Soldering & Surface Mount

Everything else in this path is downstream of one fact: a solder joint is not glue, it is a metallurgical bond, and the intermetallic layer that forms between the solder and the pad is what actually holds. That layer is why a joint that looks fine can fail six months later, why too much heat is as damaging as too little, and why 'it made contact' is not the same as 'it is soldered'. Start here or every technique later is a ritual with no reasoning behind it.

Most beginners think of flux as an optional cleaning aid and solder as the thing that matters. It is closer to the reverse: oxide on the pad prevents wetting entirely, flux removes it, and the flux window is what you are actually managing when you control heat and time. This unit separates leaded from lead-free behaviour, and the flux classifications — which decide whether residue must be cleaned off or is designed to stay, a distinction that has real reliability consequences.

🔴 The unit that decides whether the practice units are productive or frustrating, and the first with serious hazard content. Thermal mass matters more than the temperature setting, tip geometry does most of the work people attribute to skill, and a poorly tinned tip makes competent technique look incompetent. The Risk elements are real and specific: burns, mains-powered equipment on a metal bench, flux fume extraction, and lead hygiene — where the actual exposure route is hand-to-mouth, not the fume.

A Skill element, closed only by logged practice and photographed joints — never by a card. Through-hole comes before surface mount because the joint is large enough to see forming, the failure modes are visible in real time, and the feedback loop is fast. The assessable outcome is a run of joints that meet the acceptance criteria consistently, not a single good one, because consistency is the thing being trained.

Chip passives from large down to the sizes where tweezers stop being adequate, gull-wing packages, fine-pitch quad packs, and bottom-terminated parts with no accessible leads at all. Each package changes the technique, the tool and the inspection method, so this unit is the map that tells you which of the following units applies. It also sets up the central problem of the path: some packages cannot be inspected by eye at all, which is why unit A12 exists.

The second Skill unit, and the one with the highest practice requirement in the path. Tack-and-solder for placement, drag soldering for fine-pitch leads, working with flux rather than against it, and handling parts small enough that surface tension does the placement for you. Closed by logged reps and inspected results. There is no shortcut and no quiz that substitutes.

The point at which you stop heating one joint and start heating a region, which changes the whole problem — neighbouring parts move, boards warp, and thermal mass differences across the board mean a uniform air temperature does not produce a uniform board temperature. Preheat is the control for that, and it is the tool most self-taught people skip and then blame their board for.

Removing a part without destroying the board is harder than fitting one, and it is what the job actually consists of most of the time. Wick, vacuum, low-melting-point alloys, and knowing which to reach for. The Risk element is pad and trace damage: lifted pads are usually unrecoverable, they are caused by mechanical force applied while the joint is not fully molten, and the mitigation is technique rather than equipment.

How boards are actually built at any volume, and the process that most hand-soldering defects are compared against. Paste as a suspension of solder spheres in flux rather than a soft solder, stencil aperture design, and the four zones of a reflow profile with what each is doing chemically. Worth learning even for one-off work, because a hotplate or a modified oven puts it within reach and the results are more consistent than hand work.

🔴 The intellectual core of the IPC-A-610 body of knowledge, and the thing that surprises people: there is no single definition of a good joint. The same joint can be acceptable for a consumer product, a defect for an avionics one, and the standard is explicitly written in three classes to say so. Learning to inspect means learning to inspect *against a stated class*, which reframes every judgement in this path from taste to specification.

Named, recognised and — the part that matters — traced to a cause. Tombstoning from asymmetric heating, bridging, insufficient or excessive solder, cold and disturbed joints, dewetting, voiding, solder balls, and head-in-pillow. Each one has a mechanism, and knowing the mechanism is what converts inspection from spotting ugliness into diagnosing a process problem you can then fix.

🔴 The honest limit of visual inspection, and a unit that has to exist precisely because the rest of the path trains the eye. Under a ball grid array or a bottom-terminated package there is nothing to look at, and confidence has to come from somewhere else: X-ray, electrical test, process control, and profile discipline. This is also where the practical reality gets stated plainly — some rework is not viable by hand, and knowing where that line is is part of the skill.

The reliability content that gets skipped because the board works when it leaves the bench. Ionic residue drives electrochemical migration and dendrite growth under humidity and bias, which is a failure that appears in the field weeks later and is invisible at assembly. Covers which flux residues must be removed, how cleanliness is actually measured, and what conformal coating does and does not protect against.

Entirely Risk elements, gathered deliberately rather than sprinkled through the path, because each needs a hazard AND a mitigation to be assessed properly. Lead exposure and the hand-to-mouth route, flux fume and extraction, thermal injury, solvent handling, and electrostatic damage that leaves a part working but degraded. The mitigation half is what is assessed — not that ESD exists, but what you do at the bench before touching a board.

The capstone: a real assembly, built, then inspected by the builder against a stated acceptance class and written up honestly — including the joints that fail. It is a Skill element and it is deliberately self-assessed, because the transferable outcome is not producing a perfect board, it is being able to look at your own work and say truthfully which class it meets.