Investigation On Viscosity And Stability Of Lead-free Solder Paste

Surface Mounted Technology (SMT) is one of the most widely used technologies in electronics packaging industry due to the advantages of small packaging size, high reliability, strong production capacity and low cost. As the primary bonding material between electronic components and substrate in SMT process, solder paste plays a crucial role in assuring good quality for surface mounted devices. With the development of lead-free solders in electronics industries, Sn-Ag-Cu alloy is considered to be the most promising lead-free solder owing to its relatively excellent comprehensive properties, and because of which, investigations on the matched flux for Sn-Ag-Cu solder have also received much attention. Moreover, since the quality of solder paste is decided by its own properties such as viscosity and stability,, the influence of flux compositions on properties of solder paste must be considered when developing flux special for lead-free solder paste.In this paper, the viscosity and stability properties of Sn-3Ag-0.5Cu lead-free solder pastes with various fluxes have been systematically investigated by solubility test, viscosity test, slump test and thermogravimetric analysis (TGA). The stable flux formula and preparation method have been confirmed. In addition, the impacts of different filmogens, solvents and thixotropic agents on the viscosity, thixotropy and slump properties of solder paste have also been discussed and concluded.The results show that the viscosity for the corresponding solder paste is higher when the filmogen exhibits higher softening point temperature and higher viscosity. But the thixotropy of solder paste is not influenced obviously by additions of different filmogens. Compared with solder paste containing alcohol ether solvent, the solder paste with alcohols solvent displays higher viscosity but lower thixotropic coefficient. The thixotropic coefficients of solder pastes with the same category of solvents are almost similar. However, in case of the viscosity properties, the bigger of the molecular weight of solvent, the higher viscosity for the solvent and corresponding solder paste. Therefore, as the two main carrier components of flux, both the high viscosities of filmogen and solvent will lead to high viscosity for the corresponding solder pastes.The flux state, viscosity and thixotropy for solder paste are different when the thixotropic agent is added by different ways. As for hydrogenated castor oil (HCO) and 6650R, the solder paste exhibits higher viscosity and thixotropy by the method of dispersion at low temperature than by way of burn-in at high temperature. Moreover, the viscosity of solder paste could be improved by adding 6650R or EBS thixotropic agent on the basis of 6% HCO, but it decreases when EBO is added. With the addition of complex thixotropic agents, the amount of agents increase, thereby improving the thixotropy.With the higher softening point temperature of filmogen, the solder paste exhibits higher viscosity at both room and elevated temperatures, as well as better cold and thermal slump resistances. The four solder pastes with different compound solvents show the same cold slump resistance at 25℃, and their viscosity values distribute in the range of 111.7～191.5 Pa.s. However, the thermal slump resistances, which are related to the volatilization characteristics of solvent at 150’C, are different for the four solder pastes. When volatilization quantity of the solvent is larger at 150℃, the viscosity of the paste becomes higher and the thermal slump resistance goes better at high temperature. In addition, the solder paste with more HCO content shows better cold slump resistance at 25℃ than the paste with more EBS content, but worse collapse resistance at 150℃. The reason for this phenomenon is that the HCO has more advantages of viscosity than EBS at room temperature, but the heat resistance of HCO is worse than EBS thixotropic agent when the temperature rises. Overall, the solder paste with complex thixotropic agents of 5wt.%HCO and 5wt.% EBS shows the best slump resistances at room and elevated temperatures.The cold slump resistance of solder paste is mainly associated with the no shear viscosity (viscosity and thixotropy together) at 25℃. Therefore, in order to obtain good cold slump resistance, the viscosity of the solder paste should be larger than 110 Pa.s and the thixotropy coefficient larger than 0.525. As for the thermal slump resistance, it depends on the viscosity variation of solder paste at 150℃, which is affected by the softening point temperature of filmogen, volatilization characteristics of the solvent and heat resistance of the thixotropic agent.As a consequence, when choosing KE604 or complex KE604 and DERM-145 as filmogen, alcohol ether plus alcohols as solvent, and 5wt% HCO plus 5wt% EBS as thixotropic agent, the solder paste exhibits excellent cold and thermal slump resistances, and its viscosity and thixotropy meet the requirements for printing process.