iron to reduce the heat put
into the solder and so keep the pasty range as small as possible.
