Quality and reliability assessment of Cu pillar bumps for fine pitch applications

This paper presents the relations between processing, microstructure and mechanical reliability of copper pillar bumps (CuPi). Two sets of samples were manufactured: Cu/SnAg and Cu/Ni/SnAg with diameters between 15 and 20 mu m. From the microstructure point of view: at these dimensions and for simulated reflows, up to 5, intermetallic compounds (IMC) follow a classical power law with a time exponent value between 1/3 and 1/2 indicating the interfacial IMC growth is grain boundary/volume diffusion-controlled. Adding a Ni layer limits micro-voids and IMC growth (similar to 2-3 mu m wo. Ni vs. similar to 1 mu m w. Ni). From the mechanical/reliability point of view: the more reflow, the tougher the Cu pillar bumps. With 5 reflows, the pillar bump is a minima 74 % tougher for a Cu/SnAg one and 19 % for a Cu/Ni/SnAg one. At the investigated shear heights, the fracture is always in the solder without any apparent impact of IMC growth.