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92 DESIGN007 MAGAZINE I MAY 2021 For flex circuit designs with high layer counts utilizing high density outer layers, the extra area utilized for separate pads and SMT components severely limits the available space for trace fan out. By designing via-in-pad in flex and rigid-flex circuits, it can significantly increase density, utilizing vias as mounting pads. e copper- or silver-filled flat vias allow for soldering components directly on via holes. Utilizing via-in-pad will release extra surface area for routing of traces. However, like any new technology, there are a few points to be aware of when using this type of construction with flex. When manufacturers fill rigid PCB vias for SMT pads, they are typically filled with a conductive epoxy, copper plated, and then planarized flat. e final sanding leaves a very smooth pad surface allowing the assembler to process the circuit board normally through the pick-and-place cycle. e via-in-pad technology has progressed so that via fill is very common on rigid PCBs. The Designing Via-in-Pad for Higher-Density Flexible Circuits Talking Flex by John Talbot, TRAMONTO CIRCUITS

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