Failure Research Of SMT Low-Ag Lead-free Solder Joint Under Mechanical Vibration Loading

With the increasing application of portable electronic products, service condition of inner component of electronic products become more and more rigorous. Failure of interconnected solder is not only due to thermal stress and microstructure deteriorating caused by temperature and void crack caused by electronic migration, but also mechanical vibration and shock leading to microstructure damage and fracture. So vibration and shock have become gradually a main factor in the process of failure of solder joint. Based on related vibration standard in the paper, we not only designed SMT vibration experiment board but also made accelerated failure test with self-designed data acquisition system monitoring failure process of solder joint. Moreover, failure process, failure mode and failure law of solder joint were studied in the paper under vibration loading by analyzing cross-section of failed solder joint and processing datas.The reseach showed that: Solder joint voltage division circuit and visual test platform are designed in the paper. The data acquisition system real-timely collects data which is affected by high strain-rate loading effectively and precisely through selecting, displaying and storing data. At the same time, crack initiation, propagation and failure process are recorded intelligently, providing strong guaranty for analyzing data subsequently.The research showed that: The solder joints were affected by alternative stress, and its service process was divided into four stages under the condition of mechanical vibration loading. The first stage was a stable service period and this stage accounts for about half solder joint lifetime. In this stage, the solder joints did not initiate macroscopic cracks, but experienced a long process of micro-damage accumulation. Higher stress level was generated in the internal organization of solder joints because solder joints were affected by high strain-rate loading. The second stage is the forming and expanding stage of macroscopic cracks. The crack did not initiate and propagate rapidly until microscopic damage and stress level of solder joints achieved the critical condition of crack initiation. The crack area percentage reached to more 50% quickly in two seconds. The third phase is the steady propagating stage of the failure crack. After crack rapidly formation and propagation in the second stage, the solder joints came into the relatively stable period of slow expansion. The crack area stayed temporarily and propagated slowly, this process accounted for about half solder joints lifetime. The fourth stage is the total period failure of solder joints. The crack of solder joints will not expand to full open rapidly until the crack area percentage reached to more 85%.Under mechanical vibration condition, alternative stress made solder joint pressed and pulled repeatedly. Two sided solder joints of BGA component are affected by higher tensile stress leading to crack initiation and failure of solder joints in advance compared with other solder joints. Crack is initiated between bulk solder and IMC. So higher stress level contributed to crack initiation and immediately propagation. The failure mode conformed to brittle fracture characteristicsIt is an inevitable trend to develop low-Ag lead-free solder joint. The paper studied the effect of indium, Ni and Bi on melting point and wettability of SAC0705 solder alloy and select optimum alloy ratio.The result shows that indium addition has a positive effect on decreasing the melting point of Sn-0.7Ag-0.5Cu solder. With the increasing amount of indium addition, melting point of Sn-0.7Ag-0.5Cu-XIn decreases and melting range increases. Indium addition has a striking positive effect on wettability of Sn-0.7Ag-0.5Cu solder according to wettability results. Considering both melting temperature and solder wettability of two new solders comprehensively, Sn-0.7Ag-0.5Cu-1.5In solder alloy would be a better choice.The addition of Bi and Ni together also decreases the melting point and melting range increases. Proper adding of Bi and Ni addition simultaneously could improve solder wettability. Considering both melting temperature and solder wettability comprehensively, Sn-0.7Ag-0.5Cu-0.05Ni-3.5Bi solder could be a good option.