PHASE 07ASSEMBLY
Hand-build the board in the right order. Sequence is everything — the wrong order lifts parts you've already placed.
Assembly rewards patience and a plan. The parts go down in a deliberate order, every joint gets flux, and you inspect the board before you ever apply power. Rush the order and you'll spend longer reworking than you saved.
Hot-air the hard parts first, iron the rest after. Do it the other way and you knock off what you already placed.
Do the fine-pitch, heat-hungry parts first on the bare board — U1, the module, and J1, the connector — with hot air or paste-and-. Then iron-solder the passives and small discretes. Finally, fit the through-hole parts (switches, headers, test points). Work the other way and the hot-air rework for U1/J1 blows your freshly-placed 0805s right off the board.
Why solder the WROOM module before the 0805 resistors? Hot-air rework near already-placed passives blows them off — do the hot-air parts first, irons after.
▸Deep dive· Why the heavy parts go down first
U1 and J1 are the heat-hungry parts: the module is a big slab with many pads (several hidden underneath), and the USB-C connector has chunky retention tabs that drain heat away. To solder them you flood the whole area with hot air or run the board through — heat that radiates several millimeters in every direction. If the little 0805 passives are already sitting nearby, that same heat remelts their joints, and the airflow can tumble them off (or stand one up on end — ). So you place the hard, heat-hungry parts onto the bare board first, then iron the passives one at a time afterward, where the heat stays local and nothing you've already placed gets cooked twice.
Flux is the difference between a bridge and a clean joint.
Flood the pads with flux, then the fine-pitch rows: load the iron tip with fresh solder and drag it steadily along the row, letting surface tension and flux pull just the right amount onto each lead while clearing the bridges. On a lead-free board you're working in , which wants a slightly hotter tip and gives a more matte joint.
- Flood the footprint with liquid flux.
- Load the iron tip with fresh solder.
- Drag along one pad row at about 3 mm/sec.
Your drag pass leaves a bridge between two pins. First move? More flux and a clean dragged pass — flux lets surface tension pull the excess off; you rarely need wick for a small bridge.
▸Deep dive· Why dragging molten solder doesn't bridge every pin
It feels like dragging a bead of molten metal across a row of pins should short them all together — flux is what makes it not. Liquid flux strips the oxide off the copper and lowers the solder's surface tension, so molten solder wets clean metal eagerly but beads up and refuses to stick to the between pads. Drag a loaded tip along the row and surface tension pulls just enough solder onto each lead while the excess rides along; any bridge that forms gets reflowed and pulled apart by that same tension. Run out of flux and the magic stops — the oxide creeps back and solder clumps wherever it lands. On this lead-free board you're dragging , which melts hotter and dries to a more matte finish than old leaded solder, so set the iron a touch higher.
Check your work before you ever apply power.
Under magnification, hunt for solder bridges and (a passive stood up on one end). Then run a sweep with your meter: confirm the grounds are connected and — the one that matters most — confirm there is NO continuity between VBUS and GND. A short there would destroy the board the instant USB is plugged in. This is the POST_ASSEMBLY_CONTINUITY gate.
Your meter beeps continuity between VBUS and GND before power-on. Power it anyway? Never — that's a dead short; find and clear it before any power reaches the board.
Quick check — assembly
Pass the build's POST_ASSEMBLY_CONTINUITY checklist — every item checked or marked N/A.