Static Bending of Flip-Chip Structures Under Humid Conditions

The miniaturization of electronics poses challenges to reliability demands on devices. In applications where both high reliability and small size are demanded, reliability testing plays a critical role. Medical applications are one such example, and in this case, not only must the durability of electronics be considered but the safety of the device to the human body is also of vital importance. This paper considers the use of thin FR-4 substrates and thin silicon chips in medical applications. The joining is made using anisotropically conductive adhesives. The assemblies are protected using a parylene C conformal coating, which has been proved to be a biocompatible material and, thus, safe for medical applications. The assemblies are bent to a fixed radius during humidity testing. Nonbent and noncoated test lots are also studied for comparison. Two different humidity testing methods are used: the 85/85 constant humidity test and humidity cycling test. Furthermore, the test structures are simulated using finite element analysis. The results show the protective nature of parylene C coating and the differences between bent and nonbent structures. Parylene C coating seems to support the structure by lowering the shear and von Mises stresses formed, resulting in better reliability. Furthermore, the results show a clear difference in the two humidity testing methods, giving value to the standard 85/85 testing.