A LOOK AT soldering irons
Jun 1, 2000 12:00 PM,
When I was just a kid, my grandfather was a plumber, and he occasionallyhad to solder pipes. Now those were irons. A 1.5 inch (38 mm) wood handleconnected to an 8 inch (203 mm) stem connected to a pointed 1 inch (25.4mm) square by 4 inch (102 mm) long tip. This tip held heat. First, grandpaused to apply acid to the pipes to clean them. Then, he would heat the ironover the kitchen gas stove and wipe the tip of the iron with a wet rag toclean it. Finally, he would heat the pipe and the solder at the same time.Sometimes, he could only do a small amount of the joint at a time becausethe iron would cool down, and sometimes, he would have two or three ironsheating up so he could do the job faster.
Well, things have changed in 50 years. We now have many types and sizes ofirons and many types and sizes of cleaning agents. One thing, however, hasnot changed and never will – the joint to be soldered must be clean and thesolder must flow evenly around and through the joint (see “To Solder or Notto Solder” in the S&VC October 1997 issue).
Thanks to electricity, electronics, efficient batteries and propane, wehave many types and sizes of solder irons today. Probably the most commoniron used by A-V technicians is the pencil iron (see Figure 1). This ironcomes with various wattage ratings and interchangeable tips. All we do isunscrew one tip and screw in another. Is it really that simple? Pencilirons heat the tip by transferring the heat from the heating element to thescrewed-in tip. If the threads are corroded, heat will not pass through thecorrosion, so the tip never gets hot enough to melt solder. So, we yell andwe swear to no avail until we remove the tip, clean the joint, applyanti-seize compound on the threads and screw the tip back in. Now, the ironis ready to solder our connection as long as the outside of the tip isclean so the heat can transfer from it to the solder or joint. A dampsponge works well for this. Simply pull the tip of the iron through thesponge, and the dark, dull oxide tip wil!l become a shiny tool. Next, wemust determine the type of tip to use – pointed or chisel. I personallyprefer a chisel tip because it holds the heat better and gives more areafor heat transfer. If I have to get into a small area, I turn the iron 90degrees and think of it as a pointed tip. Of course, there are times thatonly a pointed tip or one of the many other types of tips will do the job,and this is the advantage of having replaceable tips. What are thedisadvantages of pencil irons? Most are limited in the wattage theydeliver, and they do take time to heat up.
A second popular iron is the soldering gun (see Figure 2). Shaped like aColt .45 pistol, the gun heats upon demand when you pull and hold thetrigger. Heating is in seconds rather than minutes, and cooling is almostas fast. Guns heat up by delivering low voltage, high current through thetip as the tip is the secondary of the transformer mounted in the handle.Guns work well out on the job where time is important and we cannot safelyset a pencil iron down. Another plus for guns is the built-in light. Mostguns have one or two spotlights that aim at the tip and, hence, the work.These lights activate whenever the trigger is pulled, making guns reallyhandy above the ceiling of a building or behind an electronics cabinet.Guns often have two levels of power controlled by the two-position trigger.This allows us to work on semi-delicate joints as well as heavy-duty ones.
Like the pencil iron, gun tips are interchangeable. The most common causeof a cold tip is the connections between the tip and the gun. Always makesure that the nuts are tight and the joints are free of oxidation. I haveoften had a gun fail when I knew the tip-to-gun joints were good only tofind out that the tip, through much use, had broken right at the tip. Weoften think the tip is good because the two pieces are touching, but whenwe put pressure on the gun and, consequently, the tip, the joint opens sothat no electricity and, therefore, no heat flows. Another disadvantage ofheat guns is their lack of controlled temperature.
Nowadays, we can even buy battery-operated pencil irons, but do not try touse them for heavy work. After all, a D flashlight cell is not designed tosupply a lot of power, and the power required to solder a joint alwaysseems to be more than the power delivered from the source. There are times,however, when these irons are a necessity, especially when a 120 V supplyis not available. Be sure you fully charge the iron before use, or you maynot have enough energy to finish the job.
Many circuit boards are ultrasonically soldered. With this method, the sideof the board to be soldered is lowered into molten solder. The ultrasonictransducer produces cavitation, and aided with the solvent action of themolten solder, scrubs and removes the surface compounds and soils on theworkpiece. The solder wets the clean metal and is deposited on it, allowingfor fluxless soldering.
One final iron is the butane torch. In this iron, the torch either soldersdirectly or heats a tip. I recommend you leave these devices to jewelersbecause electronics and flames do not usually mix.
Of course, there are many variations to the mentioned irons. Probably themost important is the thermostatically controlled pencil iron (see Figure3). With this iron, we can control the temperature of the tip from about250 degrees F to 1,000 degrees F (107 degrees C to 524 degrees C). Theseirons are most often used on the workbench and are often left on all day.It is usually better, however, to turn the iron off when it is not in useso that the tip will not oxidize. These irons usually come with a built-inholder and a sponge. This is, without a doubt, the iron of choice.
Now that we have looked at irons, how about the second part of thesoldering formula – the solder? Solder comes in various sizes and formulas.The most common for our work is probably Kester 44 inch (1.1 m) rosin-fluxcore 0.62 inch (15.7 mm) diameter 60% tin, 40% lead.
Pure metals always melt at a single temperature, while alloys melt atvarious temperatures, depending on their composition. The temperature wheresolder begins to melt is called the solidus, and where it is completelymolten is called the liquidus. One other point is called the eutecticcomposition and is where the solder both melts and solidifies at a singlepoint, as in pure metals. The 60/40 solder is solidus at 361 degrees F andliquidus at 374 degrees F (167 degrees C and 176 degrees C). If the circuitis exceptionally sensitive to heat, you can use 63/37 solder, which is bothsolidus and liquidus at 361 degrees F (167 degrees C), i.e., its eutecticcomposition. A 50/50 solder is popular for general soldering with a solidusof 361 degrees F and a liquidus of 421 degrees F (167 degrees C and 202degrees C).
All metals corrode and collect dirt. This is unacceptable for soldering;therefore, the parts must be cleaned before soldering. There are threetypes of fluxes – inorganic fluxes, which are rather active, organicfluxes, which are moderately active, and rosin fluxes, which are the leastactive and the only one we should use for soldering electronic parts. Rosinis produced from pine trees and is unique because it becomes active as aflux when heated and returns to an inactive state when cooled. This meansthe remaining flux does not have to be removed from the workpiece aftersoldering except for aesthetic reasons. Flux can either be manually appliedto the parts to be soldered or can be applied through rosin-core solder.Using the rosin core solder is usually the preferred method because itapplies the correct amount of solder for most jobs.
Whichever soldering iron and solder you use, always remember to clean, andbe sure to bring the parts up to the proper temperature to melt the solderso that it can properly flow between and around the parts you are soldering.