Through-Hole Soldering
The four-step joint, hole and pad sizes, barrel fill, ground-plane pins and a worked solder budget.
The four steps
A through-hole joint passes a component lead through a plated hole in the board and solders it on the far side. The solder flows through the hole by capillary action (see How Soldering Works) and forms a fillet on both sides. Done right it takes about three seconds.
- Clean and place: insert the lead, bend it slightly or hold the part flat against the board, and have the tip tinned.
- Heat the joint: touch the tip to both the pad and the lead at the same time, for one second, with the broad face of the tip, at an angle of about 45°.
- Feed the solder to the joint (where the lead meets the pad), on the opposite side from the tip, so that it melts from the hot work and not from the iron. A little solder should flow into the hole and spread round the pad.
- Remove the solder, then the iron, after the fillet is formed (a total of about two to three seconds), and hold the part still until the joint is bright and set (about two seconds).
A good joint is a shallow, concave cone covering the pad and wetting the lead (see Joint Quality, Inspection and Failure). If the solder balls up, the joint was cold; if it spreads and flows, it was right.
Hole, pad and lead sizes
The hole must be large enough to insert the lead and small enough for capillary action to pull the solder through. The standard guidance is a finished hole 0.2 to 0.3 mm larger than the lead (for a square pin, than its diagonal), with a pad (annular ring) at least 0.3 mm wide around it, and a lead that protrudes 1 to 1.5 mm.
| Lead | Lead diameter (diagonal) | Finished hole | Pad diameter (at least) |
|---|---|---|---|
| Resistor lead 0.5 mm | 0.50 mm | 0.70 to 0.80 mm | ≥ 1.40 mm |
| Axial diode lead 0.8 mm | 0.80 mm | 1.00 to 1.10 mm | ≥ 1.70 mm |
| Round pin 1.0 mm | 1.00 mm | 1.20 to 1.30 mm | ≥ 1.90 mm |
| Square pin 0.64 mm | 0.91 mm | 1.11 to 1.21 mm | ≥ 1.81 mm |
| Square pin 1.0 mm | 1.41 mm | 1.61 to 1.71 mm | ≥ 2.31 mm |
Worked example: why the clearance matters
- A 0.8 mm lead in a 1.0 mm hole has a radial gap of (1.0 − 0.8) ÷ 2 = 0.10 mm, which is in the 0.05 to 0.15 mm range in which capillary rise is strong (see How Soldering Works).
- A 1.4 mm hole for the same lead leaves a gap of 0.30 mm: the capillary pull falls to a third and the barrel fills poorly. A 0.85 mm hole would be too tight to insert the lead without damaging the plating.
The solder budget: barrel fill and fillets
The solder in a good joint has three parts: the barrel (the hole’s annulus around the lead), the bottom fillet on the side the iron touches and a smaller top fillet. IPC-A-610 requires at least 75 % of the barrel filled for class 2 (general electronics) and 100 % for class 3 (high reliability), counting from the bottom to the top.
Vbarrel = π/4 × (Dhole² − Dlead²) × t
Worked example: solder for one joint
- Hole 1.0 mm, lead 0.8 mm, board 1.6 mm, pad 2.4 mm. Barrel: π/4 × (1.0² − 0.8²) × 1.6 = 0.45 mm³. 75 % of this is 0.34 mm³ (a fill height of 1.2 mm of the 1.6 mm).
- Bottom fillet: a cone from the pad edge (radius 1.2 mm) up to the lead at a height of 0.9 mm. The frustum volume is π × 0.9 ÷ 3 × (1.44 + 0.48 + 0.16) = 1.96 mm³; subtract the lead inside it (0.45 mm³) to get 1.51 mm³. The top fillet is about half, 0.75 mm³.
- Total = 0.45 + 1.51 + 0.75 = 2.7 mm³.
- 0.8 mm wire has a cross-section of 0.50 mm², so the joint needs 2.7 ÷ 0.50 = 5.4 mm of wire: about 5 mm, two short touches. A ball of solder 2 mm across has a volume of π/6 × 2³ = 4.2 mm³: noticeably too much.
Heat and time
The joint must reach the solder’s melting point everywhere. The time depends on the thermal mass (see How Soldering Works):
Worked example: time per joint at 350 °C
- Heat flow at 350 °C is about 30 W (contact conductance 0.14 W/K).
- Signal pin: 7 J ÷ 30 W = 0.2 s. Pin through a ground plane: 37 J → 1.3 s. Large lug: 97 J → 3.3 s.
- The ground-plane pin’s 1.3 s is why such a pin seems to need “forever”: a thermal relief (below) or a larger tip is the fix, not a longer time.
Mounting order and technique
- Low to high: resistors and diodes first, then sockets, small capacitors, ICs and transistors, then tall parts and connectors, so that the board lies flat on the bench when you turn it over.
- Orientation first: check polarity (diodes, electrolytic capacitors, ICs) and pin 1 before you solder; unsoldering is far harder than checking.
- Hold the part flat: bend the leads slightly outward (“clinch”) for light parts, or use tape, a rubber band or a piece of foam; solder one pin of a multi-pin part, check it sits flat, then solder the rest.
- Trim after soldering, just above the fillet, leaving 1 to 1.5 mm; use flush cutters and catch the clippings (they fly, and a solder joint that is cut is stressed, so reflow it if you cut into the fillet).
- Do not drag the iron: press, solder, lift. Do not “paint” the solder on with the tip.
- Clean the flux if it is a rosin or active type (see Solder Alloys, Flux and Paste) and inspect every joint with a lens.
Worked example: a kit of 167 joints
- The kit has 167 joints (40 resistor leads, 25 capacitor and diode leads, 56 IC and socket pins, 40 header and connector pins, 6 power-part leads and tabs).
- At about 2.7 mm³ each the total is 451 mm³, or 3.8 g of solder (density 8.4 g/cm³) = 0.9 m of 0.8 mm wire.
- At 6 s per joint including placement the soldering takes 17 minutes (0.3 h). A 100 g reel of 0.8 mm wire has about 23 m, enough for 26 such kits.
Ground planes, thermal reliefs and big joints
A pin connected directly to a copper plane loses its heat into the plane as fast as the iron supplies it, so it may never reach solder temperature. Designers therefore connect such pins with thermal reliefs (four narrow spokes) that make the joint hot enough to solder while still carrying the current. If you cannot change the board:
- Use a larger tip and a higher temperature (up to 400 °C), with a short dwell and plenty of flux.
- Preheat the board from below with a hot plate or a hot-air blower to about 100 °C, which removes a large part of the energy gap (the energy to reach 250 °C from 100 °C instead of 25 °C is about one third smaller).
- Touch the tip to the lead and the pad together and add a bit of solder to the tip for a thermal bridge.
Worked example: preheat benefit
- Energy to take a mass from 25 to 250 °C is proportional to the 225 K rise; from 100 °C it is proportional to 150 K, so only 67 % of the energy is needed: the joint takes two thirds of the time.
- The ground-plane pin (37 J cold) needs about 25 J from a 100 °C preheated board: 0.8 s at 350 °C instead of 1.3 s.
Connectors, sockets and power parts
- Connectors and headers: solder one pin, check that the part sits square on the board, then do the rest; the mounting tabs or shells carry the mechanical load, so solder them as well (they are a large thermal mass and need extra time).
- IC sockets: solder two diagonal corner pins, check, then the rest; a socket avoids heat on the IC and allows replacement.
- Power parts and heat-sink tabs: mount them to the heat sink first and solder afterwards, with the screws tightened so that the leads are not stressed (see Joint Quality, Inspection and Failure).
- Electrolytic capacitors, relays, LEDs: short contact; they are heat-sensitive.
- Heat sink on the lead: a crocodile clip or tweezers clamped to the lead between the joint and the body keeps heat from the part.
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Solder does not flow through the hole | Hole too tight or too big, pad cold, no flux | Reflow with flux; check hole size; heat the pad as well as the lead |
| Solder only on one side | Iron on the lead only, or ground plane draining heat | Heat both pad and lead; preheat; bigger tip |
| Blob with the lead hidden | Too much solder | Wick off some, reflow |
| Dull, grainy fillet | Moved while freezing | Reflow with fresh flux and hold still |
| Pad lifts or burns | Heated too long or too hot | Shorter contact; preheat; repair (see Rework, Desoldering and Repair) |
| Bridge between two pads | Too much solder, tip dragged | Add flux and reflow, or wick |
| Part sits crooked | Not held flat while soldering | Reheat one pin and reposition |
Related
- Soldering: the overview of this guide.
- How Soldering Works: heat energy and capillary flow.
- Joint Quality, Inspection and Failure: judging the joint.
- PCB Design and Layout: hole and pad sizes in the design.
FAQ
How long should I hold the iron on a joint?
One to three seconds in total: about one second to heat the pad and lead, one to two to flow the solder. If it takes longer, the tip is too small or too cool, or the joint is on a ground plane.
Should I put solder on the iron or on the joint?
On the joint, on the side opposite to the tip: a pre-loaded tip only helps with heat transfer. If the solder melts when it touches the lead, the joint is hot enough.
How much solder is enough?
A concave cone that covers the pad and shows the lead outline, with the barrel filled at least 75 % for class 2. Not a ball.
Do I clip leads before or after soldering?
After, leaving 1 to 1.5 mm. Clipping a cooled joint can stress it, so reflow the joint if the cut touched the fillet.
Why won’t a pin on a ground plane take solder?
The plane carries away the heat. Use a bigger tip, a hotter setting, plenty of flux and preheat; the board designer should have given it a thermal relief.
References
- IPC-A-610, Acceptability of Electronic Assemblies (barrel fill, fillet criteria); IPC J-STD-001.
- IPC-2222 (rigid board design: hole and land sizes); Horowitz, P. and Hill, W., The Art of Electronics.
- NASA-STD-8739.3 (soldered electrical connections).
Last reviewed: October 2026. Hole, pad and fillet dimensions are typical: follow your board’s data sheet and the standard you design to.