Effect of Underfill on the Reliability of Indium-bump Flip-chip Interconnections Under Cryogenic Thermal Shock

Niobium-based single flux quantum (SFQ) technology for superconducting integrated circuits has attracted significant attention due to its advantages of high frequency and low power consumption. Flip-chip interconnect technology has been widely adopted in the packaging of superconducting integrated circuits and superconducting multi-chip modules (MCMs). However, the ultra-low-temperature (liquid helium) operating environment of superconducting SFQ circuits poses substantial challenges to the reliability of flip-chip interconnects. Underfill technology is commonly used to enhance the reliability of flip-chip interconnects. This study investigates the influence of underfill on the reliability of indium bump flip-chip interconnects under cryogenic conditions.Using finite element simulation, the stress distribution of bumps in two configurations—non-underfill and full underfill (including fillet formation)—was analyzed and optimized. The simulation results were validated through cryogenic thermal shock testing. Experimental results demonstrate that although the introduction of underfill slightly increases the stress levels in flip-chip bumps, considering comprehensive factors such as mechanical stability, moisture protection, vibration resistance, drop resistance, storage, and transportation, underfill can significantly improve the reliability of flip-chip interconnects for deep-cryogenic applications.