Metal-coated mono-sized polymer core particles for fine pitch flip-chip interconnects

The increasing demand for fine pitch flip-chip interconnection has led to significant interest in alternatives to the widely used solder joints. The use of mono-sized metal coated polymer based micro-spheres is emerging as a method to achieve more robust and reliable interconnects for ball grid arrays and chip scale packages. This paper presents results from a collaborative research project investigating whether polymer cored interconnects are a viable replacement for such ultra-fine pitch solder joints. The focus of this work centres on the selective deposition of these polymer microspheres onto fine pitch bond pads. An important advantage of using such microspheres is that their mechanical properties can be tailored to suit the needs of the application, in addition to the relatively low usage of metals. Initial trials have been carried out using 9.8 μm diameter polymer particles which have been coated with electroless nickel and then an immersion gold surface layer. In order to selectively deposit these particles on to the bond pads of an ASIC device they were required to be charged. The particles were immersed in an aqueous acid solution and after rinsing in deionised water they were then transferred to a suitable solvent. This process resulted in a net positive charge on the particle surface which allowed the particles to be deposited using electrophoretic techniques i.e. an electric field was applied to the particle suspension to drive the particles to deposit on the substrate. The effect of this immersion in the aqueous acid solution on the surface metal layer of the particles has been studied using electron microscopy. The rate of electrophoretic deposition of the particles onto a homogeneous gold coated silicon substrate was studied to determine how the length of chemical treatment affects the particle deposition. Following from this the same technique was then used to deposit the particles directly on to the bond pads of an ASIC device. Using the ASIC as the cathode in the electrophoretic setup the particles were able to be selectively deposited onto bond pads with a pitch of 170 μm.