Intermetallic growth kinetics in gold ball bonds on Al-1%Si-0.5%Cu bond pads at 175∘C

Purpose The purpose of this study is to demonstrate that isothermal intermetallic growth data for gold ball bonds can be non-parabolic with explanations of why deviation from parabolic kinetics may occur. Design/methodology/approach Intermetallic thickness measurements were made at the centre of cross-sectioned ball bonds that were isothermally annealed at 175°C. Intermetallic growth kinetics were modelled with a power law expression(x(t) − x0)2 = α1tα2. The parameters of the power law model were obtained by transformation of the response and explanatory variables followed by data fitting using simple linear regression (SLR). Findings Ball bonds made with 4 N (99.99%Au) and 3 N (99.9%Au) gold wires exhibited two consecutive time regimes of intermetallic growth denoted Regime I and Regime II. Regime I was characterised by reactive diffusion between the gold wire and the aluminium alloy bond pad, during which Al was completely consumed in the formation of Au–Al intermetallics with non-parabolic kinetics. In Regime II, the absence of a free supply of Al to sustain intermetallic growth led to the conclusion that thickening of intermetallics was caused by phase transformation of Au8Al3 to Au4Al. Ball bonds made with 2 N (99%Au) wire also exhibited non-parabolic kinetics in Regime I and negligible intermetallic thickening in Regime II. Research limitations/implications The analysis of intermetallic growth is limited to total intermetallic growth at a single temperature (175°C). Originality/value The value of this study lies in showing that the assumption that only parabolic intermetallic growth is observed in isothermally aged gold ball bonds is incorrect. Furthermore there is no need to assume parabolic growth kinetics because with an appropriate data transformation, followed by fitting the data to a power law model using SLR and with the use of statistical diagnostics, both the suitability of the kinetic model and the nature of the growth kinetics (parabolic or non-parabolic) can be determined.

Lateral Diffusion Spreading of Two Competitive Intermetallic Phases over Free Surface

The kinetics of growth and lateral spreading of intermetallic layers during surface interdiffusion in Cu – Sn system has been studied in a temperature range 160 – 200oC by the methods of optical microscopy, SEM provided with X-ray microprobe, and AFM. Lateral phase spreading over the surface is characterized by competition between two phases: Cu6Sn5 and Cu3Sn. A steady state solution for concentration distribution on the surface of growing intermetallic phases, as well as kinetic equations of lateral spreading of growing phase layers have been obtained. By comparison of experimental data on intermetallic growth kinetics with the proposed theory, the dynamic surface diffusion coefficients have been calculated.

Kinetics and Mechanisms of Intermetallic Growth by Surface Interdiffusion

ABSTRACTKinetics of growth and lateral spreading of an intermetallic layer along a free surface during surface interdiffusion in binary systems have been studied both theoretically and experimentally. By comparison of experimental data on intermetallic growth kinetics with the proposed theory, the surface diffusion constants have been calculated for intermetallics Cd21Ni5 and Cu5Zn8, and mechanisms of surface diffusion have been discussed.