Electronic – Specific solder wire/flux questions

soldering

Sometimes you have to try something and have it not go well before you even know what questions to ask. This is one of those times.

I recently soldered header pins to a 2×16 LCD display. This was my first attempt at soldering. It "worked" but it seemed harder than it should be, and I don't have a ton of confidence that the connections won't mysteriously become intermittent. I'm sure part of the problem is my technique, but I'm also wondering if I need different materials.

I have a Weller WES51 soldering station (comes with PES51 point tip iron), brand new, so that should be fine.

From reading something here, I thought I would purchase some Kester solder. It's fair to say I had no idea how many types of solder exist. Kester has like 6 families of solder wire and 10 families of flux! (Who knew?)

So I bought some "245 flux cored wire" SN63PB37 0.025in. I assumed (not sure I'm correct on this) that since this was "flux cored" I did not need to buy flux.

Sorry for the long explanation, but here are my questions:

  1. I know there are some soldering videos around, perhaps on youtube, but can someone recommend one that they feel is well done? I would have no idea how to judge the quality of the soldering info.

  2. Did I buy the right product? To be honest, I don't really understand all the different types. I know I want leaded, I've seen people here recommend 60/40 or 63/37. I like the idea of a "no clean" solder, which I believe this one is. And the 0.025 size seems ok. I guess my question is mostly about the different types of cores.

  3. Do I need flux? One of my problems is that I wanted the hot solder to surround the header pin. The best I could do was to kind of build a hill from the contact area to the pin. Would putting some flux around the pin help? And if I do want flux, which of the 10 types do I want?

Best Answer

You bought a reasonable solder. Sn63Pb37 is a (close to) eutectic alloy, so it has a bit more of the more expensive element (tin) and is a bit better than Sn60Pb40 solder. As a result, it melts at a bit lower temperature and (more importantly) it's a bit easier to use because it solidifies suddenly rather than going through a mushy phase where it's easy to create a "cold" solder joint if the parts move during cooling.

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0.025" (0.635mm) is a reasonable size for general purpose. I use 0.8mm and 0.38mm.

Personally, I prefer Kester 44 because the "no clean" flux leaves a nasty residue that is very difficult to clean off. For most applications it's just fine, but for sensitive high-impedance analog circuitry it can be a problem. I had an application with 5-50K resistors where it was a problem, because of extreme accuracy requirements. The contract assembler had used no-clean, contrary to instructions. The Rosin RMA flux may look ugly if you don't clean it, but it seems pretty inert, and it's easy to clean with solvents. I doubt you'll see any problems with the one you chose.

You say "solder station". It's temperature controlled? That helps.

You don't need to buy any extra flux for through-hole parts. You might want to buy a flux pen (eg. Kester #186) if you intend soldering surface mount parts- it helps a bit. They look like a felt-tip marker but dispense flux. Unfortunately, they seem to be a bit hard to come buy these days. Bulk flux attracts Hazmat shipping charges.

Make sure you're dealing with "fresh" parts. If they've been laying around in some surplus shop oxidizing for a decade or two, they're not going to be all that easy to solder!

If the tip is well wetted, you should be able to touch it to the two parts at once, feed the solder into that pool, and stop when you have a fully wetted joint. It should be shiny and smooth when it's done. Practice makes perfect.

I suspect if you've had problems with soldering, other than technique, your parts are the source of the difficulty rather than the solder or soldering iron.