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I'm looking to buy some lead-free solder. There appears to be two different kinds available, tin-copper alloys and tin-copper-silver alloys. What advantage does the latter have, as it is more expensive?

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  • \$\begingroup\$ I would expect that the alloys containing silver would have better conductive properties, as silver (if memory serves) is a better conductor than tin/copper. I think it might also be less susceptible to corrosion, but I don't have anything to back this up. \$\endgroup\$ – DerStrom8 Jun 22 '16 at 13:08
  • \$\begingroup\$ Very low resistance.... This solder can support high current flow without heating itself. \$\endgroup\$ – soosai steven Jun 22 '16 at 13:10
  • \$\begingroup\$ @soosaisteven How much current would be necessary to see any difference? Would it make a difference on, say, a small motor, or are you talking about maxing out an outlet kind of current? \$\endgroup\$ – Louise J. Harris Jun 22 '16 at 13:14
  • \$\begingroup\$ I remember reading somewhere that (SMD) components may have silver in their solder pads, and silver-free solder might absorb some of that silver, with detrimental effects. Adding silver to the solder would combat that. I don't remember the source, sorry. \$\endgroup\$ – marcelm Jun 22 '16 at 13:16
  • \$\begingroup\$ @marcelm: source is everywhere you google, even the wikipedia page on solder has it. In fact it has lots of annotations to various combinations of solder, since its not only about Ag or not Ag, it is also about its amount. \$\endgroup\$ – PlasmaHH Jun 22 '16 at 13:22
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In general silver makes the solder stronger and has a higher melting point. We use it in high temperature applications such as downhole. Copper in the alloy lowers the melting point and makes it somewhat easier to work, and has some chemical advantages when soldering to copper conductors. Neither alloy has sufficient resistivity to matter much as far as the actual solder joint is concerned.

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    \$\begingroup\$ Just to complement this answer, not all solder joints are electrical, so the extra strength is good for mechanical joints (e.g. stainless steel pipes). \$\endgroup\$ – user57709 Jun 22 '16 at 13:53
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There are far more than two types available. RoHS and lead-free solders including silver have the following advantages and disadvantages:

Pros:

  • Higher melting point, higher working temperature.
  • Stronger bond, less susceptible to mechanical fatigue, more reliable joint.
  • Improved resistance to fatigue from thermal cycles.
  • Addition of an impurity to tin (silver, copper) reduces chance and/or rate of tin-whisker formation. (Note that silver itself can whisker in humid, hydrogen-sulfide environments.)

Cons:

  • Too much silver can form inter-metallics that cause grittiness and formation of pimples on the solder surface.
  • Higher melting point = higher fabrication process temperatures.
  • Higher temperatures mean rework can be more difficult.
  • Stronger bond = more reliable, but also more brittle, having a lower ductility and higher Young's Modulus.
  • More expensive due to silver content.

While it is true that silver is a better conductor than most other metals, the resistivity of a typical solder joint is so low that any small gain in conductivity would matter only for very high-current applications. What usually matters more is the mechanical properties and assembly/rework-ability.

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    \$\begingroup\$ Because of RoHS we had to gold-flash all of our pc boards so the lead-free solder would wet and flow properly. We had to buy solder with a gold-silver-bismuth content and new soldering irons with alloy tips. Adapting to RoHS standards was very expensive for us. \$\endgroup\$ – Sparky256 Jun 22 '16 at 21:00
  • \$\begingroup\$ I interviewed recently and they asked for some samples, so I showed them a 4-layer PCB with mixed-mode A/D/RF. They were shocked that RoHS wasn't used. But as explained, the device required this, as the qualities of RoHS solder and fabrication greatly complicated and compromised the device integrity. They were skeptical. (I've tested them, I'm convinced!) So I can relate to this frustration. Moving to all-RoHS is a complex and costly affair, but is inevitable, as less than half of e-waste is recycled today. \$\endgroup\$ – rdtsc Jun 23 '16 at 12:19
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It is my understanding that silver solders are used purely for mechanical reasons. Any variation in conductive qualities are incidental.

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    \$\begingroup\$ You should detail the mechanical reasons you are talking about, to help the OP choose. Otherwise, this infomation is not very helpful. \$\endgroup\$ – dim Jun 28 '16 at 20:58
  • \$\begingroup\$ I mean 'mechanical' as in strength of a soldered joint. I find it hard to think what else 'mechanical' in this context could refer - although I would be genuinely very interested in learning. \$\endgroup\$ – blueflash Jul 13 '16 at 19:54
  • \$\begingroup\$ Well, indeed, the fact you're talking of the strength of the mechanical joint is obvious. But given the information you gave, we have no way to know how much stronger the silver alloy is. What would be really helpful, for example, would be to provide examples of cases where a tin-copper-silver solder is more appropriate than tin-copper solder (or the opposite). As it is, your answer adds nothing to the other answers that were already posted. \$\endgroup\$ – dim Jul 13 '16 at 21:18

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