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Meander Delay Compensation in High-Speed Digital Multilayer Packages
Keywords: simulation, package design, high speed
The routing of multi-trace digital signal buses in printed circuit boards often results in mismatches in the lengths of the lines. This results in mismatched propagation time, referred to as "timing skew" in a digital system. A common method that is used to compensate for this is to add meander sections of line to lengthen the signal path length. Many advanced circuit board design tools have the capability to perform this compensation automatically. Advanced Ball Grid Array (BGA) packages are fabricated using advanced multilayer laminate substrates or they are built up using multilayer wafer-scale processes. The design tools for these types of packages have evolved from printed circuit board tools and typically use the same methods and principles. It is very common in BGA packages for high-speed digital applications to use meander trace patterns to match the trace lengths of high speed bus interconnections either from the chip to the solder balls or between chips in a multi-chip package. However, electromagnetic simulation of these packages shows that despite the use of these techniques to match the physical length of the traces, electrical lengths often vary by as much as a factor of two. Examples of such packages will be presented and analyzed. The resulting timing skew is not a significant problem in most current applications, since the overall delay is small compared with the clock interval. But with emerging applications pushing well beyond 10Gb/s, timing skew in packages will be an important consideration. The reasons for the ineffectiveness of meander delay compensation are discussed, and are demonstrated by some simple simulations. In many applications, compensation of delay mismatch within a package is unnecessary and often compromises other important signal integrity properties. For very high-speed applications requiring delay compensation, effective design practices will be discussed.
Robert C. Frye, Consultant
RF Design Consulting LLC
Piscataway, NJ

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