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34 DESIGN007 MAGAZINE I FEBRUARY 2022 defect that is normally acceptable at lower fre- quencies may not be acceptable at mmWave frequencies. e normal variation of conduc- tor trapezoidal effects, differential etching, slight undercut, layer-to-layer alignment, are all things that can normally happen in the PCB fabrication process and are relatively harm- less to the RF performance of the PCB when operating at lower frequencies. However, at mmWave frequencies these normal variations can be problematic. Another issue that is relatively normal and related to the high frequency circuit materi- als, is the glass reinforcement layers which are used to give mechanical stability to the mate- rial, among other things. Depending on the formulation of the laminate or prepreg and the glass style used, the supporting glass fabric can cause small disturbances in the RF propagation medium of the PCB. Again, at lower frequen- cies this may not be a concern, but at mmWave frequencies the very small wavelength can eas- ily detect and be affected by the small differ- ences due to the glass and/or overall formula- tion of the laminate or prepreg. e main issue with the glass is known as the glass-weave effect, fiber-weave effect, or woven-glass-weave effect. e glass styles that have been used for decades are typically open- weave glass and, as the name implies, there are openings between the glass bundles. e glass that has been typically used is E-glass and that type of glass has a Dk of about 6 and a Df near 0.004. However, the E-glass Df will signifi- cantly increase with an increase in frequency and the Dk will decrease some as well. e basic issue for the glass-weave effect is related to the opening size in the glass fabric as well as the wavelength of the propagating wave. For example, if the average opening size in the glass fabric is 16 mils x 16 mils and operat- ing at 3.6 GHz with a wavelength of 2.1 inches (or 2100 mils), the openings in the glass fabric are such a small percentage of the propagat- ing wavelength, that the openings do not have a significant effect on the wave performance. Although, it is well known that a fraction of a wavelength can be significant and if an anom- aly is half wavelength in size it can cause wave disturbance in the propagation of the wave. If the anomaly is one-quarter wavelength it can also cause a disturbance but usually less than the half wavelength anomaly. Typically, around one-eighth wavelength or less and the anomaly will not influence the propagating wave. Continuing with the example and using the same glass with open- ings of 16 mils x 16 mils and now considering the wavelength at 77 GHz, which is 97 mils, it is obvious that the 16-mil anomaly is a larger portion of the 97-mil wavelength and can have a disturbance on the propagating wave. At this frequency, the 1/8th wavelength size is 12 mils and the 16-mil opening in the glass will likely have some influence on the wave behavior. ese mmWave prepregs will oen use open-weave glass due to its lower cost than spread-glass fabric, a better performing alter- native glass fabric. Spread-glass fabric has very few or no openings between the glass bundles and will behave as a more uniform propagat- ing medium than one using open-weave glass. Additionally, it has been found that if the lami- nate is heavily loaded with ceramic fillers, the differences of the propagating medium between the openings and the glass bundles of the glass fabric are greatly minimized. Due to concerns of the glass-weave effect, many of the legacy mmWave PCB designs used Having a very small etching defect that is normally acceptable at lower frequencies may not be acceptable at mmWave frequencies.

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