Failure Mechanism of Underfill Fillet Cracks in Flexible Wearable Electronics

This paper studies the failure mode and failure mechanism of the underfill fillet cracks of a flexible surface-mount device, where the device is produced by mounting the stiff components directly onto the surface of a flexible printed circuit board; the underfill encapsulants are used to reduce the stresses around the solder joints. The underfill fillet cracks are observed in failed devices by the scanning electron microscope. The mechanics behind how the underfill geometry and the material property affect the reliability of the device is investigated by studying the stress singularity near the interfacial edge of the underfill fillet/flexible substrate. It is found that there exists a strong stress concentration near the interfacial edge of the underfill fillet/flexible substrate, due to the large mechanical and geometry mismatch between the underfill fillet and the flexible substrate. The results present the method to modulate the level of stress singularity by changing the wedge angle of the underfill fillet and the material property of the underfill encapsulants.