Reaction competition in micro-bump and the influences on reliabilities

The generalized meaning of micro-bump is composited of under bump metallization (UBM), supporting metals and Sn based materials. Given the small dimension of it, there are many reactions can happen among each layers. Copper and Nickel are two most common metals shown in micro-bump; and they all react with Sn well. However, in the aspect of reaction competition, Sn-Cu reaction rate is much faster than Sn-Ni reaction and releasing much more gibbs free energy due to the stability of Cu-Sn intermetallic compound. Therefore, this results in preferences of Sn atoms to react with Cu instead of Ni when both elements presenting in the micro-bumps. Another fact we need to take into account for the reaction competition is the distance between Cu/Ni and Sn; if the distance is longer, the reaction will be less preferable as the atoms have to have more energy to travel in order to react to each other. Moreover, temperature is also a key factor in chemical reaction as the distance given that atomic migration rate is highly related to the temperature, especially at the temperature near the melting point of an element. In a micro-bump structure, there are two reactions; atoms can travel across intermetallic compounds (IMC) through lattice diffusion to react with each other or atoms can travel through surface diffusion. Sn surface migration rate is larger than IMC formation rate (or atomic lattice diffusion rate in IMC) at the temperature below 200 oC because the long distance Sn side-wall migration is faster than Cu/Ni traveling through to center Sn. The large void may form at the center of the micro-bump high temperature (maximum 170 °C) storage (HTS) torturing process caused by migration of Sn atoms in the micro-bump favors to Cu side-wall. The sidewall reaction can bring out malfunction of the products.