Lead-Free Solder Composition for Secondary Reflow Reliability

The patent modifies the chemical composition parameters of the solder alloy by adding specific elements (Ni: 0.1-1 wt%, Ge: 0.1-1 wt%, Mn: 0.05-0.5 wt%, P: 0.05-0.1 wt%) to the Sn-Cu base alloy. This changes the physical and chemical properties of the solder, creating a new alloy system with improved reliability under repetitive thermal cycling while maintaining appropriate melting temperature for electronic assembly.

The patent creates a composite solder alloy by combining multiple elements (Sn, Cu, Ni, Ge, Mn, P) to form a new material system. This composite alloy leverages the beneficial properties of each element: Sn provides low melting point and good wetting, Cu provides strength, Ni provides oxidation resistance and grain refinement, Ge provides hardening and liquidus temperature control, Mn provides grain refinement, and P provides deoxidation. The synergistic combination resolves the reliability issue while maintaining processing advantages.

The patent replaces the expensive AuSn alloy with a much more cost-effective Sn-Cu-based alloy containing small amounts of Ni, Ge, Mn, and P. The base metals (Sn, Cu) are abundant and inexpensive compared to gold, making the new alloy economically viable for mass production while achieving the desired high-temperature performance.

The patent adjusts the composition parameters to achieve a melting temperature suitable for high-temperature applications (above conventional SnAg but below AuSn) while maintaining ductility and processability. The specific addition of Ni (0.1-1 wt%) and Ge (0.1-1 wt%) controls the liquidus temperature and solidification behavior, creating an alloy that processes like conventional solders but performs at higher temperatures.

The patent raises the liquidus temperature of the solder alloy by adding Ni (0.1-1 wt%) and Ge (0.1-1 wt%). The Ni forms intermetallic compounds that elevate the liquidus temperature above 250°C, ensuring the solder remains solid during secondary reflow operations. The Ge addition further controls the solidification range and liquidus temperature, preventing premature melting during subsequent processing steps.

The patent proactively designs the alloy composition to anticipate and prevent future problems during secondary reflow and rework operations. By building in a sufficient temperature margin (liquidus >250°C vs. reflow at 240-250°C), the alloy provides a safety buffer that prevents die tilt and positioning errors before they can occur during later manufacturing steps.