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Rigid Flex PCB Stackup Made Easy

The rigid flex PCB stackup is an interesting combination of rigid and flexible circuit boards. As such, it requires a unique set of design specifications to build. Knowing what layers are included for both the rigid and flexible areas is essential when designing the rigid flex stackup. This article sheds light on the basics of this PCB design.

What is PCB Stackup?

A PCB stackup is a combination of different layers that make up the final printed circuit board. Each layer has a purpose and is essential to the functionality of the board. The layers can be either rigid or flex both rigid and flexible, depending on the design requirements.

Flex PCB Stackup

The flex PCB stackup typically consists of a coverlay, adhesive layer, base material, and copper circuitry. The coverlay is responsible for protecting the flexible circuitry, specifically the copper traces from corrosion and oxidation.

The adhesive layer bonds the flex circuit board to the base material and ensures that it remains in place during use. Finally, the copper circuitry provides the electrical connection between electronic components on the board.

Rigid PCB Stackup

A rigid PCB stackup comprises a core or core layers and substrates, copper traces, and solder mask. The soldermask helps protect the circuit board from environmental elements like dust and moisture.

The core material, on the other hand, is responsible for providing rigidity and structural support. Finally, the copper traces provide an electrical connection between components on the board.

Sometimes, and depending on application requirements, a PCB may combine both the rigid and flexible stackups into one. This is known as a rigid flex PCB stackup. In this case, the two stackups are connected together with an adhesive layer, unusually no flow Pre-preg, and the flexible layers placed between the rigid layers.

Other layers, such as solder mask or coverlay for protection and silkscreen layers for component reference are also included. In the below section, we take a detailed look at the rigid flex PCB stackup, highlighting the different types of layers and their purpose in the circuit board.

6 layer rigid flex PCB stackup
6 layer rigid flex PCB stackup
Resource: https://www.researchgate.ne

Rigid Flex PCB Stackup

A rigid flex circuit board is a type of PCB that has a flexible section and a rigid section or sections. The rigid section is responsible for providing the structural support, while the flex area provides flexibility to fit into tight spaces or bend without damaging any electrical connections. The typical rigid flex PCB stackup includes layers for both the stiff and flexible areas, as listed below.

Rigid Flex PCB Stackup Substrate

A PCB substrate is the base layer, the core, that’s responsible for providing structural support among other functions such as electrical insulation and heat dissipation. This layer is often FR4, which is a type of glass reinforced epoxy laminate, polyimide, or both FR4 and polyimide.

In a rigid flex PCB board, polyimide is the most used core or substrate. It forms the main layer in the flexible section (s), and runs through the middle of the rigid parts of the board. In the rigid areas, FR4 is used as additional support to make the board stronger and more durable.

Rigid Flex PCB Stackup Trace

Just above the core layer is the trace layer. This is where electric connections are made between components on the board, and signals are routed from different locations and connected to each other.

The traces need to be designed carefully and accurately as they affect how well the board functions. In a rigid flex PCB stackup, the traces are usually made from copper and plated or bonded to both the flexible and rigid areas.

The thickness of the trace layer depends on the application requirements. Note that the trace width and thickness can affect the circuit board’s impedance, and must be considered while designing the board.

In the flexible sections, also, the copper traces need to be thin enough to allow for flexibility, but strong enough to handle the current and voltage requirements of the circuit.

Rigid Flex PCB Stackup Prepreg

This layer is the insulation and bonding material that’s used to laminate the flexible and rigid layers of the board together. In a rigid flex PCB stackup, this layer is only found in the rigid area and it helps to bond and sandwich the flex layer between the rigid parts.

Pre-preg is a basically a laminate composite that has adhesive properties and forms a durable bond between the rigid and flex circuit board layers. Many properties make it a preferred material for in rigid flex circuit boards.

First, it offers excellent uniformity and strength, a quality that is important when you are dealing with two different types of circuit boards. Pre-preg is also an eco-friendly material while also offering resistance to contaminants.

Rigid Flex PCB Stackup Cover Film

This is the rigid flex PCB layer that serves as the protective cover on the board, specifically to shield the copper traces. In the flexible parts, it consists of two layers, an adhesive layer and a coverlay material, or it may be the adhesive-less type and electrically deposited to the substrate. In some cases, a thin soldermask is used.

Most often, the stiff parts of a rigid flex PCB stackup use solder masks instead of coverlay. Depending on the requirements, both coverlay and soldermask covers may also be used.

Above the coverlay or soldermask, another layer called the silkscreen is usually applied. This is the uppermost layer of any rigid flex PCB stackup. The silkscreen layer is used to add reference markings and graphics on the board, such as component names or company logos.

Conclusion

Rigid flex circuit boards offer a number of advantages over traditional PCBs, including increased durability and improved signal integrity. When designing a rigid flex PCB stackup, engineers must keep in mind the board’s application requirements and choose layer materials based on the required performance and durability features.

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