Soldering Iron Temperatures

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 ·        At 40% tin and 60% lead (40/60) the melting temperature is 238°C.  ·        At 50/50 the melting temperature is 212°C.  ·        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.

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.