Why should you use air-gaps in Rigid-Flex PCBs?

High-density circuits are supported by the rigid part and the connecting cables are provided by the flexible areas of the PCB. Technological advancement has established a growing demand for high-density circuits increasing the count of PCB layers. But these complexities have an impact on both the mechanical and electrical reliability of the Rigid-flex PCBs.

The bending capacity of a Rigid-flex PCB reduces with the increase in the number of flex layers. This affects the mechanical reliability of the PCB. The standard minimum bend radius for a 1-2 flex layers PCB is around 10 times the flex thickness. Similarly, for a 3-4 flex layers PCB the bend radius can be 30 times the flex thickness. The PCB may break due to stress on the copper tracks if the bend radius exceeds the set value. This will call for a board respin as the material specifications cannot be changed once the PCB fabrication is completed.

Most of the reliability issues for Rigid-Flex PCBs are caused due to broken copper tracks, split vias, or lifted tracks. Proper routing can help in reducing copper track damages in the circuit board.

• It is recommended to route the traces perpendicular to the bending direction to avoid stress on the traces during bending.
• Using rounded bends for traces and avoiding sharp angles may help in reducing the stress points on the rigid-flex PCBs.
• Staggered traces and hatched ground areas increase the board flexibility.
• Placing vias and plated through holes away from the flex areas is suggested to avoid cracks while frequent bending of the flex PCBs.
• Increasing the surface area of the traces on flex circuits provides more space for adhesion of coverlay and reduces trace lifting.
• In the case of vias and through holes, using anchors or increasing the annular rings can avoid track peeling.

Following these design guidelines can reduce the reliability issues of the rigid-flex PCB. But with the increased number of layers, the reliability concerns also add up.

In a Rigid-Flex PCB, the rigid area is made using the FR4 material while the flex area is a lamination made from epoxy or acrylic-based adhesives. The coefficient of thermal expansion (CTE) is different for FR4 and flex laminates. When such a rigid-flex PCB is exposed to a high temperature, the amount of expansion or contraction varies in rigid and flex sections causing damages in via plating. This results in faults like trace lifting and open circuits in the PCB.

In collaboration with the IPC association, PCB manufacturers are using a design technique of including an air-gap in the PCB stack-up. This is to isolate the flex layers as independent sets of a maximum of three layers. This air-gap method eliminates the flex adhesives in the rigid section and also improves the flexibility of the flex section. It reduces the via hole damages and increases the reliability of assembled components.

Airgaps enhance the bond strength between layers. However, maintaining sufficient clearance between two airgaps with each airgap not less than 10mm is recommended. This method averts di-electric breakdown by high voltages in PCBs. Even the arc voltage hazards are prevented by the insulation provided in the air gap construction method.

Flexibility and reliability can be significantly increased using air gap constructions in Rigid-Flex PCBs as follows:

• As the flex layers are grouped in sets of not more than three layers, the effective flex thickness is reduced as compared to the laminate thickness with all layers together.
• Each flex set can bend at its own bend radius depending on the flex thickness. Air gap construction eliminates interference from other flex groups.

If there is insufficient spacing between the air gaps, then with a U-shaped bend the external flex set can restrict the bending of the inner flex set, causing it to buckle. Avoiding such bends can help in restricting the damages of cracked circuits. On a contrary, an S-shaped bend will not cause buckling as the two associated bends help in nullifying the difference in bend radiuses between the two flex sets.

There are certain methods to detect the faults in vias or plated through holes (PTH) of an air gap construction in rigid-flex PCBs. One such technique is the Interconnect stress test. In this method, vias and PTHs are exposed to high temperatures up to 150°C, until the PCB test coupon meets the reliability standards. This test is cost-effective and can detect faults in the initial stages of the PCB fabrication process. It can analyze the circuit board and estimate the PCB durability also.

Following IPC guidelines helps the designer to understand the effect of reliability issues in Rigid-flex PCBs. IPC 2223 standard provides various techniques to improve the design methodologies in Rigid-flex PCBs to meet the demands of sophisticated electronic applications. The construction of the air gap is one such recommendation from the IPC standards. The designer has to explain the air gap construction requirements in the PCB stack-up and layout drawing notes. This method helps to achieve tighter bend capacity and reduces reliability issues.

Rigid-Flex PCBs can be bent and folded in desired shape and size. They are excellent in shock absorption and can operate reliably in harsh work environments. These features make them the best choice in a range of applications like consumer electronics, automotive, medical apparatus, and also in space equipment. But the complex design increases the cost of rigid-flex PCBs. Also, the expertise required to handle the design and fabrication of these PCBs adds to the total cost.

Building a PCB product with high reliability is possible by adhering to the IPC standards. Air gap construction is one such method to enhance circuit board reliability. To build high-performance rigid-flex PCBs, you have to communicate with your contract manufacturer (CM) from the early design stage itself. An experienced CM can work on techniques like the air-gap construction method and can assist you in building robust rigid-flex PCBs successfully.