It’s spring cleaning time at my house. I have so much old stuff in my garage, and I can never decide what to get rid of and what to keep. The stuff I keep always has some kind of sentimental value, or it’s something that I think I’ll need later. Dusty old circuits and boards that I’ve kept around, an old computer I haven’t gotten around to cleaning off that I made some of my first CAD designs on - the life of a hoarding engineer.
Annular rings in PCBs form a joint between a trace and the via its connects to. Although they typically appear on exposed board layers, these annular rings can also be placed in the interior layers along the neck of a via and form nonfunctional pads, similar to ELIC design. Much like my debates on which items to keep or give up during spring cleaning, deciding whether these nonfunctional pads should be kept or removed during manufacturing is an ongoing controversy in the PCB design community.
Annular Rings in Functional and Nonfunctional Pads
An annular ring is the ring of copper that surrounds a via hole. When used with via-in-pad or VIPPO technology, the annular ring actually forms a functional pad. SMT components can then be soldered directly onto the pad. Using via tenting, VIPPO, or the appropriate solder mask will prevent solder from wicking into the via hole and annular rings on functional pads allow tighter packing of components by eliminating the trace that would travel between the via and pad.
The aforementioned pads actually serve a useful purpose as they connect directly to an SMT component. They also serve as a drilling target during manufacturing. The copper ring at the exposed layer improves adhesion to the board and helps prevent cracking at the connection between the trace and the via. Your PCB manufacturer might be very grateful that you’ve included such particular direction for your boards.
Larger annular rings on the top and bottom layers allow for looser manufacturing tolerances when drilling via holes. Very small annular rings at the ends of a via require tight drilling tolerances for the specific hole locations. If the drill is off-center, there is a chance that the drill punches its hole through the trace leading to the via pad, thus making the board useless.
Drilling vias in a PCB
Annular rings can also be placed in the interior layers of a PCB, forming nonfunctional pads. There is still a lack of consensus among PCB designers regarding the presence of nonfunctional pads on interior PCB layers. PCB designers and engineers will argue whether or not these pads should be removed. The real answer, as with most complicated matters, is “it depends.”
Annular Rings in High-Speed PCBs
The annular rings in a via form a parallel plate capacitor, and this affects the impedance of the via. This means that more annular rings along the via barrel, whether or not they are nonfunctional, increase the overall parasitic capacitance of the via. This, in turn, reduces the via’s overall impedance. This could bring the via impedance closer to the trace impedance if the via impedance is already high. In any case, impedance matching should be performed on critical traces and components.
Any unused stub present at the end of a via has a set of resonance frequencies as high-speed digital signals and high-frequency AC signals propagate through the via. As more nonfunctional pads along the via barrel increase the capacitance, this lowers the quarter wavelength resonance frequency of the via stub. This increases the via insertion loss and degrades signal quality.
The typical strategy used to overcome via stub resonance is to backdrill the stub so it’s as small as possible. The lowest order resonance frequency of the via stub can then be set higher than the signal operating frequency. Via stub resonance is then effectively suppressed. Including nonfunctional pads along the via barrel reduces the lowest order resonance frequency and can effectively destroy the typical strategy used to overcome insertion loss problems.
In addition to degrading insertion loss, resonance results in signal distortion and bit errors in high-speed digital signals. High-speed PCBs should have the annular rings making up nonfunctional pads removed during manufacturing in order to avoid signal distortion.
Thermal Reliability and Through-hole Via Aspect Ratio
There has been much debate on how nonfunctional pads affect the reliability of plated through-hole vias. The primary argument is that, in general, the annular rings that form the non-functional pads improve thermal reliability as the pads would create an anchoring effect in each layer. During thermal expansion, it was thought that the annular rings hold the via in place and counteract expansion.
Flaming circuit board
The real answer to this controversy has to do with the via’s aspect ratio, i.e., the ratio of its length to its diameter. In a via with a large aspect ratio, the plating in the via is less uniform and thinnest in the center of the via. As the board heats up and expands, the annular rings on the nonfunctional pads concentrate the strain near the center of the barrel, especially if the pad arrangement is symmetric. Since the center of the via barrel has the thinnest plating, the via plating will crack at this point.
Removing the interior nonfunctional pads from vias with large aspect ratio doesn’t offer much improvement. In this case, the strain is concentrated throughout the barrel and there is a potential for cracks to appear anywhere. Proper design rules can help with your copper planning for manufacturing and drill holes.
In low aspect ratio vias, the interior copper plating is more uniform. Annular rings on nonfunctional pads can increase the lifetime of the via. The anchoring provided by the annular ring still causes the strain to be uniformly distributed throughout the via barrel and a more uniform via will be less prone to cracking under thermal stress.
When it comes down to it, you’ll want to be as sure as you can when designing for caution toward drilled holes in your circuit board. The built-in CAD tools in Altium Designer 18.1 make it easy to define the structure of your vias and make it easy to route connections. Don’t be afraid to take extra caution in your designs when you’re working with particular via and buried via arrangements.
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