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Design007-June2019

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74 DESIGN007 MAGAZINE I JUNE 2019 When a circuit pattern is adjacently aligned to the glass weave pattern within a laminate, there will be areas of very different Dk val- ues. However, at low frequency or low-speed digital, these Dk differences blend togeth- er and give an average Dk value. This is typi- cally not a problem for PCB electrical perfor- mance. However, at much higher frequencies, like mmWave and high-speed digital, the wave and the digital pulse can detect these Dk differ- ences, which can alter the expected electrical performance of the circuit. There are a few things to consider about the glass weave effect. One point of interest is when a laminate is using more than one layer of glass, the glass weave effect is some- what dampened due to an averaging effect of the glass weave layers with their random align- ment to each other. Another point of interest would be when a conductor is much larger than the geometry of the glass bundles, knuck- les, and open area. Then, the conductor will experience an averaging of these different Dk values from the glass weave effect. This is typi- cally not a problem for electrical performance. However, when the conductors are narrower than the glass bundles, knuckles, or open ar- eas, and assuming higher frequencies or digi- tal speeds, then the circuit can detect these Dk differences, and electrical performance of the circuit may be altered. In the last scenario, it is possible for one conductor to experience a dif- ferent Dk value than a neighboring conductor on the same circuit, as shown in the left draw- ing in Figure 1. There are two configurations where the glass weave effect can impact electrical per- formance; one is the locale trace environ- ment and the other is the periodic Dk varia- tion environment as shown in Figure 1. Rog- ers Corporation conducted a large study done last year to understand the glass weave effect, and it seems that the periodic Dk variation is probably more dominant than the local varia- tion. However, in an effort to get a well-defined study, we purposely made circuits with con- Figure 1: Top view of relationships between the glass weave pattern and circuit conductors, which can impact the Dk environment of the circuit.

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