Comparative study of micro-BGA reliability under bending stress

The micro-ball grid array (μBGA), 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 μBGA type, eutectic tin-lead solder ball bumps are used instead of plated nickel and gold (Ni/Au) bumps. Assembly and reliability of the μBGA´s PCB, which is soldered by conventional surface mount technology, has been studied in this paper. The bending cycle test (1000 με to -1000 με), is used to investigate the fatigue failure of solder joints of μBGA, PBGA, and CBGA packages reflowed with different heating factors (Q_η), defined as the integral of the measured temperature over the dwell time above liquidus (183°C). The fatigue lifetime of the μBGA assemblies firstly increases and then decreases with increasing heating factor. The greatest lifetime happens while Q_η is near 500 second-degree. The optimal Q_n range is between 300 and 750 s°C. In this range, the lifetime of the μBGA assembly is greater than 4500 cycles if the assemblies are reflowed in nitrogen ambient. SEM micrographs reveal that both μ & P-BGA assemblies fail in the solder joint at all heating factors. All fractures are near and parallel to the PCB pad. In the μBGA assemblies cracks always initiate at the point of the acute angle where the solder joint joins the PCB pad, and then propagate in the section between the Ni₃Sn₄ intermetallic compound (IMC) layer and the bulk solder. In the CBGA assembly reliability test, the failures are in the form of delamination, at the interface between the ceramic base and metallization pad.