Guides›Through-Hole Hole and Pad Size: How Big to Drill for a Component Lead

Through-Hole Hole and Pad Size: How Big to Drill for a Component Lead

Make the finished (plated) hole the maximum lead diameter plus 0.25 mm at density Level A, 0.20 mm at Level B or 0.15 mm at Level C. Make the pad the finished hole plus twice the minimum annular ring of 0.05 mm plus a fabrication allowance of 0.6, 0.5 or 0.4 mm. A 1.0 mm round lead at Level B therefore gets a 1.20 mm hole and a 1.80 mm pad. ^{[1, 6]} These are the rules the Through-Hole Padstack calculator applies. This guide shows where each number comes from, works three common leads, and covers the drilled hole, inner layers and plane connections.

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The two equations

Hole = d_lead,max + hole allowance Pad = a + 2b + c

Here d_lead,max is the largest lead diameter on the component drawing, not the nominal one. In the pad equation, a is the hole, b the minimum annular ring and c the fabrication allowance. IPC-2221 defines a as the maximum finished hole on external layers and the drilled hole on internal layers. ^{[2, 3]} The fabrication allowance is a margin for the fabricator's process tolerances, chosen by supplier capability, so that the ring left at the worst point is still at least b. ^[3]

Value used by the calculator, mm	Level A	Level B	Level C
Hole over maximum lead	0.25	0.20	0.15
Fabrication allowance, IPC-7251 basis (default)	0.60	0.50	0.40
Fabrication allowance, IPC-2221 Table 9-1 basis	0.40	0.25	0.20
Minimum annular ring, external / internal	0.05 / 0.03

Level A gives the most clearance and copper (low density); Level C gives the least (high density). The calculator rounds the hole, pads and thermal relief up to 0.05 mm by default. ^[6]

Where the numbers come from

The hole allowances are the IPC-2222 values quoted by PCB 3D. ^[1] Tom Hausherr gives the same 0.2 mm (8 mil) as the nominal lead-to-hole value of the IPC-2222 chart. ^[5] The 0.05 mm minimum annular ring is the IPC-2221 external value (2 mil). ^{[1, 2]} The calculator's 0.03 mm internal ring is from IPC-2221 (1998) Table 9-2; Altium lists 1 mil (0.025 mm). ^{[2, 6]}

The sources disagree on the fabrication allowance. PCB 3D gives 0.6, 0.5 and 0.4 mm and calls them IPC-2221 values. ^[1] Altium gives IPC-2221 Table 9-1 as 16, 10 and 8 mil (0.4, 0.25 and 0.2 mm), and NCAB quotes 0.00984 in (0.25 mm) for Level B. ^{[2, 3]} The calculator offers both. Its default, labelled IPC-7251, uses 0.6/0.5/0.4 mm and reproduces the PCB 3D example. The IPC-2221 option uses 0.4/0.25/0.2 mm. It adds the Table 9-1 notes: 0.05 mm (0.00197 in) per ounce of copper above 1 oz, and another 0.05 mm above eight layers. ^{[3, 6]}

IPC-7251 itself was never released. The IPC land pattern committee stopped work on it on 1 October 2014 and moved through-hole patterns into IPC-7351C. ^[4] Values labelled IPC-7251 in libraries and calculators, including this one, come from its draft and the charts built on it.

As a check, PCB 3D's 0.55 mm lead gives holes of 0.80, 0.75 and 0.70 mm and pads of 1.50, 1.35 and 1.20 mm at Levels A, B and C. ^[1] The calculator returns the same values, and its unit tests check them.

Check the 0.55 mm example in the Padstack calculator →

Worked examples: 0.64 mm square pin, 0.8 mm and 1.0 mm leads

Each lead was run through the calculator's code at its defaults: IPC-7251 allowance, zero hole tolerance and rounding to 0.05 mm. The annular ring shown is the nominal (pad − hole)/2. In every row the worst-case external ring, after the fabrication allowance is used up, is 0.05 mm.

Lead (maximum)	Level	Finished hole, mm	Outer pad, mm	Nominal ring, mm
0.64 × 0.64 mm square (diagonal 0.905 mm)	A	1.20	1.90	0.35
	B	1.15	1.75	0.30
	C	1.10	1.60	0.25
0.8 mm round	A	1.05	1.75	0.35
	B	1.00	1.60	0.30
	C	0.95	1.45	0.25
1.0 mm round	A	1.25	1.95	0.35
	B	1.20	1.80	0.30
	C	1.15	1.65	0.25

With the IPC-2221 allowance instead, the outer pads at Level B are 1.50 mm for the square pin, 1.35 mm for the 0.8 mm lead and 1.55 mm for the 1.0 mm lead.

Square and rectangular leads use the diagonal

A drilled hole is round, so the corners of a square or rectangular lead must clear it. The calculator therefore sizes the hole from the lead diagonal √(W² + T²). ^[6] For a 0.64 mm square pin, such as a 2.54 mm header pin, that is 0.905 mm, and the Level B hole is 1.15 mm, not 0.85 mm. Take W and T as maxima from the connector drawing. At 2.54 mm pitch the Level A, B and C pads leave 0.64, 0.79 and 0.94 mm of space between adjacent pads.

Load the 0.64 mm square pin at Level B →

Load the 0.8 mm round lead →

Load the 1.0 mm round lead →

Finished hole, drilled hole and hole tolerance

The sizes above are finished holes, after plating. The fabricator drills larger to leave room for the barrel copper. NCAB gives the drill as typically 0.004 to 0.006 in (0.10 to 0.15 mm) larger than the finished hole, depending on the finish, and 0.004 in as enough for an electroless finish such as ENIG. ^[3] The calculator assumes a 0.10 mm drill oversize. It is an editable default, not an IPC value; enter your fabricator's figure. ^[6]

IPC-2221 sizes the outer land from the maximum finished hole, so a plus tolerance on the hole adds to the pad. ^{[2, 3]} The calculator's default tolerance is zero, which matches the IPC-7251 charts. ^[6] For the 1.0 mm lead at Level B with the IPC-2221 allowance and a +0.08 mm hole tolerance, the code gives:

Quantity	Tolerance 0	Tolerance +0.08 mm
Finished hole / maximum finished hole	1.20 / 1.20 mm	1.20 / 1.28 mm
Outer pad	1.55 mm	1.65 mm
Inner pad (from the 1.30 mm drilled hole)	1.65 mm	1.70 mm
Worst-case external ring	0.050 mm	0.060 mm
Worst-case internal ring	0.050 mm	0.035 mm

The worst-case rings are above 0.05 and 0.03 mm because the pads are rounded up to 0.05 mm.

Load the 1.0 mm lead with IPC-2221 allowance and +0.08 mm tolerance →

Inner-layer pads

On internal layers IPC-2221 uses the drilled hole, which is larger than the finished hole, with the 0.03 mm internal ring. The IPC-7251 padstack default repeats the outer land on inner layers. The calculator reports the larger of the two. ^[6] For the 1.0 mm lead at Level B with the default 0.10 mm drill oversize, the IPC-2221 minimum is 1.30 + 2 × 0.03 + 0.5 = 1.86 mm. Rounded up, that is 1.90 mm, so the inner pad is 0.10 mm larger than the 1.80 mm outer pad.

Thermal relief and antipad on plane layers

Where a pin connects to a plane, the calculator draws a thermal relief. Its inner diameter (ID) is the finished hole plus 0.60, 0.40 or 0.30 mm, and its outer diameter (OD) the hole plus 1.00, 0.70 or 0.50 mm, at Levels A, B and C. The antipad on unconnected planes equals the thermal OD. These values come from the IPC-7251 padstack charts published by PCB Matrix. They are a library convention: the IPC-2221 and IPC-2222 text leaves thermal geometry to the designer. ^[6]

The spokes follow IPC-2222 §9.1.2. Their total width is 60 % of the land diameter, split among the spokes and rounded up to 0.05 mm. The combined web on one hole, over all connected planes, should not exceed 4.0 mm for 1 oz copper or 2.0 mm for 2 oz. ^[6] For the 1.0 mm lead with four spokes on two planes at 1 oz:

1.0 mm lead, mm	Level A	Level B	Level C
Thermal ID	1.85	1.60	1.45
Thermal OD = antipad	2.25	1.90	1.65
Plane-to-drill gap, (antipad − drilled hole)/2	0.45	0.30	0.20
Spoke width (4 spokes)	0.30	0.30	0.25
Total web, 2 planes	2.4	2.4	2.0

All three are within the 4.0 mm guideline. The same Level B pin on four planes of 2 oz copper has a 4.8 mm total web against a 2.0 mm guideline, and the calculator warns that the hole may be hard to solder. Use fewer or narrower spokes, or connect fewer planes.

See the web-width warning for four 2 oz planes →

Other libraries use different conventions. Hausherr's proportional padstacks set the thermal ID 0.5 mm larger than the hole, the OD equal to the antipad, and the spoke width to 75 % of the antipad divided by four. ^[5] If your fabricator or assembler has a thermal-relief rule, use it.

Annular ring by IPC class

The 0.05 mm ring is a design minimum. Whether a finished board passes is decided by IPC-6012 for the class in the procurement documents. ^[3] IPC-6012 is not openly available, so the figures here are from secondary summaries. Altium summarises Class 3 as 2 mil (0.05 mm) external and 1 mil (0.025 mm) internal. For Class 2 it lists external breakout of less than 90° and internal breakout of up to 90° as acceptable. ^[2] NCAB's Class 3 example uses the same 0.002 in and 0.001 in. ^[3] With its default allowances, the calculator keeps a 0.05 mm external and 0.03 mm internal ring at the worst case. That meets these Class 3 figures as long as the fabrication allowance covers your fabricator's real drill and registration tolerance. Confirm the class and the fabricator's capability before relying on it.

What the calculator does not check

A maximum hole size relative to the minimum lead. IPC-2222 also limits how much larger the hole may be than the lead. ^[6]
Rectangular lands for rectangular leads. It draws square lands at the calculated minimum size; a longer land is your choice. ^[6]
Non-plated holes, press-fit pins and the solder fill of the barrel.

Checklist

Take the maximum lead diameter from the component drawing. For a square or rectangular lead, use the diagonal of the maximum width and thickness.
Choose the density level. Level A gives the most clearance and copper, Level C the least.
Finished hole = maximum lead + 0.25, 0.20 or 0.15 mm, rounded up to the fabricator's drill step.
Ask the fabricator for the drill oversize and the plus tolerance of the finished hole, and enter both.
Pad = hole + 2 × 0.05 mm + fabrication allowance. Check that the worst-case ring is at least 0.05 mm outside and 0.03 mm inside.
Check the space between adjacent pads at the part's pitch.
On plane layers, check the thermal relief, the total web against the copper weight, and the antipad clearance to the drilled hole.
State the finished hole sizes and the IPC class on the fabrication drawing.

Open the Through-Hole Padstack calculator →

Vias are sized differently: no lead goes through them, so current, voltage drop and aspect ratio set the hole. See the via current guide for worked examples.

Size a via in the Via Calculator →

Tools used in this guide

Through-Hole Padstack Calculator – Plated through-hole sizing per IPC-7251 / IPC-2221 / IPC-2222: hole, outer and inner pads, antipad, thermal relief and annular ring for levels A/B/C.
Via Calculator – Via current capacity, resistance, thermal resistance, capacitance, inductance and rise-time impact.

Sources

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