PCB Drill Hole Tolerance Explained

Manufacturing is not just meeting design intent, it also must account for the unpreventable variances that designs gloss over; this challenge is as true for printed circuit boards as for anything else. Therefore, the drill hole sizes assigned to a PCB will have drill hole tolerances attached to them to allow for manufacturing variance in the lead size of the inserted components and the different conditions the PCB will encounter during fabrication. Here are some factors to consider when assigning PCB drill hole tolerances to your next project.

PCB Drill Hole Tolerance Factors
Drill wander	Drill hits aren’t perfect and even hits that are within tolerance can deviate during drilling from vibration.
Drill bit age	As the drill bits wear down due to friction, they are more likely to produce hole diameters outside of acceptable tolerance. Timely replacement is critical.

Component Leads and PCB Drill Hole Tolerances

The PCB hole sizes and tolerances we’ll focus on for this article are plated through-holes (PTH) used for leaded component insertion and soldering. Most of the holes on a circuit board are PTHs. Exceptions include holes used for mounting hardware that doesn’t have to be electrically connected (i.e., for mechanical integrity to reduce shock, vibration, etc.) and tooling holes used during manufacturing. Those are non-plated through-holes or NPTHs.

Designers must specify the holes at their finished size (i.e., post-plating for PTHs) when they document the drill hole size in drill charts, fabrication drawings, or drill files. To determine the lead size, it will be inserted into the hole. The primary source of information on the lead size of a through-hole component will be the manufacturer’s datasheet. You are looking for the maximum lead diameter or lead size tolerance (typically indicated on the datasheet).

Component leads can also be different shapes, and you need to consider that in your lead size calculation. Working with a round shape is reasonably straightforward, but you must be careful to get the maximum diagonal dimension if the lead is rectangular. If a datasheet isn’t available, you can always use a pair of calipers and take the measurement yourself. If you are going to do this, though, it’s a good idea to measure several components to find the maximum lead size.

Hole Size Tolerances for Manufacturing

Once you know your maximum lead size, you can calculate the finished hole primary concern is to make the hole large enough that the component lead can fit without issue. Although a loose and sloppy fit can be challenging to work with and solder, a hole too small for the lead can lead to huge problems if you try to enlarge it. You could end up ripping out the metal plating of the hole and or breaking the signal traces connecting to the hole.

Here are some other considerations to remember:

• Aspect ratio – This is the ratio between the thickness of the board and the drill hole diameter. For example, a board 62 mils thick with a 10-mil drill will have an aspect ratio of 6:1. Most manufacturers can reliably fabricate a board with up to a 6:1 aspect ratio, and larger ratios will increase the manufacturing cost. The constraint is because the larger the aspect ratio, the more difficult it is to plate the hole. Be careful, therefore, that you don’t specify too small of a hole that will force a larger aspect ratio.

• Drill tolerance – Mechanical drills change subtly with use, and they may vibrate slightly in the hole, causing the hole to be slightly oversized. Therefore, drill sizes are assigned a tolerance to account for that. Manufacturers usually prefer plus or minus 3 to 4 mils, but smaller tolerances are specificable for additional costs.

• Plating – The hole will be plated with metal to conduct electrical signals. The depth of the plating will vary depending on the board type and materials, but you should expect the hole size to decrease by 3 or 4 mils after plating.

To sum it up, your finished hole size should be:

Lead size + the tolerance for the maximum lead size + the drill tolerance and the extra size before plating.

The values will vary slightly depending on the manufacturer you use, and you should get their specified tolerances for accuracy. Generally, designers often use a 7-mil-over-the-lead-size rule of thumb to determine a finished hole size.

Putting It All Together

After deciding on a finished hole size, you must construct your PCB footprint pad size for the hole. You will want your pad size to be at least 14 mils over the finished hole size, which will give your pad a seven-mil annular ring. Larger pads and annular rings are better because they help to contain any drill breakout in the hole due to excessive drill wander. On the other hand, you need to maintain adequate pad-to-pad spacing and pad-to-trace spacing, so keeping the pads smaller is also good.

With so many PCB drill hole tolerances requirements, getting the best information possible is essential when building your PCB CAD footprints. One of the best resources to use is the component’s manufacturer’s recommended hole and pad size and the recommended footprint pattern from the part datasheet. Another great resource is to work with your PCB contract manufacturer before you start your design. This way, you will have the correct data upfront to help you with PCB library part construction, placement, and routing for the best manufacturing yields.