Through-hole fillet baseline
The reference joint every other through-hole technique is measured against: a 1.0 mm hole, a 0.8 mm lead, a 2.6 mm annular pad and a concave fillet that wets to the top of the pad without climbing the lead. Get this one reproducible and 80% of hand-soldering defects stop appearing.
Procedure
- 1
Clean the tip on brass wool, then tin it with a thin film of SAC305 so heat transfers through liquid metal rather than through oxide.
- 2
Place the chisel face flat against the pad and the lead at the same time, angled 45 degrees to the pad so both surfaces see the same contact area.
- 3
Hold for one second, then feed 0.8 mm SAC305 wire into the pad-to-lead junction rather than onto the tip.
- 4
Remove the wire as soon as the alloy flows around the barrel, then lift the iron straight up half a second later.
- 5
Inspect at 10x from three angles: the fillet should be concave, shiny-to-satin, and wet the top of the pad without a raised shoulder.
Workshop notes
Why the fillet shape is the acceptance criterion
IPC-A-610 class 2 asks for a concave fillet that wets the pad and the lead, with a contact angle under 90 degrees and visible evidence of wetting on both surfaces. The fillet forms because the alloy is pulled by surface tension toward the hot metal it can wet, not because you pushed solder into the hole. If you feed wire faster than the pad can heat, the alloy freezes into a convex blob that sits on top of the pad and never touches the barrel wall - electrically it may still buzz out on a continuity test, but thermally and mechanically it is a cold joint waiting to crack.
Choosing the tip and the iron setting
A 2.4 mm chisel face covers a 2.6 mm pad without touching the neighbouring pad on 2.54 mm pitch. Set the station to 340 degrees C and let it recover for a full 20 seconds after the last joint; a station that reads 340 while the tip sits at 290 will make you hold the iron far too long. Lead-free SAC305 needs roughly 30 degrees more tip temperature than Sn63Pb37 for the same dwell, so if you keep two irons on the bench, label the stands rather than trusting memory.
The three-second rule and how to count it
Feed 0.8 mm SAC305 wire into the junction of lead and pad, not onto the iron tip. Count one-thousand-one, one-thousand-two as the alloy collapses into the hole, then remove the wire first and the iron half a second later. Two seconds is the target, three is the ceiling, and anything past four seconds means the pad is being damaged rather than heated. If the joint is not finished at three seconds, the tip is too small, too cold or too oxidised - change one of those, do not lengthen the dwell.
Practice drill
Solder a 20-pin strip of 0.1 in headers into a scrap board, alternating the direction you approach each pin. Photograph the row from a low angle and compare fillet heights: a uniform row should show the same meniscus on every pin. Re-do the row until you can produce 20 identical joints without touching a single one with wick.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Convex blob, solder sitting on top of the pad | Wire fed onto the iron tip instead of the work; pad never reached wetting temperature | Wick the blob off, re-flux, and feed wire into the pad-to-lead junction with the tip held one second before the wire arrives |
| Fillet wets the pad but not the lead | Oxidised or contaminated lead plating, or the tip is parked on the pad away from the lead | Scrub the lead with a fibreglass pencil, add flux, and place the chisel so it bridges pad and lead simultaneously |
| Pinhole or blowhole in the fillet | Trapped flux volatiles escaping through freezing alloy, usually from too fast a dwell | Extend the pre-heat by one second and let the joint cool undisturbed for three seconds before moving the board |
Supplies
- 0.8 mm SAC305 rosin-cored solder wire
- ROL0 no-clean flux pen
- 99.9% isopropyl alcohol in a dispensing bottle
- Lint-free swabs
Tools
- Temperature-controlled soldering station, 340 degrees C
- 2.4 mm chisel tip
- Brass wool tip cleaner and distilled water sponge
- 10x loupe or USB microscope
Safety
Lead-free SAC305 contains no lead, but the flux core is a sensitiser: keep the bench under fume extraction and do not eat or drink at the bench. The iron idles at 340 degrees C; set it to sleep rather than leaving a hot tip face up.
Neighbouring cards
- Diagnosing and repairing a cold joint 340 °C / 2.5 s
- Hook splice on stranded wire 330 °C / 2.5 s
- Wick cleaning a through-hole pad without lifting it 350 °C / 1.5 s
- Drag soldering a 0.1 in header 360 °C / 1.2 s