Impact of thermal aging on the thermal fatigue durability of Pb-free solder joints

It is known that isothermal aging of lead-free solder materials at elevated temperatures leads to changes in microstructure. A growth of the intermetallic interface between bulk solder and substrate material can be observed as well as the growth of intermetallic particles in the bulk solder itself. Isothermal aging is used as preconditioning for reliability and mechanical tests. The idea is to bring the solder microstructure to a common baseline condition for comparison purposes and relax the stress in the solder joint coming from solidification. However, limited studies on the impact of thermal aging on the reliability of the solders are available. For solder interconnections, thermal aging will impact both the interconnect interfaces and the bulk material. Thermal aging has been shown to increase the interfacial intermetallic compounds (IMC) thickness and changes IMC composition which can result in reduced interconnect reliability. However, the thermal aging can also change the distribution of IMC particles in the bulk material that will also influence interconnect reliability. The focus of the present study is the evaluation of the temperature cycling fatigue reliability of solder joint interconnects after a exposure to extended isothermal aging at 100°C. This paper discusses the impact of isothermal aging on the thermal fatigue durability of SAC305, SAC105, and SN100C and SnPb assemblies. The test structures consist of chip resistors (CR2512 with Sn-finish) soldered to Cu/OSP-finish pads on an FR4 substrate. The test structures were subjected to isothermal aging of 100°C for 24, 100, 500 and 1000 hours. Subsequent, the aged structures were exposed to temperature cycling test conditions (-55°C to 125°C, 15 min dwell, ramp rate: 10°C/min) while monitoring electrical continuity of conductive paths formed with the specified solders. Failure identified as an electrical discontinuity was measured during the temperature cycle exp- - osure and the impact of high temperature aging was documented. Specimens aged at 100°C for 24 hours acted as the controls for the experiment. The thermal cycling reliability of the lead-free solders was compared with that of eutectic SnPb.