INTERMETALLIC PHASE-FORMATION IN THIN SOLID-LIQUID DIFFUSION COUPLES

Conducting joints with low fabrication temperatures and high thermal s tability are useful in modern electronics. This paper discusses the po tential use of intermetallic phases in making such joints. Thin interc onnection layers that consist entirely of intermetallic phases have be en produced by joining planar Cu substrates that are coated with thin films of Sn. Thin layers (1-5 mum) of intermetallic phase are produced at temperatures slightly above the melting temperature of Sn in a pro cess similar to reflow soldering. Metallography and x-ray analysis are used to characterize the formation mechanism of the intermetallic. Cu dissolves into the liquid Sn by diffusion along narrow channels betwe en grains of the growing eta-Cu6Sn5 intermetallic phase. Tensile tests were used to measure mechanical properties. The joint strength increa sed with reaction time. The joint fails in a ductile mode as long as u nreacted Sn is present, but fractures along interphase boundaries when the joint is completely intermetallic.