INTERFACIAL EFFECTS IN HIGHLY FLEXIBLE ELECTRONICS AND THIN SOLDER JOINTS FOR 3D ELECTRONICS

Yeasir Arafat

Interfaces play an important role in determining the mechanical and electrical properties of microelectronic systems. In this research, we study and engineer interfaces in two microelectronic systems. In the first system, a metal-polymer interface is modified to enable high stretchability of metal films for flexible electronic applications. In the second system, a metal-metal interface of electronic micro-bumps is studied to enhance its interconnect reliability for three dimensional (3D) packages. We note that stretchable metal interconnects are required in several applications such as micro and nanoelectromechanical systems (MEMS and NEMS), sensors, soft robotics, electronic clothing, and biomonitoring devices. A novel approach is proposed, where the metal film ductility is significantly enhanced by confining its deformation into multiple periodic narrow regions by engineering the interfaces between the metal and the polymer. This confinement delays the strain to failure for the film under a linear strain due to a reduction in its gauge length; leading to a highly enhanced ductility for the film. This approach is demonstrated on electroplated indium interconnects on polydimethylsiloxane (PDMS) substrates with a hard-discontinuous interface layer of Cr which confines the deformation of the indium film into periodic narrow regions. A linear strain of >100% without mechanical and electrical failure is obtained, which is the highest linear stretch-ability yet reported in literature. Finally, a detailed electrochemical study is undertaken for a cheaper low-melting temperature metal, Sn, to obtain equiaxed large grains suitable for flexible electronic applications. In the second study, the growth kinetics of intermetallic phases during thermal aging of a Sn-Cu micro-bump are studied. An analytical model is developed to determine the evolution of two intermetallic phases, Cu6Sn5 and Cu3Sn, at the interface of Cu and Sn held at a constant temperature where the supply of Sn is limited. The model shows that Cu3Sn dominates the structure of microbumps as they grow at the expense of Cu6Sn5 when Sn is fully consumed. These results agree reasonable well with experimental observations from literature and give the processing and operational window for the reliable usage of Sn-Cu micro-bumps.

INTERFACIAL EFFECTS IN HIGHLY FLEXIBLE ELECTRONICS AND THIN SOLDER JOINTS FOR 3D ELECTRONICS

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