Investigation of the Microstructure of Mn-doped Tin-Silver-Copper Solder Alloys Solidified with Different Cooling Rates

Due to the moderate price and the non-toxicity, manganese is considered as an ideal dopant for the SAC (SnAgCu) solder alloys. Manganese refines the grain of solder joints, yielding better thermomechanical properties. In present research, the microstructure of the manganese-doped alloys solidified with different technological parameters had been investigated. Sn/Ag0.3/Cu0.7 based solder alloy with three different Mn content (0.1, 0.4, 0.7% wt%= were reflowed on a copper substrate with tempered hot plate. They were solidified with different cooling rates from 0.3 to 4.5 K/s. Cross-sections have been prepared from the solder samples and the metallographic properties of the solder samples was investigated with optical and scanning electron microscopes. The characteristic features of the samples have been compared to conventionally used SAC305 (Sn/Ag3/Cu0.5= solder alloys, solidified with the same rates of cooling. Results showed that besides the grain refinement, the Mn content might also have effect on the evolution of intermetallic layer between the substrate and the solder alloy. The IMC grains of the layer were more elongated and more spalled grains had been observed close to the layer. However, independently of the cooling rate, the microstructure of the Mn containing solder alloys remained the same. This suggests that the macroscopic properties are also expected to be less sensitive to the cooling rate of the solidification.