Effect of palladium on copper aluminide intermetallic growth in palladium copper bonding wire

摘要

The reliability of copper bonding wire in plastic mold package has been associated with the intermetallic compound formation and growth in the presence of halogen element such as chloride in epoxy molding compound. Much have been studied on copper aluminide intermetallic compounds at the copper/aluminium interface such as CuAl, CuAl2 and particularly Cu9Al4 which is known to link with failure in copper wire bonding. Riding on the success of incorporating palladium in gold bonding wire to improve Au/Al interface reliability, same approach has been applied to add this noble metal to copper bonding wire via plating to form Pd coated copper wire and allows the Pd to dissolve into copper matrix and prevent or delay copper rich IMC formation that leads to failure. It has been observed in industry that Pd rich phase formed when Pd coated copper wire forms a free air ball but whether the Pd can completely cover the entire ball before bonding is still of question. Moreover, whether there is a consistent layer of Pd or Pd rich phase formed to separate or prevent transformation of aluminium rich IMC to copper rich IMC has yet to be seen as only partial areas in the Pd coated copper bonded ball has seen the evident. The fact that the Pd coating can be of various thickness when plated on the copper wire surface, whether the thickness will change the characteristics of Pd rich phase or provide more uniformity has yet to be seen. It was known that EFO process to form FAB has great influence in the Pd distribution and hence in the bonded ball. How the Pd thickness affects the distribution has yet to be understood. This paper examines Pd distribution in the Cu-Al intermetallic phase bonded with Pd coated wires of different Pd thickness in as bonded condition and high temperature thermal aging of 250°C. Microstructural analysis via TEM and EDS revealed presence of voids which is related to presence of Pd. Pd distribution is affected by the EF- current used to form FAB and also Pd thickness has also influence in presence of Pd across the bond interface. Apart for the known copper rich Cu9Al4 phase, Pd is also found in the Cu rich IMC which needs more detail investigation on whether a new compound is formed.