Mounting Hole Design, Plating And Clearance
A mounting hole looks like the simplest feature on a printed circuit board and is one of the most frequently mis-specified. It may be a plain hole through the laminate, a plated hole that connects the board to a chassis, a hole with a pad for a screw head, or a hole that must be isolated from everything around it. Each of those variants has a different purpose, and a drawing that does not say which one is wanted will be built to the shop’s habit.
This article describes the types of mounting hole, when each is used, how the clearance and the annular ring are chosen, and how the mechanical and electrical requirements are reconciled.
The Types Of Mounting Hole
A non plated hole is drilled through the laminate with no copper on its wall and no annular ring, and it is used where the board is held by a screw that must not make electrical contact. A plated hole has copper on its wall and is normally surrounded by a pad that connects to a net, which makes it a mechanical fixing and an electrical connection at the same time.
A hole with a pad but no plating sits between the two: the pad provides a bearing surface under the screw head, while the wall is bare laminate. It is used where a large current has to be carried into the board through a screw or a standoff, or where a screw head would otherwise damage the laminate. The distinction is not cosmetic, because the three variants behave differently under tightening, under thermal cycling and under the electrical test.
Plated Or Not Plated
The electrical decision comes first. If the hole is to connect the board ground to the chassis, it is plated and it is connected to the ground net, usually through several connections to the plane rather than one. If the hole is to be isolated, it is not plated and there is no copper within a defined distance of it, so that a screw, a washer or a metal standoff cannot touch anything.
The mechanical decision follows. A plated hole has a copper wall that is thin, and the wall is not a bearing surface: a screw that is tightened against it will deform the plating and can crack it. Where a plated hole is used for a fixing, the pad under the head carries the load and the plating only carries the current, and the pad has to be large enough to distribute the compression.
Annular Ring, Clearance And Screw Head
The clearance for the screw or the standoff is set by the fastener, not by a general rule. A metric M3 screw needs a clearance of about 3.2 to 3.4 millimetres, and a hole that is nominal or undersize will not accept the screw when both the screw and the board are at the extremes of their tolerances. The plating adds thickness to the wall, so a plated hole is drilled larger than the finished size by twice the plating thickness.
The pad diameter is set by the head of the screw, by the washer if one is used, and by the electrical requirement. A pad that is smaller than the head will be crushed into the laminate and may crack; a pad that is much larger wastes space and can create a thermal problem if it is connected to a plane, because a large connected pad is difficult to solder but easy to heat. The dimensions that the pad and the mask opening should have are described under PCB pad design standards.
Making A Chassis Ground Connection
A mounting hole that provides a chassis ground ties the board ground to a metal chassis, and it is a low impedance connection only if it is made properly. The usual arrangement is a plated hole with a pad on both sides, connected to the ground plane by several short traces or by direct connection to the plane on each layer that carries ground. A single narrow trace to the hole defeats the purpose, because the inductance of that trace appears in series with the chassis connection.
The connection also has to survive assembly. A screw tightened through a pad compresses it, and a pad that is connected to a plane through thermal relief spokes can crack at the spokes. Solid connection is preferred at mounting holes for that reason, and the general rules for connecting to planes are given under ground routing and power trace planning. Where the board edge and the chassis interact directly, the measures are described under ESD and PCB edge traces.
Mechanical Tolerances And The Panel
The position of a mounting hole has a tolerance, and the tolerance has to be compatible with the enclosure. A hole that is placed relative to the board outline inherits the outline tolerance, while one that is placed relative to the copper inherits the registration tolerance of the drilling. Where a hole has to align with a boss in a moulding, the tolerance chain has to be followed from the fixing point to the hole, and the outline rules that support this are described under board outline and mounting design.
The panel contributes as well, because a mounting hole is drilled with the same tooling as the rest of the board and its position is checked with the same coupon. If the panel is routed after drilling, the movement of the panel between the two operations appears as a shift in the hole position, and on a large panel that shift can be a significant part of the tolerance.
Inspection And Acceptance
Inspection is dimensional and visual. The hole diameter is checked with a pin gauge or with a coordinate measuring machine, the position is checked against the drawing and the plating on the wall is confirmed visually where the hole is plated. A section is used where the plating thickness matters or where a crack is suspected.
A functional check is more revealing than either. Assembling the board into its enclosure with the production screws and standoffs shows whether the clearance, the position and the pad size work together, and it shows it before a panel has been fully assembled. The first article assembly is the cheapest place to find a mounting hole that is 0.2 millimetres out of position.
Standoffs, Insulators And Solder Mask
The hardware around the hole is part of the design. A metal standoff passes through the hole and contacts the pad, so the pad must be sized for the standoff rather than for a screw head; a plastic standoff insulates and allows the pad to be omitted; and a nylon washer under a screw head spreads the load and prevents the head from cutting into the mask. Where a metal screw is used on a board that must not be grounded at that point, an insulating shoulder washer is the usual solution, and the clearance around the hole has to be wide enough for the washer rather than only for the screw.
Solder mask around the hole is a related decision. Mask over the pad prevents the screw from making contact, so a grounding hole has the mask opened over its pad, while an isolated hole usually has the mask covering the copper annulus that surrounds it. A mask opening that is larger than the pad under a screw head lets the head sit on bare laminate, and that is usually the arrangement that damages the board least.
FAQ
Should mounting holes be plated? Only where an electrical connection is required or where the wall needs protection. A plated hole that is not connected to anything still needs a clearance rule so that no other copper touches it.
How much copper clearance does an unplated hole need? Enough to cover the drilling tolerance, the position tolerance and the fastener, commonly 0.5 millimetres to the nearest copper on every layer concerned.
Can a mounting hole be used as a test point? It can, and it is often convenient, but the pad then has to be accessible and the probe has to reach it without touching the screw or the standoff, which is a reason to keep the two functions separate.
