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What is a Stiffener Board in Flex PCB?

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Flex PCB with stiffener
Flex PCB with stiffener
Resource: https://www.designworldonline.com

When specified parts of a flex PCB need strengthening, a structure called stiffener board is normally used. This board allows the supported sections to remain rigid, allowing manufacturers to assemble components, for example, or ensure board stability. This guide explains what a flex PCB stiffener means, the type of materials used to make it, and more.

What is a Stiffener Board?

In the world of printed circuit boards, a stiffener board is a sheet of supporting material that’s glued to specific parts of PCBs to make them rigid (or thicker). Note that stiffeners are initially not part of circuit boards, but add-ons that are introduced in the last stages of the manufacturing process.

Flex PCB Stiffener

Flex circuit boards are special PCBs that curve to suit an application requirement. However, their flexibility can be a disadvantage. For example, it becomes difficult to assemble such boards with electronic components, or even solder them.

A flex circuit board may also coil or bend where it must not bend such as near a connector, a solder joint, or mounted components. In such cases, a stiffener board is used to provide mechanical support and other benefits. In essence, flex PCB stiffeners perform these functions.

  • They give flexible circuit boards the mechanical support needed to prevent (or correct) warping, and other structural issues
  • Stiffener boards allow PCB manufacturers to assemble and solder components on flex circuit boards
  • Stiffeners can help manufacturers ensure the required level of thickness for a flex PCB, such as ZIF connectors would need
  • A stiffener is used to help with stabilizing flex PCBs and supporting connectors
  • In high temperature applications, flex PCB stiffeners can help dissipate heat
  • Adding a stiffener board to the top of a PCB can help protect it from damage by abrasion
  • Sometimes, a PCB stiffener material in the form of busbar offers electrical conductivity to help carry power

Flex PCB Stiffener Thickness

Flex stiffener board thickness can range from 0.01” to 0.059”. A lot depends on the required level of rigidity and other features which, in turn, are mainly dictated by the board’s specific application.

As an example, a PCB stiffener may need to be thin enough to avoid introducing unnecessary weight to a thin, flex PCB, or thick enough to provide the necessary support and thermal properties.

It’s good to note that flex PCB stiffener thickness can also be customized. This is done to cater to the varied needs when it comes to the boar’s application requirements, and which may range from the mild to the extreme.

PCB stiffener material types
PCB stiffener material types
Resource: https://www.diyaudio.com

What are the Different Types of Stiffener Boards

Different types of stiffener boards are available, and each type of board has its advantages and disadvantages, such as higher levels of rigidity or heat dissipation abilities. The types of PCB stiffener material that are beings used today include FR4, Polyimide, and metals such as stainless steel and aluminum.

FR4 Stiffener

This is the same material that’s commonly used to make rigid circuit boards. Consisting of glass fiber that’s been impregnated with epoxy, the FR4 stiffener offers is often on sections that need to support a connector.

These types of stiffener boards are also commonly used on areas that will have many components mounted, and must be rigid enough to support the weight or their assembly.

Polyimide Stiffener

Polyimide is a popular material used to manufacture of flexible circuit boards. As such, it’s usually a preferred material the thickness of a flex circuit board needs to be increased, either for stability or to meet certain requirements.

As an example, a polyimide stiffener is used when a flex PCB is to be used with a zero insertion force connect, or ZIF connector. Manufacturers also often add this type of stiffener board to flex circuit as a way of protecting them from abrasion damage.

Metal Stiffener

Metal PCB stiffeners may also be used, depending on the required qualities and other factors such as usage environments. Metal stiffeners are also typically aluminum and stainless steel.

Aluminum offers better thermal properties than steel, making it a good option when heat dissipation is a major factor. Stainless steel is used where higher rigidity is required, or where both rigidity and thinness.

Overall, metal board stiffeners are a more costly option, but necessary in applications where extra rigidity is needed. Also, applications that involve higher temperatures and the PCB must conduct heat away at a higher rate.

Flex PCB with stiffener boards added to specific sections
Flex PCB with stiffener boards added to specific sections
Resource: https://www.researchgate.net

How to Use Stiffener Board

When incorporating a stiffener board in printed circuit requires a number of considerations. These include where to place the stiffener, the type of material to use, and (more importantly) the board attachment method.

PCB Board stiffeners

PCB manufacturers use two different methods to attach stiffener boards: thermal bonding and pressure-sensitive adhesive. Attaching a PCB stiffener board using the thermal method involves using heat to cause the stiffener to adhere to the PCB.

Although costly, thermal adhesives produce stronger bonds. This method is, therefore, used in more demanding applications such as aerospace and automotive.

The pressure-sensitive adhesive method is less expensive. However, it produces a weaker bond and is often used in the circuit boards of consumer electronics.

PCB Stiffener Busbar

Sometimes, busbars are used as stiffeners. These are conducting strips or bars, and often made of copper for increased conductivity among other benefits. When used as PCB stiffeners, they strengthen the circuit board, in addition to allowing higher amounts of power to be passed.

The use of busbars as circuit board stiffeners eliminates the need for heavy duty copper tracks or traces, plus these can carry high currents of electricity than traces could. Not that stiffener boards cannot be used as busbars, only the other way round.

Conclusion

A stiffener board, when added to a flexible circuit board, makes the parts strong enough to support component assembly and weight, among other functions. These boards are also available in different materials and thickness level, which caters to the different usage needs of flexible and rigid flex PCBs.

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