Issue link: https://iconnect007.uberflip.com/i/1457913
MARCH 2022 I DESIGN007 MAGAZINE 83 en we need to look at the environment where these devices will be deployed. Are they being deployed in the middle of the des- ert, out on rough seas, in a cockpit above 30,000 feet, or is it on its way to the Interna- tional Space Station? Both target application and environment influence the material selection. What type of substrate and what type of inks will be used? Is this going to be a poly substrate or a poly- ethylene substrate? Will silver particle-free ink or nanoparticle ink be used? Perhaps not even silver at all; we may have to use copper carbon instead. Material selection in turn drives process choices: how we actually manufacture the device and what type of process we use. Is it going to be a screen printer, an extrusion printer, an aerosol jet printer, or a combina- tion of one of those along with a gravure offset printer or an ink jet printer. ese all need to be considered before we even begin the pro- cess of designing the device. Once we begin design work, we have to account for FHE being inherently different from the standard rigid PCB process. is truly is an electromechanical system, and we need to design for it as such. We need to design in how it will be bent or flexed, and we need to alleviate the mechanical stresses wherever possible to increase reliability and robustness. For example, we can apply curves instead of sharp angles, or we may apply teardrops onto the traces- to-pad junctions to help allevi- ate the stress in those areas as we bend the device. We also need to investigate pro- cess dependent features. On a normal SMT for a rigid PCB, we might have just the standard 0402 or 0603 footprint. But for our additive process, if we're using sil- ver ink, we might be using a con- ductive epoxy instead of a tin sol- der. So we need to design the pad footprints differently. ey need to allow for adhesion to both the substrate and to the ink. We may need to add a non-conductive epoxy as well, to help secure the component to the device if it undergoes any mechanical stresses. Embedding components is an option as well with FHE. If we want to use printed passives, or if we have an ultra-thin die, we can embed them between layers. Finally, encapsulation also affects how we apply and place components on the board. Are we going to use local encapsulation of the com- ponents or encapsulation of the entire device? Where do we place those components? Where do we group those components? Do we place mini-groups across the device to allow for both more rigid areas and more flexible areas? ese are all questions that come with design- ing for FHE, and it's quite a few aspects to con- sider. How do we handle these multi-layered and interdependent considerations, choices, and processes in the most efficient and speedy manner possible? Design Process Vision We start with the materials and process data- base that feeds into the whole design process chain. Design process capabilities and con- straints are fed from the materials and pro- cesses database into a FHE PDK, which is similar to an IC design PDK. e FHE PDK Figure 2: The FHE design process.