Effect of Dwell Times and Ramp Rates on the Thermal Cycling Reliability of Pb-free Wafer- Level Chip Scale Packages - Experiments and Modeling

摘要

Lead-free (Pb-free) solder has seen increasing use in interconnect systems for electronic packages due to legislative and marketing pressures. The NEMI selected eutectic Sn3.9Ag0.6Cu alloy (or a close variation near eutectic Sn3.5Agl.Ocu used in this study) is a leading Pb-free substitute for the Sn/Pb solder candidate. The reliability of this Pb-free solder alloy under accelerated thermal cycling and thermal shock testing as a function of testing parameters such as dwell time and ramp rate is critical in qualifying the performance of these Pb-free alternatives with the traditionally used Sn37Pb solder This paper presents the reliability of Pb-free solder joints in wafer level chip scale packages (WLCSPs) , which are extensions of flip-chip packaging technology to standard surface mount technology, with external dimensions equal to that of the silicon device [1]. The reliability of these packages under both liquid-to-liquid thermal shock (LLTS) testing and accelerated air-to-air thermal cycling (AATC) conditions, as a function of dwell times and ramp rates is evaluated using extensive experimental testing in combination with finite element analysis. Besides, two asymmetric cycles in which the cold and hot dwell times differ at two temperature extremes were studied. Along with the Pb-free solder, some test vehicles were built using eutectic Sn-Pb solder and evaluated for comparison purposes. Experimental results show that an increase in ramp rate does not adversely affect the solder joint reliability in the case of Pb-free solder. The reliability of lead-free WLCSPs was highly dependent upon the dwell time at the temperature extremes, with this dependency being considerably greater for the lead-free alloy than for Sn/Pb at 0°C and 100°C. Accelerated test results show that increasing the dwell time from 280 to 900 seconds reduced the N_(63.2) of the Sn/Pb samples by 12% and the Pb-free samples by 65%. Reliability during asymmetric cycles resulted in a trend that is similar in two cases studied. A predictive equation was developed to evaluate the characteristic life of the package with respect to the dwell time. Non-linear, finite element (FE) modeling was conducted using temperature dependent creep constitutive relations for the Pb-free solder to understand the experimental trends observed. The FE results predicted the same trend of the package reliability as observed experimentally, with respect to the changing dwell and ramp times. The finite element predictions demonstrated reasonable correlation with the experimental observations.