Drag soldering a 0.1 in header
Drag soldering turns a 40-pin header from a two-minute job into a twelve-second job, but only if the tip carries a reservoir of molten alloy and the row is fluxed generously enough that the solder refuses to bridge. The technique works with Sn63Pb37 far better than with lead-free, because the eutectic alloy freezes 34 degrees lower and the tip can stay in contact without the joint stiffening under it.
Procedure
- 1
Flux the full pin row with ROL1 gel, working the syringe tip between the pins so flux reaches the barrel, not just the pad surface.
- 2
Tin the hoof with a bead of Sn63Pb37 and shake off the excess so only a bright film remains in the concave face.
- 3
Set the hoof on the first two pins at 40 degrees to the row and pause for one second until the alloy flows into both barrels.
- 4
Drag steadily along the row at about 4 mm per second, letting the bead follow the tip; do not lift between pins.
- 5
At the end of the row lift straight up, then inspect at 10x for bridges and re-flux before any touch-up.
Workshop notes
Flux quantity is the whole technique
A drag joint needs enough flux that a visible liquid film sits between adjacent pins throughout the pass. Use a ROL1 mildly activated gel flux dispensed from a syringe, and apply it along the row twice: once before heating, once a third of the way through the drag if the row is longer than 25 mm. Insufficient flux is the single reason drag soldering bridges, not excess solder - the alloy follows the flux, and where the flux has burned off, the solder stays behind as a bridge.
Tip geometry and the reservoir
A 2.0 mm hoof (also sold in catalogues as a mini-wave or gull-wing tip) holds a bead of alloy in its concave face. Tin the hoof generously - a dry tip cannot drag. Hold the iron at 40 degrees to the row so the flat of the hoof contacts two pins at a time, and move at roughly 4 mm per second. Faster than that leaves unwetted pins behind, slower boils the flux away and produces bridges.
Alloy choice and when to ignore it
Sn63Pb37 at 360 degrees C tip temperature gives a bright, low-resistance joint and forgiving dwell. Lead-free SAC305 works at 385 degrees C but the window between wetting and bridging narrows considerably, and the finished row looks matte. For repair work on equipment that was assembled lead-free, staying lead-free is still the correct choice if the board may later be inspected to RoHS; for your own prototype boards the alloy choice is yours.
Practice drill
Solder a 40-pin 2.54 mm header in a single continuous drag two times: once with a 1.2 mm chisel, once with the 2.0 mm hoof. Compare bridge count. Then deliberately create three bridges by stopping mid-row and rehearse clearing each with wick in under four seconds.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Solder bridges between adjacent pins | Flux burned off mid-row, or the tip was lifted and re-placed between pins | Do not stop: add gel flux ahead of the tip and keep the bead moving; clear existing bridges with wick and fresh flux rather than by re-dragging |
| Row is matte and grainy with dull pins at the end | Tip temperature fell below 340 degrees C during the pass because the station never recovered | Split long rows into 20 mm segments with a two-second recovery pause between them, and check the tip is not loose in the handle |
| Excess solder forms a wave at the row end | The hoof reservoir was overfilled and had nowhere to discharge | Shake the tip before starting so only a film remains, and finish the last two pins with a 1.2 mm chisel and hand-fed wire |
Supplies
- 0.8 mm Sn63Pb37 rosin-cored solder
- ROL1 mildly activated gel flux in a 10 cc syringe
- Isopropyl alcohol and a stiff brush for cleanup
- Desoldering braid for bridge removal
Tools
- Temperature-controlled station set to 360 degrees C
- 2.0 mm hoof tip
- Brass wool pot
- 10x loupe
Safety
Sn63Pb37 is 37% lead by weight. Wash hands after handling, keep the wire spool off food surfaces, and collect dross and clipped leads as lead-bearing waste rather than binning them with general refuse.
Neighbouring cards
- Wick cleaning a through-hole pad without lifting it 350 °C / 1.5 s
- Diagnosing and repairing a cold joint 340 °C / 2.5 s
- Vacuum desoldering a through-hole part 380 °C / 3.0 s
- Through-hole fillet baseline 340 °C / 2.0 s