I’m not ashamed to admit that I’ve been sucked into intractable debates on technological and scientific topics with friends, family, and on internet forums in the past. Many people stick to their opinions, and you will have to move mountains to change their minds. Even the PCB design industry is not immune to this problem, and there is still debate about a simple design issue that can have big implications.
Non-functional pads are an issue in the PCB design industry that still lacks consensus. Debates about reliability and effects on signal integrity abound. While most designers and engineers agree that the debate on signal integrity is largely settled, the reliability debate still rages on behind the scenes. Since no general rules about non-functional pads exist, designers will need to determine if they should ditch non-functional pads in their specific applications.
Telegraphing and ECM Failure
The presence of non-functional pads on plated through-hole vias can lead to a condition known as “telegraphing”. When there is so much copper at the vias, the material between pads becomes resin starved. As a result, an image of the copper stack appears as peaks and valleys in the surface layers of the dielectric. In other words, the image of the copper stack is "telegraphed" to the board surface.
When the copper is thicker than the dielectric layer between pads, the condition is exacerbated and reliability is significantly reduced. The high spots create regions where epoxy can be squeezed out, leaving voids between neighboring pads. Voids usually form at the right angle formed by the pad and via barrel, eventually leading to thermal failure at the joint. The effective cross-sectional area of the joint shrinks over time, resulting in increased current density.
Electromigration in action
Void formation at via joints results in adhesion issues and allows electrochemical migration paths. This can cause growth of dendritic or fibrous structures between pads due to the slight voltage difference between them. Growth of these structures accumulates over time, eventually leading to PCB failure that is difficult to diagnose.
If dendritic structures can bridge the gap between adjacent conductors, a sudden drop in resistance occurs as these structures grow in parallel with the via. If the cross-sectional area of the dendrite is small, the current density will be high and the structure may burn out, essentially eliminating the fault. This leads to an intermittent failure behavior that is difficult to diagnose.
The Jury is Still Out on Reliability
In many situations, non-functional pads are relatively harmless. Fabricators generally prefer that non-functional pads be removed because it makes drilling easier. There are debates over reliability that relate to the via aspect ratio. Whether you should remove non-functional pads really depends on the application and operating conditions. Strain buildup in the copper plating along the via barrel during thermal cycling can lead to cracking, depending on the via aspect ratio.
In low aspect ratio vias, the interior copper plating is more uniform, and nonfunctional pads can increase the lifetime of the via. The combination of anchoring provided by the pad and the more uniform in the via barrel causes the via to be less prone to cracking. In high aspect ratio vias, the via barrel is more prone to cracking at the center due to the thinner copper coating in the center of the via barrel, regardless of the presence of non-functional pads.
Non-functional pads take up valuable real estate on the inner layers in thinner multilayer HDI boards. As long as you can be sure the board will remain stable under thermal cycling, it may be desirable to remove non-functional pads in order to improve trace routing on inner layers.
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Non-functional Pads in Rigid-flex Boards
Removing non-functional pads is a great way to gain some extra space on the inner layers of any PCB. However, caution should be used when designing flex and rigid-flex PCBs. Copper in a plated through-hole via does not bond to a flexible substrate as well as it bonds to a rigid substrate. Since copper bonding is a real issue in flexible substrates, non-functional pads now become useful.
Bonding is especially important in controlled impedance flex and rigid-flex boards, where thicker dielectric layers are used to build the material stack-up. All pads, both functional and non-functional, function as anchor points that are dispersed along the via barrel. This increases the strength of the via in a flexible board.
Some manufacturers recommend leaving at least some non-functional pads on flex and rigid-flex boards. If all non-functional pads along a via are removed, the gap between functional pads becomes very large, and the plating may start to separate from the hole wall. If possible, these pads should be evenly dispersed along the via barrel in order to distribute strain evenly. This will keep the plating from cracking or separating from the substrate.
Via structures can be easily defined with the built-in CAD tools in Altium Designer 18.1. You can download a free trial and find out if Altium Designer is right for you. Talk to an Altium expert today if you are interested in learning more.
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