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56 DESIGN007 MAGAZINE I JUNE 2021 be within a certain range and even the roughness of conductor sidewalls have shown to cause RF differences at 77 GHz. Copper plating thickness variation can also be very problematic; having tight control of this process is important for very high frequency applications. ere are several new applications using PCB technology above 77 GHz which require process- ing capabilities that exceed the current PCB tech- nology. A lot of research work is being done, for example, around 140 GHz for a particular appli- cation. e conductor width tolerance needs to be ±1 micron (0.04 mil), conductor thickness needs to be very thin (2 micron), and the thick- ness variation needs to be at ±0.5 micron. ere are some other specifications which are also extremely difficult, or maybe impossible for cur- rent PCB technology, such as very tight location tolerances between circuit features and micro- vias and layer-to-layer alignment. One of the 140 GHz applications I read about in a white paper on used a combina- Some 'Exotic' PCB Processes Could Become Commonplace Technology is advancing quickly and over the years the PCB industry has advanced its processing and capabilities extensively as well. However, due to technology demands, it is likely that some of the processes for PCB fab- rication that were previously considered exotic may soon be considered commonplace. Of course, that prediction depends on many issues. Due to the nature of new and evolving applica- tions, the PCB industry will likely need to make more advances in some of its processing. ose PCB fabricators who support 77 GHz automotive radar applications are probably very aware of the stringent requirements for these very high frequency applications. Generally, applica- tions at higher frequencies will have more sensi- tivity to several PCB processes and their normal variations. One simple example is circuit etch- ing quality, and for many mmWave applications, the conductor widths are commonly required to be held to tighter tolerances. e trapezoidal shape of a signal conductor is oen specified to Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION

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