Stiffeners are a key design element in most flex designs and have a significant impact on both the performance and reliability of the finished flex circuits. As a result, stiffeners need to be fully and accurately defined in the data set. Not doing so may result in a finished part that does not meet your requirements.
Stiffeners serve the following purposes in a flex design, each of which have unique material and design requirements:
Component/Connector Area Support
Thickness Modification to Meet ZIF Connector Specifications
The two most common stiffener materials used are FR4 and Polyimide.
This prevents the flex circuit from being bent at or adjacent to the components/connectors. Doing so would most likely cause the solder joints to be cracked or compromised. FR4 stiffener thickness can range from 0.010” to 0.059” thick with 0.020”, 0.031” and 0.039” being very common.
Polyimide stiffeners are most commonly used in designs that connect to a ZIF (Zero Insertion Force) connector. Connector specifications define that the flex circuit, at the exposed ZIF contact fingers, must be of a specific thickness to engage the connector reliably. The two most common ZIF finger thicknesses are 0.3mm and 0.2mm. Rather than unnecessarily increase the thickness of the entire flex to meet the ZIF specs, a polyimide stiffener is attached locally at the finger areas. Increasing the entire flex thickness would be costly and significantly impact the bend capabilities and mechanical bend reliability of the parts. ZIF specifications also define a very tight tolerance for both the width of the outline and the positional accuracy of the fingers. Polyimide is the only stiffener material that allows the part outline and the stiffener to be profiled and meet all the required connector specified tolerances.
Polyimide can also be used to support component/connectors areas where there is a restriction in the total part thickness below the 0.010” practical limit of FR4. While polyimide does not provide the same degree of support as FR4 having “a” stiffener is often preferred over “no” stiffener.
Aluminum And Stainless Steel
Aluminum and stainless steel is also used in some designs but come at a significant cost premium over FR4 and Polyimide. Aluminum is commonly used for applications that require heat dissipation. Stainless steel is commonly used when there is very limited space for stiffeners, but the design requires more component area support than what an equivalent thickness FR4 stiffener can provide.
Attaching Your Stiffeners
Stiffeners are attached using either a thermally bonding adhesive which is laminated under heat and pressure, or with a PSA (double sided tape) and placed by hand. The preferred method is thermal bonding adhesive, but design limitations may require use of a PSA. Thermal adhesives are the same flexible epoxy or acrylic adhesives used to attach coverlays to the flex circuit and result in a permanent bond. They are also in most cases more cost effective than PSAs.
For designs with heat dissipation requirements thermally conductive PSAs must be used. Note that these PSAs are not rated to withstand assembly reflow temperatures and require the stiffeners to be attached after component assembly.
Additional Design Requirements
There are also additional design requirements that pertain to stiffeners.
As previously mentioned, ZIF area stiffeners must result in a specific thickness as measured across the ZIF contact fingers and a tight profile width tolerance to ensure accurate engagement with the connector. There is also a minimum stiffener length defined.
ZIF Connector Specifications Example:
FR4 stiffeners, when used in designs with Plated Through Hole (PTH) components and connectors, have the following additional requirements over SMT only designs.
Attachment side must be on the side of circuit from which the PTH components are inserted from. This allows full access to the component pins and circuit solder pads for soldering purposes.
Clearance holes are necessary to allow PTH pins to pass through the stiffener. Due to manufacturing and material tolerances, the clearances holes must be oversized in relation to the PTH holes in the flex circuit, by 0.016”. This prevents any overlap between stiffener clearance holes and the plated flex holes due to accumulation of tolerances.