Flex and Rigid-Flex Boards Are in Flux

Most real devices bend somewhere.

An earbud folds its board around the battery. A smartwatch wraps its sensors around the case. A camera module sends its image data down a thin flex tail to the main board.

Today the Flux agent designs those boards too. Flux now designs pure-flex and rigid-flex boards, from the stackup to the manufacturing files.

A camera for a pair of smart glasses

Here is a scenario. Dana is a hardware engineer on a smart-glasses team. The camera must sit in the front of the frame, next to the hinge. Its image data must go 40 mm along the temple arm to the main board. The sensor board can be 10 × 10 mm at most, and the lens can be 6 mm high at most.

Dana opens a Flux project and gives the requirement to the agent:

Design a 5 MP camera module for a pair of smart glasses. Use a 4-layer rigid board for the image sensor and a 2-layer flex tail, 40 mm long, to a board-to-board connector on the main board. Add a stiffener under the connector.

The agent first asks which camera input the main board has and how much space the module gets. Dana answers: a 2-lane MIPI CSI-2 input on the application processor, and the size limits. The agent writes the requirements into a project specification. It selects a 5 MP image sensor and a board-to-board connector, captures the circuit, and reviews the circuit and fixes what the review finds.

Then the agent sets up the board. It makes a 4-layer rigid area stackup for the 10 × 10 mm sensor board and a 2-layer flex area stackup for the 40 mm tail. The two flex copper layers are the inner layers of the rigid board, so they continue from the sensor board into the tail. The agent adds a 0.4 mm FR-4 stiffener under the connector.

In about one hour, Dana has a specification, a selected sensor and connector, a reviewed circuit, and the area stackups for the rigid board and the flex tail, with a stiffener under the connector.

Next, Dana asks the agent to fold the tail 90° at the hinge. The 3D view shows the folded tail in the frame of the glasses.

What Flux does on a flex or rigid-flex board

You give the agent the requirement. The agent sets up the stackups, draws the areas, places, routes, checks and exports. You can also do each step yourself in the editor.

One board. A stackup for each area.

A rigid-flex board does not have one stackup. It has a rigid stackup in some areas and a thin flex stackup in other areas.

In Flux, you draw Stackup Areas on the board. Each area gets a named stackup, for example "Rigid 4 layer" for the sensor board and "Flex 2 layer" for the tail. All of these stackups come from one layer list. A copper layer keeps its identity when it goes from a rigid area into a flex area. A trace on the second copper layer of the rigid board is still on that layer in the flex.

The flex materials are in the stackup editor: polyimide, polyimide with adhesive, bondply, coverlay, rolled annealed and electro-deposited copper, stiffeners and shield film. Start from the Flex 2 Layer or Rigid-Flex 4 Layer template, or from a JLCPCB Flex template with 1, 2 or 4 layers.

The agent knows which area it works in

Routing, copper pours, vias, placement, design checks, impedance and simulation all follow the stackup of each area.

Route a differential pair from the rigid board into the flex tail. Impedance, crosstalk and delay for that pair use the flex dielectric in the tail and the rigid core on the board. Auto-routing stays inside the limits of each area. Copper pours in flex areas are hatched, so the tail stays flexible. Simulation uses the real layers and thicknesses of each area.

The design checks know flex. Flux checks the bend radius, coverlay openings, outer copper without coverlay, holes, vias and parts in flex areas without a stiffener, pads and holes too near a rigid-to-flex boundary, trace corners and width changes too near that boundary, traces that cross a rigid-to-flex boundary at an angle other than 90°, sharp trace corners in the flex, missing teardrops, traces stacked on top of each other on adjacent layers, pads too near a stiffener edge, solid copper pours in the flex, and copper too near the board edge in the flex.

Hatched copper where the board bends

Solid copper is stiff. When a flex area bends, a solid copper pour can crack. A hatched pour is a grid of thin copper lines with open spaces between them. It bends without cracking.

Flux hatches the copper pours in flex areas and keeps the pours in rigid areas solid. The hatch lines are 0.25 mm wide with a 0.5 mm gap, at 45°. A 0.5 mm solid border runs along the edge of the pour, and solid rings stay around pads and vias of the same net.

See it in 3D. Send it to the mechanical team.

The 3D view shows each area at its own thickness. Download the board in 3D and give it to the team that designs the frame and the housing.

Bend it, twist it, fit it

You or the agent add a Flex Joint to a flex Stackup Area. On a pure-flex board, the Flex Joint goes directly on the board. A Flex Joint has two optional rules, Bend and Twist. Set one or both.

• Bend, for example 90deg 1mm, sets the bend angle and the bend radius. The longhand rules are Bend Angle and Bend Radius.
• Twist, for example 90deg 20mm, sets the twist angle and the twist length. The longhand rules are Twist Angle and Twist Length.
• Position, Rotation and Repeated Bending complete the Flex Joint.

Flux shows the bend zone on the board and the folded board in the 3D view. The 3D downloads are folded too. Put the folded board in the enclosure in the 3D view and check that it fits.

The design checks know bends. They compare the bend radius with the minimum for the flex stackup, which is larger for repeated bending. They check the minimum twist length for the width of the tail. They report parts, pads, vias and holes in a bend or twist zone, traces that do not cross a bend at 90°, and zones that overlap.

Ask the agent:

Fold the camera tail 90° at the hinge with a 1 mm bend radius, then check that the board fits in the frame.

Twist the tail 90° over 20 mm, so that the connector faces the main board.

Ready for the fab

The manufacturing files carry the Stackup Areas. Gerber files include the coverlay and stiffener layers and a fabrication drawing that shows each area with its stackup. IPC-2581 and ODB++ exports include the Stackup Areas too.

In your Flux project, get quotes for flex boards from JLCPCB and PCBWay, and quotes for rigid-flex boards from PCBWay. Order flex boards from JLCPCB. PCBWay gives the rigid-flex price after it reviews the files.

Try it

Open a project in Flux and ask the agent for a flex or rigid-flex board. Earbuds, a wearable, a camera module, a sensor that wraps around a pipe.

Rigid where the parts sit. Flex where the board must move. One project, one agent.