Bi-Based High Temperature Lead-Free Solder Wetting And High Temperature Aging

High-temperature lead-free solders play an important role in connection,conduction and heat conduction in electronic packaging.Traditional high-lead solder contains Pb,which has great harm to human,health and environment.Therefore,the research of high temperature lead-free solder has become one of the research hot spots in electronic packaging.Alloy Bi-based alloys are considered as the most likely alternative to high lead solders for high temperature electronic packaging because of its shear modulus(12 GPa)and melting point(271 °C)are both very close to Pb-5Sn solder.However,the bond strength between the Bi-based alloy brittleness and the brazed joint is weak.Therefore,finding some methods to improve the physical properties and mechanical properties of Bi-based alloys is of great significance to its use in the microelectronics packaging industry.The spreading area method and the sessile drop method were used to characterize the effects of Cu,Zn and Ag added on the wettability of Bi-based solder on Cu and Ni coatings.SEM was used to characterize the microstructure of Bi-base solder joints before and after aging at 160°C.The wettability of the solder measured by the spread area method and the seat drop method showed that the alloying element Zn promotes the wettability of the solder on the Cu substrate,and the alloying element Cu promotes the wettability of the solder on the Ni plating layer.When 0<Cu wt.%<0.5,the Bi-Cu solder material increased from 25° to 29°as the content of Cu increases.When 0.5<Cu wt.%<1.0,wetting angle does not change.When Bi-Ag solder Ag wt.% = 0~2.5,wetting angle does not change to 25°.When Ag wt.% = 5.0,wetting angle decreases to 24°.SEM was used to characterize the interface microstructure changes before and after aging,it can be seen that the interfaces of solder Bi and Bi-0.5Cu/Cu did not change significantly after aging for 72 h and 140 h at high temperature,and there are no cracks and other defects generated.The brittle NiBi phase is formed when the interfaces of solder Bi and Bi-0.5Cu/Ni is aged at high temperature.When the thickness of the Ni plating layer is5 μm,the solder Bi-0.5Cu has a phenomenon of “eating gold”.When the Intermetalliccompounds layer(IMC)of the solder Bi-1.2Zn/Cu interface is aged at high temperature,the compound grows decomposes causing IMC to become narrower.At the interface of Bi-1.2Zn/Ni solder,there are two forms of NiZn compounds which are IMC layer and white weave,respectively.The weave grows up and then decomposes with the increase of aging time.The loss of Zn in the IMC layer leads to the formation of pores.