Understanding Flex PCB and Stiffeners

In electronic design, the ability to lay out complex circuits on a single board is very important. However, manufacturing costs associated with circuit board development have led to the use of more advanced materials and techniques. These can make it easier for the manufacture of fully flex PCBs. Flex pcb circuits are a fairly complicated subject that is sometimes difficult to understand. Whether you have been involved in flex PCB design for years or if you are just getting started, one thing you will have to learn is how to route flex pcb circuits and stiffeners properly.

Flex PCB, stiffeners, and overall flexibility have been a hot topic for several years. And the more manufacturers and designers integrate flex PCB into their designs, the more their attention has shifted towards how to stiffen the flex PCB components to control how much flex is possible within a design. This guide will help you learn more about Flex PCB stiffeners.

The Basics of Flex pcb Stiffeners

A stiffener is a layer of extra material on a flex PCB that helps prevent the board from bending and breaking. A flex pcb stiffener helps protect against mechanical damage during the handling and use of the product. A flex pcb circuit-bent too far or dropped on its edge could crack or break through its connection pads or traces. A stiffener prevents this by strengthening the board's weakest point — its edges.

Stiffeners are often used in designs that require high amounts of force or torque to be applied to the material. They are also commonly used when designing electronics that need to withstand impacts, such as those found in automotive applications. They can be made from various materials, including FR4, glass-reinforced epoxy, polyimide, or carbon fiber. The stiffness of the stiffener will depend on the material chosen and the thickness of the layer. The pcb stiffener thickness should be chosen based on the application's expected stress and deflection requirements.

Why Do Stiffeners Matter in Flex PCBs?

These constraints matter greatly because there is a design tradeoff between board stiffness, size constraints of a given enclosure or form factor, assembly considerations, and manufacturing costs. Stiffener placement and material are typically chosen to balance all these factors while satisfying EMI shielding requirements or other design-specific requirements.

A stiffener in a flex pcb circuit is common, but it is not required for all designs. If you do not use one, it can cause problems with your design because there will be no way for current to flow through your flex pcb circuit without causing interference or noise problems.

Flex PCB Stiffener Material

The flex PCB stiffener material is flexible yet rigid, providing support and rigidity to the flex pcb circuit. The flex PCB stiffener can be made from polyimide or glass-filled epoxy materials. The flex PCB stiffener material is usually coated with a non-conductive material to prevent shorting between the conductive traces and any metal component on the board's surface. The stiffener supports the traces and prevents them from bending too much when bent around tight corners or over bumps.

The pcb stiffener  thickness of the strip depends on the stiffness required for your application. For example, if you are mounting a component on top of a flex PCB with large amounts of force exerted onto it, you may need to use a thicker flex PCB stiffener than if you are mounting a component on top of a thinner piece of plastic that requires less support.

Flex PCB Stiffener: Common Materials

Depending on the pcb stiffener application you are dealing with, you must use several different types of stiffeners. Understanding the different types can help you pick the right one for your flex pcb project.

FR4 Stiffeners for your flex pcb

FR4 is a glass epoxy laminate with good mechanical properties and high-temperature resistance. It can be used in many applications where good thermal stability is required. The main disadvantage of using FR4 is its low chemical resistance compared to other materials such as polyimide (PI) or stainless steel (SS).

PI Stiffeners for your flex pcb

PI is a polyimide film laminate with excellent chemical and heat resistance compared to FR4. However, it has lower mechanical properties than FR4 due to its thin-film structure, making it more prone to cracks if subjected to large loads or stress concentrations. Additionally, PI has poor thermal conductivity compared to FR4, limiting its use as a flex PCB stiffener unless additional measures are taken, such as adding copper traces for heat dissipation.

Stainless Stiffeners for your flex pcb

Stainless steel is a sheet metal material that can be used as a stiffener in some cases because it has good mechanical properties and can be bent without deforming much (i.e., it has high yield strength). Stainless steel sheets can be cut using standard tools like shears and nibblers. In addition to being cut with standard tools, stainless steel can be laser cut into any desired shape, making it an ideal candidate for flex PCB stiffeners where many different shapes may be required for various designs.

How to Use Flex PCB Stiffeners

When choosing a flex PCB stiffener, it is important to understand how they work and how they can be used. Flex PCB stiffeners increase the board's overall stiffness, which is measured in bending stiffness (N/mm). This is often referred to as an "E" value. The higher the E value, the stiffer the board will be.

The number of layers also plays an important role in determining how much stiffness a flex PCB will have. To determine how much stiffness you need, you need to know how many layers your design requires. For example: if you have a 2-layer design with 1-ounce copper on each side with 1oz copper on both sides, then your E value should be around 600N/mm (1 oz copper). If you want it even stiffer, 4oz copper will give you an E value of 900N/mm (2 oz per layer).

Stiffeners Placement in Flex PCBs

Stiffeners on the Top Layer

Flex pcb Stiffeners on the top layer can be used to provide structural support for the entire flex pcb circuit board. These stiffeners are often used when a lot of weight is applied to one side of the flex pcb circuit, such as when it is attached to a device like an LED display. The stiffener helps transfer that weight evenly across both sides of the flex pcb circuit board.

Stiffeners on the Bottom Layer

Flex Stiffeners on the bottom layer are typically used when you need extra strength in only one area of your flex PCB board. For example, if you have an active component in one area that needs more support than other parts of your flex pcb design, you can add a stiffener here for better mechanical support and protection against fatigue damage from repeated use over time.

Using Double or Multiple Stiffeners in Flex PCBs

Multiple flex pcb stiffeners are used to provide increased rigidity and strength to flex PCBs. They are typically placed at critical locations on the board and can be composed of different materials, including metals and plastics. The vital step in designing a multiple-stiffener layout is determining which areas of your flex pcb circuit require greater stiffness than others. This can be done by performing a mechanical analysis on each area of interest or looking at where flexing tends to occur in your flex pcb product.

The main issue with using multiple stiffeners in flex PCBs is that it would not be suitable for all applications. If you have a high density of vias, you may need to use more than one stiffener. To determine if this is necessary, first look at the flex modulus of your material and compare it to the dielectric constant. If both are high (e.g., >3), you can generally get away with using a single stiffener. If they are low (e.g., <2), you will likely need multiple stiffeners to achieve sufficient mechanical support.

In addition to the flex modulus and dielectric constant, other factors influence whether or not double stiffening is required:

The Number of Vias per Square Inch (Vpsi)

This refers to how many plated through holes exist on a given area of the flex pcb board. The greater the number of vpsi, the more difficult it will be for single stiffeners to support all of them without bending too much or collapsing at some point due to overloading from lateral forces such as solder paste squeegee-ing and pick-and-place action during flex pcb assembly.

The Size of the Vias

Vias are typically larger than necessary to accommodate more current and power density requirements during the flex pcb board layout design. This can cause problems when using a single stiffener because it will be stretched beyond its elastic limit as more current flows through it. Double stiffeners can help alleviate this problem by providing more support for large vias and preventing them from buckling under pressure during flex pcb assembly processes.

The Density of Components on the Board

Components such as chipsets and memory devices can be densely packed together, which increases the risk of damage during pick-and-place operations. If double stiffening is not used, this can cause a failure in one component to negatively affect an entire package or even cause an entire row or column to fail during testing or burn-in.

Final Thoughts

When creating a flex PCB for your project, it is important to understand the additional components used in conjunction with your flex PCB. This is certainly the case with flex PCB and stiffeners. By understanding how these are used in tandem with your flex PCB, you will be better equipped to create efficient circuit boards and have a better chance of success. However, regarding flex PCB and stiffeners, you may have to ensure you are working with qualified and experienced flex circuit manufacturers like Hemeixin. Check out their site for more information about flex PCBs.