Dissolution and interfacial reactions of thin-film Ti/Ni/Ag metallizations in solder joints

The dissolution and interfacial reactions involving thin-film Ti/Ni/Ag metallizations on two semiconductor devices, diode and metal-oxide-semiconductor field-effect transistor (MOSFET), a Sn-3.0Ag-0.7Cu solder, and a Au-layer on the substrates are studied. To simulate the dissolution kinetics of the Ag-layer in liquid solder during the reflow process, the computational thermodynamics (Thermo-Cale) and kinetics (DICTRA: DIffusion Controlled TRAnsformations) tools are employed in conjunction with the assessed thermochemical and mobility data. The simulated results are found to be consistent with the observed as-reflowed microstructures and the measured Ag contents in the solder. In the as-reflowed joints two different intermetallic compounds (IMC) are found near the diode/solder interface. Both are in the form of particles of different morphologies, not a continuous layer, and are referred to as IMC-I and IMC-II. The former corresponds to Ni3Sn4 with Cu atoms residing in the Ni sublattice. It is uncertain whether IMC-II is Cu6Sn5 phase with Ni atoms residing in the Cu sublattice or a Cu-Ni-Sn ternary phase. Near the as-reflowed MOSFET/solder interface, both particles and a skeleton-like layer of Ni3Sn4 are observed. The primary microstructural dynamics during solid state aging are the coarsening of IMC particles and the reactions involving the unconsumed (after reflow) Ni- and the Ti-layer with Sri and Au. While the reaction with the Ni-layer yields only Ni3Sn4 intermetallic, the reaction involving the Ti-layer suggests the formation of Ti-Sn and AnSn-Ti intermetallics. The latter is due to the diffusion of Au from the substrate side to the die side. It is postulated that the kinetics of Au-Sn-Ti layer is primarily governed by the diffusion of Au through the Ni3Sn4 layer by a grain boundary mechanism. (C) 2001 Acta Materialia Inc-. Published by Elsevier Science Ltd. All rights reserved.