Lightning Speed Laminates: Practical Evaluations of Glass Weave Effect

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The impact of glass weave effect on PCB electrical performance is a topic that has been investigated for many years. From my perspective, the initial investigators for the glass weave effect were mostly from the high-speed digital industry. However, as more RF applications are using millimeter-wave (mmWave) frequencies, the RF industry is now pursuing more studies for the potential of glass weave to impact these applications.

The basic issue for the glass weave effect is that many laminates have a layer, or layers, of woven fiberglass used to improve mechanical properties. The raw glass used to make the woven glass reinforcement layers is typically an E-glass, which has an approximate dielectric constant (Dk) of 6. As an example, for many laminates that have an overall Dk of 3.5, the resin system must have a much lower Dk than the glass, so the overall Dk of the laminate will be roughly 3.5.

For glass-reinforced laminates, and when looking down through the laminate, in isolated areas, the Dk of the laminate will be different in an area where there are glass bundles from the woven glass layer compared to areas where there are openings in the woven glass layer. In the open areas of the glass weave pattern, the Dk will be that of the resin system, and will probably have a Dk value around 2.1 to 3.0, depending on the laminate construction. In an area of the laminate where there is a combination of resin and one layer of glass bundles, the Dk will be somewhere between 2.1 and 6. In another area where the glass bundles intersect to form the grid of glass reinforcement, there will be two layers of glass as a knuckle and also the resin system. In these knuckle areas, the Dk will be the highest for the laminate. This means that within a small area of the laminate, there can be three areas of distinctly different Dk values.

When a circuit pattern is adjacently aligned to the glass weave pattern within a laminate, there will be areas of very different Dk values. However, at low frequency or low-speed digital, these Dk differences blend together and give an average Dk value. This is typically not a problem for PCB electrical performance. However, at much higher frequencies, like mmWave and high-speed digital, the wave and the digital pulse can detect these Dk differences, which can alter the expected electrical performance of the circuit.

There are a few things to consider about glass weave effect. One point of interest is when a laminate is using more than one layer of glass, the glass weave effect is somewhat dampened due to an averaging effect of the glass weave layers with their random alignment to each other. Another point of interest would be when a conductor is much larger than the geometry of the glass bundles, knuckles, and open area. Then, the conductor will experience an averaging of these different Dk values from the glass weave effect. This is typically not a problem for electrical performance.

To read this entire column, which appeared in the June 2019 issue of Design007 Magazine, click here.



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