Drag soldering a SOIC-8 with gel flux
A SOIC-8 at 1.27 mm pitch is the easiest drag-soldering target there is: wide enough that bridges clear easily, narrow enough that a 2.0 mm hoof can span two leads. This is the technique to learn drag soldering on before attempting TSSOP.
Procedure
- 1
Align the package by pin 1 and tack opposite corners with one-second touches each, checking rotation at 10x before continuing.
- 2
Flood the whole lead row with SAC305 from a 0.5 mm wire, deliberately leaving more alloy than the joints need.
- 3
Drag the 2.0 mm hoof along the row at 5 mm per second at 340 degrees C, keeping the concave face in contact with the leads.
- 4
Add RMA flux and lay 1.5 mm braid perpendicular across the row, then sweep once with the flat of the tip to remove bridges.
- 5
Inspect each lead at 10x for a fillet that wets both the lead and the pad, and re-flux before any touch-up.
Workshop notes
Tack two opposite corners first
Align the body by its pin-1 marking, then tack pin 1 and pin 5 (or 4 on a narrow-body package) with a one-second touch each. Tacking opposite corners rather than adjacent ones lets you check rotation: if the body is rotated even two degrees, the far corner will be visibly off its pad. Reflow the tack and nudge with tweezers before you commit to the full row.
Flood, then drag, then wick
The three-step method: flood the whole row with Sn-rich solder so every lead has more alloy than it needs, drag the hoof along the row to even it out, then wick the bridges off with 1.5 mm braid. Flooding first is what makes the drag predictable - a starved row drags unevenly and half the leads end up with insufficient fillet. A 2.0 mm hoof at 340 degrees C moves at about 5 mm per second on 1.27 mm pitch.
Braid width and bridge removal
Use 1.5 mm braid for a 1.27 mm pitch row. Lay the braid across all eight leads perpendicular to the row, add RMA flux, and sweep once with the flat of the tip. Sweeping parallel to the leads pulls solder out of the joints and leaves them starved; sweeping perpendicular removes only the excess that bridges between leads. One perpendicular pass, one second, done.
Practice drill
Solder and then remove a SOIC-8 five times on the same footprint. Track how many leads need individual touch-up after the drag. A competent operator finishes with zero touch-ups on 4 out of 5 attempts; if you need touch-ups on every lead, the flood step was too light.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Leads starved of solder after the drag | The flood step was skipped, so the hoof had no reservoir to redistribute | Re-flood the row with 0.5 mm wire, drag once more, and only then remove bridges with perpendicular braid |
| Body rotated a few degrees, far leads off-pad | Adjacent corners tacked, which sets one edge but not the rotation | Tack opposite corners instead, check at 10x, and reflow the tacks with tweezers applied before committing to the row |
| Lifted pad on one lead after bridge removal | Braid swept parallel to the leads with a filled segment, so heat passed through to the adhesive | Sweep perpendicular, change braid segments every two passes, and cap each sweep at one second |
Supplies
- 0.5 mm SAC305 solder wire
- ROL1 mildly activated gel flux
- 1.5 mm desoldering braid
- Isopropyl alcohol and a stiff brush
Tools
- Station at 340 degrees C
- 2.0 mm hoof tip
- Tweezers with 0.5 mm tips
- 10x to 20x stereo microscope
Safety
Gel flux in a syringe releases solvent when it hits a hot joint; keep the plunger moving slowly to avoid a spatter. Flux residue on skin should be washed off, not wiped on a rag and forgotten.
Neighbouring cards
- Dragging a 0.5 mm pitch QFP and clearing bridges 350 °C / 1.2 s
- Two-iron removal of a 0805 chip 330 °C / 1.5 s
- Hand-placing a 0402 passive 320 °C / 1.0 s
- Fixing tombstoned 0603 and 0805 parts 310 °C / 1.0 s