One of the best lessons I’ve learned in my career in PCB design is realizing when I’ve gone too far. It’s easy to miss the forest for the trees when you’re in the weeds of placement, layout, routing, and other core tasks, but sometimes it’s necessary to take a moment to reflect on your work. Sometimes I’ll find I’ve overlooked some crucial element in the design documents or strayed from the engineer’s intent. In this case, saving the work I think is worthwhile, and undoing the rest with a backup file can seem like an easy fix.
However, undoing prior work is not as simple in the real world when performed digitally. PCB production involves complex manufacturing steps, and additional work requires more than a few keyboard shortcuts. For example, a common PCB process that gets partially “undone” is stub removal via a back drilling process to improve signal stability in high-speed designs. It offers a general performance boost to nearly every design. Here we’ll look at the back drilling process, when you should use it, and how.
Stubs and Signal Integrity: A Justification for the Back Drilling Process
Signal integrity is often composed of two distinct but interrelated aspects of the design process: layout and fabrication. Diving deeper into the latter, some designers may think of the issue only in terms of material selection relating to pre-preg aspects like dielectric/loss values and the physical design of the weave. However, the manufacturing process can render an unintended vector for signal integrity issues: stubs stemming from the through-hole plating step. The presence of stubs for some board types has a trivial impact on performance; it is primarily RF or high-speed designs where stub length can begin to affect board functionality negatively.
Stubs form naturally in the manufacturing process whenever a signal passing over the surface of a plated through-hole does not run the length of the barrel. In HDI boards, it’s common for a designer to use a via to pass through heavily routed areas before arriving on a less-densely laid out layer. The signals of these vias can travel over the entire length of the barrel, regardless of whether the ultimate destination lies on an internal layer. Depending on the fractional length of the stub to the length of the transmission line (the length of the barrel the signal passes over before reaching the stub), significant interference can occur upon reflection, up to and including complete destructive interference of the waveform for a reflection totaling π radians. More often, the stub represents some phase shift. Waveguides may utilize this ability to add capacitive or inductive reactance to a transmission line to negate a prior shift for impedance matching (recall that impedance matching requires the complex conjugate z* where z = a + bi).
Stubs pose issues to performance as it relates to signal integrity, but this effect is nonuniform. Varying stub lengths create particular resonant frequency nulls. When these resonant frequencies fall near the Nyquist frequency (half the sampling rate), the bit error rate occurs at a high enough level to completely undermine the signal. The significant distortion occurs due to the quarter-wavelength reflections—when a signal splits at the point of internal connection and stub. A stub length corresponding to a time delay of a quarter will result in a total time delay of half (total distance traveled before and after reflection), causing the most significant interference when returning to the original junction point of the signal.
The Logistics of Back Drilling
Back drilling, compared to the standard drilling process, is a bit more technical. Most importantly, a depth-drilled hole must remove the stub portion of the via barrel without impacting the rest of the length. A larger drill bit than the original hole is used to bore out the plated hole to remove the barrel. Designers must account for the additional clearance required around the back drilled through-holes to any copper features. They must also understand that stub removal is actually stub reduction. Stubs at their shortest length must be taken down to approximately 5-10 mils past the connect to avoid damaging the internal connections.
In summary, not every board will require back drilling. More specifically, not every via in designs that are especially susceptible to signal degradation will need its stub removed. While any additional processing carries some risk to yield, the general performance improvements and stability of controlled-depth drilling are likely to far outweigh any production concerns.
Diagnostics Can Help Guide Your Manufacturing Process
Stubs can cause numerous issues during testing that indicate a board may benefit from back drilling for a more efficient production process. Be on the lookout for any of the following cases that may suggest that stub length is negatively impacting performance:
- Deterministic jitter. Unlike random jitter, which arises from thermal noise impacting performance, deterministic jitter is a predictable, non-Gaussian, and bounded distribution. It creates differences between the expected periodicity of the signal and the experimental value.
- Bit error rate. A quantification of how many bits in a signal packet differ from the packet sent at transmission. At high levels, BER indicates a heavy amount of noise or distortion on the signal.
- EMI. At appreciable lengths, stubs begin to act as antennae that can not only radiate significant amounts of energy but also pick up environmental signals and introduce them to the board.
- Via-via crosstalk. Just like traces, the greater the length of distance two via barrels spend in parallel, the greater the coupling between them. Drilling down stubs minimizes the distance two nearby vias spend in parallel.
Your Contract Manufacturer Has Signal Integrity Issues Covered
For optimizing signal integrity when it counts, the back drilling process is the best bet. It reduces incidences of degradation, noise, coupling, and other undesirable issues that sap your board’s overall performance. If you have questions about how back drilling can improve your board—or other design and manufacturing topics related to signal integrity—VSE can help. We’ll lend combined professional experience to your next board production. Our goal is to design and build electronics that meet customer expectations and our high standards.
