Thermal Cycling Reliability of Lead-Free Solder Alloys using Surface Mount Resistors Considering Aging

Abstract

There have been several new solder alloys developed for high reliability applications. As with any new product, the reliability has been the primary concern even in the case of new solder alloys, and the stakes are high when it comes to critical applications where failure is not an option. Several new elements such as Bi (Bismuth), Sb (Antimony), In (Indium), Ni (Nickel) are added to SnAgCu (SAC) based systems in order to mitigate the effect of aging and improve the reliability. In this study, a thorough investigation of the performance of such three new alloys are conducted using surface mount resistors (SMR) and are then compared with SAC305 solder alloy. The test vehicle consists of 2512SMRs along with BGAs and LGAs with OSP surface finish, even though only data regarding the SMRs are analyzed in this study. Each test vehicle has six SMRs connected in series, and all alloys have five boards in each aging condition. One set of boards are aged for a period of three months at 125°C before thermal cycling. The aged and non-aged boards were then thermally cycled for 8000 cycles from -40°C to +125°C with a ramp rate of 10°C per minute and dwell times of 15 minutes at high and low temperatures.Two parameter Weibull analysis is used to quantify the performance of the different alloy materials. B10 life is also used to analyze the early failures in each case. It was found that aging had a negative impact on the fatigue life, even though the extend of this impact could vary according to the solder alloy, aging period and component, among several other factors. In most cases, a drop of about 30%-40% in characteristic life was observed.