Soldering to a live connection

Is it a bad idea to solder to a board when it is powered up? My instinct is that it is a bad idea. But is it? Aside from shorting something out with the iron is there any danger to the hardware?

I'm talking about low current (~1mA), low voltage(~3.3V), digital microprocessor connections.

Just curious.

Don't play with fire, you WILL get burnt.

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In many soldering units (like solder pencils) the tip is coupled to the AC line by stray capacitance, so you run the risk of injecting quite high voltages into a high impedance circuit.

I would never do it.

jremington: In many soldering units (like solder pencils) the tip is coupled to the AC line by stray capacitance, so you run the risk of injecting quite high voltages into a high impedance circuit.

Wouldn't that be a problem even if the circuit wasn't powered up?

It is a problem if the board is grounded, like through a USB connection. I solder unconnected boards only.

I learned that lesson a long time ago. I had installed a US$ 110 industrial grade pressure sensor into a plastic pipe, having forgotten to solder connector pins onto the sensor leads. To save some time, I thought it would be OK to solder the pins onto the leads while it was still mounted in the pipe.

Current flowing from the AC line, through the soldering pencil, into the sensor and then into the water in the pipe burned out the bridge element in the sensor.

I keep a butane iron with a small tip just for that situation.

Among the risks are:

blindness - big fat spark explodes molten solder into your face when you accidentally short the power rails (here its the power supply current capability that matters most, not the voltage)

damage to board components when you accidentally short something out, or the static charge on your iron blows the gate oxide in a CMOS chip.

So use eye protection (you should do this for very high current circuitry anyway, big sparks throw molten metal around anyway whether there's solder present or not), and be careful/lucky.

Don't put yourself in a situation where you need to do this!

jboyton: My instinct is that it is a bad idea. But is it?

Another in the series of how to torture electronics here. See his previous posts.

Only if you do the soldering underwater will this not be a problem. ;)

This is undoubtedly bad practice. The couple of times I've done it. I've seen LEDs flicker when they should be off. So far I haven't damaged anything doing this, but that doesn't mean it's a good thing to do.

Soldering a high current carrying path can explode molten solder in your face. :'(

What I had thought briefly about soldering was something drawing very little current from a coin battery, so explosions of solder wouldn't have been likely. But what I hadn't realized was that the iron itself could be a source of current. Even though I suspected soldering live was a bad idea it was still a question worth asking for that reason alone. I didn't know that a grounded circuit, even if not powered, could be a problem for soldering.

jremington: In many soldering units (like solder pencils) the tip is coupled to the AC line by stray capacitance, so you run the risk of injecting quite high voltages into a high impedance circuit.

That would be rather unusual. The tip is always supposed to be grounded through the third wire.

If the circuit is indeed completely isolated from the mains (including any USB connections) and powered by its own battery, then the risks are only that of static electricity which is actually less if the circuit is powered, signal injection and shorting out two or more points on the board, exactly the same as touching parts of a working circuit with a probe or instrument.

The tip is always supposed to be grounded through the third wire.

No. I have never used a professional grounded soldering iron.

Soldering a live circuit is just as stupid as altering the configuration of a live circuit on a bread board, making connections is just like wiring up some wrong horrible damaging circuit even if it is only temporary. By contrast dunking an Arduino board in water, while still being incredibly stupid, is comparatively sane.

The tip on my inexpensive iron is definitely not grounded.

It's interesting to see some disagreement about what I thought was a settled question. One thing we can probably agree on is that Mike is very grumpy.

I just checked the tip on my Weller WES51 soldering station, and the tip is grounded.