PCB007 Magazine

PCB-Nov2016

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18 The PCB Magazine • November 2016 by Happy Holden Introduction In today's fast-paced global, economic envi- ronment—which requires constant innovation, upskilling, and performance improvements— there is a need for increasing density. The clas- sic way to increase density is to reduce the trace and spacing. But in many high-speed board ap- plications, the copper and dielectric losses from smaller traces, or the increased crosstalk from reduced spacings, does not permit this. Four old ideas used in board design and fab- rication can offer some insight into how rout- ing density can be increased. These four ideas are: • Landless vias • Swing vias for BGA breakout • Solid paste vias for via-under-pads • Power mesh for increased layer density Getting Over the Density Wall When it comes to getting higher routing density, you only have five degrees of freedom: • Smaller traces • Traces closer together (spaces) • Smaller vias (down to microvias) • Smaller annular ring for the vias • Higher layout efficiency when routing (definition in column #10-DFM/A, June 29-2016 Newsletter) I'm talking about routing density on a single layer; more signal layers will result in more total routing distance on a board. The equation for routing density is: Equation 1 Where: N = number of traces in the channel G = routing channel dimension Dv = via diameter (FHS) Da = via's annular ring Cs = conductor spacing Cw = conductor width Innovative Use of Vias for Density Improvements FEATURE

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