Soldering a pad tied to an internal ground plane
A through-hole pad connected to an internal copper plane behaves like a heat sink with the thermal resistance of the board itself. The iron is not failing; the plane is pulling 8 to 12 watts out of the joint the moment contact is made. The fix is to add thermal mass and area, not to raise the temperature indefinitely.
Procedure
- 1
Confirm from the layout or by meter that the pad really is plane-tied, and note the thermal relief spoke width.
- 2
Pre-heat the board to 80 degrees C on a hot plate and leave it in place for the joint.
- 3
Flux the joint generously with ROL1 gel; flux burns off faster on hot copper, so apply it as you go.
- 4
Place a 3.2 mm chisel flat on the pad at 400 degrees C and hold for two seconds before feeding any wire.
- 5
Feed 1.0 mm SAC305 into the junction, count two more seconds, and withdraw wire first then iron.
- 6
Let the joint cool on the plate for five seconds before moving the board so the alloy does not crystallise under stress.
Workshop notes
Why the numbers are different
A plain 2.6 mm pad reaches wetting temperature in about two seconds on a 340 degrees C tip. The same pad tied to a 30 x 30 mm internal plane takes four seconds on a 400 degrees C tip and will still be climbing unless you cover more copper. Measure the plane on the layout first: if the thermal relief spokes are 0.3 mm wide and there are only two of them, the plane is deliberately slowing you down and the correct answer is a wider tip, not more heat.
Tip face area beats tip temperature
Moving from a 2.4 mm to a 3.2 mm chisel increases contact area by roughly 78% and drops the joint time by about a third at the same setpoint. Raising the setpoint from 340 to 400 degrees C buys less, and costs you flux life and pad adhesion margin. Keep the setpoint at or below 400 degrees C and solve the problem with face area, a pre-heat, or both.
Pre-heating is the professional answer
A 60 to 80 degrees C pre-heat from a hot plate or a preheater cuts the required dwell roughly in half and makes a ground-plane joint look like any other joint. Set a hot plate to 80 degrees C, give the board three minutes to soak, and solder while it is still on the plate. Every rework station that handles power boards has one, and it is not a luxury - it is what keeps pads on the board.
Practice drill
Find a scrap board with a large ground pour, pick six identical grounded through-holes, and solder three cold and three from a 80 degrees C pre-heat. Compare dwell time and fillet quality side by side; the difference is usually obvious enough to justify buying the preheater.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Solder freezes before it reaches the barrel bottom | Plane drawing heat out faster than the tip can replace it, usually compounded by a small tip face | Pre-heat to 80 degrees C, move up to a 3.2 mm chisel, and hold two seconds on dry copper before feeding wire |
| Pad delaminates despite a good-looking joint | Setpoint raised past 400 degrees C to compensate for the plane, cooking the adhesive from the top | Return the setpoint to 400 degrees C and solve the mass problem with pre-heat; check the tip is actually reaching setpoint with a bead-on-tip thermometer |
| Flux burns into a brown varnish around the joint | Gel flux applied to already-hot copper and left for the full dwell | Apply flux in two smaller doses, one before contact and one at the one-second mark, and clean with alcohol while the board is still warm |
Supplies
- 1.0 mm SAC305 solder wire
- ROL1 gel flux
- Isopropyl alcohol
- Kapton tape for masking adjacent parts
Tools
- Station set to 400 degrees C
- 3.2 mm chisel tip
- 80 degrees C hot plate or preheater
- Thermocouple probe for verifying plate temperature
Safety
A 400 degrees C tip on a hot plate is a two-source burn hazard, and the plate stays hot long after the station is off. Mark the plate as live whenever it is above 60 degrees C and keep the cabling clear of it.
Neighbouring cards
- Vacuum desoldering a through-hole part 380 °C / 3.0 s
- Drag soldering a 0.1 in header 360 °C / 1.2 s
- 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