Comparative study of micro-BGA reliability under bending stress

The micro-ball grid array (mu BGA), a form of chip scale package (CSP), was developed as one of the most advanced surface mount devices, which may be assembled by ordinary surface mount technology. In the latest mu BGA type, eutectic tin-lead solder ball bumps are used instead of plated nickel and g old (Ni/Au) bumps. Assembly and reliability of the mu BGA's PCB, which is s oldered by conventional surface mount technology, has been studied in this paper. The bending cycle test (1000 mu epsilon to -1000 mu epsilon), is use d to investigate the fatigue failure of solder joints of mu BGA, PBGA, and CBGA packages reflowed with different heating factors (Q(eta)), defined as the integral of the measured temperature over the dwell time above liquidus (183 degreesC), The fatigue lifetime of the mu BGA assemblies firstly incr eases and then decreases with increasing heating factor, The greatest lifet ime happens while Q(eta) is near 500 second-degree. The optimal Q(eta) rang e is between 300 and 750 s degreesC, In this range, the lifetime of the mu BGA assembly is greater than 4500 cycles if the assemblies are reflowed in nitrogen ambient. SEM micrographs reveal that both mu &P-BGA assemblies fai l in the solder joint at all heating factors, All fractures are near and pa rallel to the PCB pad, In the mu BGA assemblies cracks always initiate at t he point of the acute angle where the solder joint joins the PCB pad, and t hen propagate in the section between the Ni3Sn4 intermetallic compound (IMC ) layer and the bulk solder. In the CBGA assembly reliability test, the fai lures are in the form of delamination, at the interface between the ceramic base and metallization pad.