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SEPTEMBER 2023 I DESIGN007 MAGAZINE 15 Feature Q&A With Kris Moyer Rigid-flex assembly brings its own set of issues, but designers can do quite a bit to make things easier on their downstream brethren. We asked IPC instructor Kris Moyer to give us the lowdown on DFA for rigid-flex circuitry. Kris, what are some of the unique challenges related to assembling rigid-flex circuits? There are several challenges to assembly of rigid- flex boards. First there is the alignment issue of the different sections of the boards. With a rigid board, the individual boards can be removed from the fab- rication panel and run through the SMT assembly line without much problem, as long as the board is of sufficient size. In this case, the relative position of all the individual parts on the board remains consis- tent. But with rigid-flex, after the board is removed from the fabrication panel, the individual boards can move and rotate relative to each other within the limits of the flex material. This makes the place- ment of parts difficult to control. To overcome this, the board must be held in a restrained condition throughout the assembly process. This will require either the fabrication of a custom fixture to hold the board restrained during assembly or the design of temporary removable restraining mechanisms (rails) into the design of the board. How does rigid-flex assembly differ from rigid or flex assembly? The rigid-flex assembly requires the board be held in a restrained state during assembly. Additionally, if the intent is to place parts on both the rigid sec- tions and the flex sections there is a need to provide mechanical support under the flex sections during the placement to prevent deflection of the flex mate- rial during placement. Furthermore, if placement is to occur on the flex sections the Z-axis height differ- ence between the rigid section surface and the flex section surface needs to be considered during the pick-and-place programing to ensure proper parts placement. What do designers need to know about placing components on rigid-flex, especially near bend areas? When placing parts on rigid-flex, the designer should use the same placement rules on the rigid sections as they would on a rigid-only board, with the excep- tion of the rigid-to-flex transition areas. The place- ment guidelines should follow what is defined in IPC-2221, IPC-2223, IPC-6013, and IPC-J-STD-001. No parts should be placed in the flex transition area and I recommend adding additional margin to the clearance around the flex transition area. What DFA tips would you give designers who are new to working with rigid-flex? My main tip for good DFA for rigid-flex is to follow the IPC design, fabrication, and assembly standards for rigid-flex designs—IPC-2221, IPC-2223, IPC- 6013, and IPC-J-STD-001. Is there anything else you'd like to add? While there are several unique challenges to rigid- flex design and assembly, the benefits of success- ful rigid-flex designs to the customer will outweigh the technical challenges and hurdles facing the designer in attempting to implement a rigid-flex design. DESIGN007 Rigid-flex DFA

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