Reliability of Flex Circuit PCBs

In contrast to rigid circuit boards that are essentially woven fiberglass impregnated in epoxy resin, flex PCBs are made from more flexible plastic films. This allows them to bend without affecting electrical conductivity or mechanical integrity. These PCBs are typically used in products that require flexibility and space savings, like mobile phones (dynamic application) or to replace wire harnesses in cars, rockets, and satellites (static application). The ability to flex is especially important for these types of applications because it reduces weight and costs while also reducing assembly steps.

Regardless of application, it is critical to understand how to test the reliability of a flex circuit pcb before final product assembly. To do this, we recommend following the IPC guidelines for the appropriate level of testing to ensure the pcb meets the requirements of its end-use environment.

The first step in testing a flex circuit pcb is determining the minimum angle at which it can flex without damage. This is a crucial consideration for all flexible circuits. Conductor patterns and routing should be designed to avoid 90o bends, which can cause high strain and damage to copper traces. Ensure the minimum allowed bend radius is identified in the design phase of the project. Bending stress can also be increased by the amount of copper in the flex region of the board. Ensure the shortest possible length of copper is routed in the bend area to prevent overheating. In addition to the bend radius, it is important to consider how many layers a flex circuit should have. Having more layers generally increases the cost of a pcb, so this should be taken into account during design.

How to Test the Reliability of Flex Circuit PCBs

A good pcb surface finish is also important to consider, as it protects the copper from corrosion and provides a solderable surface. A good choice for a flex circuit is electroless nickel immersion gold (ENIG). The ENIG coating is a very thin layer that will help to maintain the optimum electrical performance of a flex circuit while preventing it from oxidizing under harsh conditions.

Another important flex circuit pcb test is to measure the glass transition temperature (Tg) of the material. This is done by exposing the circuit to an elevated temperature for a set period of time and recording its results. The resulting Tg values indicate whether the flex circuit can withstand the temperatures it will encounter in use, including heat, moisture, chemicals, and shock/vibration.

Other important flex circuit tests include measuring the CTE, or coefficient of thermal expansion, of the board. This is performed by heating the board to 105 degrees Celsius and then placing it in a DSC tester. This is a slow and gentle test that measures the rate at which the board expands or contracts in response to temperature change.

Lastly, it is important to know if your pcb will be plated using panel plating or the more common pad-only plating (button plating). With panel plating, all areas on the flex circuit will have copper, whereas with button plating only the pads and vias will be plated. Button plating is preferred for flex circuits because it improves etch yields, especially in small traces, while increasing the accuracy of copper thickness and width.