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 µm. 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 (~2-3 µm wo. Ni vs. ~1 µm 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.