New Bonding Wire for Fine Pitch Applications

Over the past few years the trend in bonding wire applications has been characterized by the minimisation of assembling components, and has in turn led to thinner wire and finer pad and pitch size. In particular for gold bonding thinner wire means less expense for precious metal. The requirements for such small wire diameters can only partially be met by the existing bonding wires types. In fact, there is a need for new materials with high conductivity and strength, but still maintaining good bondability. Such an improvement could be achieved with new, carefully chosen dopant element combinations. The impact of these elements must be achieved at low concentrations, or otherwise the electrical conductivity would decrease. The inhomogeneous distribution of some high efficient doping elements, such as Ca, Be, and other elements due to their poor solubility in Au, can be prevented by using a special alloying procedure. Higher strength and high electrical conductivity lead to a thinner wire accompanied by smaller ball diameters, this increases the feasibility for future applications with pitch size less than 35 mum. The range of achievable combinations for breaking load and elongation with the technically oriented value 290 N/mm² at 4% will be shown. Furthermore, the bonding behaviour aspects, such as pull force and break mode, will be discussed. The usability in the fine pitch application field will be considered with respect to free air ball geometry and the surface quality of the wire.