Abstract
The development of low-temperature, lead-free solders with balanced mechanical reliability, functional properties, and soldering performance remains a key challenge in electronic packaging. This study systematically investigates the mechanical, electrical, thermal, microstructural, and soldering behavior of pure Sn, a hypoeutectic Sn88Bi12 alloy, and two Sn88Bi12-based composites reinforced with Cu and graphite particles. The addition of 12 wt.% Bi to pure Sn resulted in pronounced grain refinement and substantial increases in microhardness and tensile strength, while further improvements were achieved through particulate reinforcement. These strengthening effects were accompanied by moderate reductions in electrical conductivity and thermal transport properties due to enhanced electron and phonon scattering. Differential scanning calorimetry showed that Bi addition effectively lowers the melting temperature of Sn, whereas Cu and graphite reinforcements broaden the melting interval without increasing the melting temperature. Soldering tests on Cu substrates revealed improved wettability of Sn88Bi12 compared to pure Sn, characterized by a lower contact angle, enhanced spreading, and reduced intermetallic compound (IMC) thickness. Although particle reinforcement slightly reduced spreading behavior, both Cu and graphite effectively suppressed IMC growth, with graphite providing the strongest inhibition.

Key words
Sn88Bi12 alloy, Cu and C composites, thermophysical properties, mechanical properties, soldering behavior

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