Open defects in PBGA assembly solder joints

PBGA has the advantage of being compatible with existing surface-mount process. The eutectic solder balls collapse during reflow accommodates the lack of co-planarity, the surface tension between solder ball and associated pad creates a strong self-centering property to compensate for any placement offsets. However, some problems emerged in practice due to its special features. The major challenges for manufacturer are the inspection of solder joint and to perform touch-up. In most cases, the defect of solder joint cannot be found until in-circuit or functional testing is performed. At this stage, it becomes difficult to determine whether the defect is due to a solder joint or the component itself. Hence controlling the open defect is very important in the PBGA assembling process. It was found that three kinds of defective modes are responsible for the open defect; insufficient heating in the solder melting phase, poor thermal stability of PCB and PBGA and insufficient amount of printing solder paste. Based on the defect mechanism analysis, by optimizing the soldering process, very high assembly yield was achieved.