Partial diffusion reactions and the associated volume changes in thermally exposed Au-Al ball bonds
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3/12/04
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Partial Diffusion Reactions and the Associated Volume Changes in Thermally Exposed Au-Al Ball Bonds NAREN J. NOOLU, NIKHIL M. MURDESHWAR, KEVIN J. ELY, JOHN C. LIPPOLD, and WILLIAM A. BAESLACK III Thermal exposure of Au-Al ball bonds results in the transformation of Au-Al phases across and lateral to the ball bond that affects their service life. The phase transformations across and lateral to the AuAl ball bond were described by partial reactions at each of the interphase boundaries. Based on a model for the mass distribution of Au from the Au phase or the Au-rich phases towards the Al phase or the Al-rich phases, volume changes associated with the partial reactions were calculated. Except for the Au4Al and AuAl2 phases, theoretical analysis showed that the growth of all the Au-Al phases across the ball bond results in volumetric shrinkage. The growth of the Au-Al phases across the bond is restricted due to the limited bonded Al metallization available to take part in the interdiffusion reactions. The supply limitation of the bonded metallization gives rise to reverse transformations that result in the formation of the Au4Al phase across the entire reaction zone across the ball bond. Except for the AuAl2 phase, theoretical analysis showed that the reverse transformation of all the Au-Al phases across the ball bond is associated with minimal volume change. A “Criterion for Inward Lateral Growth” that governs the growth of Au-Al phases laterally into the ball bond was developed. Theoretical analysis showed that except for AuAl2, the lateral growth of all the Au-Al phases results in volumetric shrinkage. The lateral phase transformations are essentially reverse transformations due to no-supply of Au. Theoretical analysis showed that all the lateral reverse transformations also result in volumetric shrinkage.
I. INTRODUCTION
MORE than one intermetallic phase forms in many binary diffusion couples. In a diffusion couple of arbitrary metals A and B (provided they are not completely miscible), the intermetallic phase most rich in A forms adjacent to the A phase, while the most B-rich intermetallic phase forms adjacent to the B phase.[1] The number of possible product phases present is dictated by the binary phase diagram if local equilibrium is attained at the phase boundaries.[1] However, there is considerable debate on the product phases that could potentially form, especially in thin films.[2,3] The thickness of a given phase depends upon the molar volume of each phase, its rate of growth, and the rate of its consumption by other growing phases.[1] The formation of two layered oxide scales on pure metals has been considered by a number of authors[4,5] in terms of partitioning of reactants among the product phases. Shatynski et al.[1] considered partial reactions at each of the interphase boundaries in a multiphase metal binary system and presented a quantitative prediction for the rate of growth or the consumption of each product phase. The Au and Al are two typical metals that are inter
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