Hand-build the board in the right order: the hardest parts first, on a bare board, through-hole last.
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.
Read this before you melt anything: the order is the whole game.
Warning
Safety: read before you heat anything
The hardest parts go down first, on the bare board: U1 and J1, then the passives, then the through-hole parts. Trickiest joints first, tall parts last.
Solder U1 (the module) and J1 (the USB-C connector) first, on the bare board. Both are the board's hardest jobs, and both were chosen to be iron-solderable: the module connects through castellated edge pads you drag-solder, and the connector holds itself still with through-board retention tabs while you work its pin row. Give them an empty board and your full attention. Then iron-solder the passives and small discretes, and finally fit the through-hole parts (switches, headers, test points). Using hot air instead? The order stops being a preference: hot-air rework for U1/J1 blows freshly-placed 0805s right off the board.
drag to rotate
Check yourself
Why solder the WROOM module before the 0805 resistors?
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. Hard joints want a bare board: you can prop it flat, angle the iron freely, and rework a pad without cooking a neighbour. The order matters even more on the hot-air path: heat radiates several millimeters in every direction, so 0805 passives already sitting nearby can have their joints remelt, tumble off in the airflow, or stand up on one end (). Either way the sequence holds: heat-hungry parts onto the bare board first, then the passives one at a time with the iron, where the heat stays local and nothing gets cooked twice.
Bench time: U1 and J1 first with the iron, then the rest. Hot-air alternative below if you have it.
A soldering iron sits at ~340 °C and never looks hot: it burns instantly, so it goes back in its stand the moment it leaves your hand.
Using hot air? Same hazard, moving: 300 °C+ air burns skin and scorches anything in the blast path, so mind where it points whenever it leaves the work. Work somewhere ventilated (flux fumes irritate), wear eye protection (hot flux can spit), tin the tip on a damp sponge, and treat the board, tweezers, and parts as hot for a while after. Lead-free or not, wash your hands when you're done.
The two hardest joints, first, with the tool this board was designed around. Flux does half the work; use more than feels polite.
You soldered J1's through-hole retention tabs first so it couldn't shift.
They do a second, longer job: they're the board's mechanical anchor. Every plug and yank of a USB-C cable lands its force on the connector, and the tabs are what stop it peeling the fine signal pads off the board. So don't just tack them: fill each tab hole with solder so it grips through the board. A well-anchored port outlives thousands of plug cycles; an under-soldered one lifts its pads after a dozen.
have hot air? Reflow them instead
Same order, different heat. If you own a hot-air station (the 2-in-1 bench pick covers it), paste-and-reflow puts U1 and J1 down just as well, and alignment is a little more forgiving because molten paste pulls the part true. The steps below walk that path.
U1 and J1 are down: now the iron does the rest. Passives one at a time, through-hole last.
Check yourself
A resistor sits tilted up on a solder blob instead of flat. Fix?
Flux is the difference between a bridge and a clean joint.
This is the technique for U1's and J1's rows, and for cleaning up any row that came out bridged. 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.
Check yourself
Your drag pass leaves a bridge between two pins. First move?
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.
Hunt for bridges under magnification, then prove no –GND short before any power.
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.
Check yourself
Your meter beeps continuity between VBUS and GND before power-on. Power it anyway?
Checkpoint
Quick check: assembly
7/8
Pass the build's POST_ASSEMBLY_CONTINUITY checklist: every item checked or marked N/A.
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