Soldering & Joints

Soldering is the one hands-on skill the Associate CET exam expects every technician to understand — how a good joint forms, what temperature and technique produce it, and how to spot a bad one. This guide covers the concepts and the inspection criteria you'll be tested on. It also covers the hazards at the bench, flux types, heat shunts, surface-mount and BGA handling, pad and trace repair, desoldering and its equipment, solder wick, and inspection to IPC-A-610.

What soldering actually is

Soldering joins metal parts by melting a filler metal (solder) into the gap between them, where it bonds to each surface and solidifies into an electrically and mechanically sound connection. The key idea: the solder does not act like glue sitting on top — done right, it forms a metallurgical bond called wetting, where the molten solder flows into and alloys with the surface of the base metal.

The heat comes from the iron, but a crucial rule is that you heat the joint (the lead and the pad together), not the solder. You then feed solder to the heated joint so the joint's own heat melts it. This is what lets the solder flow by capillary action and wet both surfaces.

Anatomy of a good joint

A textbook joint is shiny, smooth, and concave — the solder curves up the lead and feathers out onto the pad at a shallow angle (a "volcano" or "Hershey's Kiss" shape). That low contact angle is the visual proof of good wetting. You should still be able to see the outline of the lead through the solder. (That shine is for tin-lead solder: a good lead-free joint looks duller and more satin, so judge it by its smooth, concave, well-wetted shape.)

The soldering process, step by step

1. Clean and prep. Surfaces must be clean and oxide-free. Flux removes oxidation so the solder can wet the metal.
2. Heat the joint. Touch the iron tip to both the lead and the pad at once, so they heat together. Hold for one to two seconds before feeding solder (the whole joint takes about 1–3 seconds).
3. Feed the solder to the joint — not to the iron tip. The heated joint melts the solder, which proves it's hot enough to bond.
4. Let it flow. The solder flows around the lead and wets the pad, forming the concave fillet. A small amount is enough.
5. Remove solder, then iron. Pull the solder away first, then the iron, and hold the joint still until it solidifies (about a second).
6. Inspect. Look for a shiny, smooth, concave joint (satin rather than shiny with lead-free solder). Re-do anything grainy, blobby, or cracked.

Iron temperature & specs

There's no single "correct" temperature, but there's a usable working range. Too cool and the solder won't flow (cold joints); too hot and you damage components, lift pads, or burn off the flux before it can work.

Good joint vs. bad joint

Desoldering & rework

Removing solder is as much a skill as applying it. The two standard tools: solder wick (braided copper that wicks molten solder away) and a solder sucker (a spring-loaded vacuum pump that slurps up melted solder). Reflow a suspect joint by adding a touch of fresh flux-cored solder and reheating — that alone fixes many cold and cracked joints.

Safety

Bench hazards: burns, fire, fumes, lead and splatter

Competency 5.1 asks you to describe solder safety as it pertains to burns and potential fires or damage to facilities or customer products; 5.2 asks you to explain the cause of solder fumes and the effects of lead poisoning; and 5.3 asks you to list the causes of solder splatter and the precautions that prevent or reduce it. Section 7 gives the short rule. Kuphaldt's point is the one to remember: soldering is a relatively low-temperature process, but it is still hot enough to start fires and burn skin.

What lead does to an adult. The EPA lists decreased kidney function, heart problems and raised blood pressure, infertility in men and women, learning, memory and concentration problems, and weakness in the fingers, wrists and ankles; a pregnant worker's exposure can also affect the fetus. That is why the hand-to-mouth rules above matter as much as the fume extractor.

Flux types

Competency 5.4 asks you to explain the reasons for using flux and to describe the types. The reason is in section 3: flux removes the oxide so the solder can wet the metal. A flux's activity is how strongly it removes surface contaminants, and the more active it is, the more its residue matters. IPC J-STD-004 grades activity as L (low), M (moderate) or H (high), with a 0 for halide-free and a 1 for a flux that contains halides (Prasad, I-Connect007).

Heat shunts

Competency 5.6 asks you to explain heat shunts (clip-on heat sinks): why and how they are used. Diodes, transistors and some passive parts such as electrolytic capacitors are easily damaged by soldering heat (Kuphaldt). A heat shunt is a metal clip fastened to the lead between the joint and the body of the part, one lead at a time, so the heat flowing up the lead goes into the clip instead of the part, without cooling the joint too much (Kuphaldt). Clip it on before you heat the joint and leave it there until the joint has set and cooled. With no formal heat sink, a pair of needle-nose pliers with a rubber band around the handles clamps the lead just as well (Kuphaldt). NASA-STD-8739.3 makes it a rule: thermal shunts are used to absorb heat from part leads as needed to protect parts, insulating materials and joints already made.

Surface-mount parts, BGAs and gold fingers

Competency 5.8.1 asks you to explain the basic handling procedures for SMT parts, BGAs and gold finger contacts on a PCB.

Pad, trace and plated-through-hole repair

Competency 5.8.2 asks you to explain the basic procedures for pad repairs (SMT and BGA), plated through-holes (PTH) and PCB trace repair. The industry procedures are in IPC-7711/7721: IPC-7711 covers removing, replacing and soldering components, and IPC-7721 covers repairing the board itself, including pads, traces (conductors), laminate, vias, jumper wires and coatings, with the materials, tools, temperatures and inspection for each (Spartronics). A repair is made only with the customer's approval, using materials and methods the customer accepts (I-Connect007).

Prevention is cheaper than any of these. Hakko's desoldering-tool manual warns that a hot nozzle placed on the land itself can make it peel, so heat the lead and its solder, not the pad.

Desoldering: principles, equipment and care

Competency 5.10 asks you to explain desoldering principles, 5.11 to describe the types of desoldering equipment and how each is used, and 5.9 to describe the proper care of soldering and desoldering equipment and aids. Section 6 names the two basic tools; here is the method behind all of them, from Hakko's desoldering-tool manual. Heat the lead and its solder, not the land. Add a little fresh solder if the joint is dry, because it forms a heat bridge and melts the old solder faster. Before you pull the solder out, make sure all of it is molten: with the heat still on, move the lead gently, and if it moves freely, the joint is fully melted. Then remove the solder in one action. If some is left, resolder the joint with new solder and repeat; never force a lead that is still held.

Care of the equipment. Hakko's FR-301 manual calls cleaning "extremely important". Wipe the nozzle on a sponge moistened with distilled or deionized water, as you would a soldering tip, and run the matching cleaning pin through the nozzle and the heating core while they are still hot. Empty the collected solder as you work, keep a filter in place (desoldering without one can damage the pump), replace a filter hardened by flux, replace a nozzle that is worn or no longer holds solder, re-tin the nozzle before putting the tool away, and never strike the tool to knock solder out. Keep solder wick sealed at room temperature: fresh braid is bright copper, and dark or oxidized braid will not wick (LCSC). The iron itself stays tinned and is wiped on a wet sponge between joints (Kuphaldt).

Using solder wick, step by step

Competency 5.12 asks you to demonstrate the use of solder wick. The method, from LCSC's guide:

1. Choose and prepare the braid. Use the narrowest braid that covers the pad, and cut back to fresh, bright copper; add a little liquid flux on a stubborn joint.
2. Lay it flat. Put the braid flat on the solder to be removed, covering the whole pad, or both joints of a bridge.
3. Heat through the braid. Press the iron tip on top of the braid, over the solder, not on the pad. In 1 to 3 seconds the solder melts and the braid darkens as it fills.
4. Lift both together. Lift the braid and the iron away together while the solder is still molten. If the iron leaves first, the braid freezes to the pad and can tear it off when you pull it.
5. Cut and clean. Cut off the used braid, inspect the pad, and clean off rosin residue with isopropyl alcohol if the job calls for it.

On the job: inspecting work to IPC-A-610

Job practice, not exam content. Employers and customers judge soldered work against IPC-A-610, Acceptability of Electronic Assemblies, the post-assembly acceptance standard; its partner, IPC J-STD-001, sets the requirements for the soldering process and materials. Both are revised about every three years, and revision J came out in 2024 (IPC). The standard sorts products into three classes by what a field failure would cost (PCBCart):

Within a class, each condition is judged acceptable, a process indicator (not a reliability problem, but a sign the process needs attention) or a defect. Revision H (2020) dropped the older "target" category (IPC), and revision J kept it out, so training material that still shows target joints predates both revisions. The same joint can pass at Class 2 and be rejected at Class 3.

1. Know the class. The customer's drawing or contract states it; do not assume Class 2.
2. Look properly. Use magnification with true color and shadowless light (NASA-STD-8739.3).
3. Judge each joint. Compare it with the criteria for that class: wetting, fillet shape and solder quantity (sections 2 and 5), and damage such as a lifted pad.
4. Act on what you find. Rework defects; record process indicators so the process can be corrected.
5. Re-inspect the rework. Check the reworked joint and its neighbors again, and escalate a joint that fails rework more than once, because repeated heating weakens the pad (PCBCart).