Specific solder wire/flux questions

Question

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 2x16 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?

Answer 1

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.

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 by 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.

Answer 2

It looks like you have already got some good answers, but I'll add some basic points that helped me when I was having trouble soldering.

1. Solder flows towards heat. That is why you heat up both components and then add solder.

2. Flux helps the solder to flow to where you want it. Even if the solder has flux in it, if you are having troubles, try using flux.

3. 2 main types of flux: Acid and Rosin. Only use rosin for electronics, use acid when plumbing your toilet.

4. "Tinning the tip" adding a little solder to the tip of the soldering iron before you start to heat up the components you are going to solder helps to conduct the heat to the components and just makes it work better.

5. Using the right tip- I couldn't believe how switching to a chisel tip helped me for one specific application that I just couldn't do with the pencil tip that came with the iron.

6. Don't Move! Hold still while the solder hardens. This will only be a second or two or even less depending on the solder. A "cold joint" could form if you move the components as the solder is cooling. Cold joints are unstable and likely to cause you future problems

7. Ventilation - Lead is bad for you. Your soldering iron should not be hot enough to vaporize lead, but the flux does create some smoke which has possibly been linked to asthma in factory workers.

Hope this helps.

Edit, some more on flux:

Flux also helps with temperature control. Think of it like when you boil water on the stove - as long as there is water in the pot, the pot doesn’t burn. Flux keeps the temperature stable, and from rising too high until it boils away, helping to keep electronics or wire insulation from melting from the heat of the soldering iron.

Answer 3

You have the right equipment, now you just need the right technique. Videos on youtube are helpful, but the best thing for it is practice.

For through hole parts like pin headers, I find a chisel-shaped tip to be much easier to use.

Let the iron heat up until solder instantly melts when touched to the tip. You may have to adjust the temperature setting if your station has one. But don't turn it up any higher than needed.

If the parts are old and oxodized instead of shiny, brush them lightly with sandpaper or steel wool. Solder won't stick properly otherwise.

Put a small dab of solder on the tip. This makes sure the heat transfers to the parts well. Put the tip against both of the parts you want to solder. If only one gets hot, the solder won't stick to the cold one and won't flow properly.

Once both parts are hot (1-2 seconds for most parts, longer for big ones and large ground planes) touch the solder to the parts, not the iron. It should flow nicely. Once it has enough solder, remove the iron. Don't let the parts move until the solder has solidified.

Practice on cheap parts until you get the hang of it. After doing it for awhile, it gets much easier to know how long the heat needs to be applied and how much solder to use.

Answer 4

I stumbled upon Dave's videos a few days ago and I would say they are sufficient for what you are asking.

Did I buy the right product?

Yes it seems fine, you can either use 60/40 or 63/37 but stay away from lead-free solders because these will be hard to handle (they have a high melting point).

According to wikipedia 60/40 Tin/lead (Sn/Pb) melts at 370 °F or 188 °C while 63/37 is a eutectic alloy which has the lowest melting point (183 °C or 361.4 °F) of all the tin/lead alloys. Also the melting point (for 63/37) is truly a point — not a range so it has the benefit of solidifying instantly when the temperature drops while a 60/40 solidifies gradually and a movement of the parts at that point may produce a bad solder.

Do I need flux?

Normally no because the solder already contrails flux core(s), all that you need it to melt it on the actual joint you are trying to solder rather than on the iron tip so that the paste de-oxidizes the PCB pad and component wire.
You will probably need flux when you start soldering SMD chips, but it is usually a liquid flux which helps avoid bridges between pins (shorts).