Through-hole soldering is still one of the most dependable ways to attach leaded components to a printed circuit board. Whether you are building a power supply, an automotive control module, or a medical device, the diameter of the drilled hole has a direct effect on how well the solder joint forms, how strong the connection stays, and how cleanly the board survives the wave soldering process. Drill the hole too small and components will not ｉｎｓｅｒｔ cleanly; drill it too large and the joint can end up weak and unreliable. This article explains what actually sets the minimum hole size for reliable through-hole soldering and the design rules that keep the process predictable.
There is no single "correct" minimum hole size that applies to every board. The right value depends on three things that have to be considered together:
Because component pins are produced with real-world tolerances, the hole has to be larger than the pin. The question is how much larger.
IPC-2221, the industry standard for PCB design, defines the minimum hole size with a simple formula:
Minimum hole size = Maximum lead diameter + tolerance allowance
The standard defines three density levels, each with its own allowance:
| Density level | Typical application | Allowance added to max lead diameter |
|---|---|---|
| Level A (general purpose) | Standard-density designs, maximum manufacturability | 0.25 mm (10 mils) |
| Level B (standard) | Higher component density, balanced choice | 0.20 mm (8 mils) |
| Level C (high density) | Dense or complex boards with limited space | 0.15 mm (6 mils) |
For most designs, adding 0.20 mm to the maximum lead diameter is a safe, manufacturable starting point. Level C saves space, but it demands tighter process control from the manufacturer, so it should only be used when the board really needs it.
The tolerance on most pin diameters is roughly ±0.05 mm. A 0.20 mm allowance comfortably absorbs that variation, so the lead seats reliably while still leaving room for solder to wick up through the plated barrel. Always use the maximum pin dimension from the component datasheet, not the nominal value. Datasheets often quote typical dimensions, and relying on them is a common cause of insertion problems when batches vary.
Not every lead is round. For square or rectangular pins, the effective diameter is the diagonal of the cross-section, calculated with the Pythagorean theorem. If a pin measures 0.64 mm by 0.64 mm, the diagonal is about 0.905 mm, so a reasonable hole would be roughly 0.905 mm + 0.20 mm = 1.10 mm. If the datasheet gives a one-sided tolerance, such as 0.9 mm +0.1 mm / -0, add the full positive tolerance to get the maximum lead size before applying the allowance.
Hole size should never be considered on its own. The aspect ratio, which is the board thickness divided by the finished hole diameter, determines whether a hole can be drilled and plated reliably. Thicker boards need proportionally larger holes to keep the aspect ratio within the fabricator's capability. On a typical 1.6 mm board, a 0.30 mm hole gives an aspect ratio of roughly 5:1, which is well within standard capability. A capable PCB board making partner will confirm that the chosen hole size matches its drilling and plating limits before production starts.
The hole is only half of the story. The pad around it must be large enough to leave a reliable annular ring, because the annular ring is what actually carries the solder joint. The general formula is:
Pad diameter = hole diameter + 2 × minimum annular ring + fabrication allowance
A thin or broken annular ring is one of the most common causes of weak through-hole joints, so it is worth checking this during design review rather than discovering it on the assembly line.
Both failure modes are avoidable when the hole size is chosen from the datasheet and checked against the manufacturer's process capability before the board goes into production.
A good hole size on the drawing is only the start. Reliable through-hole soldering depends on how the board is actually made and soldered: controlled wave soldering, trained operators, inspection at every step, and functional testing before the board ships. An experienced EMS partner runs the whole chain as one process rather than as disconnected steps.
Farway Electronic, an ISO 9001, IATF 16949, and ISO 13485 certified manufacturer in Shenzhen, provides DIP through-hole plug-in processing and welding as part of a one-stop PCB and PCBA service. Its DIP line covers component forming and insertion, wave soldering, lead cutting, repair welding, board washing, and functional testing, supported by two plug-in lines, two wave-soldering machines, and 24 rear-welding stations. Assembly work follows IPC-A-610, and boards are checked with plug-in AOI, IPQC, and QA sampling before release. For designers, this means the hole sizes specified on the drawing are handled by a team that understands how they behave on the line.
The minimum hole size for reliable through-hole soldering comes down to the maximum lead diameter plus a sensible clearance, typically 0.15 mm to 0.25 mm depending on the density level you choose, checked against the fabricator's drilling and plating capability. Start with the datasheet, apply the IPC-2221 allowance, confirm the aspect ratio, and verify the pad and annular ring. When in doubt, talk to your manufacturer early, because a hole that looks fine on the drawing can still cause trouble on the line if the process is not controlled.
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