Via Aspect Ratio: Design, Signal Integrity, and Manufacturability
There is a certain beauty that exists in nature and can only be mimicked in PCB design. The golden ratio found in nature is a good example of a universal relationship that defines the aspect ratio of plants and animals. As with anything, PCBs can have layers to them: conductive layering, dielectric layers, barrier layers and with layers come drilling. And while via aspect ratios are no golden ratio, they have a beauty all their own and are extremely important in PCB design.
The aspect ratio of a via is an esoteric issue that many designers are happy to ignore. Simply put, a via’s aspect ratio is its length divided by its diameter. While it may seem like a minute detail, the aspect ratio of a via is important in various PCB applications. It’s also important to consider the aspect ratio when considering manufacturing requirements.
How Via Aspect Ratio Affects High-Speed Design
As digital signal transmission speeds continue to become faster, any impedance discontinuity will affect signal quality. Vias placed in signal lines are an often overlooked impedance discontinuity in PCBs. Vias can appear as inductive and/or capacitive discontinuities and these parasitic elements degrade signals as they pass through the via. Whether working through characteristic impedance or reducing noise, signal integrity can be a constant challenge.
The impedance of a via can be tuned without using extra capacitors or inductors to compensate for the via impedance. Compensating parasitic capacitance in high aspect ratio vias requires that any non-functional pads along the via barrel be removed. Capture pads should use the smallest possible diameter, which also helps maximize routing space. Additionally, backdrilling to remove via stubs is used to mitigate signal integrity problems. Any leftover via stubs present inductive discontinuities and cause signal reflection as a signal passes through the via.
Just like differential traces, vias for high-speed signals can be designed in a differential manner. A signal via can be placed alongside a ground return via. Ground and signal vias used for differential routing must have the same geometry, including aspect ratio, in order to maintain symmetry and take full advantage of crosstalk reduction. The added ground via also provides insertion/return loss improvements.
When it comes to understanding your board’s needs and material needs for PCB manufacturing, board thickness isn’t the only concern on your plate. Different manufacturers have different fabrication capabilities. Most manufacturers should be able to place vias with aspect ratios of 6:1 up to 10:1. IPC reliability standards also specify via aspect ratios ranging between 6:1 and 8:1. An aspect ratio of 8:1 is considered to be something of a required capability among PCB manufacturers.
PCB processing with a PCB drill
No matter if you are working with blind vias or buried vias, when it comes to via placement with mechanical drilling the critical parameter is the via diameter, not the aspect ratio. A smaller diameter requires tighter drilling tolerances. If the drill slightly misses its target, the drill can punch through the trace leading to the via, rendering the board effectively useless with a hole that simply missed its mark.
Laser drilling faces the same lateral mechanical tolerance requirements as mechanical drilling. However, laser drilling facilitates the formation of smaller via holes without subjecting the board to the stress that would be applied during mechanical drilling. This process can be a significant improvement for your printed circuit board manufacturing and vastly shift the relationship of your electronic production.
The resulting aspect ratio is proportional to the depth of focus divided by the beam waist and the aspect ratio can be somewhat controlled by controlling these two parameters. Using a laser with a smaller wavelength, like an ultrafast UV laser, further reduces the via diameter and decreases drilling time during your fabrication and manufacturing process.
Via Plating Challenges
Drilling vias will require some understanding of the intricacies that involve adding depth into your circuit and adding fabrication necessities. The aspect ratio of a via affects the difficulty with which the interior can be plated. Copper is deposited on the interior of a via using plating solution. The plating solution must be able to wick into a via hole by capillary action in order to fully plate the interior of the via.
The physics and chemistry involved in micropillar plating are very interesting. During capillary action, surface tension draws the plating solution into the via, and copper begins depositing along the wall. Due to the meniscus that forms at the surface of the solution, the copper precursor is consumed quickly from the solution in deeper regions of the via. As a result, the inner portions of a via barrel can have thinner plating than at the edges of the via.
If the aspect ratio of the via is larger, then the copper coating in the interior of the via will be thinner, and the center of the via will be more prone to cracking under thermal stress. A plating solution with a lower viscosity can more efficiently coat the inside of a high aspect ratio via. This not only improves the structural strength of the via but also improves its reliability against thermal stress.
Small diameter vias on an HDI board
The strength and reliability of a via tend to vary across different manufacturers. This happens despite the fact that the vias have the same aspect ratio. The obvious reliability difference occurs due to the weight of the copper deposited in the via and the thickness at the center due to the plating method used during fabrication.
Working through multi-layered PCBs and understanding the needs of your circuit boards and all their requisite holes and pads is a task for strong PCB design software. Altium Designer® has built-in CAD tools that make it easy to customize the structure of your vias for your circuit. Through proper use of design rules and an easy-to-alternate 3D viewer feature, you can always be aware of your via placement needs.