| In recent years,Sn-Ag-Cu(SAC)lead-free alloys have been widely used in the packaging industry due to their lower eutectic temperature and superior solderability.Although high-silver SAC lead-free solder has excellent wettability and oxidation resistance,it has high cost,high brittle joints,and poor drop resistance.However,reducing the Ag content will increase the soldering temperature and deteriorate the thermal reliability.Therefore,it is particularly important to choose a lead-free solder with strong applicability,excellent performance,preferable reliability,and affordable price in different service environments.With the gradual promotion and application of high-power devices,the heat flux density in the package is getting larger and larger,which requires the micro-joints that play the role of connection to have better thermal and thermal fatigue resistance.Besides,there is no related equipment that can simulate the thermal effects of high-power devices withstand high frequency and rapid temperature changes in the service environment.Therefore,this article used SAC lead-free solder with different Ag content(Sn-0.7Cu,Sn-0.3Ag-0.7Cu,Sn-1.0Ag-0.5Cu,Sn-3.0Ag-0.5Cu)to prepare lead-free BGA solder balls,SAC/Cu bumps and Cu/SAC/Cu solder joints.Under rapid thermal fatigue,the change of Ag content on the internal microstructure evolution and internal and external crack growth of SAC lead-free solder balls and bumps,the evolution of intermetallic compound(IMCs)at the interface of the bumps and mechanical properties,the internal crack growth laws and different constraints of the sandwich structure solder joints were studied.The main conclusions were as follows:The surface cracking of the four kinds of Ag content solder balls occurred at 650 cycles,that's the germination stage of surface cracks was about 650 cycles.As the number of thermal fatigue cycles increased,the number of cracks on the surface of the solder balls increased.Additionally,the main cracks gradually became deeper and wider,and branch cracks appeared.The secondary cracks gradually developed into the main cracks,and the cracks propagated mutually and finally showed an X-shaped network distribution.Due to increase of Ag content,the number and size of cracks on the surface of the solder balls decreased.At the same time,the degree of cracking decreased and the growth rate slowed down.Therefore,the surface of SAC305 solder balls had better resistance to thermal fatigue cracking.The internal cracks of solder balls were mainly concentrated near the heat source and the left(right)corner of solder balls.In the low fatigue cycles,the number and size of internal cracks increased with the increase of the number of cycles.After 3500 cycles,the number and size of internal cracks tended to decrease.When Ag content was increased,the number and size of internal cracks in the solder balls decreased during the low fatigue cycles.However,the number of internal cracks at the end of thermal fatigue increased,and the degree of cracking became larger.When rapid thermal fatigue cycles were increased,the number and size of surface cracks on the bumps increased.After 3500 cycles,the growth rate slowed down,and the surface cracks were mainly concentrated on the lower side with larger curvature.With the increase of Ag content,the crack growth rate on the bump surface decreased.Compared with the surface cracks of the solder balls,that of the bumps with the same solder composition under the same cycle had a smaller degree of cracking.In addition,the incubation period of surface cracks initiation was longer,and the growth rate was smaller.With the increase of the rapid thermal fatigue cycle,the internal cracks were mainly concentrated at the corners of the bottom of bumps,and randomly distributed inside the solder of bumps.When the rapid thermal fatigue cycles were increased,the cracking path gradually transferred from the solder / IMC to the interfacial layer.In the same cycle,the interface crack width of bumps first decreased and then increased with the increase of Ag content.Accordingly,the cracking degree of interface cracks of SAC105/Cu bump was the smallest.Compared with the internal cracks of solder balls and that of sandwich solder joints,the internal cracks of the bumps germinated between 800-900 cycles,and the internal cracks of solder balls germinate between 1000-1500 cycles,and the germination period of internal cracks in sandwich solder joints was between 600-700 cycles.With the increase of thermal fatigue cycle,the thickness of interfacial IMC increased and the morphology of interfacial IMC changed from sawtooth shape to flat shape and finally tended to be flat.The growth rate of IMC showed a trend of first increasing,then flattening and finally increasing.At the end of thermal fatigue,the maximum thickness of IMC was only about 3?m,and the main component of IMC was still Cu6Sn5.As the Ag content decreased,the smoothness of interfacial IMC increased and finally became flat.In the middle of thermal fatigue(2500 cycles-3500 cycles),the growth rate of IMC thickness of all bump interfaces was negative or little change.In the initial stage of thermal fatigue,the shear strength of bumps had a large reduction rate and the mechanical properties drop sharply.After that,reduction rate was reduced,and the mechanical properties of the bumps were extremely unstable(discrete).At the end of thermal fatigue,the interfacial bonding strength of SAC305/Cu bump decreased,which was 63.2%,and the interfacial bonding strength of Sn0.7Cu/Cu bump was lower,which was 61.89%,and the interfacial bonding strength of SAC0307/Cu bump was lowest,which is 46.26%.Therefore,SAC0307/Cu had better shear resistance under rapid thermal fatigue. |
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