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36 DESIGN007 MAGAZINE I JULY 2021 Lee Ritchey: It is not at all difficult to control crosstalk. Methods for doing this have been in place for four decades, at least. Eric Bogatin: Yes, crosstalk is a major problem in many designs. It is manageable when there is a continuous return plane adjacent to sig- nals. It is hardest to control in the via field of a BGA breakout field. Using a 3D full wave solver is essential in evaluating different design options. e general guideline is using return vias when possible, and back-drilling via stubs. Heidi Barnes: Not a lot of information in your question, but yes, crosstalk only gets worse as edge speeds go up and power delivery volt- age margins decrease with low voltage power delivery. EM simulation is oen the best way to see what is going on with the PCB parasit- ics. One possible solution would be to reduce the dielectric thickness between the power and ground layers and check the impedance of the power rails. Cherie Litson: Crosstalk can happen for many reasons. It's difficult to say what exactly is cre- ating the issue without looking at your layout. Hire any one of us and we can help pinpoint it with you. DESIGN007 Carl Schattke: As you start to use less conven- tional technology, costs tend to go up. at's why it's less conventional. Aer all, if it was easy everyone would be doing it. Lee Ritchey: To connect the two stacks in a build-up design such as that described in this question, it is necessary to laminate them together first. Electrical connections will be made between them using the conventional through-hole drilled and plated holes. DESIGN007 With a 1+N+N+1 stackup, how should we connect the two stacks? We're having an office disagreement. Is ALIVH our only option? Are there any low-cost fab processes that will work here? Q

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