Grain Morphology of Intermetallic Compounds at Solder Joints
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The grain morphology of the intermetallic compound (IMC) that forms at the interface between liquid solders and solid-metal substrates was observed at solder joints. Cu6Sn5 grains on Cu substrates were rough or rounded, and Ni3Sn4 grains on Ni substrates were faceted. Through the energy-based calculations, the relationship between the IMC grain morphology and Jackson’s parameter ␣ was explained. The Jackson’s parameter of the IMC grain with a rough surface is smaller than 2 while it is larger than 2 for faceted grains.
I. INTRODUCTION
Two largely different types of grain morphology of intermetallic compounds (IMCs) are observed at the interface between Sn-based solders and metal substrates during the liquid soldering. The IMC grains are mostly scallop-shaped and rough (rounded) on Cu while it is polyhedral-shaped and faceted on Ni.1–6 In general, the morphology of the solid phase that grows into the liquid at the interface is divided into a rough or a faceted shape.7 Thus the grain morphology of the IMC at the interface in the liquid soldering is similar to the morphology of the solid growing into the liquid. According to the classical solidification theory of unary materials proposed by Jackson,8 the geometry of the surface that separates the two types of atoms of liquid and solid may be faceted or meandering (rough), which is highly correlated with the Jackson’s parameter. The Jackson’s parameter of crystals with rough surfaces is smaller than 2 while it is larger than 2 for faceted grains. In this study, this solidification theory of unary materials is extended to growth of multicomponent IMCs between liquid solders and solid metals.
II. THEORY ON GRAIN MORPHOLOGY
According to the classical solidification theory of unary materials, the atomic position of atoms in the crystal at the interface is considered to be in a perfect crystallographic position. Using a nearest-neighbor bond
a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 3, Mar 2002
http://journals.cambridge.org
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model and assuming that a random arrangement of atoms is added to an atomically planar crystal surface, a criterion was proposed; that is, Jackson’s parameter ␣: ⌬G = ␣ xa 共1 − xa兲 + xa ln xa + 共1 − xa兲 ln 共1 − xa兲 , RTm (1) with ␣=
⌬Hm ⭈ RTm
,
(2)
where ⌬G is a change of Gibbs free energy at the growing interface, R is a gas constant, xa is an atomic fraction of possible sites occupied by solid atoms at the growing interface, is a fraction of the total number of nearest neighbors in a plane parallel to the interface under consideration and is always much smaller than 1, ⌬Hm is a molar latent heat, and Tm is an equilibrium melting temperature. The Jackson’s parameter ␣ is an indication of the degree of attachment of atoms to the growing interface, and it determines whether the interface becomes rough (when ␣ is smaller than 2) or faceted (when ␣ is larger than 2). According to the theory, the growing interface becomes rougher as the value of ␣ decrease
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