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36 DESIGN007 MAGAZINE I JANUARY 2021 Feature by Eric Bogatin UNIVERSITY OF COLORADO, BOULDER If interconnects were transparent, the stackup in your board would just be about how many layers you would need to route all the connec- tions between components. In fact, some test boards that are strictly testing continuity and isolation do exactly this. Each layer is a dense packing of narrow signal traces connecting an array of pogo pin pads on the top side to an array of connectors to an ATE on the bottom side. But rarely do we have the luxury of designing a board just for connectivity. When intercon- nects are not transparent, we must engineer them to reduce the noise they can generate. This is where design for signal integrity, power integrity and EMC—collectively high-speed digital engineering—are so important. Seven Tips for Stackup 1. An important element in reducing the noise contributions from the interconnects comes from the stackup of the board and the layer assignments. The very first step is to engi- neer all signal layers with at least one adjacent plane as the return path. This will reduce the crosstalk between the signal-return paths: the microstrip traces on the outer layers and strip- line traces on the inner layers. 2. The striplines can be either one signal layer between two planes or two signal lay- ers between two planes. With two signal lay- ers between two planes, there is the danger of excessive crosstalk if signals on adjacent layers are routed broadside to each other. 3. To avoid this problem, it is best to route the adjacent signal layers in dual stripline stackups orthogonally. One signal layer is routed in the x-direction, the other in the y-direction. 4. When interconnects must distribute 10 A of current or less, traces as wide as 200 mils can carry the 10 A of current in 1 oz copper with an acceptable temperature rise. But, with larger currents, like 20 A or more, it may be necessary to use wide planes to distribute the current from the power generators to the power consumers on a board. This is when some of Seven Tips for Your Next Stackup Design

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