Assembly & reflow
Assembly attaches the components, which come in two styles. Surface-mount (SMT) parts sit on pads on the board’s surface. They’re small, cheap, and suited to machine placement, which is why nearly everything modern is SMT. Through-hole (THT) parts push leads through plated holes and get soldered from the far side. They’re bulkier but mechanically strong, so they stay popular for connectors and anything a person will yank on.
SMT boards are soldered by reflow. A laser-cut stencil squeegees solder paste, powdered solder suspended in flux, onto every pad. Pick-and-place machines set each component into the paste. Then the whole board rides through a reflow oven on a controlled temperature profile: preheat, soak, a brief spike past solder’s melting point, and gradual cooling. Every joint on the board forms at once, and surface tension in the molten solder even pulls slightly misplaced parts into line.
Through-hole parts go on afterwards, soldered by hand or by wave soldering, where the board’s underside skims across a standing wave of molten solder. For prototypes, hand assembly with a decent iron, flux, and patience is entirely realistic for most SMT work. Learning to do it also makes you much faster at spotting bad joints on other people’s boards.
Key points
- SMT parts sit on surface pads and dominate modern boards. THT is stronger for connectors.
- Reflow: stencil the paste, place the parts, then an oven profile melts every joint at once.
- Surface tension in molten solder self-aligns slightly misplaced SMT parts.
- THT gets soldered by hand or wave after reflow. Hand SMT assembly is realistic for prototypes.
Practice
0 of 2 answered · Not startedWrong answers just let you try again, and hints are there if you want them. Answering every question first time, without hints, is what earns mastery.
- 1
What causes tombstoning on a small two-terminal part?
- 2
Why does a reflow profile ramp gradually rather than heating straight to peak?
