While most PCB drilling is for vias and through-hole components, some drilling does not encompass electrical connectivity. The board’s placement within the enclosure requires mechanical drilling for fasteners, and some large components, like connectors, may utilize support pins for additional vibration dampening. While simple features overall, PCB mounting hole sizes require a basic understanding of geometric dimensioning and tolerancing and how pins and holes can diverge from true or ideal measurements.
PCB Mounting Hole Size Chart for US Screw Sizes |
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|---|---|---|---|
| US Screw Size | Measurement (mils / mm) | Nearest* drill bit (in) | Nearest* drill bit (mm) |
| #0 | 60 / 1.52 | 1/16th | 1.6 mm |
| #1 | 73 / 1.85 | 5/64th | 1.9 mm |
| #2 | 86 / 2.18 | 3/32nd | 2.2 mm |
| #3 | 99 / 2.51 | 7/64th | 2.6 mm |
| #4 | 112 / 2.84 | 1/8th | 2.9 mm |
| #5 | 125 / 3.18 | 1/8th | 3.2 mm |
| #6 | 138 / 3.51 | 9/64th | 3.6 mm |
| #8 | 164 / 4.17 | 3/16th | 4.2 mm |
| #10 | 190 / 4.83 | 3/16th | 4.9 mm |
| #12 | 216 / 5.49 | 7/32nd | 5.5 mm |
| #14 | 250 / 6.35 | 1/4th | 6.4 mm |
*: Nearest mounting hole size greater than or equal to screw size.
Accommodating PCB Mounting Hole Size for Tolerance, Processing
Mounting holes are purely mechanical (i.e., non-plated through-hole or NPTH) that support larger/bulkier pins where mass distribution away from the solder joints can cause joint failure. Since these pins do not communicate any electrical data, some designers treat them as an afterthought with little consideration for how hole sizing can affect the long-term reliability of the assembly. Key to understanding this aspect of component fit (and lot variance in general) is the tolerance of the pin sizing – tolerance exists for all dimensions of components and the PCB due to the limitations of machining precision. However, mechanical pins do not have the soldering process to fall back on. An incorrect electrical pin-pad connection may still function after soldering (albeit with reduced reliability), while a mechanical union has less leeway due to the absence of any fill material.
As communicated on datasheets and production documentation, tolerance increases the minimum hole size to ensure the largest possible produced lead (according to the manufacturer) can fit in the smallest drilled hole. Hole fabrication contains a unique tolerance, and the manufacturer’s and fabricator’s tolerances combine. Notably, pins with smaller nominal values are relatively more susceptible to tolerance – in other words, there’s a constraint on the lower limit of mounting hole sizes due to process variations. For the fabricator, this value reflects:
- Vibration during drilling can cause the mounting hole size to increase beyond its theoretical minimum tolerance.
- Drill wander, or the tendency of the drill bit to drift off its center position and rotation axis (especially true as hole depth increases).
- Drill bits wear during normal operations, shrinking the size of the hole as the surface of the bit abrades. The difference is most extreme when the lot size/hole count is large enough to require multiple drill bits or when the lot size/hole count is small and bit replacement occurs mid-production.
Therefore, sizing holes means designers must account for the max dimensions (i.e., nominal diameter plus positive tolerance) and a constant drilling tolerance, usually in the realm of an additional ~5 mils to the diameter. The shape of the pin also affects the tolerance calculation. Consider the difference between a pin with a square or circular cross-sectional area: the former, when compressed in a circular hole, has some ability to deform, whereas the latter does not, as it fills all directions of the hole equally. Designers can slightly shrink the hole tolerance for square through-hole pins using the diagonal as the diameter analog when board space is at a premium. Conversely, circular through-hole pins require a slight hole expansion to accommodate the shape and prevent installation issues.
Documentation should indicate the standard hole sizing to prevent ambiguity issues: when hole size callouts don’t align with standard sizing, the press operator has to decide between the next drill bit up or down. Going down a size means the pin may be too large to fit in the hole (or this depends on a case-by-case tolerance basis), while going up a size ensures fit but could intersect a copper feature, resulting in board scrap. Alternatively, the manufacturer may simply choose to reject the fabrication outright. Designers should accommodate the standard hole sizing in their design by resizing any “tweener” hole sizes to the next largest hole size.
Placement: An Additional Mounting Hole Consideration
It’s also essential to keep the placement of mounting holes in mind to prevent an undue buildup of stress on the connectors or board that accelerates material wear:
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- Mounting holes should be placed well back from the board edge, especially those used to house off-board connectors. The issue with placing connectors too close to the edge is that the pin/plug housing can protrude toward the direction of the connection. Since the pin/plug housing has to pass through the enclosure panel, the board’s placement comes closer to the edge of the enclosure, which may result in difficulties during the manufacturing or assembly of the fitted enclosure. A 375-500 mil gap will provide the requisite space to prevent poor enclosure fitting.
- Similarly, mounting hole placement must prevent flushness with the board edge for external connectors. This configuration means the edge of the board must butt up against the vertical side of the enclosure, leaving exceedingly little room for design tolerance.
See How Your Contract Manufacturer Sizes Up
PCB mounting hole sizes will differ according to design needs, but designers will want to work alongside their manufacturers to ensure design hole sizing accommodates manufacturing, not vice versa. This sizing will be critical to understanding how pins and hole size tolerances allow for some “slop” (an engineering term of endearment) in the assembly. Here at VSE, our engineers are committed to building electronics for our customers, including a thorough mechanical documentation review to prepare for manufacturing. We’ve been realizing life-changing and life-saving electronic devices alongside our valued manufacturing partners for over forty years.
