Flexible PCBs: Ductility in Circuitry

Key Takeaways

• Flexible circuits offer numerous mechanical advantages over regular rigid boards but have density and manufacturing limitations.

• Design rules must account for the difference in material properties between flex and rigid substrates.

• Flex circuits can take the place of complex wire harnesses.

It wasn’t long ago that rigid circuit boards were the only possible electronic system method available. Whether it was entertainment or industrious, the shape of most devices was an extension of an intractable rigid board. More recently, the tide has turned: as technology has developed and matured, the enclosure is shaping the electronics inside them. Despite additional manufacturing challenges, flexible PCBs (or flex printed circuits) are enticing to product designers due to a nearly unlimited range of possibilities compared to rigid board constraints.

Rigid and Flexible PCBs At a Glance

Why Flexible PCB Has Risen To Prominence

Flexible PCBs are a misnomer: how can a rigid board exhibit flexibility? It’s valuable to understand that the rigidity of printed circuits evolved out of workplace convenience and is not necessarily a design demand (although it certainly can be): early etched circuits used simple insulative materials like plywood as the backing for single-sided proto-PCBs. In its nascent stage, manufacturing and electronics research were heavily intertwined, with advancements in one usually pushing forward the capabilities of the other. As performance demands increased, researchers developed and introduced materials more suitable for mass production. With the packaging of components still constrained to through-hole soldering (SMT would not become the dominant packaging/assembly technique until well into the 90s), substrates had little need for more robust integration methods.

Today, miniaturization at both the component and device level is the dominant factor in design constraints: continued materials R&D created discrete packages magnitudes smaller than earlier generations that consume significantly less power, allowing greater functionality and performance at a smaller footprint. Demand for smaller, portable devices at the consumer end is also a factor, as these devices offer long-lasting uptime from a battery charge. While miniaturization alone doesn’t preclude rigid boards, compounding ergonomics or enclosure considerations can make rigidity a liability. For example, a laptop vs. a desktop computer: the former’s inclusion of a screen into a foldable design provides significantly more challenge than a standalone monitor that interfaces to the tower with a static plug.

The Advantages of Flexible PCB Designs

Flexible printed circuits can accommodate a wide range of enclosure designs and device applications, but they come with greater manufacturing complexity. They can even combine with rigid PCBs in a flex-rigid construction that melds the physical attributes. Designers must remain conscientious that many practices honed during layouts for rigid PCBs may require some adaptation for flexible circuits:

• Three-dimensional layout - While the layout will only occur in two dimensions, a flex circuit's ability to fold and bend can simplify the stackup and assembly by eliminating unnecessary layers and components. Any flexible discrete portion of the circuit intended to bend (or stay bent) during operation should be slightly longer than the targeted dimension to support bending and fulfill flex-printed circuits' general looser tolerance requirements.

• Pad design - Adhesion between flex substrates and conductive materials is often poorer than rigid substrates. To compensate, designers should add tabs in the padstack to provide a greater surface for adhesion and a more rugged connection. Transitional fillets from trace to pad will also improve material strength by avoiding acute angles that act as stress concentrators. 

• Panelization - Because rigid PCBs have a fixed shape at the time of production, the only optimization possible for panelization is rotating the board within the panel to maximize space (short of a redesign that reduces/changes the dimensions of the board outline). However, technicians can panelize flex printed circuits using a bent or “unraveled” shape, depending on which arrangement saves more space. 

• Design segmentation - Layout may benefit from a circuit arrangement that uses discrete stackup layer routing; individual layers do not necessarily occupy the entire circuit area. However, separating layers in a rigid-flex configuration complicates manufacturing the printed circuit. As a more extreme example, manufacturing may prefer to produce a complex circuit across multiple production runs – this can alleviate manufacturing challenges and reduce costs.

Arguably, the greatest convenience and purpose for flex printed circuits is the replacement of the standard wire harness. Rarely does a board exist outside of some level of system integration – boards need to communicate with other boards and I/O within an enclosure that protects the board. Historically, system-level communications used a wiring or cable harness to interface the many connectors in a rugged and reliable setup. Connector plugs and receptacles take considerable space within an enclosure, and design constraints may orient connectors suboptimally, placing significant strain on the connecting wires before becoming a failure point.

Flex trivializes many issues with wire harnesses by effectively combining the function of the circuit and connector into a unified package. Since manufacturers have more control over the flex printed circuit's capabilities than any cable harness (albeit at a substantially higher cost), even the most challenging system-level designs may have a more feasible implementation.

Cadence Software Offers Flexible Solutions

Flexible PCBs offer extra leeway for realizing circuit applications where traditional rigid boards may be unsuitable. Flex circuits are a tremendous manufacturing undertaking that blends the capabilities of circuits and wire harnesses in a single fabrication. Yet, their compact and practical design endears them to many circuits where rigid would suffer greatly from performance or reliability issues—Cadence’s PCB Design and Analysis Software suite is a comprehensive ECAD environment that can rapidly simulate, model, and develop electronics with a comprehensive toolset. Together with the powerful and easy-to-use OrCAD PCB Designer, DFM product development has never been more precise.

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