Polyimide Stiffener

Polyimide is a common material used as a stiffener in flexible printed circuit boards (FPCBs). It provides rigidity and enables the FPCB to adhere to connector-specific tolerances, allowing for precise alignment and reliable connections. Polyimide can also be utilized to support components and connectors in areas where the total thickness of the FPCB is restricted.

On the other hand, FR4, a type of fiberglass material commonly used in rigid PCBs, has certain limitations when it comes to providing support and flexibility in FPCBs. Its practical thickness limit is 0.10 mm (or 0.010 inches). While polyimide does not offer the same level of support as FR4, having some form of stiffener is often preferred over having no stiffener at all in FPCBs.

Stiffeners play a crucial role in the performance and reliability of finished flexible circuits. It is essential to accurately define and include stiffeners in the design data set to ensure that the final product meets the required specifications. In many designs that involve connecting to a Zero Insertion Force (ZIF) connector, polyimide stiffeners are commonly used.

Connector specifications often dictate that the flex circuit at the exposed ZIF contact fingers should have a specific thickness to ensure reliable engagement with the connector. The most popular ZIF finger thicknesses are 0.3mm and 0.2mm. Rather than increasing the overall thickness of the entire flex circuit, which would be costly and negatively impact the flexibility and mechanical reliability of the bends, a polyimide stiffener is locally attached only at the finger areas to meet the ZIF specifications.

Furthermore, ZIF specifications also require tight tolerances for the positional accuracy and outline width of the fingers. These requirements ensure precise alignment and reliable connectivity.

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