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Rigid-flex PCB assembly – The Basics and How it Works

The basic process for rigid flex PCB assembly begins with a polyimide dielectric film cladded with rolled copper, which is more flexible than conventional copper foil. You then drill the cladded base material, and selective plating is applied to the plated holes to isolate the conducting layers. You then insulate the flex stackup with Bondply, a polyimide film with adhesive on either side. Finally, the Ribbon extends from the flex stackup section to a section of the rigid PCB.

What is a Rigid-Flex PCB?

A rigid-flex PCB is a flexible circuit board that has a rigid core and an interchanging structure of two types of PCB layers. This type of circuit board provides advantages over traditional PCBs in several areas, including ease of installation and packaging. It is also very cost-effective, making it a good choice for high-density applications. Rigid-flex PCBs have a range of advantages that make them an ideal choice for many applications.

A Rigid-flex PCB is ideal for industrial and commercial applications, where it can maximize space and improve reliability by eliminating discrete wiring and flexible cables. The circuits are integrated into the overall construction of the PCB, providing higher electrical performance and improved service reliability. Rigid-flex PCBs are the best choice for high-reliability applications.

Flexible-flex PCBs are great for printing, but rigid-flex PCBs can be complicated to design. Design them meticulously to ensure that everything is in its proper position. The rigid-flex PCB has layers that you must align carefully, so paying close attention to every detail is necessary. However, the advantages of rigid-flex PCBs are worth the extra cost and complexity.

Types of Rigid Flexible PCBs

We categorize the types depending on the process of joining the rigid and the flex PCBs. They fall into two groups:

  • Rigid-Flexible Composite PCB – You combine the rigid and the flexible circuit boards. You will note a common blind & buried via design between the two boards. This type will have a higher-density circuit design. Blind vias are those that connect two layers but do not penetrate the outer layers. Buried vias are useful for trace routing and augment circuit space by allowing different component pads to connect to each other. Rigid-flex PCBs should be designed with as few vias as possible and avoid via placement.
  • Rigid-Flexible PCB – You produce the rigid and flexible circuit boards separately. You then laminate them together. The through-hole design doesn’t apply here.

The production process for a rigid-flex PCB is more complex, requiring more materials and manpower. However, it is worth the extra cost because rigid-flex PCBs are good in high-performance applications like TVs and medical devices. These circuit boards are also widely used in low-tech, bendable applications, such as kitchen cabinets and LED lighting. Their flexibility allows them to fit tightly into small spaces and minimize layout flaws.

The main difference between flexible and rigid PCBs is the type of support. Flexible PCBs are more pliable, while rigid ones are more durable. In addition, rigid PCBs can handle extreme temperatures, while flexible ones can withstand a range of temperatures. Both types are ideal for high-end products, but rigid ones are usually heavier. In most cases, rigid-flex PCBs are better suited for consumer electronics.

Rigid Flex PCB Assembly – Step by Step Process

Rigid Flex PCBA Assembly

Rigid-flex PCB assembly consists of several steps to follow to complete a successful assembly. For instance, the substrate used for the assembly must be of the right material to ensure durability and strength. It also has to be resistant to heat and chemicals. Additionally, the substrate must be flexible enough to allow easy access to circuitry areas. Combined with the substrate, flexible film and adhesives are made for cover lays, which provide all-inclusive protection and accessibility to circuit pads.

Step 1: Preparing the rigid-flex PCB material

Before manufacturing our rigid-flex PCB, we clean our copper-clad laminates (CCL) and cut polyimide (PI), coverlay, prepreg, stiffeners, and the chemically cleaned CCL into the appropriate sizes.

Step 2: Creating the flex section’s inner core

We will need an internal flex board to manufacture rigid-flex PCB. In addition, we must generate the inner core of the flex to manufacture the flex board. However, if our rigid-flex design has a single flex layer on the PI, we must wrap a thin copper foil. Similarly, if there are two or more flex layers in the rigid-flex, we need to laminate a couple of external copper foils on the PI.

Step 3: Creating the flex inner core circuits

In this case, we aim to leave a particular pattern of copper traces on the foil and eliminate the rest copper composition. We first coat the copper foil with a curable photoresist in case of UV irradiation. Next, we use a nontransparent film and draw the transparent PCB connections on the copper foil. Turning on UV light, the cured dry film covers and protects the copper traces in our PCB circuit pattern. Next, using our chemical solution, we now wash away the uncured photoresist. Finally, we dissolve the exposed copper using NaOH solution, leaving only our copper circuit traces on the inner core.

Step 4: Laminating, generating the circuits, and drilling holes on the flex Section

We laminate the PI layers depending on the number of flexible layers in the rigid-flex design. Therefore, if the number of flexible layers in the rigid-flex design exceeds two, we laminate alternating copper foil and PI layers on our flex inner core. Next, we start creating the circuits. On the other hand, if there are two or only a single flexible layer, we proceed to circuit generating.

Generating the circuit on the external flex layers is our last step. However, in this case, we retain the rest of the copper and remove the copper traces in the PCB circuit. We first use copper to electroplate the circuit area before electroplating it with tin. Next, having protected the circuit area with tin, we remove the photoresist and wash away the copper remaining in the non-circuit area using NaOH. We now remove the tin and keep the electroplated copper traces on the external flex layers. Next, we drill through holes on the plate using a laser before laminating the coverlay on the flex layers. Finally, we’ve completed the flex region.

Step 5: Rigid layer lamination, circuit generation, and drilling

We laminate alternating prepreg layers and copper foil if our rigid-flex PCB has plated-through holes. Next, we generate the circuit and mechanically drill through holes on the board. On the other hand, we use a laser to drill alternating prepreg layers and copper oil if the rigid-flex PCB has HDI requirements. Having laminated alternating flex layers, we finally generate the circuits on the boards’ rigid layers.

Step 6: Using the laser to cut off excess prepreg in off the flex region

To expose the flex section we need, we cut off the prepreg area outside the circuit using a laser.

Step 7: Finalize our PCB manufacturing process and test its functioning

On the surface of the rigid-flex PCB, we apply our surface finish, silkscreen, and solder mask before treating the panel’s edge half holes and V-cut. Finally, we carry out the nail-in-bed and flying probe, the main tests. In addition, for components applicable in the military, medicine, aerospace, and automotive industries, we must carry out the four-terminal sensing test. We have completed the manufacturing process.

Applications for Rigid Flex circuits

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The advantages of rigid-flex circuits are their reliability, compact size, and low weight. In addition, you can build them to precisely fit the device they fit into. Manufacturers are under pressure to fit higher-end technology into smaller spaces, so rigid-flex circuits are a great choice. They allow for a denser device population with lighter conductors while maintaining a high level of flexibility.

There are several other benefits of rigid-flex circuits. These include being less expensive to manufacture and providing better connection reliability, polarity, and flexibility in packaging. Moreover, rigid-flex circuits are highly reliable and can easily integrate into various applications. They are also less costly than conventional circuit boards and can be useful in high-density applications.

In addition to the many benefits of rigid-flex circuits, you can manufacture these circuits in various sizes and densities. For example, polyimide circuit boards can have a high density of connection points, allowing for high-density circuit routing. Rigid-flex circuits also minimize the overall weight of a system. As a result, they are ideal for high-shock and vibration applications.

Other areas of application include:

  • Medical wearables – to identify and track breathing patterns and heart rates
  • Industrial applications – Like power distribution circuits and RF devices
  • eCommerce – For scanning and tracking shipment