A straight line is not always the shortest route in between 2 points in digital products: Thanks to rigid flex PCB architecture, circuits can be folded up onto themselves with 180º bends– laid over at minimal height– thus reducing product dimensions. In addition, if a product has moving areas with electronic devices embedded, rigid flex PCB building is the ticket.
Credit rating rigid flex PCB architecture for the presence of smartphones and various other pocket-sized electronic wonders. However, to understand the advantages of rigid flex PCB building (consisting of light-weight settings up) specific design constraints use specifically to the flex PCB layers in a stackup. Do not attempt your first (or second, or 3rd) rigid flex PCB design before you consult your prototype PCB manufacturer.
Utilizing Flex in Rigid Flex PCB Assembly
Unlike a traditional PCB stackup, foil building could not be used for flex layers. The flex layers in a rigid flex PCB assembly are built from unreinforced base substrates usually containing polyimide dielectric film, clad with rolled stiff copper.
Therefore, the clothed base material is first drilled, holes are uniquely layered, then the traces and pads are engraved. Bondply, a layer of polyimide film with sticky layer on both sides, isolates that conductor layer from the following, etc.
Flex PCB materials are flexible under all conditions, consisting of processing. Throughout the final lamination of the rigid flex stack, they are much less dimensionally secure compared to 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 stacks, preferably a minimum of 50 mils from the side, yet absolutely no less than 30 mils. This rule is the one most broken in rigid flex PCB designs.
Look for assistance to develop your stackup and design rules. Differing coefficients of thermal expansion amongst the flex base material, adhesives, prepreg, and rigid cores calls for a really careful balance of densities, especially for impedance-controlled designs. There can be many layers of flex in a rigid flex design, relying on the bend distance of the bow portion and whether it will continue to be stationary after assembly. Flex layer count must be limited in vibrant applications. Consult your PCB manufacturer. If greater than four flex layers are needed, bonding adhesive should be absent in the areas that are made to bend. The bend radius need to be no less than 12 times greater than the circuit density.
Properly Making use of Trace Routing
Trace routing in the ribbon location will be bent, not angled, to boost peel toughness. This suggestion is opposite the transmitting technique for rigid PCB boards.
To enhance ribbon flexibility, aircrafts ought to be cross-hatched; nevertheless, the cross-hatch complicates impedance control. Again, a cautious equilibrium is needed. In some applications, a large, solid strip under vital traces suffices. Traces on different layers must be surprised vertically, not put atop each other, to increase ribbon adaptability.
Annular rings need to be as huge as possible in flex-only areas to reduce the danger of peeling, and the change from the annular ring to the trace need to be teardrop-shaped for the very same factor. Adding tabs or anchors also aids to avoid peeling.
The stiffeners can be laminated flooring when the cover-coat is bound and are the preferred technique to stop splits. The very best technique is to avoid making use of sharp edges in a flex PCB design.
An extremely standard list for rigid flex PCB designs includes these transmitting factors to consider:
– Stagger flex traces up and down layer to layer
– Turns must be gradual
– Vias needs to be no closer to the side of the rigid board than 30 mils at the flex change
– Minimize flex PCB layers
Keep in mind, rigid flex PCB designs might be expensive to produce, however they could save prices throughout system assembly. Such design usually is the only way to squeeze the required product functions within the target package volume. It’s far better to get in touch with a PCB manufacturer during the drawing board.