The Mechanical Behavior Of BGA Solder Joints Under Different Loading Modes

The reliability of solder joints is important to life of electronic device.Inmost of the electronic device, the solder joint bear the dual role of mechanicalconnections and electrical connections. More than70%of the electronic devicefailure is caused by the solder joints failure. As an important part of thepackaging device, the reliability of solder joints is closely related to therealization of the function of packaging device.With the development of the SMT (Surface Mounted Technology)therequirements of the new lead-free solder need to comparable to or even surpass63Sn37Pb solder on comprehensive performance such as mechanical behavior,solderability and solder joint reliability for serving long-term in various harshenvironment. Therefore the analysis and research of the reliability of the solderjoints is crucial. At present, the research on reliability of BGA solder joints underdifferent loading modes are mainly focus on temperature cycle, shock andvibration, such as thermal cycling test, drop test, vibration test. In this paper,loading unloading, step loading unloading, cycle loading unloading, step cycleloading unloading were used to analysis the reliability of the BGA solder joint.In this paper, the mechanical behavior of BGA solder joints under differentloading modes was investigated. The results indicated that the failure locationunder different loading modes is the same，the stress concentration area locatedat the loading side of the IMC, the plastic strain concentration area located at theunloading side of the solder ball neck. The residual stress of BGA solder jointsunder the cycle loading is largest, which is most easy to cause the unstability ofthe crack propagation. The plastic strain of BGA solder joints under the stepcycle loading is largest and it is the most easy to produce fatigue crack becauseof plastic strain concentration.Through the research on step loading-unloading test indicated that with the increase of maximum load, the plastic strain become the main reason for thefailure. With the increase of loading rate, the equivalent plastic strain areachanges from the center to the neck but the stress concentration area remains thesame. And the stress becomes the main reason for the failure. With the increaseof loading step, the crack caused by the plastic strain concentration is more likelyto occur.The research on the cycle loading-unloading test indicated that with theincrease of maximum load, the plastic strain becomes the main reason for thefailure and the fatigue life will reduce. The increase of load time could only makethe process of creep happen adequately, but could not produce extra creep.Holding time had small influence on the fatigue life of solder joints. The increaseof the cycle number could make the process of plastic deformation adequatelyhappen and the damage accumulation increase. With the increase of the cyclenumber, the influence of the number on fatigue life is smaller and smaller.