Coupling effect of thermomigration and cross-interaction on evolution of intermetallic compounds in Cu/Sn/Ni ultrafine i
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Luqiao Yin Key Laboratory of Advanced Display and System Applications (Shanghai University), Ministry of Education, Shanghai 200072, China
Chingping Wong School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta 30332, Georgia, USA (Received 13 February 2017; accepted 13 April 2017)
By reflowing Cu/Sn/Ni ultrafine interconnects under a temperature gradient, a new transient liquid phase (TLP) bonding process was proposed for three-dimensional packaging applications. The evolution of the dominant (Cu,Ni)6Sn5 intermetallic compounds depends strongly on the temperature gradient. The essential cause of such dependence is attributed to the different amounts of Cu and Ni atomic fluxes being introduced into the liquid solder. Under the coupling effect of thermomigration and Cu–Ni cross-interaction, the total atomic flux of Cu and Ni is promoted. As a result, the growth of dense (Cu,Ni)6Sn5 is significantly accelerated and the formation of Cu3Sn is eliminated. The new TLP bonding process consumes only a limited amount of the Ni substrate, but much more from the Cu substrate. The mechanism for the new TLP bonding process is discussed and experimentally verified in this study.
I. INTRODUCTION
Transient liquid phase (TLP) bonding is a very promising technology for chip bonding and chip stacking in a three-dimensional (3D) packaging process.1 This bonding process, also known as solid–liquid interdiffusion (SLID) bonding, can be processed at relatively low temperatures while resulting in higher re-melting temperatures of the produced joints which fully consist of intermetallic compounds (IMCs).2,3 On heating, the interlayer melts and solidifies as a result of interdiffusion with the substrates. It has the advantage of not requiring high pressures needed in typical solid state diffusion bonding processes. Several TLP bonding schemes have been developed to join base metals Cu, Au, Ag, Ni, Pd, Zr, and Pt using Sn or In foils of 5–100 lm in thickness as interlayers.1–9 Among them, Cu–Sn and Ni–Sn TLP bonding schemes have distinct advantages over the others, such as forming high re-melting temperature IMCs, cost effective, compatible with manufacturing, and commonly used elements in electronic packaging. However, an inherent drawback for the conventional TLP processes is that they all necessitate a very long bonding Contributing Editor: C. Robert Kao Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2017.171
time for complete consumption of interlayer melts, especially for the Ni–Sn system. Moreover, the Cu–Sn system would experience the transformation process from high-temperature hexagonal g-Cu6Sn5 to lowtemperature monoclinic g9-Cu6Sn5 at equilibrium temperature 186 °C and from Cu6Sn5 to Cu3Sn which brings in serious reliability concern for the formation of massive Kirkendall voids.9–11 In recent years, Cu–Ni cross-interaction in solder joints has attracted a wide attention.11–13 Studies have indicated that the interfacial reactions in
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