Soldering, Good and Bad

By David Herres

Frequently the cause of electronic equipment failure turns out to be one or more poor solder joints. In fact, when conventional troubleshooting techniques fail to reveal a bad component, it is sometimes possible to restore operation by touching every joint in a suspected area with the soldering iron and a small amount of flux.

A solder joint, improperly made during manufacture, may remain conductive for days or weeks until, because of thermal cycling or vibration, it abruptly goes down. Proper soldering technique will prevent these cold joints, either at the manufacturing level or for rework and making prototypes in the shop. We’ll take a look at this latter type of process. The insights may be useful in designing or improving a manufacturing process that is yielding less than optimum results.

The basic requirement for good soldering is to provide the right amount – not too little and not too much – of heat. Clean tools and work are also necessary. The correct tools and materials are prerequisites for success as well.

If the soldering iron or the pieces to be joined are not clean — coated with contaminants or a layer of oxide, say — heat transfer will be poor. Specifically, an oxide coating, seen as a dull coloration on the soldering iron tip, will act as a thermal barrier. It can prevent the solder and the work from getting hot enough. The solder will be seen to break and crumble, falling away from the intended joint. The remedy: Keep the iron and objects to be joined clean – free of oxide – immediately before soldering.

Oxide will form gradually on metal objects in storage, and it forms instantly at an elevated temperature during soldering. Excess contaminant will readily scrape off with a knife. Avoid the use of sandpaper as it may leave embedded granules that will eventually reduce the ampacity of the joint, creating hot spots. Then, immediately prior to soldering, apply a small amount of flux. No matter how clean the metal appears, the flux is always necessary to prevent oxide formation at the elevated temperature just prior to melting in the solder.

The flux for electronic soldering should be rosin rather than the acid flux used by plumbers and in automotive radiator work. The reason is that any acid residue will remain in place to corrode the joint and attract dirt and moisture, shorting out closely spaced traces on a circuit board. If you suspect that acid flux has been used, the area may be cleaned with isopropyl alcohol. Flux may be contained in the core of the solder, or solid wire solder may be used, the flux applied manually.

(It should be noted that rosin flux used for electronics is actually made up of mild organic acids though it is distinguished from the “acid” flux used in plumbing. Also, there are several different grades of rosin flux used in electronic soldering. But the distinctions are beyond the scope of this article. Wikipedia has a page on rosin and rosin solder. )

The soldering iron tip must be clean prior to soldering. You typically clean the tip by tinning it. This involves coating the tip with a shiny blob of solder that prevents oxide from forming. Bring the tip up to temperature and wipe both sides on a damp sponge, then power down the iron and apply solder as it cools. The tip should be tinned before each soldering operation and at the end of the work session so that it is protected during storage.

The solder should be a low-temperature alloy. If the joint is heated excessively, the heat can jeopardize nearby semiconductors and potentially lift PCB traces in close proximity from the circuit board. Heat sinks attached to component leads are useful in preventing heat migration.

The solder, soldering iron power, and tip must be sized to match the work. This is more a question of experience and intuition than adhering to specific guidelines. Keep a variety of sizes handy, and use what works.

Soldering is unlike welding, where the materials melt and mix to some depth and filler is added to strengthen the joint. In soldering only the solder melts, not the items to be joined. The solder is absorbed to only a short depth below the surface. This suffices to make a good solder joint that will remain electrically conductive and strong for years.