Issue link: https://iconnect007.uberflip.com/i/1515637
52 DESIGN007 MAGAZINE I FEBRUARY 2024 2. e residual PDN noise from the first event reflects in the cavity (which resem- bles an unterminated transmission line) a couple of nanoseconds later, back to the driver. If at that precise moment, the driver switches a second time, both pulses (first and second) are superimposed. If the phases are added and the driver has a repetitive pulse (as clocks do), the reflected pulse may build significantly into a standing wave. One could avoid this potential failure by com- paring the round-trip delay across the plane in question to the clock period. If it is close, then an adjustment in the plane size may be an appro- priate solution. is may not eliminate all plane resonances but can serve to shi the resonances to other frequencies. Also, adding stitching vias or terminating RC networks in appropriate locations can reduce the extent to which the sig- nal energy spreads through the plane cavity and raises the frequency of structural resonances. Replacing conventional power/ground planes with an embedded capacitance layer allows for tighter component density, reduced via count, and thus increased signal routing channels. ECMs provide higher capacitance in a PDN, resulting in lower AC impedance and greater damping for power bus ripple. is leads to less intense power plane cavity resonances at gigahertz frequencies due to the material's higher dielectric constant and loss tangent. This lossy property is what makes these materials so useful for power integrity, even when there are insufficient decoupling capacitors. It dampens signal propagation beginning at a lower frequency cut-off and can reduce the signal level if the core voltage is low and/or the routing channel is long. Key Points • Embedded capacitance material can improve power integrity dramatically by reducing AC impedance and generally enhancing the per- formance of the product. • Decoupling capacitors are only effective up to a few hundred megahertz. Above that, only on-die capacitors or planar capacitance can reduce the impedance significantly due to their low inductance. • ECM comprises copper-clad laminates with very thin dielectric thickness and high dielec- tric constant and loss. ese materials can replace standard power and ground planes. • e high Dk increases capacitance and the high Df dampens electromagnetic energy through the relatively high loss of the material. • ese ultra-thin laminates replace conven- tional power and ground planes. • Placing the embedded capacitor layer closer to the surface of the stackup will reduce via inductance and make the capacitance mate- rial more effective. • e distinction between a lumped element and a distributed system involves the rela- tionship between the time delay of the system and the rise time of the signals. • If the plane is very small, then the driver per- ceives the VCC and GND structure as a dis- tributed object with significant delay. • If the phases add and the driver has a repeti- tive pulse (as clocks do), the reflected pulse may build significantly into a standing wave. Resources 1. Beyond Design by Barry Olney: "The 10 Fun- damental Rules of High-Speed PCB Design Part 3," "Plane Crazy, Part 2." 2. "Fabrication of Embedded Capacitance Printed Circuit Boards," by Joel S. Peiffer, 3M. 3. "Embedded Capacitance Material Properties," by Cadence PCB Solutions. Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board- level simulation. The company developed the iCD Design Integ- rity software incorporating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded at www.icd.com.au. To read past columns, click here.