What is a Flex PCB and Its Design Functions?
Flexible printed circuits or flex pcb are made of polyimide-based substrates with copper etched electrical circuits that provide packaging designers with many degrees of design freedom for 3D applications. Bending of a flex circuit or flex pcb is possible because of:
- The unique mechanical properties of the polyimide film.
- The ductility of copper foil (either when laminated with flexible adhesives or when coated through direct adhesive-less processes).
Bending can be classified into static or dynamic applications.
- Static flexing is either a one-time bending to fit in the packaging design or less than fifteen bends during the life of the product. Typical static applications include packaging designs that require multiple planes for the flex circuit to traverse during installation or when an attached component (e.g. switches, LEDs, sensors, etc.) must fit into a holder or a pocket that is not planar to the FPC.
- Dynamic flexing is when the FPC must be bent multiple times as part of the functionality of the end product and generally will require tens to hundreds of thousands of cycles. Applications can range from a single point bend location to a rolling bend, from a few degrees of bend to more than 360 degrees. The design constraints on the flex circuits in dynamic applications are more restrictive than in static applications.
Failure mode (understanding the material mechanical limitations)
The failure of a material caused by bending occurs when exceeding the material strain capability. Strain is a deformation metric and identifies the percentage of dimensional change a material has before it reaches a plastic state. Once the plastic state is reached, the material does not return to its original state when the stress is removed (continuing application of the stress will result in a rupture or fracture of the material).
Stress is the force applied to the material to pull it apart (tension), compress it and/or shear it. Shear stresses are parallel forces applied to the material but in opposite directions. The bending process produces all three of these stresses but engineers tend to only consider the tension stress when evaluating the impact of bending. Such a limited view can cause an unexpected failure mode to appear by the compressive and/or shear stress.
Design features and operating conditions that determine flexing endurance
- Design features
- The design of the flex circuit in the bending area is critical to its ability to survive the application. When considering a flex application, it is imperative to identify the copper layer count, and the copper thickness and electrical properties of the circuit.
- The objective of the design is to create a laminate structure that does not exceed stress levels beyond the yield stress point, which is that moment where the copper goes from elastic to plastic state. While the configuration of the flex circuit in the bend area is critical, other features in close proximity to the bend area can result in failures. As an example, thickness transitions areas can cause stress dams, which result in exceeding bend radius limits as well as FPC clamping.
- Operating conditions
- If it is a dynamic application, focus on the number of bend cycles and the size of the bend radius, both of which will ultimately determine the type and level of stress.
- Even in static flexing, bend radius is important when the copper layer count exceeds one.
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