Aspect Ratios and Their Importance to Multi-Layered PCBs

March 12, 2018 Altium Designer

The golden ratio



There’s a certain level of gratification that comes with doing something challenging. In my off time, I tinker with carving spoons and other utensils. They seem incredibly simple to make but the difficulty comes in making the curve just right, smoothing out the wood, and adding any ornate finishes you want on the handle.


When embarking on the manufacturing journey post PCB design, you may be one of the skilled few who have taken on the challenge of incorporating vias throughout your design. Although you will soon be reaping the benefits of adding this technology into your board’s layout, there are inevitably going to be a handful of hurdles you must jump through prior, however, such is life.


Aspect ratios play a crucial role in the manufacturing process of PCBs and should not be taken lightly. If neglected, they can leave you with a tattered board barely able to form proper connection points for your via plating, or even component placement.

Foundations: Vias Before Aspect Ratios for PCBs

Before diving into aspect ratios, let’s take a step back and re-review what PCB vias and annular rings are and why they play important roles in your design.


Vias are holes that are drilled through the traces of PCB layers with the sole purpose of connecting to another trace on another layer. They are often present in HDI multi-layered PCBs which require each layer to be connected in one way or another. There are a few variations that come in the form of blind, buried, and through-hole vias.


Blind Vias: They connect an outer layer of the PCB to an internal layer of the PCB but do not go any further. So if we have a four-layered PCB, the first two layers will have holes drilled through the traces, but not the third or fourth.


Buried vias: These connect two or more internal layers to one another. Again, in our four-layered PCB, the second and third layers will be drilled and connected, while the outer layers, the first and fourth, won’t show any holes and will simply look like a blank spot on the board.


Through-hole vias: As you may now have deciphered, these are drilled literally ‘through’ the entire board connecting the outer first and fourth layers together (or other combinations of connecting the four layers together).


After these vias are properly designed into the PCB, it’s now time to drill through each via location. Enter annular rings. The annular ring is the copper ring left around the drilled portion of the via which gives us our connective surface in which to either mount components onto or fill in to give us a nice plated via. The larger the annular ring, the larger the connection surface we have. A simple, yet very crucial, piece to the manufacturing puzzle.


PCB copper layout snapshot

Aspect ratios will affect your trace placements



The Sweet, Sweet Numbers of Aspect Ratios

The aspect ratio is simply defined as the board thickness to the diameter of the drilled via. This is an important ratio due to its effect on the plating that is within the vias (as also affected by the annular rings).


Say you have a board with a thickness of 0.2” and a via drill diameter of 0.02”. The aspect ratio would be 10:1. As this ratio increases, more plating will be surrounding the via compared to the inside drilled portion and is therefore at greater risk of cracking due to z-axis expansion while soldering. Keeping your aspect ratio lower will ensure consistent plating throughout the via and will allow for greater strength throughout the life of the PCB.



Drilling PCB panels

Aspect ratios are often affected by drilling limitations


Most manufacturing shops can attain an aspect ratio of 6:1. With the smallest practical drilling diameter floating around 0.013”, that leaves you with a max board size of around 0.078”.


Keeping both the board thickness and the smallest drill diameter in mind when considering a lowered aspect ratio may just save you that one long and confusing headache down the manufacturing road. Additionally, your vias will be much stronger and will leave you with a greater connection to mount your components onto.


Using great software like Altium Designer can greatly assist in figuring proper via strategies, annular rings, and aspect ratios alike. If you would like to see what Altium’s software can do for your aspect ratios, talk to an Altium expert today.


About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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