LIBRARYSoldering and assembly basics.

A finished board is bare copper until the parts are on it. Assembly is either hand-soldering with an iron and solder, which is fine for through-hole and larger surface-mount parts, or reflow, where solder paste and heat melt every joint at once for dense boards. Know which your board needs before you order the parts.

Through-hole and surface-mount assembly

Through-hole parts drop into plated holes and solder on the far side; they are forgiving and strong, good for connectors and anything hand-built. Surface-mount parts sit on top of pads and range from easy 0805 chips down to fine-pitch packages that fight a hand iron. The part sizes on your board decide how you will assemble it.

Hand-soldering

Hand-soldering needs a temperature-controlled iron, solder, and flux, plus solder wick to undo mistakes. You heat the pad and the pin together, feed solder into the joint, and let it flow into a shiny fillet. Flux is what makes it easy: it cleans the metal so solder wets instead of balling up. Larger surface-mount parts hand-solder fine with a fine tip and patience.

Reflow soldering

For dense surface-mount boards, reflow does every joint at once. You apply solder paste (a mix of tiny solder balls and flux) to the pads, usually through a stencil, place the parts on the wet paste, and heat the whole board on a hotplate or in a reflow oven. The paste melts and pulls each part into place by surface tension, which is why reflow is the way to assemble crowded boards.

The heat follows a profile with four stages: a preheat ramp, a soak that evens the temperature and activates the flux, a reflow spike above the solder's melting point, and a controlled cool-down. Common lead-free paste (the SAC305 alloy) melts around 217 to 219 C and is taken to a peak near 245 to 255 C, held only briefly (CompuPhase). The paste datasheet gives the exact profile to follow.

Common defects

A few faults show up again and again. A solder bridge is stray solder shorting two pins, cleared with wick and flux. Tombstoning is a small two-pad part standing up on one end because one side reflowed first. A cold joint is dull and grainy from too little heat and can fail intermittently. Learn to spot these under a loupe and most assembly problems are quick fixes.