Thermal behaviour and microstructural analysis of Sn-40Pb alloy and Sn-40Pb soldered on electroless nickel/immersion gold

Due to the toxicity of lead solder, the lead-free solder has been developing and improved in order to replace the lead solder. However, the behaviour and properties of lead solder is still better than the lead-free solder, especially in application when soldering on substrate. Hence, a common lead solder, Sn-40Pb is studied in this research by comparing with Sn-40Pb soldering on electroless nickel/immersion gold (ENIG) substrate. The thermal behaviour by differential scanning calorimeter (DSC) and microstructure formation with elemental analysis by scanning electron microscope (SEM) were carried out for Sn-40Pb solder alloy and Sn-40Pb soldered on ENIG substrate in this study. The result showed that the pasty range of Sn-40Pb was lower than Sn-40Pb/ENIG while the undercooling was higher. The diffusion and dissolution of Ni and Cu elements from ENIG substrate into the solder, forming the lead-rich phase with Ni elements and interfacial (Cu,Ni)6Sn5 with Pb elements in Sn-40Pb/ENIG. The diffusion and dissolution of elements from substrate into the solder affects the thermal behaviour and microstructural of solder.