In Soldering 101 last fall, I went through the basics of tools, tips and technique on getting good soldering skills. In this soldering 102 session, I still want to keep it basic, but show more examples and commonly encountered soldering tasks.
Splices
Even the best intentions cannot avoid the need for occasionally splicing cables together. Whether it is a falling cymbal chopping an audio cable, or having the bad fortune of quickly making an adapter cable to bring two previously unimagined things together in a pinch, splicing cable conductors together is a basic skill for anyone dealing with electronic system maintenance.
Back in the late 1800s, soldering was virtually unheard of; and all splicing was done mechanically and typically to repair broken telegraph or telephone wires. To make rugged splices that would not easily pull apart with tension, the Western Union splice was invented to solve the lineman’s splicing dilemma. Figure 1 shows pictorially the three-step process to making a quality Western Union splice. The main secret to the splice’s success was a stiff solid conductor copper wire, which could be wrapped easily with your fingers, but axial tension could not easily unwrap. The Western Union splice is still good with stranded wires, but soldering is needed to make up for the flexibility of stranded wires. By simply tinning up the wraps and letting the solder joint cool, is the best way to go about it. Of course, multi-conductor wiring should be individually taped after soldering to prevent shorts.
The second, and intuitively obvious splice, is the pigtail splice. As shown in Figure 2, it is nothing more than two wires placed next to each other and twisted together. Electricians use wire nuts on solid or stranded conductors to make pigtail splices in commercial and residential premises wiring, but audio connections should have more reliability and tension relief by soldering after twisting pigtail splices. Again, nothing more than tinning up the wound conductors, and a complete and light application of solder, should wet up the whole pigtail wrap.
Component Soldering
In the rare occasion that you need to replace electronic components on printed circuit boards (PCBs), some additional soldering tips should be acquired. Figure 3 shows typical “leaded” components attached to a circuit board and soldered. The first tip is that solderable leads should not be bent in right angles very near the component body. Doing it right becomes old hat after lots of practice, but you initially need to be aware that bends really close to the component body may fracture the connections just inside the body.
Another teachable tip is that leads going through the holes on the circuit board should be slightly bent (either inward, or preferably outward) after fully seating the component part on board. This mechanically holds the part in place while your two hands handle the soldering process that comes next. Note that the depicted good solder filets are modest in solder amount, and will be on both sides of the circuit board if the printed circuit traces are also on both sides. To repeat from Soldering 101, these solder joints are done quickly and take only a few seconds each when the soldering iron is properly tinned and at the right temperature. Once soldered, do not forget to nip off the excess lead lengths before going on to other tasks.
{mosimage} Soldering Now
If you get the chance to see many modern printed circuit boards, you will note that not only are the components shrinking, but many are missing leads and just have solder dipped ends to make solder joints to. These surface-mount technology components (SMT parts) are here to stay, and are shrinking size and solder joint sizes beyond normal soldering iron usage. For the larger SMT parts, reflow of solder is shown in Figure 4, and generally done by automation in factories.
To replace an SMT part, or even a normal (through-hole) component, soldering wick or braid; or special purpose soldering/suction iron is used to first remove the old solder filets and pluck the part away. Then with SMT parts, the component must be placed back precisely and held in place by a glue dot beneath the part or by mild hold-down pressure on top. Then each solder joint is remade. You may need three hands for this if the glue dot technique is not available.
In The Future
In the future, SMT parts/components will no longer have easily visible solder contacts, and your cell phone guts are a good glimpse of that future. As components morph to tiny chip-scale packaging with 50 to 100 micron diameter solder ball connections, the only viable component replacement method are precision hot-air guns with gentle warm-up and cool-down jets of air precisely placed on one part at a time. I work with these parts daily, and I feel the pain of the layman whose hands and tools are not shrinking, but the soldering surfaces are.
