ABSTRACT
Sn58Bi-xGa alloys are potential candidates to use as a low melting temperature lead free solders. The interfacial reactions between Sn58Bi-xGa (x = 0.25, 0.5, 1.0, 2.0, 3.0 wt. %) molten solders and Cu substrates have been are investigated to understand the effects of Ga addition upon the Sn58Bi/Cu interfacial reactions.
There are four intermetallic compounds θ-phase (CuGa2), γ-phase (Cu9Ga4), η-phase (Cu6Sn5) and ε-phase (Cu3Sn) observed in the interfacial reaction region. The growth rate of γ-phase follow diffusion controlled mechanism at 200 degree celsius while it comply interfacial reaction controlled mechanism at 230 degree celsius.
Two abnormal phenomena were observed at these Sn58Bi-2Ga/Cu couples, i.e. (1) the formation of the bulky grains of single θ-phase or the γ/θ core-shell structure depending on the reaction time, and (2) the thinning of the interfacial θ-layer as reaction time increased. The γ-layers grows thicker for longer reaction time, which is likely resulted from interfacial reactions. However, the θ-phase become thinner with longer reaction time imply that θ-phase transferred to γ-phase, in addition, it was precipitated from the molten solder during solidification at the end of reaction. This assumption is agreement with the connection of tie-line between liquid phase and θ-phase, between liquid phase and γ-phase in 200 degree celsius isothermal section of Sn58Bi-Cu-Ga extrapolated by using CALPHAD method based on thermodynamic parameters of six binary systems, Sn-Bi, Sn-Cu, Sn-Ga, Bi-Cu, Bi-Ga, Cu-Ga and ternary interaction parameters of liquid phase in Bi-Cu-Sn system. Thus, the diffusion path for shorter reaction time is L/θ/γ/Cu and L/γ/Cu.
With longer reaction time or the Ga content in solder was depleted, θ-phase completely transfer to γ-phase, then ε-phase formed between γ-phase (Cu9Ga4) and Cu, after that η-phase formed between γ-phase and ε-phase. Finally, γ-phase (Cu9Ga4) was consumed in η-phase and ε-phase. Therefore, the diffusion path form L/θ/γ/Cu shifted to L/γ/Cu, and for prolonged reaction time the diffusion path form L/γ/η/ε/Cu shifted to L/η/ε/Cu. These observations are consistent with the presuming base on three dimensional 200 degree celsius isothermal section of Cu-Ga-Bi-Sn quaternary system with two isothermal sections of Cu-Ga-Sn58Bi and Cu-Ga-Sn ternary system extrapolated by using CALPHAD method based on thermodynamic parameters of six binary systems, Sn-Bi, Sn-Cu, Sn-Ga, Bi-Cu, Bi-Ga, Cu-Ga and ternary interaction parameters of liquid phase of Bi-Cu-Sn system.
The γ-phase formed at the interfacial reaction can act as a diffusion barrier to limit the overgrowth of η-phase and ε-phase. The effect of Ga addition upon the Sn58Bi/Cu interfacial reactions is decrease the total thickness of IMCs at the interfacial reactions.
Keywords: Interfacial reactions; Low temperature solder; Phase equilibrial; ternary system.

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