Alloyed copper bonding wire with homogeneous microstructure

Abstract

The new alloyed copper (Cu) wire exhibited homogeneous free air ball (FAB) formation and better electrochemical corrosion resistance than that of bare Cu wire. It also has comparable electrochemical corrosion resistance as palladium (Pd) coated Cu wire. The softness of the alloyed Cu wire and its FAB are uncompromised though the alloy composition is of 1N purity. This is attributed to the innovative process design to make the alloyed Cu wire properties close to bare Cu wire. The tensile behavior of alloyed Cu wire and grain structure of wire/FAB is also close to that of bare Cu wire for comparable and easy application. The looping performance is also observed to be similar to bare Cu wire and is within the defined dimensional tolerances required by the semiconductor packaging industries. In addition, it has a wider 2nd bond process window compared to bare Cu wire. On thermal ageing at 175°C for 2000hrs, the alloyed Cu wire bond showed no ball lift failure, wire break at neck or near to neck region, as expected. The rate of growth of intermetallics at the interface of alloyed Cu wire bond is slower than bare Cu wire bond and certain intermetallic phase is absent. The reaction rate calculated from Arrhenius plot showed lower value for an alloyed Cu wire bond than a bare Cu wire bond, which indicates slower interfacial diffusion with alloyed Cu wire bond. BHAST testing of molded device using green mold compound and bonded with alloyed Cu wire showed no failure until 168hrs at 130°C under +20V biased voltage.