Issue link: https://iconnect007.uberflip.com/i/1408619
SEPTEMBER 2021 I DESIGN007 MAGAZINE 77 construction techniques used in flex circuits can shrink, expand, stretch, and compress in different ways than rigid PCB counterparts. Relaxing tolerances allows for these events. 10. Thermal expansion Most adhesive-based flex base materials are unfilled, which may result in unchecked thermal Z-axis expansion. is makes vias and other features prone to delamination and causes potential intermittent responses aer components have been soldered to the surface of the flex material. To combat this, include a more robust pad stack, as compared to the rigid areas, for components soldered on the flex material. Releasing to Manufacturing Once designs are completed and validated, the product model data must be sent to the fabricator and assembler with specific manu- facturing instructions. ese must be included at a minimum to receive a correct board: • Class of product • Materials to be used for construction • Base material and coverlayers • Metal foil type and thickness • Definition of holes, hole position and hole size • Stackup showing number of layers and cross-sectional view • Coverlayer or covercoat opening locations • Board outline with datum and dimensions • Marking requirements, materials and locations • Bend and flex locations and direction of bend • Stiffener locations and bonding requirements • Special process and/or finish requirements • Test point locations • Special testing requirements Figure 4: Pads that are larger than coverlay openings add strength. Figure 5: Conductor spacing should be 5 mm between the rigid PCB and bend. Also, traces should not change direction within the bend area.