Designers tend to make PCBs out of a rigid substrate on which various components are affixed and connected via races to create a completed circuit. They are solid and unbendable by nature. However, more advanced electronics have caused printed circuit board assembly companies to evolve their techniques.

From this evolution, we soon developed flexible PCBs – traces sunk into flexible strips that can connect multiple solid boards in any position or direction. But, as useful as these flexible PCB assemblies are, they require a specific component to act as a bridge between the flexible board and the solid one: a PCB stiffener.

PCB Stiffeners

(delphinmedia / pixabay)

What is a PCB Stiffener?

While flexible PCBs are, by nature, a part of the overall functioning circuit, PCB stiffeners are not. These components provide a rigid connection from the flexible portion to the solid board. This solidity makes applying solder for interconnects or components easier and completes the circuit run through the flexible part.

PCB stiffeners also serve a practical purpose in PCB construction by increasing abrasion resistance and strengthening solder joints. Additionally, they provide a greater degree of handling for machine pickers during the pick and place process of assembly.

A flexible PCB needs a stiffener when:

  • The Flex needs to connect to another board or power source.
  • You need to attach a component to the flexible material of the PCB.
  • The attached components will place too much weight on the flexible PCB.
  • Connectors requiring multiple insertions might stress the connection pads.

Types of PCB Stiffeners

The type of stiffener your PCB needs depends on many factors. There are three types of stiffeners, each with its pros and cons. Let’s take a look at each one.

1. FR4 Stiffeners

FR4 is the same substrate that you typically use to build PCBs. It is a solid material made of epoxy resin, reinforced fiberglass, and copper. For this reason, it is also the most widely-used material for stiffeners. The main benefits of FR4 stiffeners are:

  • They provide solid, flat support during the pick and place and reflow assembly processes.
  • Thanks to their solid, conductive material, they offer ease of application for Plated Through Hole (PTH) connectors.
  • Are a cheaper option that allows the flexible PCB to blend easily into other solid PCBs made of the same material.

2. Polyamide (PI)

Polyamide stiffeners are made by creating thin, firm layers with high-temperature polymers, such as Kapton. These polymer layers can be stacked up to create a much thicker, firmer stiffener, depending on the requirements of the final product. The main reasons for using a PI stiffener are:

  • Great for creating solder resistance and high bond strength.
  • Higher numbers of layers can limit the bending ability of PCBs in specific areas, making them easier to manipulate during the assembly process.
  • Provides additional wear resistance over hole locations, limiting damage over time.

Most often, PI stiffeners are used on the backside of gold insertion fingers due to the controlled flexibility of the stiffener when compared to FR4. This makes them better for the frequent insertion and removal of these components, limiting the wear and tear. You can also pick them apart if any further insertions, such as ZIF connectors or sockets, need to be added.

3. Aluminum/ Stainless Steel

Though much more expensive than either PI or FR4, stainless Steel and aluminum provide many benefits that make them ideal for challenging circuits. Some of these benefits include:

  • Much tougher and more robust security in the connector
  • Resistance to corrosion
  • Higher capability
  • Greater formability for components

Stiffener Thickness

The thickness of your desired stiffener is a critical consideration. The thicker the required stiffener, the more it will affect the overall design of the PCB and circuit. Each type of stiffener has a typical range of thicknesses available.

  • Polyamide is usually available in thicknesses of 0.001”, 0.002”, 0.003”, and 0.005”
  • FR4 ranges in thickness between 0.003” and 0.125”
  • Aluminum/Stainless Steel varies between 0.1mm and 0.45mm

The rest of the PCB needs to be designed according to the thickness and mass of the stiffener.

How to Attach a Stiffener

Stiffeners are attached to a flexible PCB on the same side as the components when using plated through hole assembly components. This allows access to the solder pads underneath the connector without requiring disassembly, making it easier to attach a second stiffener on the other side, if necessary.

The stiffeners are then sealed in place using either thermal bonding or pressure-sensitive adhesives.

  • Thermal bonding: The stiffener effectively melts in place using heat and pressure without damaging any components. This provides a much stronger bond making them more secure when used. However, this method is a more expensive and time-consuming option. You cannot remove the stiffener without significant damage to the PCB and its components.
  • Pressure Sensitive-Adhesives: This method only uses pressure to affix the stiffener, which is cheaper and quicker, though less permanent. As a result, this option is ideal for consumer electronics, and you can remove them fairly easily if issues arise.

The Difference Between Rigid-Flex and Rigidized Flex PCBs

How you use stiffeners will affect the type of PCB you get. These are known as either Rigid-Flex or Rigidized Flex PCBs. The differences between these are:

  • Rigid-Flex – All rigid sections of the PCB include traces connected via holes. As a result, the stiffener helps to provide an electrical connection to the PCB and becomes a functional part of the whole.
  • Rigidized Flex – Only certain PCB parts are made rigid with stiffeners but serve no purpose other than mechanical support. The stiffened areas help make the assembly process more manageable and the overall PCB stronger, but they do not affect the PCBs functionality.

Improving the rigidity of flexible PCBs with stiffeners is a complicated process that requires a good bit of forethought. Knowing what you need the PCB assembly to do, how you wish to support it, and what you will use it for is critical in determining what type you should use. Ultimately, the options cover so many specific areas that it shouldn’t be difficult to find the ideal stiffener for your task.