Oct 30, 2018|Blog

Having a rigid flex PCB is both a luxury and a necessity for many situations. By using flexible PCBs, it allows the manufacturer to install the board in smaller, more confined areas. The added flexibility enables the manufacturer to maneuver it into the correct position. Without the flexibility, it would be complicated, almost impossible in some applications, to install a PCB.

Without the right knowledge to design a rigid flex PCB however, these boards could prove to be a challenge. That is why we have our rigid flex PCB design guidelines to break down the steps of creating a rigid flex PCB and what it all entails.

General Design Rules for Flexible PCBs

When you begin to design your rigid flex PCB, there are a few rules you want to adhere to as best as possible:

  • Try not to bend at the corners. When making a bend, keep the bend curved rather than sharp.
  • Gradually change the width of the traces. Abruptly changing the trace width could result in a weak spot.
  • Try to keep the flex layers to a minimum of two for increased mechanical flexibility and reduced costs.

Construction Examples of Flexible PCBs

The layer count of a flexible board varies between one and six. Anything over three layers, though, will be thicker and harder to bend.

A one-layer flex is commonly used for connector to connector flex or functional circuits. A two-layer flex is for surface microstrip controlled impedance, and a multilayered flex is for radio frequency or electromagnetic interference shielding.

Layout Guidelines

Use the following layout guidelines to ensure you successfully create your rigid flex PCB:

  • When it comes to the trace width and spacing within the flexible parts, keep them as wide as possible. Use the trace width calculator for accurate results.
  • The connections of the solder pads and tracks should be done in a rounded, or tear-drop-like style.
  • The annular rings and soldering surfaces should be as large as possible.
  • Using stiffeners will help achieve a final thickness of 0.2mm to 1mm.

Covering of Flexible PCBs

A coverlay is recommended for the flexible circuit board. Adding a coverlay helps protect and insulate the outer surface conductors. The coverlay also will restrain and hold the pads in place during soldering. Using a polyimide coverlay is quite common.

Pads and Vias on Flexible PCBs

The pads used for your rigid flex PCB should have anchoring spurs, also known as tie-downs. The anchoring spurs help to ensure there is no separation between the base material and copper during the assembly. The pads should also be filleted as to reduce the stress points, as well as the chance of breakage during flexing.

When calculating the pad size, it will depend on your board design and pad requirements specific to your application. Use the following formula to calculate the pad size required:

Hole Size (finished) + Required Tolerances + All Flex Tolerance = Pad Size Required

Plated vias on the PCB are metalized through holes that connect the conductive layers on a flexible circuit. There are also blind vias that connect the outer layer to the adjacent inner layer without going through the circuit. Lastly, you also can use buried vias, which connect all the internal layers but do not connect the out layers. The different via types can increase the space in the circuit to add component pads or more trace routing.

When it comes to using vias for the flex circuit, it is best to try and avoid them. However, if you must use them, place the vias in areas where there will be no bends.

Calculation of the Bending Radius

When it comes to bending the rigid flex PCB, you must be careful with your calculations to ensure accuracy and no breakage. The bend radius should be at a minimum .05” away from the plated through hole, and 10 times the material thickness. If the thickness follows the minimum measurements, you have a better chance of the circuit functioning as intended.

Design of Bending Areas

When you’re bending your board, it’s best to avoid bending at the corners. With copper traces, they work best when placed at a right angle to the flexible circuit bend. However, with curved traces, avoid placing at hard right angle traces and 45-degree hard corners. This could increase the stress on the copper traces during bending.

A sharp bend on the board can cause wrinkles in the cover coat. The sharp bend can also cause stretching, which could lead to tears in the cover material, and even broken conductors on the outside of the bend.

To increase the reliability of the bend, try and reduce the overall thickness of the flex area. Start with the base copper weight or reduce the dielectric thickness. You can also try to make the circuit more robust by balancing the weight of the conductors and the material thickness on either side of the bend axis. Do not stack the conductors on top of one another on each layer. Instead, stagger them on every layer.

Ground Plane

If the design allows it, try to have the ground areas crosshatched. This will help to reduce the weight, as well as increase circuit flexibility. As the openings in the ground plane, the size will be determined by the requirements for shielding or to control the impedance level. If the openings are too large, you could lose shielding effects depending on the frequency.

When rastering the continuous ground planes, do so with the copper balance in mind. The ground planes must be rastered in the flexible bending area.

Processing Guidelines for Flexible PCBs

You must dry your rigid flex PCB (roughly four hours at 120 degrees Celsius) to dry out any moisture absorbed by the polyimide. After drying, you can then go through the placement and soldering process within eight hours.

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