| Solders are used for connecting device chip and components in manufacturingelectronic product industry, and solder joints act as mechanical support, electricconnection and thermal conductivity. The Au-20Sn (weight percentage) alloy wasselected as a candidate of the high temperature lead free solder alloy in this paper. Theformation of the solidification structures under different cooling rate wereinvestigated by the microstructure observation, scanning electron microscope, X-raydiffraction and differential thermal analysis. Thermal aging treatments between theoperating temperature ranges were employed to simulate the high-temperature serviceconditions and the relevant microstructural stabilities of solders were explored.Finally, the influences of the cooling rates on the Vickers hardness of the solder weremeasured before and after aging treatment. More details were given as follows.The influences of cooling rates on the room-temperature microstructure of weightpercentage composition Au-20Sn were investigated by changing the cooling medium.The results indicate that the solidification process of furnace-cooled alloy is close toequilibrium solidification, and the room-temperature microstructure, which consists ofthe primary phase ζ, fine lamellar eutectic microstructure (ζ’+δ) and coarse eutecticmicrostructure. The microstructure of water-cooled alloy presents the dendriticmicrostructure in the room-temperature microstructure which results from the highcooling rate.Thermal stability of solder alloy under operating temperature was investigated byhigh-temperature aging treatment. For this purpose, the alloys were aged at a lowertemperature,150℃from10hours up to100hours and compared with aging at200℃for respective durations. The results show coarsening of the microstructure at150℃,while the microstructure aging at200℃for respective durations occurs phasetransition. The same situation also happens in the water-cooled alloys with thedifference in phase transition in the microstructure obviously observed until the agingtime up to100h at200℃.According to the results of the Vickers hardness measurement, the hardnesses ofthe furnace-cooled alloy increase after the thermal aging at150℃because of thecoursing of hard Au5Sn phase, while the hardness of the furnace-cooled alloy decrease aged at200℃due to phase transition. The hardnesses of the water-cooled alloyincrease after thermal aging for the aggregation of the hard Au5Sn phase; the phasetransition occur aged at200℃for100h, leading to the decrease of hardness. |
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