Issue link: https://iconnect007.uberflip.com/i/1507356
54 DESIGN007 MAGAZINE I SEPTEMBER 2023 should be built in specialized flex shops. For a PCB designer, it may not seem like a big deal either way, but understanding the difference will help in choosing the correct supplier and result in better quality and lead times. When it comes to rigid-flex, both flexible and rigid PCB manufacturers bring their own strengths and weaknesses, so I will analyze the different aspects of the processes, how the two types are differentiated, and how designers can adapt to these shops. Rigid PCB Shops Printed circuit board shops typically provide a multilayer FR-4 solution. About 95% of their circuits are rectangular-shaped substrates that are not required to bend. e product is rela- tively simple to panelize and, in many cases, the material stacks are off the shelf. PCB shops are very good at complex stacks, high-density interconnect (HDI) for blind and buried vias, and multiple lamination cycles. ey have a stable, repeatable process, and PCB designers have continued using the same design rules for years. Still, as rigid boards get more complex, the available real estate shrinks while the num- ber of components has increased dramatically. Flex Shops Flex shops differ from rigid board shops because they provide a largely custom mechan- ical solution that bends, but does not break. Once you add in circuit electrical character- istics, component placement and support, obscure outlines, copper grain direction, and coverlays, you realize that rigid-flex is some- thing completely new. e design and manu- facturing rules for a flex board are different than a rigid PCB. Flex manufacturing is more manually inten- sive than rigid board manufacturing, and mate- rials are differentiated for every job. Materials Suppliers for rigid PCBs and flex are very different. That includes base materials and adhesives, and processing and handling the material. A PCB shop conventionally uses FR-4 and prepregs along with electrodepos- ited (ED) copper. If they are supplying HDI boards, the material choices can include pho- tosensitive dielectrics, resins, and dry films. Flex uses pr imar ily polyimide dielectr ic materials, adhesives, and rolled annealed (RA) copper foils. Very thin materials are used for flexibility. Copper Plating A PCB shop uses panel or pattern plating, a process that has always existed with PCBs. Copper is plated on all aspects of the panels and vias, then etched away, leaving a circuit design and plated holes. A flex shop uses pad or but- ton plating in a process that puts copper only on the pads and in the plated through-holes. If you plate up—add copper to—the traces on a section of flex that is designed to bend, there's a high potential for cracked traces. If your rigid-flex has a tight bend radius, make sure your vendor is using button plating in the flex portion of your rigid-flex. Design Considerations PCB and flex circuits require their own design considerations. For example, there are many rules regarding vias. Figure 1 shows what happens when a via is placed in a bend area of the flex. Figure 1: This is what happens when vias are placed in a bend area of flex.