A quantitative method of reliability estimation for surface mount solder joints based on heating factor Qη

A novel method of reliability analysis on thermal fatigue failure for surface mount solder joints, based on the heating factor Qη, is presented, by which quantitative reliability estimation and prediction of solder joints suffering from cyclic thermal stress can be done. Based on the typical lifetime data of thermal cycling test, the relationship of the mean time to failure (MTTF) as well as the reliability of solder joints as an explicit function of Qη is deduced and presented in a unified mathematic form. Numerical calculations are performed, and the result shows that the MTTF decreases quickly with the increases in heating factor and then slowly approximates to a constant value when Qη ⩾ 1500 s °C. The solder joint reliability in terms of thermal cycle degrades in an analogical fashion for different heating factors. For any given thermal cycle, calculation suggests that to obtain a higher reliability, a lower heating factor should be controlled during soldering. The presented method gives an applicable solution and can be used for online reflow control in industry. On the one hand, an ideal reflow profile can be achieved by properly controlling heating factor during soldering to meet the given reliability goal. On the other hand, the life expectancy of solder joints can be approximately estimated and predicted from a known reflow profile with a specified heating factor. Finally, for a specified reliability goal, how to properly choose and control heating factor during soldering to achieve reliable solder joints is discussed.