Sunday, July 23rd

It wasn’t that long ago a printed circuit board was drilled using a simple drill press.  The operator had to manually move the panel to the correct x and y coordinate and then pull the lever to drill an individual hole.  This process was repeated over and over again until all the holes were drilled.

Today, it’s not uncommon for a PCB to have more than 10,000 drill hits of varying sizes.  Can you imagine how long it would take to complete a project like this drilling one hole at a time? Worker Drilling Drill Press Retro Circle

Now, we can all be thankful that drilling a PCB is a fairly simple process that utilizes an automated drilling machine controlled by a computer.  Similar to a CNC machining process, a CNC drill machine with one or many drill heads can drill all the different hole sizes needed at a rapid rate.  Machines themselves vary by speed, drill sizes and even x-ray optimization.

When designing a PCB there are a couple of considerations as it relates to drilling; that is, the size of the holes, the type of hole and its final treatment.

Drill Sizes

First, as it relates to hole size, consider the aspect ratio relative to the manufacture of the PCB.  Aspect ratio is the ratio between the thickness of the board and the size of the drilled hole before plating and is used as a guide to ensure that a manufacturer does not exceed the mechanical capability of the drilling equipment or the available drill sizes.

For example, a PCB that is 0.062″ thick that has a drilled hole size of 0.020″ will have an aspect ratio of 3 to 1 (3:1).  At Bay Area Circuits our minimum capabilities for aspect ratio is 10:1 and based on this here are some common material thicknesses and corresponding hole sizes:

0.031″ Thick – Smallest Hole 0.006″ (smallest BAC can do mechanically)
0.062″ Thick – Smallest Hole 0.006″
0.093″ Thick – Smallest Hole 0.009″
0.125″ Thick – Smallest Hole 0.012″


After considering the aspect ratio there are a variety of ways that a PCB via (plated through hole) is created and/or modified to meet the project specifications.

If a PCB via smaller than 0.006″ is required, laser drilling is a solid option.  Using laser technology a hole can be drilled as small as 0.001″ in size.  In most cases these holes will need to be plated and are called microvias.  Microvias are usually needed for very tight designs with a large number of traces and connections to be made.

With a multilayer PCB, blind vias only go partially through. For instance, in a 6 layer board the via may go from layer 1 to layer 3. A buried via hides inside the internal layers; for example layers 2-4 on a 6 layer board.  The process to create these vias requires that the holes are drilled and then plated on the layer stacks before all of the layers are laminated together.  It may require multiple drill, plate and lamination processes to manufacture the final PCB.

Another option that may be needed is to drill partially through the PCB.  Like it sounds this is the process of controlling the depth of the drill so it only goes to a set point through the final PCB.  Also called back-drilling where part of a previously plated hole is drilled out but some of the plated through hole will remain.

Vias can also be filled with conductive or non-conductive materials.  Typically non-conductive fill is done using soldermask to fill the vias as well as cover the top and bottom of the via and copper pad.  Alternatively, a conductive material would be used where high heat or current requirements exist.

It’s also possible to “tent” the vias where soldermask covers the top side of the via on the PCB but the bottom side of the via is not covered in order to allow for electrical testing. And, in some cases, it may be required to plug the vias by plating them shut with copper during the copper plate process.

While these are some of the more common uses of drilling and vias there are many more options available. If you have any questions about how any of these could impact cost or reliability of your next PCB design, please comment below or contact us.



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2 Responses

  • Jean Metsch June 26th, 2017

    Please send how i can do the calculation of “aspect ratio”.
    Same examples;

    Thanks and B.R


    • Brian Paper June 28th, 2017

      Hi Jean, the aspect ratio is the ratio between the thickness of the board and the size of the drilled hole before plating. To calculate aspect ration, you could take the board thickness (ex. 0.062″) and divide by the drilled hole size (ex. 0.020″). With this example, the resulting aspect ratio would be 3:1.

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