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Eye On Electronics

By Mike Dale | July 2011

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Reliable, durable solder joints don’t just happen. In the second of a two-part column, Mike explains the materials, equipment and techniques necessary to produce quality solder joints and long-lasting repairs.

It seems simple and straightforward enough: a couple of wires, some solder and some heat. Follow that up with a wad of electrical tape and you should have a good, reliable solder joint, right?

You already know the answer from looking at work done by others that comes through your shop. When it’s done correctly, soldering can produce good, reliable connections. When done poorly, it can put more trouble into the system than it had before the repair. Worse yet, it can lead to intermittents that are harder to solve than the original problem.

Not all metals are compatible with the typical 60/40 or 63/37 lead-tin solders. Copper, tin and nickel do solder well. Brass and steel fall into the more difficult-to-solder category and, of course, aluminum doesn’t solder at all with conventional techniques (but can be joined with zinc-based solders).

A good solder joint starts with an understanding of the metals involved. Metals like copper, tin and nickel seldom exist in their totally pure state. These metals like to combine with oxygen to form metal oxides. The speed with which and degree to which this oxidation occurs depends on the presence of oxygen and the temperature of the metal. The presence of electrolytes, including contaminated water (distilled water won’t do it), can also speed the reaction.

When this happens to steel, we call it rust. When it happens to the metals we’re talking about here, it forms different kinds of oxide compounds. The point is that metal oxides do not solder. Before we can make a good solder joint, we have to have metal free of any oxides that may have formed.

The term flux refers to a variety of materials, including acids, pine tree-based rosins and other materials, whose goal is to remove the surface oxides so the solder can wet to the metals being joined. (Wetting refers to the amount of surface contact between the metals being soldered.) The oxides that can prevent a good solder joint occur at two different rates and at two different times. The metal slowly begins to oxidize as it leaves the smelting plant. During the heating of the metals to be soldered, the rate of oxidation accelerates greatly. You can start with “clean” metal and wind up with unsolderable metal if it’s overheated in the absence of a protective flux. This kind of oxidation is probably the one to be most concerned about.

Choosing a flux requires the balancing of different goals. You want to remove the oxides, but you also want to prevent corrosion and contamination of the completed solder joint. The flux used must not be electrically conductive, it cannot contain dangerous or polluting chemicals and, of course, it must be easy to apply.

Preventing corrosion after the soldering is complete is a critical issue in electronics and wire harness work. The dimensions of the wire and electronic components generally are small. If a solder joint were left with corrosive residues on it, the result might be that over time the joint could be attacked, weakened or destroyed.

For typical automotive wiring repair work, the traditional flux material has been rosin. There are “no-clean” fluxes and “water-soluble” fluxes. The no-clean fluxes might not be strong enough for wire harness work, while the water-soluble fluxes are more active but require cleaning after the joint is complete.

You can buy solder with rosin built into the center. Some solders will have a single tube, or core, of flux in the middle of the wire while others will have as many as five. The difference is in how the flux is distributed into the joint during the soldering operation. Either can work, but the multicore type is better. You can get solder without the built-in flux, but generally it’s harder to use.

The strength of the rosin comes in three grades—nonactivated, rosin mildly activated (RMA) and rosin activated (RA). The nonactivated rosin will clean most wire and leaves a residue that will not affect the joint later on. It uses an “R” suffix after the alloy number (e.g., 63/37 R). The RMA and RA grades contain activating agents that are usually some form of acid that’s used to improve the wettability of the solder. While the RMA residue can be left on the joint after soldering, RA residue must be cleaned to prevent corrosion later on. For most routine work, the ordinary R rosin flux is the best choice.

In terms of your solder results, there are other forms of contamination to be aware of. The black goo adhesive on electrical tape is a prime example. This type of contamination cannot be removed by the flux in the solder. Similar contaminants include residue from old wire harness insulation, plus oil and grease. Sometimes mineral spirits, brake cleaner or acetone must be used to clean off the goo before a solder joint should even be attempted. You even may have to mechanically scrape the wire to remove the goo.

Various equipment, both new and traditional, is used to heat the metals for a solder joint. The goal is to heat the metals quickly to minimize oxidation. The faster the heating occurs, the less likely the heat will travel to nearby heat-sensitive components. Excess heat can damage electronic components, wire insulation and wire harness connectors. Excess heat can also damage the “spring” properties of brass terminals and wire connectors.

Electronics technicians often use alligator clip leads or even needle-nose pliers to keep heat from getting into sensitive components. The alligator clips work well because they don’t require a third hand. Just clip them onto the wire between the component and the solder joint. They aren’t always necessary, but they can be used as a safeguard.

For most repair work, a soldering iron or solder gun is most often used. It’s important, though, that the size of the iron be matched to the job. Not enough heat results in slow work and possibly cold solder joints that are weak and resistive; too much heat and you wind up with the damage just noted.

The easiest solders to use are the 60/40 and 63/37 alloys with a rosin core. These alloys have a narrow temperature difference between the liquid phase and the solid phase. This helps minimize the time needed to hold wires still while the solder cools.

If you were to ask “where do most techs go wrong” with solder repairs, the answer would be in failing to “design” the solder joint. You need to give thought to how the wires will enter the joint and how the joint will be supported afterward. As mentioned earlier, solder is not a strong metal. It will move or creep if it’s left under a strain.

One of the most important steps to making a good solder joint is to ensure that it’s a mechanically sound joint in the first place that does not depend on the strength of the solder itself. Usually, this amounts to twisting or coupling the wires together first and then applying the solder to fill in and around them.

Protecting the completed solder joint is the final step in the process. In the case of solder joints, almost always the poorest choice is electrical tape. The tape is not pliable enough to closely match the surface profile of the solder joint. It doesn’t stick very well. It also doesn’t do a very good job of keeping out water or dirt. Most importantly, electrical tape doesn’t mechanically support the joint.

The preferred solution is shrink-tubing. A small piece of the right size tubing can be put on one of the wires first. Then the wires can be mechanically linked together, the joint heated and then the rosin core solder applied. After the joint cools, the shrink tube is pulled over it and shrunk with a hot air gun. Do not use other heat sources like matches or soldering irons to shrink the tubing. You need to apply uniform heat so the shrink tube will pull down hard and tight around the joint. Hot air guns can be purchased for roughly $30 and are useful for other purposes around the shop as well.

Making a good solder joint starts with clean, solder-compatible metal. You want to mechanically link the two sides, apply the correct heat, apply the correct flux (unless you’re using rosin core solder) and then apply the solder to the heated metal (not to the tip of the soldering tool). If you do this correctly, the metal being soldered will pull the solder into the joint. When the solder has cooled, pull the shrink tube over the joint and then shrink it with a hot air gun. If you do all of this, you, the solder joint and your customer will be good to go.