Issue link: https://iconnect007.uberflip.com/i/1327102
90 DESIGN007 MAGAZINE I JANUARY 2021 materials may be in stock, so working with your fabricator as you are creating the stackup to understand what is in stock is very beneficial. If you are able to use materials in stock, that can greatly reduce your lead time and if you identify early in the process that there is a material that does need to be ordered, that material can be pre-ordered while the design is being completed. Dunn: I agree, working with your fabricator early in the design is always a good idea. Flex- ible circuit stackups also need to include any polyimide or FR-4 stiffeners that are being used. Can you explain the function of each? Martin: Sure. Stiffeners are used to create a support area where the flex will not bend and will support components and connectors. An unclad piece of FR-4 is a common solution. Another common type of stiffener is polyimide. This, too, can add support to component areas and is also often used to build up the termi- nation area with ZIF terminations, thickening that specific area without increasing the over- all thickness of the flex. Dunn: We have been speaking primarily about flexible circuits, what are the key things to keep in mind regarding materials and stackups for rigid-flex designs? Martin: First, as I mentioned earlier, check with your board manufacturer to see what material they stock. This will help when you are con- sidering your stackup. With rigid-flex it is very important to keep acrylic adhesive outside of any rigid area that will have plated through- holes. So, selecting adhesiveless flex materi- als, and using bikini cut with the coverlays is highly recommended. From there, keep in mind the form, fit and function of the circuit. How will it mount? Where will it flex? How often does it flex? What will it plug into? And so on. dielectric. Breaking that down, the laminate will consist of a layer of rolled-annealed copper bonded to the polyimide and this is done in two different ways. Adhesive-based cores are made up of a polyimide core, a layer of adhesive and a layer of copper which is bonded together using heat and pressure. The adhesive commonly used is either epoxy or acrylic. A typical adhesive thickness is one-half to 1 mil. Adhesiveless based cores have the copper directly attached to the polyimide core without the use of any adhesives. Adhe- siveless cores are typically recommended for applications that have a thickness constraint and also when working with rigid-flex construc- tions. Dunn: If the adhesive thickness is one-half to 1 mil, what thicknesses are available for the cop- per and polyimide? Martin: Although you can likely get nearly any combination of dielectric and copper thick- ness, the laminates will typically have some combination of half-ounce to 2-ounce copper, with anywhere from half-mil to 5-mil poly- imide. When you factor in options for adhe- siveless construction or adhesive-based, with or without flame retardant formulations, there is a wide variety to select from. Dunn: I imagine that can be overwhelming for those new to flex design and trying to select the best materials. Are there certain materials that are more common than others and likely to be in stock with a fabricator? Martin: This one is a little difficult to answer because it really does vary from fabricator to fabricator, but in general constructions using half-ounce and 1-ounce copper and 1 and 2 mils of adhesive are probably the most stocked materials. If a fabricator builds a lot of rigid- flex, they may be more likely to stock adhe- siveless materials. It is risky to assume that Jeff Martin