Issue link: https://iconnect007.uberflip.com/i/1507356
18 DESIGN007 MAGAZINE I SEPTEMBER 2023 sions or "degrees of freedom." A rigid stackup, for example, can easily be viewed in 2D, and that world is relatively easy for most electrical hardware engineers to understand and man- age. You have impedance, frequency, and loss and then physical and electrical parameters that drive them. When flex substacks come into the picture, the fact that the flex portion needs to bend turns it into a 3D mechanical engineering problem that takes some time to learn. is is where you really need to lean on a flex fabricator's expertise. Toward that end, I decided to poll a few "flexperts," including Mark Finstad from Flexible Circuit Technolo- gies and Nick Koop from TTM. Now I'll weave their thoughts in with some of my own. Mark emphasized that rigid-flex stackups have to blend both rigid and flexible base mate- rials into the final product, and that these mate- rials all stretch and shrink at different rates due to processes like etching and lamination. Addi- tionally, flex materials can stretch and shrink just due to temperature and humidity changes. To further complicate things, these materials typically don't move at the same rate in the X and Y axes. Most manufacturers have a good history with many materials, so they know how much to scale their artwork and programs to account for these dimensional instabilities. Also, laser direct imaging and smart laser and mechanical drills help with a lot of these issues, but the need to use those processing tools does come at a cost. What do rigid board PCB designers need to understand about rigid-flex design and stackups? I don't know if this will catch on, but I've started referring to rigid-flex design as "design- ing in 3D" or "3D design." I doubt that some- one who's been designing rigid-flex boards (in 3D) for 20 years would learn much from my brief commentary here, so I'll throw out a challenge to the rest of the designers who may benefit from this. Along with HDI, rigid-flex PCBs have been trending up in recent years. Younger engineers would do well to start adding rigid-flex design skills to their repertoire in order to open up career opportunities. Maybe start with the rigid-flex chapter in my book on stackups and branch out from there. For signal integrity and EMI control, we like to have nearby reference planes for the current return path. But that Figure 1: Loose-leaf designs are most flexible for 90-degree and 180-degree bends. They are the simplest to manufacture but not recommended for impedance control. Bonded is less flexible, thus requiring a more generous bend radius.