A diffusion-kinetic model for predicting solder/conductor interactions in high density interconnections
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I.
INTRODUCTION
JOINING dissimilar materials involves different bonding techniques, such as diffusion bonding, brazing, and soldering. The prediction and control of the microstructural and compositional changes in the joint region are not only important during these bonding operations but also during testing and in use of the assemblies. This is most pronounced in the advanced microjoining technologies, such as Flip Chip, that provide high-density/fine-pitch interconnections, where increasing joint densities (finer pitch) are associated with decreasing solder-joint volumes and thinner metallization layers. One or more of the interconnection elements, either by diffusing, by dissolving into, and/or by reacting with the mating materials in the solder, bump, metallizations, and substrate, can be entirely consumed during a joining operation or in use of the assemblies, which leads to marked changes in the microstructures. Accordingly, it is evident that more emphasis has to be placed on better understanding and control of the energetics and kinetics of the intermaterial dissolution and the diffusion and reaction processes. Long-term microstructural changes, including the solidstate nucleation and growth of intermetallic compounds, occur during solid-state diffusion bonding or between the metallization layers, for example, in the Flip-Chip bonding. This also occurs, regardless of the joining technique, during cycling testing and when using the electronic assemblies at elevated temperatures, which can be as high as 150 7C in harsh automotive and avionics application environments. In conventional industrial soldering or brazing operations, the joint regions spend a relatively short time at elevated temperatures and only a few seconds in the liquid state, where ¨ NKA ¨ , Design Manager, is with Interconnection Technology, K.J. RO Nokia Telecommunications, Fixed Access Systems, Operations, 90831 Haukipudas, Finland. F.J.J. VAN LOO, Professor, is with the Laboratory for Solid State and Materials Science, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. J.K. KIVILAHTI, Professor, is with the Laboratory of Electronics Production Technology, Helsinki University of Technology, 02015 TKK, Finland. Manuscript submitted July 15, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
the conductor metal or substrate to be joined is in contact with molten solders or brazes. Despite the short time interval, marked dissolution of the materials into the liquid fillers will occur, causing a subsequently fast precipitation of intermetallic compound(s) at the liquid filler/substrate interfaces.[1,2,3] Thus, the dissolution and precipitation processes are expected to play an important role in determining the wetting of the materials to be joined by the liquid fillers.[3] In diffusion bonding, the materials to be joined typically have relatively large solubility regions, and no intermetallic compounds are formed at the joint interface. The bonding operation involves only solid-state diffusion across the joint interface, and
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