Impact of Cu content on the Sn-Ag-Cu interconnects

The effect of Cu component content on the thermal-mechanical performances of the lead free (LF) Sn-Ag-Cu interconnects was investigated through a series of board level tests. Four compositions of Sn-Ag-Cu solder alloys (Ag =3.5 = constant; Cu = 0.0, 0.5, 1.0 and 2.0) were used in the studies. The tests were composed of voiding assessment of the LF assembly, die shear test and fracture mode investigation, interfacial microstructure analysis, and thermal cycling test. To directly assess the Cu content influence on the LF interconnects of electronic devices, wafer-level chip scale packaging (WL-CSP) devices were used as the test vehicles. To eliminate the biased-influence of the LF paste, the vehicles were divided into two groups using two different assembling approaches, LF pasted and non-pasted (dip flux). Die shear tester, SEM and EDX were utilized to conduct the analysis. The results indicate: i) more voiding in the LF interconnects with using LF paste than using dip flux. ii) little difference in the die shear strength for the WL-CSP devices. iii) fracture modes varied for the LF interconnects, depending on the Cu content. iv) The microstructures of the LF interconnects were largely influenced by the Cu content. The influences were reflected in: a) the interfacial intermetallic compound (IMC) identity, b) the growth of the P-rich layer and depletion of the E-less Ni finishing of the board pad, and c) the nucleation and growth of the Ag3Sn platelets. The initial results of the thermal cycling of the LF interconnects were compatible with those reported.