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In a typical interconnect, there lie multiple places where capacitance plays a factor in the signal integrity. This includes the driver and receiver output/input capacitance, as well as the packages, vias, and the transmission lines. Failing to optimize these parameters can often lead to unwanted reflections, excessive radiated and or conducted emissions, and sometimes failure of components and systems.
Reflections can occur anytime there is an impedance mismatch on the line. Sources of mismatches are plentiful and include trace width changes, vias, stubs, reference plane changes, and even the so-called fiber weave effect. In this case, a trace can encounter a different dielectric constant depending on whether it is routed over glass or the epoxy resin in the dielectric material.
In this investigation, it is the capacitive contribution of the different components that are of interest, and how they affect the characteristic impedance the driver sees.
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Chang Fei Yee, Keysight Technologies
In electronic systems, signal transmission exists in a closed-loop form. The forward current propagates from transmitter to receiver through the signal trace. Meanwhile, the return current travels backward from receiver to transmitter through the power or ground plane directly underneath the signal trace that serves as the reference or return path. The path of forward current and return current forms a loop inductance. It is important to route the high-speed signal on a continuous reference plane so that the return current can propagate on the desired path beneath the signal trace.
Andy Shaughnessy, PCBDesign007
I had the opportunity to talk with our contributor Doug Brooks recently. He has been doing some research on temperature effects on PCB traces over the last few years, and I wanted to check the status of his latest thermal efforts. He discussed his work with Dr. Johannes Adam, why temperature charts based on a trace in isolation are inaccurate, and how the industry remained so wrong about PCB temperatures for so long.
Craig Armenti, Mentor
A key aspect of HDI technology is the use of microvias. For reference, the IPC HDI Design Committee has identified microvias as any hole equal to or less than 150 microns. Multiple types of HDI stack-ups associated with blind and buried microvias can be used to meet the density and cost requirements for today’s products. Design teams should develop stack-ups in conjunction with the fabricator to minimize cost and meet signal integrity requirements.