Hand-placing a 0402 passive
A 0402 chip is 1.0 mm long and weighs about 0.6 mg, which means the iron's own magnetic field and the flux vapour are both strong enough to move it. The technique is to stop trying to hold the part and let surface tension centre it instead.
Procedure
- 1
Tin the outer pad with a thin film of SAC305, leaving the inner pad clean and dry.
- 2
Pick the 0402 body with a vacuum pen and position it so one termination sits on the tinned pad.
- 3
Touch the 0.4 mm conical tip to the tinned joint for one second at 320 degrees C; the chip will self-centre.
- 4
Solder the second termination with a one-second dwell, adding 0.3 mm wire only if the fillet is starved.
- 5
Inspect at 20x for tombstoning, end-shift greater than 0.1 mm, and termination leaching.
Workshop notes
Tin one pad only
Flow a small amount of SAC305 onto one pad - the left one if you are right-handed, so your iron approaches from the outside. Do not tin both pads; a doubly tinned footprint gives you no freedom to slide the part, and the second pad's solder will wick up the side of the chip and tombstone it. A 0.5 mm pad takes a film of solder roughly 0.15 mm thick, which is what a 0.4 mm conical tip leaves after one touch with 0.5 mm wire.
Place with vacuum, not tweezers pressure
Use a vacuum pickup pen rather than tweezers if you have one: tweezers store elastic energy and release it as a flick when the chip slips. Place the part so its termination sits on the tinned pad and the other end rests flat on the dry pad. Then touch the tinned joint for one second - the solder wets the termination and pulls the chip into alignment by itself, which is why the placement tolerance is 0.2 mm and not 0.02 mm.
Second joint and the dwell limit
Solder the untinned pad with a one-second dwell at 320 degrees C, feeding 0.3 mm wire only if the joint is visibly starved. A 0402 termination is typically 0.25 mm thick nickel barrier over the ceramic; more than 1.5 seconds of contact and you will see the termination begin to leach, which shows up as a silver-grey smear on the iron tip. Cap the dwell at one second and move on.
Practice drill
Place twenty 0402 resistors on a scrap footprint field using only a 0.4 mm conical tip and a vacuum pen. Inspect under 20x and count tombstoned, shifted and leached parts. Ten clean placements out of twenty is a good first session; the failure mode you should see going away first is shifting, not tombstoning.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Component stands on one end (tombstones) | Solder on both pads melted unevenly, and the first fillet pulled the chip upright | Wick the inner pad dry, pre-tin only the outer pad, and solder the second joint in a single one-second contact |
| Termination leaches, tip shows a grey smear | Dwell over 1.5 seconds at 320 degrees C, dissolving the nickel barrier into the alloy | Cap the dwell at one second, drop the setpoint to 300 degrees C, and stop feeding wire once the fillet has formed |
| Chip drifts after the iron is lifted | Flux vapour jet from the pen pushed the part, or the board was moved before the alloy solidified | Apply flux before placement rather than after, and hold the board still for two seconds after the second joint |
Supplies
- 0.3 mm SAC305 solder wire
- 0.5 mm SAC305 wire for pre-tinning
- ROL0 no-clean flux pen with a 0.5 mm nozzle
- Isopropyl alcohol and a fine brush
Tools
- Station set to 320 degrees C
- 0.4 mm conical tip
- Vacuum pickup pen with 0.3 mm cups
- 20x stereo microscope
Safety
At 320 degrees C a 0402 chip gets hot enough to blister skin on contact, and flicked parts are a genuine eye hazard at bench height. Wear safety glasses for this job, not only for the desoldering ones.
Neighbouring cards
- Two-iron removal of a 0805 chip 330 °C / 1.5 s
- Fixing tombstoned 0603 and 0805 parts 310 °C / 1.0 s
- Dead-bug wiring a TSSOP 300 °C / 0.8 s
- Tip tinning, storage and end-of-day care 300 °C / 0.5 s