No matter what weekend project that you are trying to build, you can usually find an online video to help. Those videos can be lifesavers, but I have one complaint about them: They usually show all the parts easily fitting together, allowing the builder to complete their project without any problems. The reality, though, is that these projects never seem to be that easy, and there is often a lot of frustration involved in trying to make misfit parts work together.
The truth is that the size and dimensions of those parts that are to be built together are not etched in stone, and they can and will vary depending on various conditions. This is as true for printed circuit boards as it is for anything else. The hole sizes assigned to a PCB, therefore, will have tolerances attached to them to allow for variations in the lead size of the components to be inserted, as well as for the different conditions the PCB will encounter during manufacturing. Here are some of those factors to consider as you assign PCB hole size tolerances to your next project.
Component Leads and PCB Hole Size Tolerances
The PCB hole sizes and tolerances we’ll focus on for purposes of 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 those holes that are used for mounting hardware that doesn’t have to be electrically connected and tooling holes used during the manufacturing process. Those are non-plated through-holes, or NPTHs.
When you document the hole size in drill charts, fabrication drawings, or drill files, you need to specify the holes at their finished size. We’ll talk more about how the hole is finished later, but first, we need to find out what the lead size is that 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. What you are looking for is the maximum lead diameter, or the lead size tolerance, which should be noted on the datasheet.
Component leads can also be different shapes, and you need to take that into account in your lead size calculation. Working with a round shape is fairly straightforward, but you must be careful to get the maximum diagonal dimension if the lead is a rectangular shape. And, 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 begin to calculate the finished hole size. The primary concern is to make the hole large enough that the component lead can be inserted without any problems. Although a loose and sloppy fit can be difficult to work with and solder, a hole that is 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 that is 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. This 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 will change subtly with use, and they may vibrate slightly in the hole, causing the hole to be a little 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 can be specified for additional costs.
- Plating: To conduct electrical signals through the hole, it will be plated with metal. 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 all vary slightly depending on the manufacturer that you use, and you should get their specified tolerances for accuracy. In general, though, a 7-mil-over-the-lead-size rule of thumb is often used by designers for determining a finished hole size.
Putting It All Together
Once you’ve decided on a finished hole size, you will need to 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 7 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 and pad-to-trace spacing, so keeping the pads smaller is also good.
With so many requirements on the PCB hole sizes and tolerances, it’s important to get the best information possible when building your PCB CAD footprints. One of the best resources is to use is the component’s manufacturer’s recommended hole and pad size as well as the recommended footprint pattern from the part datasheet.
Another great resource that is available to you is to work together 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, as well as placing and routing, so you have the best manufacturing yields.