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I’m trying to repair my espresso maker with a new thermostat but am unable to get the solder to flow. I’m a beginner but have had some experience soldering my own guitar pedals using stripboards in the past so I have some base knowledge.

I tried using my cheapo Weller 25w soldering iron originally but didn’t get anywhere with it so I bought an 85w temperature controlled soldering iron, soldering flux, and tip tinner hoping that would be what I needed. Unfortunately, I wasn't able to get the soldering flowing onto the component even then.

I'm sure there's some obvious thing I'm doing wrong but I cleaned the two metal tabs with rubbing alcohol, tried a few increments between 250-350°C to heat up the tabs but am also not sure how hot I can get before damaging the component.

Below is a picture of the component. I could not remove the tab from the wire on the original thermostat so I'm attempting to solder the old tab to the new thermostat tab. Excuse the copper wire, it's my DIY way of keeping the two components in place.

Espresso maker thermostat

Top of original connection

Bottom of original connection

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    \$\begingroup\$ Two things could be going on here. 1) You may need a bigger tip on your soldering iron, to better heat up the tab. 2) Tabs on high temperature devices like that thermostat may not be solderable. You're expected to use a push on crimp-type connector. \$\endgroup\$
    – SteveSh
    Apr 11 at 23:36
  • \$\begingroup\$ I've had to remove a threaded rod from a 50A power supply eval board once. I mounted it on a vice, mounted a hot air gun to continually hit it from one side. And used the biggest soldering iron tip to heat up the other side. It took a few minutes for the solder to flow and was able to remove the rod. Sometimes you need to give it more time. I was unable to touch the entire PCB because it was so hot after that. So be mindful of that. \$\endgroup\$
    – Michael
    Apr 11 at 23:49
  • \$\begingroup\$ Okay that’s good to know. I was not entirely sure how the circular tab (name?) was connected to the thermostat tab originally as I didn’t see any solder around it. Could you point me in the right direction where I’d find some crimp-type connectors? \$\endgroup\$ Apr 11 at 23:49
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    \$\begingroup\$ Are you sure that tab is solderable? Stuff that's expected to get to high temperatures (where solder might melt under normal operating conditions) doesn't usually have solderable tabs; you might be expected to use a crimp connector or rivet to connect it. \$\endgroup\$
    – Hearth
    Apr 12 at 0:13
  • \$\begingroup\$ Yeah, quite possible it's not supposed to be soldered then. Here's a picture of the original connection between the circular tab (name?) and the original thermostat tab top and bottom. Not sure how the two are bonded together. tylorjreimer.com/clients/espresso/Connect-Top.jpeg tylorjreimer.com/clients/espresso/Connect-Bottom.jpeg \$\endgroup\$ Apr 12 at 0:23

3 Answers 3

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As others have mentioned in comments, the metal is probably not easy to solder to. It could be stainless steel which needs special flux (acid flux - which is a no-no for wires as the flux will go up the insulation and corrode the wire after some time) and silver bearing solder.

High-temperature thermostats should never use solder for connections as the solder could melt and you may end up with a wire disconnecting or solder dripping causing a hazard. These sorts of connections require crimp terminals or a screw terminal connection.

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  • \$\begingroup\$ A small note: You can use solder in high-temperature environments, just not standard 60/40 or SAC305. Tin-lead solders with a high proportion of lead (like 10/90 or 20/80) have solidus points near 300 °C, and eutectic gold-silicon and gold-germanium alloys (m.p. over 350 °C) are sometimes used to attach silicon dice to leadframes. There's also pure zinc or high-zinc alloys used for soldering aluminum that have melting points close to 400 °C. Any of those would probably work here, if they can be convinced to wet the terminals, but they're all pretty exotic (and pricy) solders. \$\endgroup\$
    – Hearth
    Apr 12 at 16:45
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These are called Spade connectors, and you might try any auto parts place as they are used extensively on cars. That is what is on the other terminal of the thermostat, although that has a long heat-shrink insulator. A new one will have a shorter push-on clear plastic cover, which you slide back along the wire first while you are soldering or crimping. It does not look as if there is anything it could contact, but winding on some electrical insulating tape for an inch or so would be OK.

You have plenty of slack in the blue cable, so you should cut off that current end, as close to the tag as you can. A new spade will either solder or crimp directly onto the cable (do that first so it does not heat the delicate component). You can crimp with normal pliers, but for mains I would prefer soldering.

The spade end should be the right thickness for the terminal, and is springy, but you can gently adjust the gap. It should only need a firm push to engage (again, to avoid stressing the thermo unduly).

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  • \$\begingroup\$ The updated photos make it look like they're not actually spade connectors--not sure what they are, though. \$\endgroup\$
    – Hearth
    Apr 12 at 16:47
  • \$\begingroup\$ @Hearth The original cable terminations look as if they were riveted (for the round part), and maybe a grub-screw fitting or even a spot-weld for the inboard hole, and it now looks awful. I would expect the stub on the thermostat would fit a suitable spade connector, but I can't guess the max amps rating needed (my Volvo push-fit fuses rate up to 15 amps). I would pragmatically settle for clamping the bare wire end directly to the terminal with a 2mm set-screw, nut and washer, but that's my accepted level of risk, not for general use. \$\endgroup\$ Apr 12 at 23:17
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Be sure to use solder for electronics and not for plumbing or jewelry. Also, use the soldering iron to heat up the connection for a while and then apply the solder. If you don't so that, the solder will just stick on the tip. If that doesn't work, replace the connector.

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