Mechanical Evaluation
A mechanical evaluation method subjects flexible circuit boards to repeated bending cycles to determine the operational lifespan of the conductive traces under dynamic strain. Performing flexure fatigue testing involves mounting a test specimen between a stationary and a reciprocating clamp to flex the assembly through a defined bend radius. This testing reveals the durability of both the copper traces and the polyimide substrate under simulated operational wear.
Cyclic Mechanism
Repetitive bending induces alternating tensile and compressive stresses within the metallic layers of the flexible circuit. In flexure fatigue testing, the test apparatus runs continuously at a specified frequency while a low-voltage electrical monitoring circuit tracks the resistance of the conductive traces. Micro-cracks begin to form in the copper grains as the cycle count rises, gradually increasing the measured resistance.
The test runs until a predefined resistance threshold is exceeded, which signifies a fatigue failure of the conductor. This method ensures that the chosen copper foil type can survive the physical demands of applications such as folding hinge mechanisms or sliding wearable devices.
Stress Limit
Design parameters like the bend radius and copper thickness determine the number of cycles a board can survive before failure. Minimizing trace thickness and placing the copper foil on the neutral bend axis of the laminate increases the fatigue life of the assembly. Test results help design engineers select between electrodeposited and rolled-annealed copper foils for dynamic applications.
High-reliability flexible boards must achieve millions of cycles to pass qualification.
