Failure Analysis Of Lead-free Solder Joints Under Temperature-vibration Coupling

With the wide application of electronic products, the reliability and theservice life of them is high demanded increasingly. Thermal shock resistanceperformance of Sn-Ag-Cu-Bi-Ni （SACBN07）, SAC305and SAC0307isconducted and compared in this study. This experiment also takes the SAC305solder as carrier to study the failure of lead-free solder joint under mechanicalvibration/thermal-mechanical loading. This Article uses independent designdigital data acquisition system to indicate the failure process of solder joint andcompares life of micro solder joint in different locations of PCB board at thesame time.Research shows that SACBN07shows the best thermal shock resistancecompared to two other solder. And three types of solders exhibit different crackpropagation paths during thermal shock test. Crack for SAC305initiates andpropagates in the bulk solder. Crack for SACBN07propagates from the bulksolder to IMC layer. And crack for SAC0307is observed between IMC andcopper pad. In addition, addition of Bi and Ni elements into Sn-Ag-Cu soldereffectively suppresses the IMC growth. Under the same test condition, the layerthickness of IMC for SACBN07is the thinnest in three solders. The hardnessvariation of SACBN07subjected to thermal shock loading is the slightest. Andthe hardness of SACBN07, SAC305and SAC0307decrease8.6%,12.5%and28.3%respectively after thermal shock test.Research shows, under the vibration load effect, micro solder joint mainlypresents three kinds of failure modes: the crack spreads in the bulk solder, thecrack spreads in the bulk solder and IMC interface, the crack spreads in IMC andcopper pad interface. In the mechanical vibration loading conditions, microsolder joint crack is easy to produce in bulk solder and IMC interface. According to Failure data, the failure process of solder joints is represented by using datacurve, failure was divided into four phases: reliable connection, crack initiation,crack propagation and complete failure. Under the mechanical vibration loads,with the increase of vibration acceleration, the lifetime of solder joints is reduced;under the same acceleration, PCB U1and U3position component life below theU2position of the component life.The temperature–vibration coupling test was designed. The methods of two-parameter weibull distribution and failure analysis were applied. The effect oftemperature（25℃,65℃and105℃）on solder interconnects subjected tovibration loading was studied. Results indicate that the temperature increaseimproves solder interconnects life. The solder joints at25℃and105℃always exhibit three fracture modes, interfacial crack between IMC（Intermetallic Compound）/Cu pad, crack within the bulk solder and mixedcrack（both interfacial crack and crack within the bulk solder）. However, themain failure mode at two temperatures is obviously different. Results indicatethat the main failure mode at25℃is the crack between IMC and copper pad,while the main failure mode at105℃is the crack within the bulk solder. Thefailure mechanism is transformed from brittle fracture to ductile fracture withtemperature rise from25℃to105℃.