High Aspect Ratio Be Recycled

Aspect ratios are an essential part of PCB design. A good via aspect ratio will minimize the impedance of the copper network in a through-hole or plated through-hole, and improve the consistency of deposited copper. However, if the aspect ratio is too large, the copper layer in the via can fracture along the neck of the hole during soldering and reflow, causing the circuit board to fail. Ideally, you want to avoid this type of failure and keep the aspect ratio low.

Choosing the right via size for your PCB starts with looking at your stackup and layer count. Typically, a standard through-hole via will have a diameter of 10 mils, which allows for sufficient space to route signals and power. However, as technology progresses and component density increases, designers will push the size of the via to smaller diameters. This will require the use of HDI techniques and microvias to meet performance and reliability requirements.

Another factor that influences the viability of a via structure is how easily its interior can be plated. Plating is the process by which copper is deposited on a metal via hole, usually with an electrochemical solution. A higher via aspect ratio will make it more difficult to plate the interior of a via because the copper will not be able to “wick” into the hole through capillary action.

Can a High Aspect Ratio Be Recycled?

Aside from this difficulty in plating, a high aspect ratio may also result in signal integrity issues when routing through the via structure. This is because the via stubs that are left behind from mechanical drilling can cause inductive discontinuities in the signal path. To mitigate this, backdrilling is performed to remove these stubs from the via barrel.

As a general rule of thumb, the minimum aspect ratio for a mechanically drilled through-hole via is 1:10. This is because it is easier to fit a 6mil finished hole on a PCB than an 8mil drill size. However, many manufacturers will specify a maximum aspect ratio for their products, so it is important to double-check with your manufacturer before designing a PCB that exceeds this maximum.

One way to solve the problem of a high aspect ratio is to implement a via-in-pad (VIPPO) structure. VIPPO is a variation on the conventional via structure wherein the plated through-hole is replaced by a pad and the pad is filled and capped with conductive or non-conductive epoxy. The advantage of this is that it helps to improve thermal management and makes the via invisible from a visual standpoint. This type of via is also known as a’resin hole’ or IPC-4761 Type 7 and is often referred to by street names like ‘filled and capped’ or ‘vipo’. Using this type of via in your PCB will help you meet your performance and reliability goals without having to sacrifice a robust design.

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