Electronic – ny problem soldering a device while it’s powered on

I've said this before, and I'll say it again: How shiny the solder looks is not a reliable indication of the quality of the solder joint.

Even before lead-free solder the shine wasn't a reliable indicator. More reliable than lead-free solder, but not reliable enough for most people.

Here are some things that I look for when evaluating the quality of the solder:

• Consistency: Do all the solder joints look the same? If not, it indicates a variation in the solder process and thus it is more likely to have problems.
• Wetting/Wicking: Did the solder melt evenly and flow onto the mating surfaces, or does it look like the solder beaded up like water on a newly waxed car? Solder that is beaded up could have problems, and hidden cracks under the bead.
• Smooth finish: I'm not asking if it is dull or shiny, but rather is it smooth or are there lumps in it? Lumps are a sign of uneven or incomplete melting.
• Conductive Flux: This one is rare, but important. Some types of flux are conductive, but not everyone is aware of this. Sometimes a board will be reworked with conductive flux but the flux will not be cleaned off correctly (water for water soluble flux, etc.). Check that the proper flux was used in the proper way. Note: Some flux leaves a residue and this is OK as long as the residue is not conductive even though it might look bad.
• Cracked Solder Joints: Often this can only be seen using a microscope, and sometimes not even then.

I think that using anything to dissipate the heat when doing soldering is just some very nasty folklore, best forgotten unless fully understood.

On surface mount parts, you want to do exactly the opposite. When soldering or desoldering, you ideally want all parts to be at the same temperature: that's why people use preheat plates. When all things are at the same temperature, there's minimal thermal stress on the terminals.

On through-hole parts with just a few terminals and good access, you may find it possible to grab all the pins at once with pliers to sink some heat away. You'll find that in practice you're more likely to mechanically damage your part by doing that, than just quickly desoldering without any heat sinking. For desoldering leaded parts, the important things are:

1. Have a high-powered iron. 50W would be bare minimum. Do not use fine tips. The wider the tip, the lower the thermal resistance between the heater and the tip, and the closer the tip temperature will be to your setpoint.
2. Have an iron with temperature control based on feedback close to the tip. Otherwise your tip, in presence of a heat sink, will be cold even though you think the temperature is right.
3. Ensure that you have good thermal contact with all the pins at the same time. This means that you need to heat up a big blob of solder with your iron, and then roll it over the pins of the device. In less than a second you should be able to pull it out.
4. If your device has leads far apart, you will need multiple soldering irons, no way around. Yes, you can try and pull leads out one at a time, but it's a hit or miss if you want to preserve the part.
5. Use flux-cored solder with electronics-compatible flux. That really means a rosin-core flux. I find that water based fluxes have too low boiling point for rework, and no-clean fluxes are irrelevant since you'll have to clean stuff anyway.
6. If you can avoid it, do not play with pulling the solder out of the holes before you desolder the part. It's easy to damage the PCB and/or the part while doing so. Apply the heat, pull the part out, and only then remove the solder from the holes so that you can replace the part.