Microstructure And Properties Of Sn-ag-cu Lead-free Solder Containing Rare Earth La

Sn-3.0Ag-0.5Cu solder alloys with 0,0.05,0.1 and 0.4(wt.%)La additions were prepared by ingot metallurgy processing.Four solder joints with different La content were prepared by reflow process.The mechanical properties,melting behaviors, electrical conductivities,wetting behaviors of Sn-3.0Ag-0.4Cu solder alloys with different La content were investigated.Optical microscopy(OM)and scanning electron microscopy(SEM)were utilized to observe the microstructures of four solder alloys,solder joints and inside the solders after soldering and aging.The effects of La on mechanical properties,physical properties,and microstructures of Sn-3.0Ag-0.5Cu solder alloy were investigated,as well as the effects of La on the formation and growth behaviors of IMC of Sn-3.0Ag-0.5Cu solder joints.The existing form and alloying mechanism of La in Sn-3.0Ag0.5Cu and solder joints were discussed.The conclusions can be summarized as follows:(1)The microstructure of Sn-3.0Ag-0.5Cu solder alloy consists of brightβ-Sn grains surrounded by eutectic networks.The addition of La can refine the microstructure of Sn-3.0Ag-0.5Cu solder alloy by deepening constitutional supercooling.Adding 0.4%La to Sn-3.0Ag-0.4Cu solder alloy can form coarse and primary LaSn₃ dendrite phases.(2)The ultimate tensile strength,yield strength,and elongation of Sn-3.0Ag-0.5Cu have been improved by adding 0.05%and 0.1%La.Adding 0.4%will decrease the elongation.The additions of La improve mechanical properties by reducing the impurities.0.4%La decreases the mechanical property due to the weak combination between LaSn₃ phases andβ-Sn.(3)The Vickers microhardnesses ofβ-Sn and eutectic area are both enhanced with the addition of La and the microhardness of LaSn₃ is much larger than those ofβ-Sn and eutectic area.The solidus temperature,melting temperature and melting range are enhanced slightly with increasing the La addition.The electrical conductivities of Sn-3.0Ag-0.5Cu solder alloys are decreased slightly as increasing the La content.(4)Adding 0.05%and 0.1%La to Sn-3.0Ag-0.5Cu solder alloy can obviously improve the wetting behavior by decreasing the wetting angle,while adding 0.4%La will deteriorate the wetting behavior due to the oxidation of LaSn₃ phases.(5)A scallop Cu₆Sn₅ layer is formed at the interface when the four solder alloys soldered with copper substrate at 260℃for 5min.The thickness and diameter of Cu₆Sn₅ IMC decrease as increasing the La content.There are some voids at the interface of solder alloy with 0.4%La/copper due to the existing of LaSn₃ phases.(6)A continuous planar Cu₃Sn layer is formed between Cu₆Sn₅ layer and copper substrate when aging at 150℃for 100h,while Some Ag₃Sn particles are embedded in the Cu₆Sn₅ layer.The total IMC thickness increases with aging time,and decreases as increasing La content under the same aging condition.Increasing the La content alter the growth mechanism of IMC from grain-boundary diffusion to bulk diffusion.(7)Ag elements exist in eutectic structures after soldering,and solid stick and hollow stick Cu₆Sn₅ IMC have formed in the solder alloys.Ag₃Sn particles are precipitated along the eutectic bonds after aging,and coarsen as increasing the aging time.The LaSn₃ phases formed during casting in Sn-3.0Ag-0.5Cu solder with 0.4% La keep the coarse dendrite shape during soldering and aging process.