Using Air-Gaps in Rigid-Flex PCBs

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Rush PCB Inc. makes rigid-flex printed circuit boards with the distinct advantage of being both rigid and flexible at the same time. The rigid portion allows mounting of components while the flexible part allows the printed circuit to bend and follow a contour.

With the advancement of technology, users are increasingly requiring rigid-flex PCBs with higher interconnection and smaller form factors. This is not only pushing up the requirement of higher circuit densities but also increasing the layer counts of rigid-flex boards. These changes are impacting the design of rigid-flex circuit boards with potential reliability issues both mechanically and electrically.

Rush PCB Inc. addresses these mechanical and electrical concerns of rigid-flex circuit boards by adding air-gaps in their design and construction methods. Typically, higher layer counts are not a problem in rigid circuit boards, as fabricators laminate them into one finished structure. However, this does not work for rigid-flex circuit board designs, as they create both mechanical and electrical reliability issues.

Mechanical Reliability Issues

Rigid-flex circuit boards must bend at their flexible parts. However, this capability to bend reduces with higher number of layers creating a thicker flex area. Mechanical reliability issues like added strain on copper circuitry increases as the flex area becomes thicker.

For instance, in a static application, a common minimum bend guideline for a 1- or 2-layer flex board is 10-times the flex thickness. As the flex thickness increases to 4 layers, the minimum bend guideline increases to 20- or 30-times. Exceeding the minimum bend guideline or capacity cracks the copper circuitry, creating intermittent or permanent open circuits. As a manufacturer cannot modify the physical properties of the material making up the rigid-flex boards, the only option for addressing the mechanical bend concerns is by changing the design and construction.

Electrical Reliability Issues

Materials necessary for construction of the rigid-flex printed circuit boards creates the electrical reliability issues. Although the rigid and pre-preg layers use materials based on FR4, epoxy- or acrylic-based adhesives laminate the layers in the flexible areas. However, as the construction of the board allows the flex layers to extend throughout the rigid areas, so do the laminated adhesives.

Compared to FR4 or the Polyimide materials, both the flex layers and the laminated adhesives have a much higher coefficient of thermal expansion. When the assembly of the rigid-flex circuitry takes a long time, or the temperature fluctuates in the field, the adhesive expands and contracts. This place vertical stress on the plating on via holes, resulting in crack formation in the copper plating causing intermittent or complete open circuits.

Another electrical reliability issue relates to the dielectric breakdown in multi-layered PCBs that must operate at high voltages.

Air-Gaps in Rigid-Flex Boards

To address the above reliability issues, the PCB industry in consultation with the IPC, has developed a design and construction method for rigid-flex boards, known as the air-gap construction.

In the air-gap construction, the fabricator retains the flex layers as multiple individual sets of not more than three layers per set. This contrasts with the regular construction where one lamination contains all the layers. Although IPC 2223 allows three layers, as a design may require controlled impedance lines in the flex area—the fabricator can configure one layer as a strip-line with two adjacent layers acting as shields. The flex layer may have a surface micro strip configuration that requires only two layers. Most common designs use only two sets of flex layers, but there can be more. Higher number of flex layer sets ultimately constrain the bend requirements of the flex area.

Electrical Reliability

Introducing an air gap in the construction addresses the via reliability concerns completely. The air-gap construction makes the via structure as reliable as a rigid printed circuit board. As the flex layers are now in independent pairs or sets, the FR4 based pre-pregs in the rigid areas are the only separations between the flex layers.

Not only does this construction improve the bond strength between layers, it also helps to eliminate all flexible adhesives in the rigid areas. This also solves the issue of high coefficient of expansion.

Introducing a minimum clearance distance between air gaps and ensuring no air gap is less than 10 mm helps to prevent any dielectric breakdown in the printed circuit board due to high voltage. While the air gap acts as a reinforced insulation between components on the PCB, it also prevents any type of hazardous voltage arcing. The gaps further help to prevent the accumulation of dirt or dust on the PCB surface, thereby maintaining the service life of the PCB.

Mechanical Reliability

An air-gap construction significantly increases the bend capability and reliability. The improvement comes from the combination of two elements. The first is the reduction of flex thickness for each individual flex pair as compared to a completely laminated construction—in effect, reducing the I-Beam effect.

The second is each flex pair can now bend on its own, depending on its own specific thickness. This significantly reduces the interference from other flex pairs. When bending, each individual flex pair can take their own natural radius.

At some point, however, a flex pair may touch another as the air-gap spacing between them reduces. When there is a severe U-shaped bend, the slight difference in bend radius between pairs may cause the flex pair on the outside of the bend to restrict the inner flex pair, and it may start to buckle. The designer must avoid this situation as buckling or a flex pair means it is exceeding the material capabilities and may consequently result in a cracked circuit.

On the other hand, an S-shaped bend does not have the above problem, as the two bends comprising the S shape help to cancel out the differences in bend radius between the two flex pairs.

Conclusion

Rush PCB Inc. offers higher layer count rigid-flex PCB designs with air-gap construction as the preferred solution for higher reliability boards. Our rigid-flex circuit boards achieve tighter bend requirements with greater reliability while eliminating concerns for via plating, mechanical bending, and electrical stability. We offer a wide variety of air-gap configurations for addressing several different design challenges.

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