Background Theory > PCB Design > Nominal Hole Size versus available Drill Bit Sizes

When designing a printed circuit board (PCB), the engineer or designer specifies a nominal hole size for each hole that needs to be drilled. This nominal hole size is the desired finished size of the hole after all manufacturing processes are complete.

However, the actual sizes of drill bits available from a PCB manufacturer may not correspond exactly to the nominal hole sizes specified in the design. Drill bit sizes are usually standardized, with a fixed set of sizes available. The selection of available drill bit sizes can vary depending on the manufacturer and the specific manufacturing process used.

If the desired hole size is not available, the manufacturer will typically use the next larger drill bit size. This is because subsequent processes, such as plating, will decrease the hole size slightly. For example, if a design calls for a finished hole size of 0.020" and the manufacturer has drill bit sizes of 0.019" and 0.021", the manufacturer would likely use the 0.021" drill bit. After plating, the final hole size would be closer to the desired 0.020".

However, this might vary based on the manufacturer's process capabilities and the specific tolerances required in the design. In some cases, it might be necessary to have a custom drill bit made, or to adjust the design to use a standard drill bit size.

As always, it's advisable to work closely with your PCB manufacturer to understand their capabilities and processes, and to ensure that your design can be accurately and reliably produced.