New-Tech Europe | November 2016 | Digital edition

glass bundles used to weave the cloth are much larger than the width of the traces. Figure 2 is a photo of a common glass weave cloth (1080) with a 3.5 mil (89 micron) wire superimposed on it to show the difference in scale of traces and glass weaves. Notice that for part of the time the wire is on top of a glass bundle with a dielectric constant of about 6 and part of the time it is in between glass bundles in pure resin that has a dielectric constant of about 3. Signals will travel faster when they are in pure resin than when they are traveling over glass. This variation in velocity is what causes differences in travel time between the two sides of a differential pair. We have seen differences in travel time over 14 inch (35 cm) differential pairs on 1080 glass as high as 62 ps. That is 60% of a bit period at 10 Gb/s and two bit periods at 32 Gb/s. Many techniques have been proposed to minimize the effect of skew. The two most common methods in use are: • Routing the signals at an angle to the glass weave hoping that the irregularities are evened out between the two sides of the pair. • Using a glass weave style that has the glass evenly spread out to minimize variation in the glass weave over which signals travel. The first of these two methods has taken two forms. One is to route the signals in the PCB at an angle so that when the PCB is fabricated the traces run at an angle to the glass cloth weave. This has proved impractical in most products because the large number of connector and component pins impose an X-Y grid on routing. Alternatively, the PCB or backplane is routed on an X-Y orientation and the artwork is placed at an angle on the fabrication panel resulting in the traces running at an angle to the glass weave. This method wastes substantial amounts of board material

Figure 3. 1067 glass weave and a 4 mil trace

Figure 4. Loss vs. Frequency of materials available for high speed

minimized. However, when receive signals can be only 2 or 3% the amplitude of transmit signals this becomes mechanically very difficult to accomplish when routing signals on the same layer in the vicinity of the connectors.

The third of these (skew or difference in travel time in the two sides of a differential pair), is a result of the uneven distribution of the glass in the woven cloth and the resin used to bind the composite together. This unevenness is due to the fact that the

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