Printed circuit boards (PCBs) are a key component of many electronic devices and systems. They come in many variations, including in regard to design, material, and more to suit the requirements of different industrial applications. Among these variations are FR4-based boards, as well as rigid-flex PCBs. The following blog post overviews the differences between PCBs made from FR4 or other rigid materials, and rigid-flex PCBs which are hybrid systems of flexible circuits and laminates bonded together with no-flow prepreg. 

FR4 Rigid Boards

Rigid PCBs —referred to as rigid boards or hardboards—are made from inflexible substrates that prevent them from bending. Due to their relatively low cost, wide availability,  and reliability, they are one of the most popular types of PCBs. 

The most common substrate used for rigid PCBs is FR4 epoxy laminate and prepreg, which are made from glass cloth impregnated with epoxy resin. The term “FR4” refers to the designation given to materials that meet certain requirements as defined by NEMA LI 1-1998 standards. The materials generally demonstrate good physical, mechanical, thermal, and electrical properties. Additionally, they are often the lowest cost material for PCBs. These aspects make FR4 an excellent material for rigid PCBs for many applications. 

FR4 material is commonly used to produce single-sided and double-sided PCBs, as well as multilayered PCBs, typically with lower layer counts (less than 14 layers). While it is a cost-effective option for these PCB builds, it becomes increasingly less cost-effective with higher layer counts and more advanced materials. 

Other common resin systems and materials for building rigid PCBs include epoxy blends to achieve desired mechanical or electrical performance properties, polyimide, Teflon and Teflon blends.  Each offer enhance performance attributes for today’s electronics designer.

Rigid-Flex Boards

Rigid-flex PCBs incorporate features of both rigid and flexible PCBs. They have flexible and inflexible sections. The flexible sections assure electronic connection, but still allow the board to be folded into place. The inflexible sections provide the mechanical and electrical connectivity for the electronic components such as semiconductors, resistors, capacitors, etc.. These qualities make them ideal for use in more complex electronic designs. 

One of the most common – and necessary – materials used in the construction of rigid-flex PCBs is no-flow and low-flow prepregs, which is a prepreg material that has undergone a longer curing period. The result is a prepreg sheet that has a lower rate of resin flow. This ensures that the resin material can flow to the edge of the rigid sections without overflowing into the flexible sections, which is critical to ensuring the rigidity of the rigid sections without affecting the flexibility of the flex sections. 

No-flow prepregs are also commonly used to bond components to PCBs (e.g., heat sinks to rigid sections and stiffeners to flexible sections). They are not ideal for use with heavier copper weights – such as three ounce copper and above, as the resin generally does not have sufficient flow to encapsulate the circuitry. 

Key Differences Between FR4 Rigid Boards and No-Flow Rigid-Flex Boards

Rigid PCBs made from FR4 and rigid-flex PCBs both offer unique advantages and disadvantages that make them suitable for particular applications. For example: 

  • FR4 rigid boards are much less expensive than rigid flex PCBs. This makes FR4 rigid boards a common choice for consumer electronics in everything from appliances, to computers, gaming systems and even a large amount of automotive electronics, where cost is more of a concern.

  • Rigid-flex boards are flexible in some areas and rigid in others. As a result, rigid-flex boards are ideal in high reliability applications such as environments with high levels of shock or vibration where conventional connectors with flexible cabling will fail.  They are also ideal for dynamic flex applications where the flexible sections are bent hundreds of thousands of cycles without failure.  These features make rigid flex ideal for “never fail” electronics applications typically found in medical, military, aerospace and industrial applications.

Choose Rigid-Flex PCBs at Printed Circuits

At Printed Circuits, we specialize in the design and fabrication of rigid-flex printed circuit boards. With over 40 years of PCB experience and state-of-the-art PCB manufacturing equipment, we produce high-quality PCB solutions to our customers’ exact requirements. We maintain ISO 9001:2015 certification, ITAR registration, and IPC membership. 

Looking to learn more about rigid-flex PCBs? Check out this list of FAQs about rigid-flex PCBs compiled by the Printed Circuits team!

To learn more about the differences between FR4 boards and rigid-flex boards or find out more about our capabilities, reach out to us today. To discuss specific PCB requirements with one of our team members, request a quote.

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