Lead-free soldering alloys: microstructure optimization for electronic applications
thesisposted on 07.05.2010, 10:23 by Sergey Belyakov
As a result of environmental issues, manufacturing of lead-free electronics has become a global trend since July 1, 2006. Due to the considerable toxicity of lead leading to health and environmental concerns, new legislations have been imposed in its use. Efforts have been made to replace the traditional soldering alloys with new compositions, but the reliability of the new materials requires further investigation. The purpose of this work is investigations of: 1. Growth of intermetallic phases (IMCs) during soldering and service life. 2. Investigation of effects of solder compositions, cooling kinetics and ultrasound vibration on the microstructure of crystallizing solders. 3. Nucleation and growth of bulk intermetallic precipitates and interfacial intermetallic layer in the "solder-joint" system. 4. Mechanisms of solder-joint failure. 5. Stability of microstructures over time. 6. Crystallographically-faceted void formation phenomenon. The solder alloy compositions used: SAC405 (Sn95.5–Ag4.0–Cu0.5), SAC305 (Sn96.5–Ag3.0–Cu0.5), CASTIN (Sn96.2–Ag2.5–Cu0.8–Sb0.5) and SN100C (Sn–Cu0.7–Ni0.05+Ge) were chosen due to their preferability for lead-free electronics applications. Investigations carried out revealed a strong dependence of the lead-free alloy microstructure on composition and solidification conditions (cooling rates, ultrasound). Lower amounts of alloying elements (< 2.5% of Ag) result in an increase in solder microstructural integrity. Cooling rates and ultrasound modify phase size and distribution. For instance, a cooling rate of 24 °C/sec (SAC405 alloy) lowered the maximum IMC needles length to 2μm compared to 44 μm at 1 °C/sec. Ultrasound of 30 kHz increases the microstructural homogeneity of bulk solders and lowers the amount of undercooling (from 24 °C to 7 °C for SAC 405 alloy). However, it deteriorates the strength characteristics of a solder-joint due to intensification of interfacial diffusional process, causing more intensive growth of the interfacial IMC layer. It was shown that solder-joint tensile strength is highly dependent on the IMC layer which has an optimal width of about 2μm. Thermal-cycling revealed differences in evolution of solder-joints and an average 7% strength reduction was obtained for samples subjected to 100 thermal cycles. Ageing resulted in rapid coarsening of a solder-joint microstructure and preferable IMC growth along the β-Sn grain boundaries. The interfacial IMC layer separates into two strata: Cu3Sn and Cu6Sn5 as it develops into the solder. Crystallographically-facetted voiding was observed in solder-joints in the as 'soldered' state and during subsequent ageing. The voids were found to be strongly dependent on the width of Cu substrate. A fundamental goal of this research is to help to meet the future requirements of reliable lead-free electronic products.