PCBs in an electric product for automotive or industrial equipment are exposed to harsh working environments with significant temperature differences and in some cases, SAC305 solder alloy cannot provide sufficient reliability.
Temperature cycling causes stress in the solder joint due to the components and PCB to expansion and contraction, and the stress causes cracking in the solder joint.

Therefore, a solder alloy which withstands thermal-cycle stress is required.

Dislocations (irregularities in the crystal structure) occur in metals including Tin (Sn), the bulk material of the solder. Dislocations eventually cause plastic deformation and cracking in the solder joint. Bi and In do not form compounds with Sn but replace the Sn atoms in their lattice positions (form a solid solution). As the atomic radius of Bi and In are significantly larger than Sn, they form distortions within the matrix which prevent propagation and accumulation of dislocations. In addition, this alloy has a lower melting point than SAC305 and reduces the thermal stress to the components during reflow.

Solid solution strengthening offers high thermal-fatigue resistance as it does not fail even in a harsh temperature condition, such as -40˚C ~+150˚C.

Thermal-fatigue resistance contributes to an extension of product life of automotive electronics, failure of which may result in safety issues and for industrial equipment the product is expected to be longer-lasting.