To Solder or not to solder:

Oct 1, 1997 12:00 PM, Glen Ballou

We have all heard the adage that a chain is no stronger than its weakest link. Certainly, none of us, I hope, would stand under a 500 pound loudspeaker that is being suspended above the ground by a 1,000 pound chain with one link of that chain's being cut two thirds of the way through. I also can't imagine any of us connecting a 1,000 W amplifier to the AC mains with #24 wire. Of course, that would eliminate the need for a fuse, but it would not circumvent the requirement for safety glasses as the wire explodes.

But how often do we connect two wires together - or connect wires to terminals - without considering the consequences of a bad joint? Probably more that we're willing to admit.

We can join wire together by twisting them, crimping them, or encapsulating them. These joints may be perfectly good - after all, wire twisted to wire does allow electricity to conduct through the joint with minimum resistance. Or does it? If the wrap is tight, the wire is clean, the insulation is off, and so on, then the joint will be good, and maximum current will flow with minimum loss. If any of the conditions are not met, however, then the doing could be physically weak and/or electrically poor. So why would we ever just twist wires together? We probably wouldn't. What we might do is twist them together using a "wire nut." The wire nut holds the twisted wires together in close proximity and insulates the joint. Some wire nuts are made only of non-conductive material and therefore only physically hold the wires together while others have conductive threads in them, which enhances the current carrying capacity of the joint. Certainly a twisted joint using a wire nut is adequate where space is not at a premium and current and vibration is not high, such as in the average 120 V house wiring.

OK, if we don't use wire nuts, what's next? Crimping. We've all crimped lugs to wire and used in-line crimps to connect two wires together, so why not always use crimped connectors? In a crimped connector, the current flows from one wire through a mechanical joint through the connector, which is probably another material and a chance for a dirty joint, through a second mechanical joint to the second wire. If the wire is clean, if the inside of the connector is clean, if the two metals do not affect each other, if there is low vibration and movement, and if the crimp is solid, then a crimped joint is fine. But what if just one of the above requirements is not met? What then?

Maybe it's time to think about soldering, but not just solder and wire, solder as part of a connection. We've probably all tried to butt two wires together and solder them because we cut the wire only to find out we shouldn't have. Of course, the simple solution would be to butt the two wires together and melt solder on the joint. But what is wrong with this joint? First, the wires are not touching each other; second, there is no mechanical bonding and third, we have a "cold-solder joint." A cold solder joint is a joint where we did not heat the material enough for the solder to flow to it and around it and make a solid physical and electrical contact. This can be seen as the bubble of solder appears to be just sitting on the wire, which is exactly what it is doing. If the joint were proper, the solder would be around the wire and built up smoothly. This joint at best would be a step gap fix as it is not strong and requires all of the current to flow through the solder.

What we should have done was to twist the wires together as an in-line splice, or with a right-angle turn as we did with a wire nut. The first method is neater and easier to insulate, but the second method would also work OK as long as we don't bend the wire too sharply or bend it back on itself.

Solder is not glue or cement. It doesn't stick metals together. When molten solder is applied to a properly cleaned and heated joint, it actually dissolves some of the copper or steel or whatever the base metal is, even though it is several hundred degrees below the melting temperature of the base metal. The base metal actually goes into solution with the solder. When the solder solidifies the base metal and solder remains dissolved, and at the area of contact, form a new alloy. This forms a continuous metallic path from conductor base metal to solder to conductor base metal.

To produce a proper joint, we just first strip and clean the wire (tin it) and then mechanically connect it to the mating part. It is then ready for soldering.

To properly "tin" a wire, be sure both it and the soldering iron tip are clean. Put the iron against one side of the wire and put the solder to the other side of the wire. This will assure us the wire is hot enough for the solder to flow completely around the wire and produce even coverage.

Yes, the soldering is important, but the mechanical joint is just as important. Never rely on solder to hold the joint together! When connecting the wire to a turret or a lug, wrap the wire one turn around the turret or lug tightly. Never wrap more than one turn. it isn't necessary, and it makes it very difficult to remove if required. Be sure that the insulation is not touching the lug (it should be 1/16" to 1/8" away). Also, be certain the wire is wrapped to the terminal in such a way that any tension on the wire will be transmitted to the terminal, not to the solder. When soldering component to the terminal with other wires, connect the component above the wires to facilitate components removal. Soldering to printed circuit boards requires the same steps as soldering to terminals, plus requires more care to assure us that we don't damage the board or other components by applying too much heat. Again, clean boards, clean parts, and good mechanical parts are imperative.

After making a good mechanical connection, we can again apply heat to one side of the joint and solder to the other side until the joint has a thin layer of solder covering it. Often, we find ourselves applying more and more solder until the joint is completely saturated so that the wire becomes invisible and the joint is twice as big as before we apply the solder. This is a no no. Apply enough solder to cover the lug and wire with a smooth and shiny cover. You should be able to see the outline of the wire beneath the joint. Remember, the solder is there to conduct electricity and hold the wire and base together and eliminate oxidation of the joint.

Of course, the joint isn't finished until we insulate it. This can be accomplished with electrical tape or shrink tubing. Shrink tubing is probably the method of choice, but be sure that you use the correct size and that you put it over one of the wires before twisting them together. Be sure that the tube is far enough away from the joint so that the soldering heat does not shrink it before you finish soldering the joint. Finally, place the tubing over the joint and shrink it.

We can also solder crimped lugs and in-line connectors. I guess it is preferable to crimp them first and then solder them. If we don't have a crimping tool and the wire-to-connector is good, however, we can just solder them together. Remember however, that this puts us in the same position as wire to connector to wire; we have three metals for the electricity to go through - copper wire, solder, the connector, solder and back to copper wire.

Always make the mechanical bond first, if possible. Then why add solder? First, it strengthens the joint as it eliminates single-area contact of the crimp, it gives more area for current to flow through, and (this is very important) it protects the bare wire from oxidation and corrosion.

Anybody that has a boat on saltwater can attest to the fact that copper wire that is not tinned, turns green, deteriorates and eventually disintegrates. A tinned wire and solder joint eliminate this problem.

While we only talked about soldering two wires together or to a connector or printed circuit board, we can solder a multitude of things together. But always remember, make a good mechanical connection. Clean, tin, solder, and you will have a good joint.

Of course, we need the proper soldering iron and solder to make good joints, so if you want to know about these tools, keep your name on the S&VC mailing list, and look for the following article, "An Iron is an Iron is an Iron... or is it?" in Ballou's Toolbox.

Bibliography: Don and Carolyn Davis, "Sound System Engineering" second edition, 1987 Howard W. Sams & Co.