Should You Use Flex or Rigid PCBs?

Although rigid PCBs or printed circuit boards are still the norm in most electronic equipment, flexible PCBs are swiftly making serious inroads in the industry. This is mostly because flexible or flex boards offer several mechanical advantages over their rigid counterparts, one of them being a replacement for complex wire harnesses. Naturally, there is confusion among users about whether to use flex or rigid boards for an application. Rush PCB Inc. helps in making a suitable choice.

Until recently, the industry was using rigid circuit boards as the only possible method available for assembling electronic equipment. Whether an industrial equipment or an entertainment device, the shape was primarily governed by an intractable rigid circuit board. However, the trend is now in reverse; with the development and maturity of the technology, it is mainly the enclosure that is shaping the electronics inside. Although this is leading to additional manufacturing challenges, flex printed circuits or flexible PCBs are the choices of most product designers as they offer nearly unlimited possibilities in comparison to the constraints of rigid boards.

Comparing Rigid, Flex, and Rigid-Flex Boards

∙ Rigid boards are inexpensive as they are readily available from most manufacturers. They perform well in designs where flexibility or form is not a factor.

∙ Flexible boards offer better convenience, such as where space is a limitation, as a replacement for complex wire harnesses, and for designs that require compactness.

∙ Rigid-flex boards are a union between discrete flex and rigid elements, offering the best features of both technologies, especially as designers can place them exactly where necessary. However, because of high manufacturing complexity and a flex-rigid interface, these boards are susceptible to stress and failure.

Popularity of Flexible Circuit Boards

Contrary to popular belief, rigid boards are also flexible, only their extent of flexibility is low. So far, workplace convenience has defined the rigidity of printed circuits rather than being a demand from designers. For instance, early etched circuits were often made from simple insulating materials like plywood and Bakelite and were mostly single-sided.

Most research and manufacturing in electronics were intertwined, with developments in one typically forwarding the capabilities of the other. With an increase in performance demands, research in PCBs brought new developments and introduced better materials that suited mass production. As most electronic components were packaged for through-hole soldering, a rigid substrate was more of a necessity.

With the advent of SMT or Surface Mount Technology, miniaturization is the dominant factor in design at both the component and device level. Component manufacturers now make electronic components with discrete packages many magnitudes smaller than their earlier generations. These tiny components not only consume significantly less power but also allow greater performance and functionality with a smaller footprint.

This has led to a demand for smaller and more portable devices from consumers, with these devices also offering longer-lasting uptimes for a battery charge. Although miniaturization alone does not mean foregoing rigid boards, the compounding requirements of ergonomics and enclosure recommendations are making rigidity a liability. The comparison between a desktop computer and a modern laptop may be an example here. The former almost always uses a standalone monitor, a tower design, and a static plug, while the latter may have a foldable design that includes a screen whose design is a more significant challenge.

Flexible PCB Design Advantage

Although designers can accommodate various device applications and enclosure designs using flexible printed circuit boards, they are considerably more complex to manufacture. If necessary, such flex boards can combine with rigid PCBs in a construction, melding the physical attributes of both. Designers may have to make certain changes in their design when adapting flexible circuits:

3-D Layout

For a rigid board, its layout can exist only in two dimensions. On the other hand, the ability of a flex PCB to bend and fold renders the existence of its layout to three dimensions. The same attributes of the flex board function simplify its stack-up design and assembly, eliminating unnecessary components and layers. To support bending, the designer must make the discrete portion intended to intermittently bend or remain bent somewhat longer than its targeted dimension. They must maintain the flex circuits’ general tolerance requirements on the looser side.

Improving Pad Strength

In comparison with rigid circuits, flex boards tend to have a poorer adhesion between the substrate and conductive materials. Designers must compensate for this by adding tabs in the pad stack to increase the surface for adhesion, thereby creating a more rugged connection. They should also avoid acute angles when designing traces, as these act as stress concentrators. Adding transitional fillets as the trace culminates in a pad can improve its material adhesion strength.

Panelization

The fixed shape of rigid PCBs allows optimization during panelization by rotating the board within the panel to maximize space. The alternative would be to redesign the board to change or reduce its dimensions. On the other hand, panelizing flex printed circuits is easier as technicians can use a bent shape, or an unraveled shape, depending on the arrangement, which leads to greater saving of space.

Segmenting the Design

Routing using discrete stack-up layers can benefit the layout in specific circuit arrangements, as individual layers hardly ever occupy the entire area of the circuit. However, maintaining separate layers in such a rigid-flex configuration tends to complicate the fabrication of the circuit board. For instance, manufacturers may find it simpler and less expensive to manufacture complex circuits in multiple production runs.

Replacing Wire Harnesses

One of the greatest benefits and conveniences of the flex circuit board is it conveniently replaces standard wire harnesses. Most complex systems have multiple boards communicating with each other and with external equipment through the protecting enclosure. It is customary to have a cable or wiring harness for interfacing multiple connectors in a system-level communication setup. Most connectors have receptacles and plugs that take up considerable space within the enclosure, and Designers may have to place these connectors suboptimally to cater to design constraints, leading to a significant strain on the connecting wires and subsequent failure.

By combining the function of the connector and the circuit in a single package, a flex circuit reduces or eliminates several issues associated with wire harnesses. Although more expensive, the flex circuit gives the manufacturer more control over its capabilities. With flex, ultimately, even challenging system-level implementations may come down to a more feasible design.

Conclusion

Flexible circuits offer designers tremendous leeway in realizing circuit applications where they find the more traditional rigid boards to be unsuitable. According to Rush PCB Inc., flex circuits require more complicated and stricter manufacturing processes as they blend the capabilities of rigid and flexible circuits into a single fabrication. However, their practical but compact design is an attraction that many find unavoidable. They can outperform rigid circuits in terms of reliability and quality issues. Furthermore, most modern PCB design software offers a comprehensive ECAD environment for rapidly simulating, modeling, and developing electronic systems with flexible circuits.