The optimization and engineering value of back drilling technology for Signal Integrity of high-frequency and high-speed printed circuit boards

With the wide deployment of 5G technology, digital signal transmission is rapidly evolving towards large capacity, low latency and high speed. The rising edge of the signal is constantly shortening, and the transmission frequency has gradually increased from below 3 GHz in the past to above 24 GHz. This poses more stringent requirements for the circuit boards used in mobile communication terminals. High-speed printed circuit boards, through modified copper foil, glass fiber cloth, resin and fillers and other materials, have achieved high signal transmission speeds of 10 to 50 Gbps, high characteristic impedance accuracy and low dielectric loss coefficients between 0.0005 and 0.01 at high frequencies, thus meeting the basic requirements of high-frequency and high-speed signal transmission. However, these modified materials generally have high hardness and high heat generation characteristics, which bring obvious difficulties to processing and manufacturing. Meanwhile, to adapt to the development trend of high integration and miniaturization of communication products, high-speed boards are constantly evolving towards higher multi-layering. The diameter and spacing of micro-via holes used for inter-layer interconnection are continuously shrinking, and the wiring density is significantly increasing. The electromagnetic coupling between micro-holes themselves and between micro-holes can interfere with signals, leading to increasingly prominent signal integrity issues such as signal delay, crosstalk, and transmission line effects. It can be said that the complexity of high-speed boards in terms of structure and materials not only increases manufacturing costs and process difficulty, but also poses a severe challenge to the reliability of high-frequency and high-speed signal transmission.

In the interlayer interconnection structure of printed circuit boards, micro-holes are the key channels for achieving vertical signal transmission. Their fabrication typically involves two steps: micro-hole drilling and hole metallization. In the copper-plated through holes after metallization, not all copper-plated sections are involved in the effective signal transmission. Taking the typical signal transmission from port 1 to port 2 as an example, the section of the copper-plated through hole that does not participate in signal transmission is called a stub. Although stuBs do not directly undertake the main transmission path of signals, they form parasitic structures in electromagnetic behavior and have a non-negligible impact on high-speed signals. Generally speaking, when the signal transmission rate exceeds 5 Gbps, the signal distortion problems caused by through-hole structures and stubs begin to emerge. The higher the transmission rate and frequency, the more obvious the phenomena such as signal delay, reflection and resonance will be. Therefore, how to effectively suppress the negative effects brought by stuBs has become one of the core issues that must be addressed in the design of high-speed boards and the transmission of high-frequency signals.

In response to the signal integrity issues caused by through holes and stuBs, the engineering and academic communities have developed a variety of optimization ideas. PCB layout design is a relatively traditional method. It often connects R, RL or RC terminals at the end of through-holes to improve impedance matching and enhance signal integrity. However, this method has certain limitations and relatively high energy consumption. Pulse modulation replaces PAM2 coding with PAM4 coding, effectively keeping the fundamental frequency away from the resonant frequency generated by the stub under the premise of high-speed signal transmission, thereby improving signal quality. However, this method has high requirements for signal control. When there are multiple through-hole stuBs, signals tend to interfere with each other, which is not conducive to wide promotion and application. In contrast, mechanical drilling, with its advantages of high processing efficiency, high processing accuracy, low cost and low energy consumption, has become the main method for micro-hole processing of circuit boards. Back drilling technology is precisely an important extension of mechanical drilling in the field of micro-hole processing. The process is that after the through-hole drilling and copper plating are completed, a micro drill bit slightly larger in diameter than the through-hole is used to drill from the reverse side of the sheet to the target layer, removing the ineffective copper-plated sections that are not conducive to signal transmission. This significantly shortens the stub length while retaining the part of the through-hole that is truly used for high-speed signal interlayer switching. Back drilling technology is characterized by its simple design, low cost, high yield and efficiency, and the ability to effectively enhance signal integrity. Therefore, it has been increasingly widely applied in the field of high-speed plate micro-hole processing.

From a physical mechanism perspective, the improvement of signal integrity by back drilling is mainly reflected in three aspects: parasitic capacitance, impedance matching, and resonant characteristics. Through-hole structures typically exhibit capacitive characteristics, and their characteristic impedance is often lower than the typical characteristic impedance of traces. When high-speed signals are transmitted from traces to vias, they will experience sudden impedance changes, thereby causing reflections, resulting in return loss and time-domain reflection, and reducing signal quality. The existence of stuBs further intensifies this effect: when high-speed signals reach the end of the stub, it is equivalent to encountering an open-circuit terminal, and the signals are reflected back to the branch point and oscillate back and forth between the branch point and the stub end. Back drilling directly reduces the parasitic capacitance of the via by shortening the stub length, improving the impedance matching degree between the via and the transmission line, thereby effectively alleviating signal delay, reflection and resonance problems. Existing studies have shown that back drilling to reduce stubs is the most effective in lowering the parasitic capacitance of through-holes. Quantitative tests also confirm this point: the impedance change of a common via is approximately 2.63%, while that of a via after back drilling is only 1.35%, indicating that back drilling significantly improves the impedance continuity during the transmission process and reduces the reflection coefficient. Further analysis shows that the shorter the stub length, the smaller the return loss. When the stub length is controlled below 12 mil, the return loss is all less than -10 dB. At this time, the signal reflection is minimal and the transmission quality is the most ideal.

In addition to the improvement in impedance matching, the back drill has also significantly altered the resonant behavior of the stub. The stub end is equivalent to an open circuit, which will form a quarter-wavelength resonator, causing a sharp increase in attenuation of the signal transmission channel at the resonant frequency point, and is manifested as a distinct downward rush on the insertion loss curve. The shorter the stub length, the higher the frequency of the resonant point, and the higher the frequency of the signal that can be completely transmitted. Back drilling raises the resonant frequency of the signal by reducing the length of the stub. As long as the resonant point generated by the stub is limited below the signal transmission frequency, the high-speed transmission requirements can be met. Experimental research also supports this conclusion: In the signal integrity test of MEMS probe cards, the insertion loss of PCB components after back drilling steadily decreases with frequency, while the signal that has not undergone back drilling shows a significant downstroke at approximately 4 GHz, proving that back drilling has a positive effect on increasing the resonant frequency of the signal. This fully demonstrates that the back drill not only improves impedance matching but also broadens the effective transmission window for high-frequency signals, creating conditions for the transmission of signals at rates above 24 GHz and even higher.

Overall, the essence of back drilling technology lies in precisely removing the residual pile copper plating in the through hole that does not participate in signal transmission through mechanical means, achieving a reduction in parasitic capacitance, optimization of impedance matching, and an increase in resonant frequency with relatively low process complexity and cost. As high-speed boards continue to develop towards higher layers, smaller hole diameters and higher wiring densities, the drilling quality, depth control, hole wall integrity of back drills and their process compatibility with high-hardness modified materials will directly affect the upper limit of signal integrity optimization. Therefore, conducting in-depth theoretical and applied research on back drilling technology is of great practical significance and engineering value for enhancing the efficiency and quality of back drilling and supporting the development of high-frequency and high-speed circuit boards.