Introduction
Are you creating electronics for a small or uniquely shaped product? A flexible PCB is your ideal solution. The key to a successful design is its foundation: the FPC stackup. This guide will take you through the crucial FPC stackup design guidelines to assist you in creating a stable and high-performing circuit.
What Kinds of Stiffeners on the Flexible PCBs?
As you design a flexible PCB stackup, you might need to reinforce certain areas. That’s where stiffeners come in, offering local mechanical support to your board. They make it much simpler to mount connectors and solder components without damaging the flexible portions. Stiffeners are not electrically active; their sole purpose is to provide strength and stability.
This added support is an essential component of your FPC stackup design guidelines, enhancing handling for automated assembly and increasing abrasion resistance. Though they occupy valuable space, stiffeners in most cases are necessary for the success of your design. The most popular FPC stackup materials applied to stiffeners are Polyimide (PI), FR4, and stainless steel.
Polyimide (PI) Stiffener
You can use PI stiffeners as an affordable means of adding strength to your FPC. They are particularly good for increasing the FPC stackup thickness at ZIF connector areas, to allow a good, secure fit.
This material also allows you to determine where the circuit will bend and supports high-wear areas. With good solder resistance and a strong bond strength, PI is an all-around selection for your FPC stackup materials. It is frequently applied to support connector fingers during insertion.
FR4 Stiffener
FR4 is the same rigid material used in standard PCBs. In a flexible PCB stackup, you employ FR4 to maintain the board flat during automated reflow soldering and pick-and-place.
This offers important support for mounting through-hole components by providing you with a stable flat surface. As an example of an FPC stackup, a 2-layer FPC stackup can use an FR4 frame in order to reduce assembly costs by getting rid of costly SMT carriers.
Stainless Steel / Aluminum Stiffener
Metal stiffeners are what you should look at when your design must be as durable and corrosion resistant as possible. Using stainless steel or aluminum will provide great strength, formability, and durability to your circuit. You usually attach them to finger connectors to reinforce their connectivity and regular loading.
These FPC stackup materials will guarantee that your flexible circuit is capable of operating in harsh environments and being handled. It is an excellent method of making your design more durable.
Using Multilayer PCB Stiffeners
Follow important FPC stackup design rules for reliability when applying stiffeners. Always slightly overlap the stiffener with the coverlay to reduce stress at the transition. If your design, for example, a multilayer flex circuit stackup, incorporates multiple stiffeners, maintain their same thickness. This creates an even profile for assembly. These actions reinforce solder joints and enhance overall abrasion resistance.
What is FPC and Rigid-Flex PCBs Stackup?
1. FPC Stackup
A flexible printed circuit, or FPC, is a dedicated board built on a flexible substrate. Unlike rigid boards, you can fold, twist, and bend an FPC to be placed into custom configurations, which makes it a perfect fit for dynamic applications or low-space products. With this flexibility, you can use the same kinds of electronic components and obtain the same performance as if you were using a rigid PCB.
Classification Based on Configurations
Rigid-Flex PCBs
A rigid-flex stackup combines rigid and flexible circuits. You have the stability of rigid sections for component mounting and the flexibility of flex for interconnections.
HDI Flexible PCBs
HDI, or High-Density Interconnect, circuits are used for high-performance applications. They employ micro-vias and thinner substrates to fit more circuitry in a smaller area.
Classification Based on Layers
Single-Sided Flexible Circuit Boards
This is a single-layer circuit with one copper conductor. The circuit is accessible from one side of the flexible polyimide film.
Single-Sided Flexible Circuit Boards with Dual Access
This board consists of a single copper layer similar to a normal single-sided FPC. However, you can access the conductor from both sides for more connection options.
Double-Sided Flexible Circuit Boards
A 2 layer FPC stackup consists of two copper layers on opposite side of the base film. Plated through-holes provide connections between them.
Multi-Layered Flexible Circuits
For very complex designs, a multilayer flex circuit stackup is used. It combines several circuits into a single laminated and interconnected assembly.
2. Rigid-Flex PCBs Stackup
A rigid-flex stackup is a hybrid printed circuit board that combines both rigid and flexible substrate materials into one assembly. This type of structure is built up of many layers laminated together, with the rigid sections providing strength and a solid platform for mounting components through surface mounting. At the same time, the flexible circuits serve as integrated connectors, enabling you to route traces around a corner or shape them to the distinct form of your product’s enclosure.
Additionally, with thin flexible films, a rigid-flex stackup has the potential to reach incredibly thin FPC stackup thickness, with profiles as thin as 0.2-0.4mm. This technology provides you with a compact, robust and highly flexible advanced electronics.
What are the Features of Rigid Flex PCBs Stackup?
A rigid-flex stackup has many important benefits over a standard circuit board. These improved features enhance performance, increase reliability, and minimize the risk of frequent manufacturing errors. Let’s explore these features in more detail.
Reduced Solder Joints and Connectors
With a rigid-flex stackup, you have fewer solder joints and connectors in your design. This is a huge benefit, as it reduces the potential for expensive soldering mistakes. Fewer connections means, fewer potential points of failure. By simplifying the connections within the board itself, you directly enhance the overall reliability of your finished circuit. This combined approach makes your product more durable.
Optimal Board Thickness
Rigid-flex technology enables you to design extremely thin and durable boards. With the use of thin film substrates, you can drastically minimize the overall FPC stackup thickness without compromising on quality. These stackups can be from as low as 0.2mm up to 0.4mm thick. This will make your design more durable and compact. You have a functional, dependable board in a much smaller profile.
Improved Thermal Strength
A rigid-flex PCB also excels at heat dissipation. It has much greater thermal strength compared to a standard rigid board, protecting your components from heat damage. This improved thermal management helps your circuit function steadily without overheating issues. Thanks to this, you can minimize worries about heat damage considerably. Your board will remain efficient and have a longer lifespan.
High Aspect Ratio
Rigid-flex boards also have a high aspect ratio, i.e., they have a greater width than height. This creates more room between components on your board. The additional space enhances the performance of high-speed signals. It also makes it easier to route traces and fit other components. This streamlines your design and assembly process significantly.
Effective Solder Masking
Your rigid-flex board requires protection provided by solder masking. It is a type of polymer coating and it covers your copper traces and only exposes the solder pads. This avoids the formation of bridges by solder between pads, the cause of short circuits during assembly. The solder mask also protects your circuit against abrasion and electrostatic discharge (ESD), guaranteeing long-term reliability of your components.
Why Use Rigid Flex PCB?
When you select a rigid-flex PCB, you will have a number of design benefits. In addition to 3D design freedom, rigid-flex boards provide greater reliability and a more consistent electrical profile. This enables you to develop compact, more durable, and more efficient products. A rigid-flex stackup is a great choice for a wide range of today’s electronic applications.
1. 3D Freedom and Flexibility
A rigid-flex design enables you to design three-dimensional circuit layouts. You can bend and shape the board for insertion into tight spaces. This flexibility enables your circuit to fit perfectly into your product’s housing. It is the perfect solution for dense and complex electronic designs, providing you with more flexibility than a standard PCB.
2. Decreased Maintenance
You can reduce the number of connectors and cables with a rigid-flex stackup. With fewer connections, there are fewer components to wear out or fail over time. This design option directly enhances the long-term reliability of your product. It leads to a device that needs less maintenance, saving you money and enhancing customer satisfaction.
3. Uniform Electrical Profile
Rigid-flex PCBs provide a more consistent electrical profile for your signals. The rigid and flexible layers combined assist you in optimizing signal integrity. This reduces impedance variations across the board. The result is better performance and less signal loss. This is an important consideration for high-speed applications where FPC impedance control is key.
4. Improved Housing Solutions
You can design more efficient housing solutions by incorporating the PCB as part of your product’s structure. This design saves you considerable space. It also enhances the structural integrity of your device as a whole. When you design the board and the housing together, you end up with a more streamlined and efficient final product.
5. Durability and Flexibility
Your design may be both flexible and tough with a rigid-flex board. These types of PCBs are designed to endure lots of mechanical stress. The combination of rigid and flexible materials gives the required bendability without sacrificing structural integrity. This makes them ideal for uses involving repeated flexing or movement.
6. Space Saving
Rigid-flex boards are also very good at saving space. Due to their thin profile, you can have complex circuitry without compromising efficiency. With the incorporation of more components in one compact configuration, you are able to reduce manufacturing expenses. This space-saving feature is essential in producing smaller yet high-performance electronic devices.
7. Single Point Connection
A single point of connection can simplify your entire system. A rigid-flex board allows a solid and stable connection between your electronics and supporting structure. This simplifies fabrication and troubleshooting. It also eliminates electromagnetic compatibility (EMC) problems induced by cables and reduces assembly errors.
8. Lightweight Construction
Rigid-flex PCBs are very lightweight. They use low-density flexible substrates to minimize weight while providing substantial weight savings over standard rigid boards. This positions them as a great fit for industries such as aerospace, automotive, and portable electronics. You can minimize your product’s total weight without compromising on strength or reliability.
9. Resistance to Vibration and Shock
These boards are quite suitable for harsh environments. A rigid-flex stackup is mechanically stronger and distributes stress efficiently. This makes your device extremely resistant to shocks, vibrations, and sudden impacts. It is designed to protect your components and is more reliable when it matters the most.
10. Improved Reliability
You achieve greater reliability than assemblies based on rigid boards and cables. The flexible portions are resistant to bending, twisting, and vibration without failing. By having fewer connectors, you eliminate the possibility of loose or intermittent connections. You ensure your product will work consistently throughout its entire lifespan.
11. Ease of Assembly
You can streamline your whole assembly process with a rigid-flex PCB. With fewer interconnections and solder joints, manufacturing is significantly quicker. That results in a smoother process with less chance of assembly mistakes. The outcome is higher production yields and an earlier time to market for your product.
What Types of Material Used for Rigid-Flex Stackup / Structure?
Your rigid-flex stackup integrates rigid and flexible materials into one single, completed circuit board. Understanding these basic FPC stackup materials is critical to the success of your design. The major materials you will be working with are copper foil, FR4, prepreg, and specialty adhesives to bond everything together.
Copper Foil
You use copper foil as your main conductor throughout your rigid-flex stackup. It is a great electrical conductor for transporting signals and power along your circuit. It’s very flexible, making it the ideal material for the flexible sections of your design. You will see it in the rigid and flex areas, creating the pathways necessary for your circuit.
FR4
FR4 is your standard material you use for the rigid sections of your board. FR4 is a fiberglass-reinforced epoxy laminate that has superior mechanical strength and electrical insulation. As part of your rigid-flex stackup, FR4 gives you the stable, rigid support your components needs for mounting components. It provides you with the structural integrity and durability of your final PCB assembly, providing you with a solid base for your design.
Prepreg
You use prepreg to bond the different layers of your board together. It is a material composed of reinforcing fibers that have already been pre-impregnated with a resin. In your FPC layer stackup, prepreg serves as an insulating layer between copper foils and rigid FR4 sections. It supplies both the adhesion and the electrical insulation required to construct a reliable multilayer board.
Adhesive
Adhesive is also very important in keeping your flexible PCB stackup intact. You apply it to join different layers, and this helps in avoiding delamination or separation. The adhesive selected should have a good bond while at the same time being flexible, where bending is required. This maintains the long-term structural and reliability of your whole rigid-flex PCB.
What FPC Thicknesses?
Selecting the appropriate FPC stackup thickness is important to your design’s success. This final thickness is the total of all materials in your FPC layer stackup. Choices vary from ultra-thin for small devices to thicker builds that need additional strength. Let’s see the common thicknesses available.
The overall thickness you choose will depend on your application’s requirements for flexibility, durability, and space. The rule of thumb is that the thinner the board, the more flexible it will be.
- Ultra-Thin FPC: With a thickness of less than 0.1mm, you can apply this to the lightest and smallest electronic devices. It is the ideal selection when meeting tight space and weight limitations is your top priority.
- Standard Thin FPC: Between 0.14mm and 0.2mm, this thickness is suitable for most thin electronic devices. It offers good flexibility with easy fitting in compact form factors.
- Thick FPC: For stronger and more stable applications, you may choose a thickness of 0.5mm to 1.0mm.
Your final FPC stackup thickness is based on the overall thickness of the core FPC stackup materials. To build your own flexible PCB stackup, you can choose from these common material options:
- Copper Foil: 1/3 oz to 3 oz (Offered in 12µm, 18µm, 35µm, 70µm, or 105µm)
- Polyimide (PI) Film:5µm, 25µm, 50µm, or 75µm
- Adhesive: Usually 20µm
When completing your design, remember to consider more than just the FPC itself. The ideal project thickness will depend on your component selection, connectors, and even the housing you use in the product.
How to Fabricate FPC
Now you know what materials go into your FPC stackup. The manufacturing process transforms raw FPC stackup components and turns them into a completed flexible circuit. It involves precise series of steps, ranging from the creation of the first laminate to end quality testing. Let’s walk through the most important phases of assembling your custom board.
- Material Preparation: We first prepare the base materials for your FPC layer stackup. We press the copper foil, polyimide (PI), and adhesive to your desired FPC stackup thickness. This creates the core laminate and protective coverlay for your board.
- Drilling and Plating: We then drill and plate all the through-holes. This process forms the plated through-holes (PTH) that establish stable electrical connections between layers. It is a very important process, particularly for a multilayer flex circuit stackup.
- Circuit Imaging and Etching: We then establish your circuit pattern. We transfer your design image onto the laminate and remove unwanted copper through an etching process. This leaves only the precise conductive traces that make your circuit.
- Lamination: After an initial electrical test, we align and stack all the layers of the circuits and coverlays. The whole FPC stackup is then loaded into a lamination press. Pressure and heat cause all the layers to be bonded together permanently into a single, unified board
- Surface Finishing: We now apply the finishing touches. We print the silkscreen for identifying components and add a surface finish, such as ENIG or OSP, to protect the copper pads. This provides good solderability and oxidation protection.
- Testing and Profiling: Lastly, we test for quality and complete the board. We do a final electrical check to ensure it works. Next, we cut out individual FPCs from the production panel, do a final visual check, and package them according to your specifications.
Conclusion
Understanding your flexible PCB stackup is the first step to a successful project. From material selection to following essential design guidelines, all the layers play a role. You now have the basic knowledge to design a more efficient and dependable flexible circuit.
MV Flex Circuit is a specialist manufacturer of flexible (FPC) and rigid-flex PCBs, providing custom solutions for medical, aerospace, and other high-reliability applications. We follow IPC Class 2/3 standards and perform AOI, electrical, and functional testing to ensure consistent quality. Send your Gerber files and basic requirements to sales@flexiblepcbboard.com for a free DFM check and prompt quotation.
