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 weakestlink. Certainly, none of us, I hope, would stand under a 500 poundloudspeaker that is being suspended above the ground by a 1,000 pound chainwith one link of that chain's being cut two thirds of the way through. Ialso can't imagine any of us connecting a 1,000 W amplifier to the AC mainswith #24 wire. Of course, that would eliminate the need for a fuse, but itwould not circumvent the requirement for safety glasses as the wireexplodes.
But how often do we connect two wires together - or connect wires toterminals - without considering the consequences of a bad joint? Probablymore that we're willing to admit.
We can join wire together by twisting them, crimping them, or encapsulatingthem. These joints may be perfectly good - after all, wire twisted to wiredoes allow electricity to conduct through the joint with minimumresistance. Or does it? If the wrap is tight, the wire is clean, theinsulation is off, and so on, then the joint will be good, and maximumcurrent 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. Whatwe might do is twist them together using a "wire nut." The wire nut holdsthe twisted wires together in close proximity and insulates the joint. Somewire nuts are made only of non-conductive material and therefore onlyphysically hold the wires together while others have conductive threads inthem, which enhances the current carrying capacity of the joint. Certainlya twisted joint using a wire nut is adequate where space is not at apremium and current and vibration is not high, such as in the average 120 Vhouse wiring.
OK, if we don't use wire nuts, what's next? Crimping. We've all crimpedlugs to wire and used in-line crimps to connect two wires together, so whynot always use crimped connectors? In a crimped connector, the currentflows from one wire through a mechanical joint through the connector, whichis probably another material and a chance for a dirty joint, through asecond mechanical joint to the second wire. If the wire is clean, if theinside of the connector is clean, if the two metals do not affect eachother, 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 aboverequirements 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 wirestogether and solder them because we cut the wire only to find out weshouldn't have. Of course, the simple solution would be to butt the twowires together and melt solder on the joint. But what is wrong with thisjoint? First, the wires are not touching each other; second, there is nomechanical bonding and third, we have a "cold-solder joint." A cold solderjoint is a joint where we did not heat the material enough for the solderto flow to it and around it and make a solid physical and electricalcontact. This can be seen as the bubble of solder appears to be justsitting on the wire, which is exactly what it is doing. If the joint wereproper, the solder would be around the wire and built up smoothly. Thisjoint at best would be a step gap fix as it is not strong and requires allof the current to flow through the solder.
What we should have done was to twist the wires together as an in-linesplice, or with a right-angle turn as we did with a wire nut. The firstmethod is neater and easier to insulate, but the second method would alsowork OK as long as we don't bend the wire too sharply or bend it back onitself.
Solder is not glue or cement. It doesn't stick metals together. When moltensolder is applied to a properly cleaned and heated joint, it actuallydissolves some of the copper or steel or whatever the base metal is, eventhough it is several hundred degrees below the melting temperature of thebase metal. The base metal actually goes into solution with the solder.When the solder solidifies the base metal and solder remains dissolved, andat the area of contact, form a new alloy. This forms a continuous metallicpath 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 forsoldering.
To properly "tin" a wire, be sure both it and the soldering iron tip areclean. Put the iron against one side of the wire and put the solder to theother side of the wire. This will assure us the wire is hot enough for thesolder to flow completely around the wire and produce even coverage.
Yes, the soldering is important, but the mechanical joint is just asimportant. Never rely on solder to hold the joint together! When connectingthe wire to a turret or a lug, wrap the wire one turn around the turret orlug tightly. Never wrap more than one turn. it isn't necessary, and itmakes it very difficult to remove if required. Be sure that the insulationis not touching the lug (it should be 1/16" to 1/8" away). Also, be certainthe wire is wrapped to the terminal in such a way that any tension on thewire will be transmitted to the terminal, not to the solder. When solderingcomponent to the terminal with other wires, connect the component above thewires to facilitate components removal. Soldering to printed circuit boardsrequires the same steps as soldering to terminals, plus requires more careto assure us that we don't damage the board or other components by applyingtoo much heat. Again, clean boards, clean parts, and good mechanical partsare imperative.
After making a good mechanical connection, we can again apply heat to oneside of the joint and solder to the other side until the joint has a thinlayer of solder covering it. Often, we find ourselves applying more andmore solder until the joint is completely saturated so that the wirebecomes invisible and the joint is twice as big as before we apply thesolder. This is a no no. Apply enough solder to cover the lug and wire witha smooth and shiny cover. You should be able to see the outline of the wirebeneath the joint. Remember, the solder is there to conduct electricity andhold the wire and base together and eliminate oxidation of the joint.
Of course, the joint isn't finished until we insulate it. This can beaccomplished with electrical tape or shrink tubing. Shrink tubing isprobably the method of choice, but be sure that you use the correct sizeand that you put it over one of the wires before twisting them together. Besure that the tube is far enough away from the joint so that the solderingheat 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 ispreferable to crimp them first and then solder them. If we don't have acrimping tool and the wire-to-connector is good, however, we can justsolder them together. Remember however, that this puts us in the sameposition as wire to connector to wire; we have three metals for theelectricity to go through - copper wire, solder, the connector, solder andback 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 thecrimp, it gives more area for current to flow through, and (this is veryimportant) it protects the bare wire from oxidation and corrosion.
Anybody that has a boat on saltwater can attest to the fact that copperwire that is not tinned, turns green, deteriorates and eventuallydisintegrates. A tinned wire and solder joint eliminate this problem.
While we only talked about soldering two wires together or to a connectoror printed circuit board, we can solder a multitude of things together. Butalways remember, make a good mechanical connection. Clean, tin, solder, andyou will have a good joint.
Of course, we need the proper soldering iron and solder to make goodjoints, so if you want to know about these tools, keep your name on theS&VC mailing list, and look for the following article, "An Iron is an Ironis an Iron... or is it?" in Ballou's Toolbox.
Bibliography:Don and Carolyn Davis, "Sound System Engineering" second edition, 1987Howard W. Sams & Co.