Tin-lead (SnPb) solder reaction in flip chip technology

Solder reactions between SnPb and one of the four metals, Cu, Ni, Au, and P d have been reviewed on the basis of the available data of morphology, ther modynamics, and kinetics. The reactions on both bulk: and thin film forms o f these metals have been considered and compared. Also the two kinds of rea ctions, above and below the melting point of the solder, have been consider ed and compared. The rate of intermetallic compound formation in wetting re actions between the molten solder and the metals is three to four orders of magnitude faster than those between the solid state solder and the metals. The rate is controlled by the morphology of intermetallic compound formati on. In the wetting reaction between molten SnPb and Cu or Ni, the intermeta llic compound formation has a scallop-type morphology, but in solid state a ging, it has a layer-type morphology. There are channels between the scallo ps, which allow rapid diffusion and rapid rate of compound formation. In th e layer-type morphology, the compound layer itself becomes a diffusion barr ier to slow down the reaction. Similar morphological changes occur between SnPb and Au or Pd. The stability of scallop-type morphology in wetting reac tion and layer-type morphology in solid stare aging have been explained by minimization of sur face and interfacial energies. The unusually high rate of scallop-type intermetallic compound formation has been explained by the gain of rate of free energy change rather than free energy change. Also inc luded in the review is the use of a stack of thin films as under-bump-metal lization, such as Cr/Cu/nu, Al/Ni(V)/Cu, and Cu/Ni alloyed thin films. (C) 2001 Elsevier Science B.V. All rights reserved.