Reliability Assessment of Indium Micro Bumps for 2.5D/3D Electronic Packaging through Cryo-Argon Milling Technique

Abstract

Indium bumps are widely used in electronic packaging for interconnecting microelectronic devices due to their low melting point of 156.6°C, excellent bonding capabilities at room temperature, high thermal conductivity, ductility at extremely low temperatures, and ability to compensate for the mismatched coefficient of thermal expansion (CTE) between different materials. However, cross-sectioning indium bumps can be challenging due to the softness and low melting point of indium. The cutting tools and techniques used must be optimized for the specific sample being analyzed, and specialized preparation techniques such as cryogenic freezing or chemical etching may be required, which can be expensive and time-consuming. Cryo-Focused Ion Beam (FIB) is also a technically demanding process that requires careful optimization of milling parameters, multiple iterations of milling and imaging, specialized equipment, and expertise, which can make it too expensive. It should be noted that the maximum achievable cross-sectional area with cryo-FIB is limited, typically in the range of hundreds of micrometers. Cryo-Argon milling is an effective technique for obtaining high-quality cross-sections of indium bumps arrays and other materials prone to deformation or melting during traditional cross-sectioning. This process involves freezing the sample using liquid nitrogen and bombarding it with a stream of Argon gas ions, which reduces the deformation and melting of the indium. This paper presents challenges related to sample preparation and mechanical polishing for Ar milling technique, and we provide key solutions to these challenges to obtain millimeter-wide and damage-free cross-sections of arrays of indium micro bumps.