I’m extremely prone to moments of forgetfulness. Whether I’m working at the house or out on backcountry trails, it’s very easy for my head to get lost in the literal or figurative clouds. Regardless if I’ve set my phone (or my keys, or my wallet) somewhere I can’t remember or I’ve wandered past a junction onto trails unknown, I’ve learned to accept and adapt: I’ve mastered the art of retracing my steps. While it can be varying levels of annoyance to reverse the work you’ve done, it’s sometimes a necessary step. Back drilling in PCBs operates under a similar guise. To correct a fabrication error – a reality of the limitations of the manufacturing and design process – it’s necessary to undo the work established in an earlier step. As an additional processing step, back drilling increases the cost of a design on a per board basis. However, it offers a relatively low-tech and reliable performance solution for modern-day high-speed designs.
What is Back Drilling in PCBs, and When is it Encountered?
The idea behind back drilling is a relatively simple one among the vast topics and processes of PCB manufacturing. Plated through holes in designs are formed by a combination of drilling and then, perhaps unsurprisingly, a plating process. As plating arises from an electrolysis-driven solution, plating occurs uniformly from top to bottom. With current technological sophistication, it’s not feasible to dictate where the plating builds. Moreover, many board functions are flexible enough that plating spanning a through hole is not necessarily an issue, and may even be a requirement based on the design.
However, a problem arises during high-speed designs: superfluous plating that extends past the last layer necessary for routing forms a stub. Stubs act as both antennae and receivers, potentially leading to EMI issues and errors during runtime. To remove this excess copper, fabricators employ a secondary drilling step known as back drilling. Back drilling uses a larger drill bit to bore out the extraneous length of the copper barrel, ensuring the length of the plating extends only to the depth necessary for the interconnects (with some reasonable tolerance on the stub). Stubs can form bi-directionally from the plated through hole and may require back drilling on both the top and bottom. In effect, back drilling recreates a completed plated through hole as a blind or buried via without the extra fabrication run-up during lamination. This is not to say that back drilling serves as a replacement: alternate via structures are pre-planned during the fabrication process, while back drilling exists to correct issues encountered during testing.
The Physics of Stubs, Reflections, and Transmission Line Losses
When a signal travels down the length of a plated barrel and encounters a stub of significant length, the signal splits along its propagation path, and some of it is reflected with a phase shift based on the physical characteristics of the stub. A λ4 (where λ is the wavelength) transmission line presents an interesting case where a line will reflect a sinusoidal wave towards the source of the transmission without inversion of the amplitude. After traveling the length of the λ4 transmission line, the wave returns to its source 𝜋2 radians out of phase, resulting in complete destructive interference of the waves at all frequencies for any transmission line odd multiple of the length λ4. While this is an extreme example and resistive losses are not represented in this idealized model, it doesn’t take much for a stub to cause a significant phase shift to the source signal. Changes to the phase of the signal can then create timing issues as the high- and low-level voltage cutoffs on the waveform have shifted meaningfully in the periodicity of the signal.
Switching gears to analyzing the real losses in the signal, there are two parameters responsible for the overall energy losses in the transmission line:
- Frequency: As frequency in a line increases, so does the average energy of the electrons traveling the line. This excitation results in more heat energy as oscillations per period increase, raising the resistance per equivalent length of the transmission line. Mathematically, this frequency-dependency exhibits a square root relationship, indicating a potential for unbounded growth.
- Length: As the frequency losses are calculated on a per length rate basis, designers simply need to measure the length the stub extends beyond the main transmission line. There exists a direct correlation between the length of the stub and the energy loss at a particular frequency.
Because the magnitude of the losses increases with the length of the stub, stubs need to be drilled down to an acceptable length that minimizes losses without jeopardizing the integrity of the interconnect structure. While this value depends on the speed of the signal in question, a measurement of 10 mils is generally seen as an acceptable stub length within the industry.
Your Contract Manufacturer is Equipped to Handle Back Drilling, and Much More
Back drilling in PCBs is a simple fabrication process in the grand scheme of PCB design. It requires minimal adjustments to the design rules and via structures in a board file alongside the associated drill file and documentation to realize massive performance and reliability gains in today’s high-speed designs. Your contract manufacturer must be able to handle the eccentricities of high-speed designs, and those found in microwave/RF, HDI, and additional board styles. Luckily here at VSE, we have a team of engineers who are committed to building electronics for our customers, no matter what complexities the design calls for. Alongside our valued partners, we’re able to enact a high standard of excellence in the design and manufacture of your PCB.
