Reliability of SnAgCu Composite Solder Joints in a Corrosive Environment

The mechanical and electrical structure of the microelectronic circuits is still based on solder joints, so their reliability is essential, mainly in applications exposed to harsh environments, like automotive electronics. The latest solution to improve the solder alloys’ properties is adding nano-sized ceramic reinforcement particles (like TiO2, ZrO2, Al2O3, etc.) into them and making “composite” solder joints [1]. The reinforcement particles improve the solder joints’ mechanical properties (shear and yield strength, microhardness, etc.), with some Kelvin of change in the liquidus temperature of the alloy. The mechanical improvement is caused by dispersion-strengthening mechanisms. The ceramic nano-particles are not soluble in the Sn solders, so they incorporate at the Sn and intermetallic (Cu6Sn5, Cu3Sn, and Ag3Sn) grain boundaries, where they promote heterogeneous nucleation. This results in the suppression of grain growth and decreases dislocation motions [2]. So, composite solder alloys could be the future of automotive electronics, but their reliability parameters are less known. In the present study, the reliability of six composite solder alloys was investigated in a corrosive environment. Sn99Ag0.3Cu0.7 (SAC0307) solder alloy was reinforced by 0.25-0.5wt% Al2O3, SiC, ZrO2, CuO, ZnO, or TiO2 nano-particles. Primary particle sizes <100nm were used. The nano-particles were mixed into the solder paste homogeneously using a YX solder paste mixer for 10 min at 400 rpm. Power FETs and chip resistors were assembled using convectional surface mounting technology (SMT) on FR4- and IMS-base printed circuit boards (PCB) with different composite solder alloys. The assemblies were kept in 85°C / 85 %RH conditions for 4000 hours. The shear strength and the thermal parameters of the joints were measured before and after the aging test. Furthermore, the corrosion resistance of the different alloys was studied. The microstructure of the solder joints was observed on metallographic cross-sections by a focused ion beam scanning electron microscope (FIB-SIM). The results showed that most of the ceramic nano-particles enhanced the initial mechanical and thermal parameters of the solder joints. However, some of them increased while others decreased the corrosion resistance of the composite solder joints. So, only an appropriate selection of applied ceramics can ensure the further reliability development of automotive electronics. Detailed results of the incorporation mechanism of the different nano-particles into the solder matrix and their effect on the reliability of the composite solder joints will be discussed in the presentation.