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Some background here: I'm an electrical engineer, working on a project that is likely going to require liquid cooling. As part of the installation of this liquid cooling system, a joint needs to be made between copper pipe and a pipe fitting. This will carry only coolant mixtures, not anything potable, so lead content is not an issue.

Is there any reason that I should not use electrical solder (of which I already have plenty) to solder this joint? I know that plumbing solder usually doesn't have a flux core like electrical solder does.

If it is fine, will I still need to use a separate flux paste? I imagine I probably will, but in the interest of having a complete answer that's useful to other people, the opinion of someone who knows more of what they're doing with plumbing than I do is valuable here.

  • what is the temperature and pressure extremes that the cooling solution will experience and are the fittings and piping the same material. Also, make sure that the flux will not contaminate the solution – d.george Mar 4 at 16:39
  • @d.george The fitting and pipe are both copper. Pressure up to 30psig, but probably lower in general use. Temperature... I'm still working on that. Probably not much more than 0°-50°C. – Hearth Mar 4 at 20:38
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My understanding is that plumbing flux is more aggressive (acidic) and will damage electronics, but that you can go the other way and use flux-core solder on plumbing in a pinch. A few caveats:

  • Electrical solder is often a softer alloy, so it won't withstand pressure as well.
  • Definitely flux the pipe fittings as you normally would. You won't get good coverage with the flux in the solder.
  • You may struggle to get enough material in the joint in a timely manner. Plumbing solder is about 5 times the diameter, meaning about 20 times the volume. You might fold the electrical solder back on itself several times and give it a twist in preparation for quicker flow.
  • Aha, that third part didn't even occur to me. It's true, plumbing solder is far thicker. I may end up using plumbing solder anyway, as I can probably get a small roll from home depot or something, and I'd have to go there anyway to get the flux; our lab doesn't have any flux that would sufficiently active for this. – Hearth Mar 4 at 14:54
  • Once the joint has cooled wipe it off with a wet rag - wipes off the excess flux - which can corrode the pipe over time... – Solar Mike Mar 4 at 15:13
  • I my experience with copper plumbing new and old, the flux tends to evaporate or burn off, and I've never seen damage from flux. – isherwood Mar 4 at 15:14
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    @isherwood was taught by a plumber / qualified gas fitter and he knew... and especially with powerflux... Also, I have a piece of pipe that I kept as part of a failed pipe in my house - pin-hole due to corrosion... The new copper pipe systems in our build were all flushed before being put into service... – Solar Mike Mar 4 at 15:23
  • Fair enough. I'm wary of Chicken Little advice on DIYSE, but your concern seems legitimate. Thanks. – isherwood Mar 4 at 15:24
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Plumbing application solders usually use mineral salts which can be corrosive if not rinsed off. Electrical solders use "resin' fluxes which are not as corrosive so do not need to be rinsed off which would be more of a problem in electric applications. both use the same range of lead/tin alloys. 20 % tin has a wide temperature range where is is "mushy" and can be worked. 50:50 freezes quickly in a narrow temperature range. 30 tin:70 lead used to be very common,maybe still is. Many new plumbing solders contain no lead for political correctness , so compromise on various properties compared to tin/lead.

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    Political correctness, eh? Here I thought it was to prevent birth defects in humans. – isherwood Mar 4 at 17:01
  • 60/40 Rosen core solder was the standard solder prior to ROH requirements I have used this many times for repairs on low pressure cooling loops. I have also used tin , silver, copper but found the lead solder flowed easier. – Ed Beal Mar 4 at 19:38
  • As an electrical engineer, I've never seen any tin/lead ratios other than 60/40 or 63/37 in common use. Eutectic solder alloys are desirable in the context of microelectronics. – Hearth Mar 4 at 20:41
  • Freezing temperature ranges can be selected from about 150 F range to 0 F range ( eutectic) to fit an application. I agree- 60/40 is very common : I am afraid I was thinking of auto body work from over 60 years ago where a wide freezing range was used to work the 30/70 solder. Lead started to be removed form plumbing solder in the US about 20 years ago, but I never heard of anyone replacing all the soldered copper plumbing out of a house to be lead free for health. Although I expect most of the lead pipes from very long ago have been replaced. – blacksmith37 Mar 5 at 1:19

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