One of my favorite team-building activities from school was constructing a stable tower from marshmallows and dry pasta under a time limit. This brainstorming exercise challenged teams to construct the tallest structure without external support. Though some people used it as an excuse to sneak some sweets during class, it highlights the importance of building a solid foundation for stability. It’s an excellent engineering instruction that is intuitive to even the youngest students.
However, I’ve since learned the reverse is also true: don’t excavate to depths without an appropriately-sized base. While most people think of the stability of holes as trivial compared to that of a structure rising against gravity, similar ideas are at play, though the underlying forces are different. The PCB via aspect ratio is of great importance to relate the opening size and depth of a hole to the producibility of the board, and it is used extensively as a check against overambitious designs.
The Drawbacks of Higher Aspect Ratios
The aspect ratio relates the diameter of a hole’s opening to its depth, the significance being that holes with small openings and large depths are less mechanically reliable than those with smaller depths. This ratio can be calculated for through-holes by dividing the board thickness by the drilled hole diameter; a larger value indicates a small diameter for the thickness and vice versa. While there is no physical lower limit on the aspect ratio, hole diameters are likely to be limited by the total space available on the board, especially for components on the outer layers. On the other hand, drilling holes above the upper limit of an acceptable aspect ratio (typically 8:1~10:1) can result in fabrication and reliability issues:
- Coefficient of thermal expansion (CTE): As board materials are subjected to extreme temperatures, materials along all axes lengthen due to an inherent property known as CTE. While this is often an issue during lamination due to the mismatch between prepreg and copper layers, manufacturers and designers should be mindful of the cases where larger aspect ratio holes experience large temperature swings, like passing through a reflow oven. The larger the aspect ratio, the more likely a board will experience a mechanical failure due to thermal expansion. This failure is because the plating towards the middle of the via barrel is often thinner than in a lower aspect ratio plated through holes.
- Plating: Plating impacts the quality and reliability of the via barrel due to its deposition action. Essentially, as the electroplating fluid enters the drilled hole, some of it is deposited at or close to the opening, restricting the flow of liquid towards the center of the drilled hole. While this effect is gradual and inconsequential in drilled holes of moderate aspect ratio, higher aspect ratio holes may experience partial or incomplete plating that leads to mechanically weak barrels or connectivity issues. In extreme cases, a barrel can plate shut towards either of the openings before plating across the entire length of the drilled hole, leading to additional drilling, plating, and design revisions.
Microvias Offer Some Exceptions To PCB Via Aspect Ratio Constraints
Drilling is not the only way to create holes in a printed circuit board. Instead of a drill bit that bores through the substrate, laser ablation can vaporize the board material while reducing the amount of mechanical stress the board experiences. Another notable advantage of laser-drilled microvias is the achievable aspect ratio, as low as 1:1 with a depth of less than .25mm/10 mils. This quality would imply an identical board thickness, yet laser drilling is common on HDI boards with much greater thicknesses.
Though it may seem contradictory, the key to understanding this disconnect is the term microvias: microvias are vias that pass through one or two layers at a time. Individually, this means microvias cannot span the width of common board thicknesses. Still, multiple microvias can be combined to form a functional through-hole with diameters far smaller than traditional drilling methods. Combined microvias can either be stacked (centers aligned on the z-axis) or staggered (microvias connected on internal layer routing).
If microvias can reach diameters much smaller than bored vias, why haven’t they superseded the latter? As always in engineering, there’s no such thing as a free lunch: laser drilling is more expensive than regular drilling, requiring many additional processing steps and potentially reducing yield. As microvias are drilled at either one or two layers simultaneously, drilling and plating become several repetitions instead of a single shot. Because the drilling occurs before the final lamination of the board, registration errors can creep in that would otherwise have been avoided by drilling a single solid body.
That said, laser drilling is widespread in designs where space is at a premium, allowing greater flexibility for the designer during routing. Additionally, some inherent drawbacks to normal drilling can be completely sidestepped:
- Drill wander: As the depth of the hole increases, so does the tendency for a drill bit to exit the hole off-center from where it entered. This scenario causes issues when drilling removes internal traces or pour, creating disconnects between the via barrel and adjacent copper feature.
- Smear: Resin that covers internal copper edges, interrupting conductivity paths between the via barrel and inner-layer routing. The cause of this can be a mechanical transfer of the resin for high infeed rates or a thermal action from excessive friction owing to a dull drill bit.
- Burring: The incomplete removal of material from a drilled hole, which often manifests as hanging bits. Boards must enter a deburring machine to remove these imperfections and prevent downstream manufacturing issues.
Your Contract Manufacturer Can Drill To The Core of Your Design
A PCB via aspect ratio is itself not a complex calculation. Still, there are a wide variety of factors that designers and manufacturers weigh to balance ease of layout and efficient production. Drilling is a crucial step in manufacturing. The greater the collaboration between the designers and the shop floor, the higher the chance that a design avoids lengthy and costly revision processes, rework, and other setbacks.
If your board needs some additional review to help overcome this DFM bottleneck, contact us at VSE. We’re a team of experienced engineers, and we’re eager to lend our expertise to your product. Alongside our valued manufacturing partners, we can provide best-in-class consultation from design review to final installation.