December 1, 2006
Keep the following lessons in mind when grabbing for your solder iron.
I was fresh out of college with my third degree, this time in something I could get a job with - industrial technology. So when I got my real, professional job as a factory technical trainer, I was rarin’ to go.
“You know how to solder?” demanded my new boss, the chief industrial engineer. He was talking about soldering small electrical controls - thermostats, transformers, limits, valves, printed circuit boards - not pipe joints.
“Uh, well, no I don’t.” There sure weren’t any college courses in that. And I hadn’t had to solder in my two years on night shift assembly in the terminal strip and fuse block factory.
“Well, you better learn,” he responded, “because you’re the solder trainer. The guy who’s been teaching wants out of it. He’s a time study guy. You’re the trainer now, so it’s your baby.”
“Where do I learn?” I asked.
“Go to Larry’s soldering class, I guess,” he grumbled, and walked away.
I soon found out that Larry held a soldering class, oh, about once every 15 years.
So I learned to solder electrical components from the gals in the factory. Most had been there longer than it had been since the last class. I watched and asked questions. I listened to them complain that there was never a class. “Oh, there will be,” I assured them, “soon as you get me trained.”
So here’s what I learned and what I taught when I held monthly soldering classes for new-hires:
The purpose of soldering is to make sure there’s a path for electricity to flow through when a wire is connected to a terminal.
Solder looks like bright silver wire on a spool. Actually it’s about 97 percent tin. It used to be combined with lead. Now it’s combined with a small amount of silver and copper.
You have a soldering iron, which looks a lot like a screwdriver with an electrical cord coming out the end of the handle. The idea is for the solder iron to be hot enough to melt the solder.
The tip of the soldering iron can be various shapes. It’s usually a point for small electrical connections, so that you can get the heat in exactly the small right place. For bigger connections, though, the tip can be shaped like a screwdriver. You can use the flat of the blade to transfer a lot of heat to a larger surface.
In the middle of the solder is a core of “flux.” The purpose of flux is to clean the surface being soldered before the solder goes on. The trick of good soldering is to get the flux to melt onto the connection before the solder gets there. If the connection is dirty - with lubricants, corrosion or even fingerprints - solder may not stick. Flux smells like burning pine pitch - not all that bad at first.
The right way to solder is to first heat the metal that’s to be soldered. You want the solder to connect the wire both to the screw and to the terminal that the screw’s screwed into. So here’s the thing: you touch the solder iron tip to all three of those things at the same time - the wire, the screw and the terminal.
Hold the solder iron tip alone there for just a second or so, until the metal starts to heat, but certainly not until it gets red. Now, touch the end of the solder only to the connection. Don’t touch the solder to the solder iron. The reason is that if you touch the solder to the iron, the flux burns onto the tip of the solder iron and doesn’t get to the wire or terminal.
It’s In The Flow: The goal of soldering is to get the molten solder to flow. Molten solder will follow heat.
So, heat the joint, touch the end of the solid solder to the hot joint, keep the solder iron on the joint, remove the solid solder from the joint, and pull the puddle of solder to cover the whole joint.
The edges of a completely flowed solder joint blend, or feather, into the metal. There isn’t a rounded edge.
Don’t drip. It’s a lot of fun to melt solder directly onto the tip of the iron and let it drip onto the connection. That’s the wrong way to do it because:
The flux, whose purpose is to clean the joint before the solder hits it, heats to boiling point and evaporates before it hits the joint it’s supposed to clean. It also fills the air with smoke.
Instead of landing on the metal freshly cleaned by flux, the solder lands on whatever is on the surface of the metal - probably lubricants from the manufacturing process. Everyone knows that nothing sticks to lubricants.
When the liquid solder hits the cold metal joint, it instantly cools where it lands, without flowing to cover the joint thoroughly. This is called “cold solder.”
The purpose of solder is more for electrical conductivity than for strength. So it doesn’t take much to get the job done. You should be able to still see the outline of the wire in the joint, though it should have a fine coating of solder over it.
Multistrand or stranded wire is made of several small wires twisted together. Its purpose is flexibility. Soldering stranded wire is a special challenge because solder easily travels upwards among the little wires. This is called “wicking.” The problem with wicking is that solder defeats the purpose of the flexible wire. So don’t let the solder go very far up the wire.
Have you ever read transformer specs and seen the word “tinned”? Tinned means that the ends of stranded wire have a coating of solder on them to fasten the ends of the wire together. This keeps the wire from fraying. It’s easy to tin your own wires - just heat the end of the wire and touch the solder to it.
Tinning is also a way to prepare a new solder iron tip.
Have you ever looked at the underside of a printed circuit board and wondered how anyone had the patience to solder all those little connections? Well mostly no one does. The soldering of printed circuit boards is done on a machine called a wave solder. There is a vat of molten solder. A conveyor moves printed circuit boards along the surface of the solder so that the underside of the boards touches the solder.
Personal Payoff: Here’s where being the solder trainer and knowing some techniques really paid off. It was January in Minnesota and we were having a cold snap - something like 10 below in the middle of the day, with the sun shining. My Dodge Tradesman van wouldn’t start because the battery cable connection had snapped.
Ah, it was a perfect time to put soldering principles to the test. I got out my biggest soldering iron and plugged it in the garage outlet next to the engine heater. This iron usually heated up really hot and fast, but this time it was in slow motion. About the time I was ready to give up, there was that familiar smell of the flux starting to melt. Then there was the little sizzle of melting flux, and finally the lovely silver flow of solder over hot metal. Soon the truck was running.
Not far into my career as the solder expert, my boss came to me and asked a new question. “Do you know now to silver solder?”
“Sometimes it’s called brazing. Do you know how to do that?”
“Well, you’d better learn because you’re the silver solder trainer.”
So, I went to the silver solder production line. I watched and asked the operators questions. After a while, the supervisor, a rough-around-the-edges gal with a gravely voice, came up and growled, “Are you one of them new lady engineers?”
“No, I, um, I’m a ...”
“I don’t care what you are,” she interrupted. “I just wanted to tell you, your zipper’s down.” And she stomped away.
It turns out the same principles apply to brazing as to regular soldering, but with a torch and liquid flux applied with a small brush. Sounds a lot like soldering pipe, doesn''t it? Heat the metal first. Don’t get it too hot. Apply the flux. Apply the solder. Use the heat to draw the solder into the joint.
And make sure your zipper’s up.