A straight line is not constantly the shortest route between 2 factors in digital products: Thanks to rigid flex PCB style, circuits can be folded up onto themselves with 180º bends– laid over at minimum elevation– therefore diminishing item measurements. In addition, if a product has moving areas with electronics embedded, rigid flex PCB building is the ticket.
Credit rating rigid flex PCB architecture for the existence of smart devices and various other pocket-sized digital marvels. However, to realize the benefits of rigid flex PCB construction (including lightweight settings up) particular design restrictions apply particularly to the flex PCB layers in a stackup. Do not attempt your initial (or second, or 3rd) rigid flex PCB design prior to you consult your prototype PCB manufacturer.
Using Flex in Rigid Flex PCB Assembly
Unlike a traditional PCB stackup, foil building and construction could not be used for flex layers. The flex layers in a rigid flex PCB assembly are built from unreinforced base substrates normally containing polyimide dielectric film, outfitted with rolled stiff copper.
Consequently, the clothed base material is first pierced, openings are selectively plated, after that the traces and pads are engraved. Bondply, a layer of polyimide film with glue layer on both sides, isolates that conductor layer from the next, and so forth.
Flex PCB materials are flexible under all situations, including processing. During the last lamination of the rigid flex stack, they are much less dimensionally steady than the rigid core and prepreg materials that sandwich them. Vias should be farther from the side of the rigid area adjoining the flex bow than the minimum distance in rigid-only heaps, ideally at the very least 50 mils from the edge, but absolutely no less than 30 mils. This rule is the one most broken in rigid flex PCB designs.
Look for guidance to create your stackup and design guidelines. Differing coefficients of thermal expansion among the flex base material, adhesives, prepreg, and rigid cores needs an extremely careful equilibrium of thicknesses, specifically for impedance-controlled designs. There can be lots of layers of flex in a rigid flex design, depending upon the bend radius of the ribbon section and whether it will stay fixed after assembly. Flex layer count should be restricted in dynamic applications. Consult your PCB manufacturer. If more than four flex layers are required, bonding adhesive must be absent in the sections that are designed to bend. The bend radius will be no less than 12 times more than the circuit thickness.
Successfully Using Trace Routing
Trace routing in the ribbon location should be bent, not angled, to raise peel strength. This referral is other the transmitting technique for rigid PCB boards.
To increase ribbon versatility, airplanes ought to be cross-hatched; nevertheless, the cross-hatch makes complex impedance control. Once more, a cautious equilibrium is required. In some applications, a large, solid strip under critical traces suffices. Traces on various layers should be surprised vertically, not placed atop each other, to boost ribbon flexibility.
Annular rings should be as big as possible in flex-only regions to reduce the threat of peeling, and the change from the annular ring to the trace should be teardrop-shaped for the exact same factor. Including tabs or supports additionally helps to stop peeling.
The stiffeners can be laminated when the cover-coat is bonded and is the recommended approach to avoid tears. The most effective strategy is to avoid utilizing sharp corners in a flex PCB design.
An extremely basic checklist for rigid flex PCB designs consists of these routing factors to consider:
– Stagger flex traces up and down layer to layer
– Transforms should be progressive
– Vias needs to be no closer to the side of the rigid board compared to 30 mils at the flex shift
– Minimize flex PCB layers
Keep in mind, rigid flex PCB designs may be costly to produce, however they can conserve expenses during system assembly. Such design often is the only way to put the required product functions within the target package volume. It’s much better to get in touch with a rigid flex PCB manufacturer during the drawing board.