Scallop formation and dissolution of Cu–Sn intermetallic compound during solder reflow

Abstract

In an effort to elucidate the mechanism of scallop formation during reflow of solder/Cu joints in flip-chip and ball-grid array technologies, a planar intermetallic compound layer (mostly Cu6Sn5) was initially formed by solid-state annealing of SnPb/Cu joints at 150 °C for ten days. Upon subsequent reflow, dissolution of the intermetallics and formation of scallops were observed. Detailed investigation has indicated that a thermal grooving process due to dissolution of the intermetallic compound during the initial reflow cycles contributes to the formation of scalloped structures, the morphology of which is dictated by the force equilibrium condition. Upon subjecting the solder joints to several reflows, the average intermetallic layer thickness was found to first decrease and then increase, which is shown to be in good agreement with the prediction made by a model based on simultaneous dissolution and growth of the intermetallic compound during reflow. The kinetics of dissolution-growth process during reflow has also been shown to depend on the initial intermetallic thickness and the shape of the solder cap.

Preferred orientation relationship between Cu6Sn5 scallop-type grains and Cu substrate in reactions between molten Sn-based solders and Cu

“…2 However, the interface between the scallops and Cu has been investigated, except that it was assumed that the interfacial diffusion of Cu is fast and the diffusion is not a rate limiting step. In spite of the very large body of literature published on the subject of solder/Cu reaction, [3][4][5][6][7][8][9][10][11][12] the structural informa-tion of the interface and the crystallographic orientation reaPresent address: Jet Propulsion Laboratory, Pasadena, California; electronic mail: Jong-ook.Suh@jpl.nasa.gov lationship between the IMC and Cu is still missing. It is the purpose of this study to determine the orientation relationship and the statistical distribution experimentally.…”

Continuous epitaxial growth of extremely strong Cu6Sn5 textures at liquid-Sn/(111)Cu interface under temperature gradient

“…Several studies have confirmed that the growth and dissolution of interfacial IMCs occur simultaneously during Sn/ Cu liquid-solid reaction. 20,22 In the initial state during an isothermal soldering, IMC grains on the (111) Cu substrate are relatively small and grain boundary diffusion is remarkable; sufficient Cu atomic flux is provided for the anisotropic growth of faceted prism-type g-Cu 6 Sn 5 grains. With the increasing reaction time, bigger IMC grains form and grain boundary diffusion is weakened; insufficient Cu atomic flux is provided for the g-Cu 6 Sn 5 IMC growth.…”

Premelting behavior and interfacial reaction of the Sn/Cu and Sn/Ag soldering systems during the reflow process

“…1)°C/30 min respectively, the interfacial pure ''Sn'' (actually the Sn-Cu alloy) was melted, and the interfacial IMC grew up gradually and exhibited scallop-type morphology. A previous study [20] indicated that in a small range of temperature near the melting point of pure Sn, the grain boundary energy of the Cu 6 Sn 5 IMC can remain the same value; however, the interfacial energy at the interface between the liquid-Sn and Cu 6 Sn 5 IMC was significantly less than that at the interface between the solid-Sn and Cu 6 Sn 5 IMC. Thus, the high interfacial energy at the interface of the solid-Sn/Cu 6 Sn 5 IMC results in the formation of atomically flat interface and presence of planar (faceted) surface at the microscopic scale; in contrast, the low interfacial energy at the interface of the liquid-Sn/Cu 6 Sn 5 IMC leads to the formation of atomically rough interface and scallop-type (non-faceted) surface morphology at the microscopic scale.…”