Impact of PCB laminates on high-speed data rates (Part 2)

As a brief overview, differential signalling has become the principal way in which electronic components are connected. For any and all traffic on the Internet, differential signalling is the way in which data is moved from one place to another over the copper interconnects.

From the PCB laminate perspective, very small variations in the laminates that comprise PCBs can destroy entire data paths if care is not taken relative to how these paths are designed and fabricated. The key contributors to skew come from several sources including differences in the lengths of two sides of a differential pair as well as differences in the velocity of the two sides of a differential pair. With modern layout tools, the mechanical length of the two sides of a differential pair can be easily matched to less than one picosecond, so this should not be a significant source of skew.

A subtle, potential contributor to skew is a result of the glass weaves used in PCB laminates. Primarily, weave-induced skew is a result of the ways in which glass reinforcement is woven. For instance, if one member of a differential pair travels more on a glass bundle and the other travels more on resin between glass bundles, the result is a different velocity for each side of the pair, resulting in skew. And, this skew is often of a fairly high number of picoseconds. While there is no standard amount of skew that a differential link can tolerate, the generally accepted amount is one quarter UI (unit interval) before the signal quality is degraded to the point that data is lost. A UI is the length of one data bit. Common date rates and their unit intervals are listed in table 1.

Table 1: Common data rates and their acceptable unit intervals.

In Part 1 of this series, I showed the differences in skew from identical test PCBs with two different glass weave styles (1080 and 3313). Also in Part 1, I described the various numerical designations for glass cloth styles, and I noted that while there are standards for how many threads per inch and the size of the threads for each glass style, there are no standards for how the glass bundles are formed. And, it's the formation of the glass bundles that has the greatest effect on the quality of a high-speed differential pair and whether or not skew is induced.

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