Aging studies of Cu-Sn intermetallic compounds in annealed surface mount solder joints

This is the second in a series of investigations aimed at studying the effect of Cu-Sn intermetallic compound on the reliability of surface mount solder joints. Our previous investigation revealed the formation kinetics and characteristics of Cu-Sn intermetallic compounds in LCCC surface mount solder joints during IR-reflow soldering. The present study focuses on the solid state growth of the interfacial Cu-Sn intermetallic compound in LCCC surface mount solder joints under prolonged annealing at elevated temperature. A thick Cu-Sn IMC layer at the Sn-Pb solder/Cu interface of a surface mount solder joint (which can be achieved by prolonged storage at high temperature or after long term operation of modern SMT electronic assemblies) makes the interface more sensitive to stress and may eventually lead to fatigue failure of all SMT solder joint. The microstructural morphology of the Cu-Sn IMC layer at the solder/Cu pad interface in all annealed LCCC surface mount solder joints is duplex and consists of η-phase Cu₆Sn₅ and ε-phase Cu₃Sn IMC. Both Cu-Sn IMC phases thicken as the aging time increases. On the other hand, at the interface close to the component metallization, the growth of both the η- and ε-phase were showed to be suppressed, with more pronounced effect on ε-phase, by Ni originating from the metallization. The mean total layer thickness was found to increase linearly with the square root of aging time and the growth was faster for higher annealing temperature. The activation energy for the growth of interfacial Cu-Sn IMC layers and the pre-exponential factor, D_o , for diffusion were calculated to be 1.09 eV and 1.68×10 ^-4 m²/s respectively for the 0805 LCCC surface mount solder joint using eutectic Sn-Pb solder. These values are useful in predicting the thickness of interfacial Cu-Sn IMC layers in real surface mount solder joints in electronic assemblies after prolonged operation and hence the reliability factor of the solder joint attributed to the interfacial Cu-Sn IMC layer. Finally, the pad size and quantity of Sn-Pb solder employed in LCCC surface mount solder joints were shown to have an insignificant effect on the solid state growth rate for the interfacial Cu-Sn IMC layers