The Phase Equilibria Of The Cu-Sn-Bi Ternary System

In the industry of electronics packaging and assembly, Sn-Pb solder alloy with excellent comprehensive performance is widely used. It is well known that the lead as a toxicity threatens the human health and environment seriously. With the reinforcement of environmental awareness, many countries have issued various legislations about banning the use of leaded solder. Researching and developing the lead-free solder has become an unstoppable trend. Up to now, the most promising lead-free alloys which have been developed are as follows: Sn-Bi, Sn- Zn, Sn-Sb, Sn-Ag and Sn-In systems. These lead-free solder have own advantages and disadvantages.Sn-Bi alloy has a high tensile strength, good wettability and high creep resistance. But the large solubility of Bi will decrease the ductility of Sn-Bi alloy. As Bi phase has bigger brittleness, the eutectic organization of Sn-Bi alloy that is layer shape will cause alloys prone to cracking and reduce the thermal conductivity, thus decrease the material performance. The study found that adding a certain amount of Cu in Sn-Bi alloy can inhibit the Bi turning bulky, enhance thermal fatigue performance and promot the comprehensive performance of alloy. Therefore, it is more likely to develop Cu-Sn-Bi lead-free solder alloy. In the field of materials research, the phase diagram is a very effective tool and has a very high reference value in the direction of designing and developing materials.In this experimental study, the 250、450 and 700℃ isothermal sections and liquidus projection of Cu-Sn-Bi ternary system have been determined experimentally by means of equilibrated alloys and thermal analysis method. X-ray diffraction, Differential scanning calorimeter, scanning electron microscopy and energy-dispersive spectrometry analysis were used to obtain the experimental result.Three three-phase regions and four two-phase regions have been determined experimentally in 250℃isothermal section, i.e., ε-Cu3 Sn +(Cu) +(Bi), ε-Cu3 Sn +(Bi) + L, η-Cu6Sn5 + ε-Cu3 Sn + L, η-Cu6Sn5 + L, ε-Cu3 Sn + L, ε-Cu3 Sn +(Bi),(Cu) +(Bi). Two three-phase regions and three two-phase regions have been determined experimentally in 450℃isothermal section, i.e., L + δ-Cu41Sn11 + ε-Cu3 Sn,(Cu) + δ-Cu41Sn11 + L, ε-Cu3 Sn + L,(Cu) + L, δ-Cu41Sn11 + L. One three-phase regions and two two-phase regions have been determined experimentally in 450℃isothermal section, i.e., γ-bcc + L +(Cu), L +(Cu), L + γ-bcc. Four primary phases have been determined experimentally in liquidus projection, i.e.,(Cu)、η、ε and γ. Four reactions were found in the process of solidification。...