Saturday, July 23, 2005


Low melting point solder

Common eutectic solder, 63/37 Sn/Pb, melts at 183 C, 361 F.

Indium Corporation has a spreadsheet of solder compositions here in HTML or here in an Excel spreadsheet . It lists 62.5/36.1/1.4 Sn/Pb/Ag as melting eutectically at 179 C, 354 F. It doesn't list exactly 62/36/2 Sn/Pb/Ag (e.g., Radio Shack 64-013 0.022" wire solder, 1.5 oz for about $4) but the alloy selection section of this guide from Indium mentions that it has a melting range of 179 - 188 C, or 354 - 370 F.

That's only a 4 C or 7 F difference in favor of 62.5/36.1/1.4, and 62/36/2 is actually WORSE, since it is still slushy up to 5 C / 9 F hotter than the melting point of 63/37.

52/30/18 Bi/Pb/Sn melts at just 96 C, 205 F. That's a 87 C, 156 F difference from 63/37. That seems interesting, although the only reference to it in electronics that I have found was in an explanation of why Bi/Sn/Cu lead-free solders are on hold for the next decade or so: if there were any lead at all on the board it would dissolve into the solder to form this very 52/30/18 Bi/Pb/Sn alloy, and melting at 96 C is seen as a problem.

(There are much much lower melting points available in indium-based solders, but they form brittle alloys with copper, making them useless for electronics; see the substrate metallization section in this guide from Indium.)

I haven't been able to find much about solder availability in small quantities of anything more exotic than 62/36/2, though. Bummers.

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