Through-Hole Pad and Hole Size Design

Through-hole parts are easier to justify than to design. The decisions that matter are small numbers — the hole diameter, the land around it and the distance between two holes — and each of them is a compromise between making the part easy to insert and making the joint easy to solder. The values below are the ones that have settled into common practice.

Standard Hole Sizes

The first economy is to use standard drill sizes rather than a size chosen for each part individually. The usual series runs in steps of a tenth of a millimetre from 0.6 mm upward, with 0.6, 0.7, 0.8, 0.9 and 1.0 mm covering the great majority of leaded components.

Restricting a design to this series reduces the number of drill changes on the machine, which reduces cost and improves positional consistency. Where a part does not fit the series exactly, the nearest standard size that gives adequate clearance is a better answer than a bespoke hole.

Clearance Around a Round Lead

For a cylindrical lead, the difference between the hole diameter and the lead diameter is the hole size allowance, and it is what makes the part insertable.

A working range is a difference of four tenths of a millimetre to six tenths of a millimetre, which corresponds to a gap of two tenths to three tenths of a millimetre between the lead and the barrel wall on each side. Two things justify the allowance. The first is assembly: leads are not perfectly straight, they are cut and formed, and the hole has to accommodate the resulting variation without the lead scraping the plating. The second is soldering: the gap is where the solder travels into the barrel to form the joint, and a gap that is too small prevents it from entering while too large a gap requires more solder than the fillet will hold.

Below that range the part is difficult to insert and the barrel plating is at risk of being scraped and exposed. Above it the joint forms around the lead rather than through the barrel, and the strength of the connection falls.

Rectangular Leads

Leads with a rectangular cross section are a different case, because the dimension that has to clear the hole is the diagonal of the section rather than either side.

The usual rule is that the hole diameter exceeds the diagonal of the lead by two to two and a half tenths of a millimetre, which leaves roughly one to one and a third tenths of a millimetre between the barrel wall and each corner of the lead. Measuring against the diagonal rather than the width is what prevents the corners from binding, and the corners are also where the scraping damage occurs if the allowance is too small.

Pad Diameter and Annular Ring

The land around a leaded hole has to do three things: provide enough copper for the solder fillet that will surround the lead, hold enough copper to resist the mechanical load applied when the component is inserted and when the product is used, and leave an annular ring wide enough that drilling tolerance cannot break out of the pad.

A common relationship is that the pad diameter is between one and a half and two times the hole diameter, which is usually expressed as a fixed increment: for holes up to six tenths of a millimetre, the pad is about four tenths of a millimetre larger than the hole; for holes up to a millimetre, about eight tenths larger. The increment stays roughly the same as the hole grows, which keeps the ring width constant while the pad area increases with the hole.

Undersizing the land is the mistake that costs most. A pad that is only marginally larger than the hole produces a thin fillet, which is difficult to inspect and easy to crack, and it leaves almost no tolerance for the drill to land off centre.

Hole Spacing for Leaded Components

The distance between two holes is set by the component rather than by preference, but the design still has a choice about which standard pitch to use.

For axially leaded parts, where the body lies parallel to the board and the leads bend down into the holes, the spacing should be longer than the body by several millimetres so that the leads are formed rather than forced. Forming the lead close to the body creates stress at the seal and is a common cause of early failure in service. Where the body length does not match a standard pitch, the next size up is the right answer.

For radially leaded parts, where the leads enter the board directly, the spacing should match the lead spacing of the component itself, and the standard pitches are chosen to match the common spacings so that the part sits flat without stress on the leads.

Using the metric series of standard pitches keeps the choice simple and keeps the drill programme consistent, since several different components can then share the same pattern.

Board Thickness and Other Constraints

Three further factors modify the numbers above.

Board thickness affects insertion. On a thick board, the barrel is long, the lead has to travel further through it, and the same clearance that works on a thin board may not be enough. The allowance is normally increased for thick boards rather than kept constant.

Mechanical strength sets a lower bound on the hole size, because holes placed too close together or too close to the board edge reduce the material available to resist the force of assembly. The strength of the finished connection also depends on the pad, since the copper is what carries the load from the component into the board.

And the drilling equipment has its own limits. Very small holes in thick material, or holes arranged in tight clusters, increase the risk of the drill wandering and of the material breaking out between adjacent holes. Where a design calls for either, the pattern is worth reviewing with the fabricator rather than assuming it will be produced as drawn.

What a Poor Choice Produces

The consequences are consistent enough to be diagnostic. A hole that is too tight produces components that cannot be seated without force, scraped barrels and, in the worst case, lifted pads where the lead was driven through. A hole that is too loose produces joints that form around the lead instead of filling the barrel, which look acceptable from above and are weak underneath. A pad that is too small produces thin fillets and a pad that lifts when a connector is mated and unmated repeatedly.

All three are visible at inspection if the inspection is looking for them, which is why the acceptance criteria for leaded joints include fillet shape and barrel fill rather than presence alone. The general rules for vias and lands elsewhere on the board are described in the article on via design rules, the related case of holes formed at a board edge is covered in the note on half-hole and stamp-hole boards, and the acceptance testing that verifies the joints sits inside our quality management overview.

FAQ

How much clearance does a lead need? Around two to three tenths of a millimetre of gap on each side for a round lead, and about one to one and a third tenths of a millimetre at each corner for a rectangular one.

How large should the pad be? Between one and a half and two times the hole diameter, which is usually expressed as a fixed increment of four tenths of a millimetre up to a half-millimetre hole and eight tenths up to a millimetre.

Why use standard hole sizes? Because fewer drill changes mean lower cost and better positional consistency, and the standard series covers the clearance requirements of most leaded parts.