IPC International Community magazine an association member publication
Issue link: https://iconnect007.uberflip.com/i/1544975
84 I-CONNECT007 MAGAZINE I MAY 2026 is essential for optimizing high-speed digital de- signs, ensuring reliable performance, and minimiz- ing electromagnetic interference. This approach shifts the design paradigm from simple routing to sophisticated electromagnetic management, cru- cial for modern high-speed electronic systems. Arguably, the most critical factor in high-speed PCB design is the impedance of the interconnect. Controlled impedance is the foundation of the de- sign's integrity. It forms the essential baseline, as reflections, terminations, return-current behavior, and even plane interactions all assume a stable, controlled impedance along the interconnect. Impedance is at the core of the methodology that is used to solve signal integrity issues: 1. Signal quality issues arise because voltage signals reflect and are distorted whenever the impedance changes along a transmission line. 2. Crosstalk arises from the coupling of elec- tric and magnetic fields between adjacent traces or between traces and return paths. The inductance and capacitance between the traces establish an impedance that deter- mines the amount of coupling. 3. Differential-mode propagation can be con- verted to common mode by parasitic capaci- tance or by any imbalance caused by im- pedance variation, signal skew, rise/fall-time mismatch, or channel asymmetry. Common- mode currents are the main source of electro- magnetic radiation. The iCD Stackup Planner, in Figure 1, illustrates the three most common transmission line struc- tures of a multilayer PCB. For embedded microstrip (solder mask-coated microstrip), the electromag- netic field propagates partially in the dielectric material, the solder mask, and the air. Whereas, in both stripline structures, the electromagnetic field propagates in the dielectric material sandwiched between the planes. A characteristic impedance of 40–60 ohms is typically used for a digital design. However, this value becomes more critical as the edge rates become faster. Also, different technologies have their specific impedance requirements. For ex- ample, Ethernet is 100 ohms differential, USB is 90 ohms differential, DDR2 memory is 50/100 ohms single-ended/differential impedance, and DDR3-5 is 40/80 ohms single-ended/differential imped- ance. So, controlling impedance simultaneously across each signal layer with multiple technologies can be challenging. Also, as operating voltages are reduced, the associated noise margins are also reduced, making it even more important to match the impedance. Once we define the characteristic impedance, the next challenge is ensuring the signal actually sees that impedance along its entire path. When a transmission line is perfectly matched to the driver and load, the signals propagating electromagnetic (EM) energy are totally absorbed by the load. This is the perfect scenario that all electronics design- ers strive for. Figure 1: Microstrip, asymmetric and dual symmetric stripline configuration. (Source: iCD Design Integrity) B E YO N D D ES I G N