Modeling of smoothening effect on morphologies of annealed submicron nickel particles used for electrically conductive adhesives

Abstract

Microelectronic packaging has been accelerating towards adoption of solutions that offer lower cost, higher electrical performance, and better reliability. Flip chip technology lends itself excellently to these goals. The development of anisotropic and isotropic conductive adhesives (ACA and ICA, respectively) as an alternative to solder bumps has received extensive attention in flip chip packaging as it offers an array of advantages such as finer pitch interconnects, green processes, low cost, and low temperature processing. Nickel, with its lower cost than that of silver and better thermal stability than that of copper, appears to be a viable candidate material for ICA and ACA applications. In the present study, a spiky surface morphology of nickel particles synthesized by means of a hydrothermal reduction method was clearly observed. The spiky morphology may have a detrimental effect on the conductivity of the ICA and ACA flip chip interconnect due to its smaller contact surface and propensity towards oxidation. The morphology of the nickel surface can be changed by annealing the particles between 270 and 360°C for 4–12h. A smoothening effect leading to more or less smooth spherical particles along with an increase in conductivity was observed after annealing at 360°C for up to 12h. An analytical model considering surface diffusion is established here to describe the temperature activated and energy minimization driven smoothening of the Ni particles.