An overview of the grain boundary strengthening mechanism of Sn-Bi-Ag low-temperature solder paste

The crystal boundary strengthening mechanism of Sn-Bi-Ag low-temperature solder paste is a multifactorial synergistic process, which is described in detail below from three perspectives: the specific role of each mechanism, research cases and data support, and synergistic effect:

I. Grain boundary precipitation and pinning of Ag3Sn intermetallic compounds (core strengthening mechanism)

Precipitation strengthening

In Sn-Bi melts, Ag reacts with Sn to form fine Ag₃Sn particles.These particles tend to be enriched at the grain boundaries of the Sn grains, as they are fast pathways for atomic diffusion and preferred locations for second phase segregation.

Pinning effect

Diffusely distributed hard Ag₃Sn particles impede grain boundary migration and grain growth through Zener pinning effect.Experiments have shown that Ag₃Sn particles can effectively prevent the slip and accumulation of dislocations at grain boundaries and significantly enhance the "strength" or "stability" of grain boundaries.

Data Support

In the study of Sn3.8Ag0.7Cu alloys, fine particles of Ag₃Sn were distributed on the grain boundaries, hindering the sliding of the grain boundaries and strengthening them.Similarly, in Sn-Bi-Ag alloys, the pinning effect of Ag₃Sn kept the interfacial IMC layer thin and uncracked after isothermal aging of the solder joints at 120 °C for 2000 h, inhibiting the appearance of Kirkendall voids.

II. Inhibition of Bi bias aggregation and embrittlement at grain boundaries (key improvement mechanism)

Problem of Bi bias aggregation

In Sn-Bi eutectic alloys, Sn atoms react metallurgically with Cu atoms in the pads to produce the intermetallic compound Cu6Sn5, Sn atoms are constantly consumed, and precipitated Bi atoms are prone to bias polymerisation at the grain boundaries, forming a continuous brittle layer, which becomes a crack extension path, leading to a brittle alloy.

Mechanism of action of Ag

Physical occupation: Ag₃Sn particles occupy the position of grain boundaries, dilute Bi concentration and reduce the formation of continuous brittle layer.

Kinetic modification: Ag may alter the bias behaviour of Bi, e.g. by lowering its equilibrium concentration or slowing down the bias rate.

Cases and data

The low temperature FL series alloys developed by Shenzhen Fuyingda Company itself, by adding micro-nano enhanced particles of Ag, Cu and other elements, the Bi rich phenomenon near the IMC of Sn-Bi alloy solder joints has been improved, the toughness of the solder joints has been increased by 30%, and the elongation at break is more than 12%.

III. Grain refinement (basic strengthening mechanism)

Heterogeneous nucleation effect

Ag₃Sn particles act as non-uniform nucleation sites for Sn grain crystallisation, increasing the nucleation rate and refining the grain size.

Hall-Petch effect

Grain refinement hinders dislocation motion more effectively by increasing the grain boundary area.The fine grain organisation also uniformly disperses stresses and reduces stress concentration.

Experimental evidence

In Sn-Bi-Ag alloys, Ag₃Sn refinement makes the solder joints remain fine grained after aging, and the interface IMC layer is thin and stable.

IV. Solid solution strengthening (minor contribution)

Solid solution of Ag

Ag solid solution in Sn is limited, direct solid solution strengthening role is weak, can add trace antimony Sb elements, play a solid solution strengthening role is more obvious.

Indirect effect

The formation of Ag₃Sn consumes part of Sn and may slightly change the matrix composition, but the main strengthening effect still comes from Ag₃Sn particles.

V. Synergies and integrated performance enhancement

Multi-mechanism synergy

Ag₃Sn pinning: directly enhance grain boundary resistance to deformation.

Inhibition of Bi bias: solving the problem of Sn-Bi alloy grain boundary brittleness.

Grain refinement: Provide base strength through Hall-Petch effect and increase effective grain boundary area.

Performance Advantages

Strength and toughness: Sn-Bi-Ag alloy has a tensile strength of over 80MPa and an elongation of 16%-30%, which is significantly better than Sn-Bi alloy.

Thermal stability: after aging at 120°C for 2000 hours, the interface IMC layer is thin and uncracked, with excellent thermal fatigue performance.

Process compatibility: low-temperature welding (e.g., 150°C) to reduce thermal damage, suitable for LED packaging, flexible circuit boards and other scenarios.

VI. Conclusion

The addition of Ag effectively strengthens the grain boundaries of Sn-Bi low-temperature solder by forming Ag₃Sn and modulating the microstructure.This multi-mechanism synergy significantly enhanced the mechanical properties (strength, creep resistance) and reliability (thermal fatigue resistance) of the solder joints and overcame the inherent defects of Sn-Bi alloys.In practical applications, Sn-Bi-Ag alloy has become the material of choice for demanding scenarios such as the welding of new energy vehicle battery lugs and the packaging of 5G base stations, demonstrating the potential for transformation from an "alternative solution" to a "mainstream choice".