Rigid‑Flex Printed Circuit Boards, commonly referred to as Rigid‑Flex, are a circuit structure that incorporates both rigid substrates and flexible circuits on a single board.
Driven by the demand for shorter, smaller, lighter, and thinner electronic products, Rigid‑Flex is an important direction in circuit substrate technology, propelled by the trends towards miniaturisation, low power consumption, and high performance.
As modern electronic products become increasingly powerful, their internal structures and interconnections grow more complex, placing higher demands on circuit boards. Traditional rigid PCBs or substrates often struggle to meet product requirements, leading to the natural emergence of Rigid‑Flex circuits.
Rigid‑Flex consists of rigid substrates and flexible circuits, with the flexible circuits directly connected to the rigid substrates without the need for connectors.
Because the flexible portion of Rigid‑Flex can be bent freely, components mounted on Rigid‑Flex can be arranged in multiple spatial orientations, offering greater flexibility than traditional electronic integration methods. This added degree of freedom is often likened to an extra dimension – hence the term "4D" integration.
In electronic integration technology, integration that involves substrate bending and folding is defined as "4D" integration. For more details, refer to the previously published article: "A Comprehensive Analysis of Electronic Integration Technology."
Rigid‑Flex circuits are lightweight, compact, and capable of flexible bending, making them suitable for applications with complex spatial structures. They are widely used in modern electronic devices and have become an ideal choice for mobile communication products. To properly design and apply Rigid‑Flex circuits, it is essential to have a clear understanding of their basic characteristics. I have summarised these fundamental characteristics into three key aspects: Multi‑bending, Multi‑stackup, and Multi‑zone.
Multi‑bending
Multi‑bending is the most intuitive feature of Rigid‑Flex circuits. The primary purpose of using flexible circuits is to enable bending, and Rigid‑Flex can be easily bent, folded, and twisted.
Since it is defined as Multi‑bending, it goes beyond just a few bends. Complex Multi‑bending designs can incorporate dozens of bends, with a wide range of bending radii and angles. The smallest bend radius may be only slightly larger than the substrate thickness, appearing almost like a folded flexible circuit. The angle of each bend can be flexibly designed from 0 to 180 degrees.
Complex Rigid‑Flex circuits can contain dozens of bends, intertwining like a python, making it difficult to distinguish the top and bottom surfaces of the substrate. If we imagine walking along such a Rigid‑Flex circuit, the up‑down and left‑right orientations would constantly shift – it truly feels like entering a "4D" space.
Regardless of how complex the bends may be, one principle must not be violated: the various parts of the circuit must not interfere with each other in space. Therefore, when designing bends, multiple factors must be considered, including bend location, orientation, angle, and radius. Additionally, material thickness and type are also important factors that constrain the bending capability.
Furthermore, it is generally prohibited to place vias or components in the bending areas – these are known as Via keep‑out and Component keep‑out.
Multi‑stackup
Multi‑stackup refers to the fact that different areas of the substrate may have different layer counts and thicknesses – meaning the substrate must support multiple stackup configurations.
Every substrate has at least one stackup. For example, a typical 4‑layer, 6‑layer, or 8‑layer board corresponds to a single stackup, which we call a Single stackup, as shown in the figure below.
For Rigid‑Flex substrates, multiple stackups (Multi‑stackup) are often used – that is, different areas of the substrate have different thicknesses and layer configurations, as shown below. The thicker stackup is typically used for the rigid portions, while the flexible circuit portion uses a thinner stackup suitable for bending and folding.
Multi‑stackup designs may include up to a dozen or more stackups. Whenever the layer thickness differs, or the material composition changes, a new stackup must be defined.
Among all these stackups, we can add new stackups or delete existing ones. However, one stackup is always present and cannot be deleted – this is called the Primary stackup.
The Primary stackup exists in both Single‑stackup and Multi‑stackup configurations. In Single‑stackup, there is only the Primary stackup; in Multi‑stackup, it is Primary + other stackups.
Multi‑zone
In the previous section, we described Multi‑stackup, where different areas of the substrate use different layer counts and thicknesses. How are these different stackups applied to specific physical areas? This leads us to the concept of Multi‑zone.
For the entire Rigid‑Flex circuit, it is divided into different zones according to functional requirements. Each zone is assigned a specific stackup – either the same or different – so that the various stackups are mapped to the corresponding areas.
To define zones, zone shapes are drawn on the substrate, and then stackups are assigned to them. For areas of the substrate where no zone shape is drawn, the default stackup is the Primary stackup.
Thus, we understand that not every area of a Rigid‑Flex circuit needs to be explicitly zoned. Areas without a defined zone default to the Primary stackup.
From a physical structure perspective, Multi‑stackup is defined along the Z‑axis of the substrate, while Multi‑zone is defined on the XY plane. They are relatively independent yet interdependent.
Typically, we define the Multi‑stackup first, then the Multi‑zone, and finally map the zones to the stackups. Multi‑stackup and Multi‑zone are interdependent – although they can exist independently in design tools, without one, the other loses its original purpose.
Summary
Multi‑bending, Multi‑stackup, and Multi‑zone are the three most fundamental characteristics of Rigid‑Flex circuits.
With the rapid development of artificial intelligence technology, robots will inevitably enter households and become a new growth area for human society. Since robots need to move like humans, and every joint requires circuit‑driven control, Rigid‑Flex circuits will be indispensable. The market demand for Rigid‑Flex is expected to grow explosively.
Therefore, engineers and technicians must learn more about Rigid‑Flex knowledge to prepare for the technological demands of the future.
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