Study of micro-BGA solder joint reliability

A new re¯ow parameter, heating factor (Q g), which is de®ned as the integral of the measured temperature over the dwell time above liquidus, has been proposed in this report. It can suitably represent the combined e€ect of both temperature and time in usual re¯ow process. Relationship between reliability of the micro-ball grid array (micro-BGA) package and heating factor has been discussed. The fatigue failure of micro-BGA solder joints re¯owed with di€erent heating factor in nitrogen ambient has been investigated using the bending cycle test. The fatigue lifetime of the micro-BGA assemblies ®rstly increases and then decreases with increasing heating factor. The greatest lifetime happens at Q g near 500 s °C. The optimal Q g range is between 300 and 750 s °C. In this range, the lifetime of the micro-BGA assemblies is greater than 4500 cycles. SEM micrographs reveal that cracks always initiate at the point of the acute angle where the solder joint joins the PCB pad.