Stress and Reliability Challenges of Underfills in Large-Size Fan-Out Multichip Module Packages

This study explored the impact of additives on the application of underfill in large-size fan-out multichip module packages. The experimental results of a high-temperature storage test demonstrated that the selected underfill developed cracks. Optical microscope examination of a cross section of the cracks revealed an oxidation layer. Through addition of an anti-oxidation agent to the underfill, both forming of the oxidation layer and cracking in the underfill resulting from thermal oxidation were prevented. By contrast, when an underfill without an added flexibilizer was used in the large-size fan-out multichip module package and subjected to a temperature cycling test, cracks were observed in the chip corners, which had formed because of excessive stress in the package. When both the antioxidant and flexibilizer were added to the underfill, the generation of thermal oxidation-induced and stress-induced cracks in the package was prevented.