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14 DESIGN007 MAGAZINE I SEPTEMBER 2023 much rigid-flex at all. What are some of the hurdles in rigid-flex assembly? When it comes to assembling flex only, you must have some kind of stiffener backer behind the material. If it's just a pure flex material going through the assembly equip- ment, when the pick-and-place machine is trying to put those ICs on the board, the flex- ible things move around with rigid-flex. Typi- cally, we'll design around the board some throwaway rigid board material with little tabs that will connect to the individual little rigid boards to hold them in place and restrain them mechanically in the X, Y, and Z axes. N o w, i f w e w a n t to tr y to do a s s e m b l y w i t h o u t t h e designed array, then the assembler has to have a fabricated mechani- cal pallet of some kind with some method of restraining and aligning all the boards relative to each other again. Assembly can definitely be more challenging. Shaughnessy: Is the flex material itself still primarily polyimide? Yes, polyimide is still the most common with acrylic-based adhesives. Acrylic is becoming the more popular adhesive for non-metallic layers, such as the adhesive on the coverlay backing material. en the coverlay strip that will be on the outer surface replaces solder mask, which tends to be more acrylic-based resins. Speaking of material, there is actually another technique out there: flexible rigid. Let's say you had your traces on top of a rigid board. ey would go in and mill out the mate- rial, basically making their original material so thin that you could do a one-time bend to install it in the product. If you need low-cost bending with flexible rigid, you're bending the board into shape and relying on the resin and the copper to act as your material to do a one- time bend like that. Shaughnessy: Your class focuses on rigid-flex stackup design for signal integrity. With the whole "mixed media" structure, signal integ- rity could get pretty interesting in rigid-flex. Yes, absolutely. ere are EE tradeoffs and EE techniques, as well as manufacturing, mechan- ical, and design tradeoffs involved in planning your rigid-flex stackup. One of the biggest chal- lenges with signal integrity in rigid-flex is the fact that 99% of the time, my transmission line structure changes when I go from the rigid to the flex section. On the rigid board, I'm usually on an internal layer, a stripline struc- ture. As soon as I transition out onto the flex structure, it's typically a single layer or one plane below it. Now we're microstrip, so now all my geom- etries need to change, and I have to set up the rules accordingly. Now I microstrip across the flex and then turn back into stripline when I get to the other side. Shaughnessy: What advice would you give a rigid board designer who's just getting into rigid-flex design? First, take my class. Don't fear rigid-flex. It has many benefits and value-adds to your design and your system that you might not see ini- tially. Keep your options open. Consider all of your options in order to get the optimal solu- tion for your design. Shaughnessy: Thanks, Kris. This was really good. ank you. DESIGN007 When it comes to assembling flex only, you must have some kind of stiffener backer behind the material.

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