How to Specify Stiffener Requirements in Flex PCB Design

17 November 2023
New

Stiffeners are used in a wide range of flex circuit designs. The most popular use of stiffeners is to provide mechanical support to flex PCBs and are not part of the circuit itself.

Adding stiffeners on flex circuits can require several lamination cycles that can add to the overall cost of the design.

Despite not having any electrical role in a flex circuit, stiffeners can have unique requirements in terms of design and material.

Main Purposes of Using Flexible PCB Stiffeners

Stiffeners can complete a range of functions, including the following.

Rigidization of flex circuits
Meeting ZIF (zero insertion force) requirements
Creating localized bend constraints
Creating a rigid surface for the installation of electronic components and SMT pads
Alleviate the stress of component placement
Help with heat dissipation (these stiffeners are usually made of steel or aluminum)
Make the circuit rigid enough to go through the assembly process without requiring manual input

Polyimide Stiffeners

Polyimide stiffeners are used in circuits that connect to a ZIF connector. Certain connector specifications require flex circuits to have a very particular thickness to work with the connector.

The most common ZIF connector thicknesses are 0.2mm and 0.3mm. A polyimide stiffener is attached to increase the thickness at a localized area instead of across the entire board. Making the entire board thicker can be very expensive and hurt the reliability of the circuit.

Polyimide is especially useful because it meets the tight tolerance specifications of ZIF connectors. There are very few materials that come close to polyimide in terms of meeting all the tolerances of the required connector.

FR4 Stiffeners

FR4 stiffeners create rigid parts in a flexible circuit. They provide support to components and any attached connectors.

Adding FR4 stiffeners prevents the flex circuit from being bent. Note that adding stiffeners can increase the thickness of the board and contribute as much as 0.059”.

Aluminum and Stainless Steel

Aluminum and stainless steel stiffeners are mostly used to improve the heat dissipation capabilities of circuits. Both materials are significantly more expensive than polyimide and FR4. Aluminum is the material of choice for heat dissipation. Stainless steel may be used if there isn’t a lot of space on the board.

Adding Stiffeners

Stiffeners can be attached to flex PCBs with the help of PSA (double-sided tape) by hand or laminated under heat and pressure (discussed below). Designers prefer to use lamination under heat and pressure, but they may be forced to use PSA depending on circuit limitations.

Thermal adhesives can create a permanent bond between stiffeners and circuits. They are more affordable to designers than PSAs.

Specifying Stiffener Requirements in Flex PCB Design

Let’s look at various scenarios where a stiffener may be required and the preferred method.

Scenario 1:

Applying rigidity across the board where components are attached
Protect solder joints by adding rigidity in the component area

Method and material

Materials like polyimide, aluminum, stainless steel, and FR4 may be used depending on their budget.
Stiffener thickness is up to the designer’s discretion
Stiffeners may be attached using pressure-sensitive adhesive or thermally bonded with flex adhesive

Scenario 2:

Using ZIF connectors to increase board thickness at a localized level to meet ZIF connector specifications.

Method and material

The only choice of material for ZIF connectors is polyimide.
Designers can choose from a variety of available thicknesses to meet design requirements

Scenario 3:

Creating rigid parts in flex circuits to achieve specific bend requirements, help with board assembly, or other requirements.

Method and material

The two most popular choices are polyimide or FR4
Designers can choose from a variety of available thicknesses
The two methods of application are pressure-sensitive adhesive (PSA) and heat bonding with flex adhesives

Scenario 4:

Flex PCB heat dissipation to ensure component integrity and performance

Method and materials

Aluminum and stainless steel are two great choices for dissipating heat from electronic components and the flex circuit. Stainless steel can be used in smaller circuits due to design restrictions.
Designers can choose from various available thicknesses depending on their budget.

Differences Between PSA and Thermal Bonding

Designers have two ways of attaching PCB stiffeners to flex PCBs. The first is pressure-sensitive adhesives (PSA), and the second is heat bonding. The two have major differences that are summarized in the table below:

Heat Bonding	Pressure Sensitive Adhesives (PSA)
Stiffeners are attached to flex PCBs using pressure and heat	Stiffeners are connected to flex PCBs with the help of pressure
They are used in high-priority projects related to the avionics industry and military equipment.	Used in consumer electronics to bond IPC class 2 products such as smartphones, laptops, and TVs.
They form a permanent and strong bond	The bond is relatively weak
This method is very expensive	The method is affordable
Thermal bonding increases processing time for flex PCBs	Pressure-sensitive adhesives are quick to work

There are a few factors that you can take into account before choosing the right adhesive. These include:

Chemical properties of the adhesive
Heat properties of the board
The capacity of the adhesive as it relates to the materials used on your PCB board

A Few Considerations When Using PCB Stiffeners

A useful design tip is for stiffeners to overlap a minimum of 0.030” to reduce processing time in fabrication and avoid stress thickness.

Before choosing a stiffener material for flex and rigid-flex PCBs, try to stick to common laminate thicknesses such as 0.010”, 0.047”, and 0.062”. Note that each manufacturer will have varying stock of different products. This is why it is important to determine stiffener thickness ahead of ordering.

Wrapping Up

The biggest advantage of PCB stiffeners is to protect components and connectors. This protects your circuit integrity and joints.

Hemeixin PCB has years of experience in everything related to flex PCB. Get in touch with the Hemeixin PCB team here for consultation and gain access to products that align with industry standards.