PCB Drill Rules: Hole Tolerance, Aspect Ratio, and Other DFM Considerations

February 8, 2018 , in Blog

Considering a number of potential issues before submission can greatly impact your board’s turnaround time, quality and cost. An important issue that sometimes doesn’t get the attention it warrants is PCB drill holes. Most designers diligently consider spacing and size as they relate to their performance objectives, but might not consider how they relate to the manufacturing process. Applying some simple PCB design for manufacturing (DFM) rules and guidelines for drill holes can have a surprisingly significant impact.

The most important DFM considerations for drill holes are generally PCB drill hole tolerance and PCB drill aspect ratio. When determining these, there is much more to consider than just the actual drill hole size.


Drill Holes With and Without Annular Rings

Drill Hole Tolerance Requirements

Basically, there are two types of drill holes: non-plated through holes (NPTHs) and plated through holes (PTHs). NPTHs are holes that do not carry current, and therefore do not need conductive plating. Mounting holes are the most common type of NPTH. PTHs, on the other hand, are usually signal carriers or ground returns. They carry current and require conductive plating. PTHs are usually vias between outer and inner layers, inner layers only or surface-to-surface.

There are some general rules that should be applied for the NPTHs and PTHs of your design. Not employing the following PCB drill hole rules in your DFM can significantly impact the turnaround time for your board:

Drill Hole Rules

Non-Plated Through Hole (NPTH)

  • Minimum finished hole size = 0.006″
  • Minimum edge to edge clearance (from any other surface element) = 0.005″

Plated Through Hole (PTH)

  • Minimum finished hole size = 0.006″
  • Minimum annular ring size = 0.004”
  • Minimum edge to edge clearance (from any other surface element) = 0.009″


Drill Hole Minimum Dimensions

If your board needs to be IPC Class 2, then the annular ring must be at least 0.004” larger than the drilled hole at every point. For example, if your hole is 0.006”, then the minimum pad size for the mechanically drilled hole would be 0.014”.

PCB Drill Aspect Ratio Limitations

When drilling holes, whether mounting or vias, board fabricators are limited by board thickness, as well as the diameter and depth capabilities of their milling machines, lasers or other equipment. These limitations are generally expressed in terms of the aspect ratio, which expresses the proportionality relationship between the hole depth and diameter.

For through holes, the aspect ratio can be calculated with the following equation:

For vias, where the hole may not extend through the entire board, the aspect ratio can be determined as shown below:

The aspect ratio for standard drilling is limited to 10:1. At Tempo, advanced drilling is used with an aspect ratio of 20:1. If the hole size is below 0.006” then laser drilling is required, which has an aspect ratio of 2:1. This aspect ratio limits their usage to the outer two layers or requires sequential construction which increases fabrication time and cost.

Other DFM Considerations

In addition to the drill hole sizes, spacing and aspect ratios, you should also consider the type of via used. For example, if saving space is a primary concern, you might employ via-in-pads. However, if turnaround time is most important, you may want to explore alternative via options, as via-in-pad typically adds two days to the PCB manufacturing process.

Good DFM involves more than simply deciding where to put drill holes. However, if you follow these PCB drill rules, your design will be manufacturing-friendly and should result in well-constructed and reliable boards.

PCB Finished Hole Rules for DFM

General Rule Benefit

Keep via sizes > .010”

Ease of fabrication

Employ aspect ratio of 10:1

Eliminate extra turnaround time

Keep drill holes at least 0.009” from pads

Ease of assembly

As shown in the table above, good DFM for PCB drill holes includes three key aspects. First, ensuring via sizes are greater than .010” to reduce difficulty during fabrication. Second, utilizing an aspect ratio of no more than 10:1 to prevent additional turnaround time. Third, and finally, spacing holes at least 0.009” away from component pads to minimize assembly issues. Employing these rules and other DFM tips will promote quicker product development by speeding up the PCB manufacturing process.

If you’re interested in learning more about PCB drill rules that you should incorporate to assist with our DFM verification, then contact us for help in making the best decisions for your design.

Or are you ready to put our industry-leading software, advanced robotic factory, skilled professionals and proven fast turnkey process to work? Use our quote tool to upload your project’s CAD and BOM files and get started.


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