In situ TEM Characterization of Phase Transformations and Kirkendall Void Formation During Annealing of a Cu-Au-Sn-Cu Diffusion Bonding Joint

Diffusion bonding with Au, Cu and Sn is a technique that can be used to manufacture printed circuit boards. The mechanisms of reactive diffusion and Kirkendall void formation were studied in the Cu-Au-Sn-Cu sandwich system. The initial thickness of the Au and Sn layers was a few micrometers, corresponding to 57 mol.% Au and 43 mol.% Sn. The experiments were conducted in situ in a transmission electron microscope (TEM). The TEM thin diffusion couple was heated to 240 degrees C at a rate of 6.5 degrees C/min and then held at 240 degrees C for approximately 2 h. The Au-Sn interaction and progressive enrichment in Cu induce the nucleation and growth of different phases. After heating to 230 degrees C, only binary intermetallic compounds are formed. Between 230 degrees C and 240 degrees C, the ternary phase B develops, replacing the other phases. Over longer periods at 240 degrees C, phase B is progressively consumed on each interface with Cu and replaced with AuCu3. The diffusion path theory is used in the ternary Au-Cu-Sn diagram to analyze the chemical evolution of the system and the flux of elements through the interfaces. The nucleation and growth of Kirkendall voids are observed in parallel with reactive diffusion at both interfaces. The formation of Kirkendall voids appears to be primarily related to the growth of phase B. Mechanisms based on different diffusion rates for the elements are proposed to explain the formation of these voids.

Automated summary

Diffusion bonding of Au, Cu and Sn for manufacturing printed circuit boards was studied. The mechanisms of reactive diffusion and Kirkendall void formation were explored in the Cu-Au-Sn-Cu sandwich system. The study revealed the formation of different phases and compounds during heating, as well as the growth of Kirkendall voids at the interfaces. The chemical evolution of the system was analyzed using diffusion path theory.