Effect of Cerium Addition on Board Level Reliability of Sn-Ag-Cu Solder Joint

In order to study the effect of rare earth doping on temperature cycle (TC) and drop reliabilities of the SnAgCu solder joints, cerium (Ce) was added to Sn2.5AgO.5Cu solder by the amount of 300, 500, 1000 ppm, respectively. Solder joints were formed. TC and drop tests were conducted. Results show that 500 ppm Ce addition has the least influence on TC reliability of the solder joints, while less or more addition of cerium may deteriorates the TC performance. Most cracks in TC tests occurred in solder bulk or between the solder bulk and the interfacial IMC layer. 300 ppm cerium doped solder joint shows best drop reliability among all samples. Cracks mainly occurred between the interfacial IMC layer and board pad. Drop reliabilities were also tested for samples after high temperature aging (125degC, 300 h). It can be inferred that the more Ce was added, the better thermal stability was achieved in drop tests. Among all high temperature treated samples, 1000 ppm Ce added sample exhibits best drop reliability. Cross-section view of the solder joints show that Ce addition refined the micro structure of the solder joint. Ce tends to segregate on grain boundaries, which restraints the growth of beta-Sn grains and Ag₃Sn IMC and in result improves mechanical properties of the solder. Ce also tends to accumulate at the interface between bulk solder and the Cu₆Sn₅ IMC layer and reacts with Sn, thus depress the interfacial IMC layer growth.