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Comment by gus_massa | Hacker News Reader

I can solder. It's just that I have to redo 1/3 of them and most of the times the solder joint is too cold. (I have a Chemistry specialization in secondary school. We learned a bit about electric circuits including soldering, but it's not my specialization.)

Anyway, I suspect that non-soldering projects may have a bigger audience.

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Some advice I'd have appreciated early on, because it took me a long while to figure out on my own:

Use a tiny dab of solder on the iron tip for better heat transfer, and heat the joint for longer than it feels like you should need to. Melt the solder wire against the joint, not the iron - the iron will always melt solder, but that's not what it's there for; it's there to heat the joint until that melts the solder, which is what needs to happen for proper wetting and bonding to occur.

Also, resist the temptation to use too much solder; you don't want a blob, but rather a clean, concave flow from wire to pad. That's how you avoid cold joints.

> heat the joint for longer than it feels like you should need to.

You shouldn’t heat the joint for more than about 2 seconds before feeding solder into it. If you need longer than that, you probably don’t have a properly tinned tip or a properly cleaned joint, or your tip is too small, or perhaps your iron has low heat capacity.

I also highly recommend using lead-based flux-core solder, e.g. 63/37 Kester 44. The lead-free stuff is awful to work with.

Anyone trying to learn to solder should watch these videos, especially #1: https://www.youtube.com/playlist?list=PL926EC0F1F93C1837

I run my iron cool to avoid overheating joints, a nasty habit left over from years of not knowing what I was doing. It takes six or eight seconds to heat a joint. Running at a higher temperature would probably obviate the need for the extra dab of solder on the tip, too; I add a tiny drop and roll it against the joint, but with a higher setting, I probably wouldn't need to.

There's probably not a huge risk running it cooler, but I would definitely try to learn the correct way to save time. I have always tried to explain it like this: the idea is that the iron gets both areas to above the melting point of the solder, so that when solder is applied it cleanly wicks into both the pin and the pad.

Try and place the iron so that it heats both the pin and the pad at the same time. It helps to have a tiny bit of solder on the tip. Then, after around 1 second (unless the pad is on a ground plane, then maybe much longer...), feed in a bit of solder. It should melt and flow immediately on application. You don't need a lot to get a good connection, it should make a little 'hill' on the PCB. Remove the solder, then remove the iron. The joint should cool and leave a nice shiny connection (depends on the solder.)

Tip for using lead-free solder: when you are done soldering, always leave a big blob of solder on the tip before turning the iron off. This way, the tip doesn't oxidize in air while cooling, and you don't have to clean the tip all the time. Next time you solder, just one wipe of the tip after it heats up and it will be shiny and ready to go.

The important thing is not the tip temperature per se, but the heat flow. Try watching that linked video, it’s great!

the most important thing you need to know about thu hole soldering is that you have to get the wire and the PCB pad both hot enough that solder will melt on both them - dont melt the solder onto the iron and depend on it flowing onto the rest - melt it onto the things you want to solder - feed it into the component lead and pad opposite from the iron.

Do that and you'll never get a cold joint

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