A Method of Optimizing Characteristic Impedance Compensation Using Cut-Outs in High-Density PCB Designs

Abstract

The modern era of technology contains a myriad of high-speed standards and proprietary serial digital protocols, which evolve alongside the microwave and RF realm. The increasing data rate push the requirements for hardware design, including modern printed circuit boards (PCB). One of these requirements for modern high-speed PCB interfaces are a homogenous track impedance all the way from the source to the load. Even though some high-speed interfaces don’t require any external components embedded into the interconnects, there are others which require either passive or active components—or both. Usually, component package land-pads are of fixed size, thus, if not addressed, they create discontinuities and degrade the transmitted signal. To solve this problem, impedance compensation techniques such as reference plane cut-out are employed for multiple case studies covering this topic. This paper presents an original method of finding the optimal cut-out size for the maximum characteristic impedance compensation in high-density multilayer PCB designs, which has been verified via theoretical estimation, computer simulation, and practical measurement results. Track-to-discontinuity ratios of 1:1.75, 1:2.5, and 1:5.0 were selected in order to resemble most practical design scenarios on a 6-layer standard thickness PCB. The measurements and simulations revealed that the compensated impedance saturation occurs at (150–250%) cut-out widths for a 50 Ω microstrip.

Analysis of SMT component land pad discontinuity effect on the overall transmission line impedance in high-speed applications

“…5(c), the impedance drop is reduced in half when using a single reference plane cut-out. This is because the capacitance of such a large pad has to drop even more, hence this could be done by introducing another reference plane cutout in the successive layer below [12]. Moreover, even though the length of the discussed discontinuities is around 17% of the length of the whole 50 Ω microstrip transmission line, the introduced reflections affect and worsen the impedance the segment of the line following it.…”

“…2(b). Simply applying a reference plane cut-out as a rule of thumb without taking into account the stackup details might lead to overcompensation, which might not be crucial if the resulting absolute deviation from the target impedance is smaller than that without applying the cutout compensation [12].…”

Applying Characteristic Impedance Compensation Cut-Outs to Full Radio Frequency Chains in Multi-Layer Printed Circuit Board Designs

“…This has been reported in various papers and design notes [ 8 , 9 , 10 , 11 , 12 ], but they are only single case studies addressing passive surface-mount device components such as 0201 or 0402 package size capacitors. Moreover, impedance compensation techniques, including reference plane cut-out introduction, have been thoroughly analyzed and improvements have been proposed in previous papers [ 13 , 14 ] on the latter topic.…”

“…Thus, the third copper layer becomes the reference layer for these pads. Different stack-ups have been thoroughly analyzed in [ 14 ], while obtaining the optimal size of the cutout has been discussed in [ 13 ].…”

Tree-Based Machine Learning Models with Optuna in Predicting Impedance Values for Circuit Analysis

“…Incorrect single-ended or differential impedance causes signal reflection within the track when performing PCB layout. Thus, the loss of signal quality, lower operating frequency, and unwanted electromagnetic interference occur [ 5 , 6 ]. Therefore, controlling the circuit impedance within certain ranges when designing the PCB is critical in stabilizing PCB functions.…”