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SEPTEMBER 2021 I DESIGN007 MAGAZINE 41 Always indicate where the thickness mea- surement is to be taken, for example: – Glass to glass at rail guides – Over gold contacts – Over solder mask Pure Build vs. Hybrid Build e term "pure build" refers to a multilayer PCB material construction that is composed of the same type of material throughout the stackup, such as a construction entirely of FR-4, or one entirely of PTFE or other high- frequency material. A hybrid multilayer PCB uses materials with significantly different critical properties than those associated with a traditional "pure" multilayer PCB. e rea- sons for using a hybrid construction in RF multilayer PCB designs are typically driven by cost, reliability, and/or electrical perfor- mance. Balanced Stackup Structure Regardless of the material used, maintaining a balanced construction (lay-up, stackup, etc.) in relation to the Z-axis median of the board will assure minimum bow and twist. is bal- ance includes the following: dielectric thick- ness of layer, copper thickness of layers and its distribution and location of circuit, and plane layers. Whenever possible, planes should be balanced around the Z-axis median line of the lay-up, and ideally located internal to the board. Understanding the cost drivers in PCB fab- rication and early engagement between the designer and the fabricator are crucial ele- ments that lead to cost-effective design suc- cess. Following your fabricator's DFM guide- lines is the first place to start. DESIGN007 Anaya Vardya is president and CEO of American Standard Circuits; co-author of The Printed Circuit Designer's Guide to… Fundamentals of RF/ Microwave PCBs and Flex and Rigid-Flex Fundamentals; and author of Thermal Management: A Fabricator's Perspective. Visit to download these and other free, educational titles. He also co-authored Fundamentals of Printed Circuit Board Technologies. Figure 3: A balanced construction is important with any PCB design, but is even more critical with a hybrid material set using mixed dielectrics.

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