Scallop-like Grains Research Articles

Microstructure evolution in Sn–Bi and Sn–Bi–Cu solder joints under thermal aging

Abstract Binary eutectic Sn–Bi and ternary Sn–Bi–Cu alloys were fabricated and soldered to conductor metallized substrates to study the interfacial morphology and microstructural evolution of the joints under thermal aging. Different types of intermetallic compound (IMC), such as Cu 6 Sn 5 , (Cu, Ni) 6 Sn 5 and Ag 3 Sn, are observed among various metallized substrates including Cu/FR4, Ni/Cu/FR4, Cu/Al 2 O 3 and Pt–Ag/Al 2 O 3 . Three major effects of aging on the microstructure of the solder joints are observed. First, the coarsening rate of the Bi-rich phase in the solder joint under thermal aging at 120°C is reduced by adding 1 wt.% Cu to the 42Sn–58Bi solder. Second, the Ag 3 Sn IMC forms planar layer at the solder/PtAg interface, while the Cu 6 Sn 5 intermetallic grows as scallop-like grains into the solder at the Cu/solder interface. Third, the growth of Ag 3 Sn intermetallic in the SnBi–1Cu/PtAg solder joint is slower than that in the 42Sn–58Bi/PtAg joint after thermal aging at 120°C.

Kinetic analysis of the soldering reaction between eutectic SnPb alloy and Cu accompanied by ripening.

The wetting reaction of molten eutectic SnPb on Cu leads to Cu-Sn intermetallic compound formation, but not compounds of Cu-Pb since they do not exist. Cu-Sn compounds do not form layered structures between the solder and Cu, rather the ${\mathrm{Cu}}_{6}$${\mathrm{Sn}}_{5}$ phase has grown as scalloplike grains into the molten solder. The scalloplike grains grow larger but fewer with time, indicating that a ripening reaction has occurred among them. However, the ripening is not a constant volume process since it is accompanied by the soldering reaction. Between the scallop grains, there are molten solder channels extending nearly all the way to the Cu interface. In aging, these channels serve as fast diffusion and dissolution paths of Cu in the solder to feed the reaction. A kinetic analysis of the soldering reaction accompanied by the ripening reaction is given. In the analysis, the growth of the scalloplike grains is supplied by two fluxes: the flux of interfacial reaction and the flux of ripening. The latter was formulated by the Gibbs-Thomson equation. The former was obtained by measuring the rate of consumption of Cu in the reaction. The measurement was carried out by determining the change of the total volume of scalloplike grains (and in turn, the Cu content in the grains) as a function of time and temperature. A reasonable agreement has been obtained between the calculated scallop-grain growth based on kinetic analysis and the experimentally measured values. A discussion of the Cu consumption rate is given, since it is important for applications in electronic packaging technology. \textcopyright{} 1996 The American Physical Society.

Rate of consumption of Cu in soldering accompanied by ripening

What is the rate of consumption of Cu in soldering reactions has been a critical question in electronic packaging technology. The Cu films are consumed by Cu–Sn compound formation. Because the Cu thickness is limited and the rework of a solder joint requires a layer of unreacted Cu, the loss of Cu in soldering must be under control. At the solder interface, Cu–Sn intermetallic compounds do not form layered structures. Rather, the Cu6Sn5 phase grows as scalloplike grains into the molten solder and ripening occurs between the grains. Therefore, it has been difficult to determine the compound growth rate, and in turn the Cu consumption rate. Using cross-sectional and top-polished samples, we have measured the total volume of Cu–Sn intermetallic compounds formed between eutectic SnPb alloy and Cu substrate as a function of reflow time and temperature. We have deduced that after 1 min reflow, for example, the thickness of Cu consumed was about 0.36, 0.47, and 0.69 μm at 200, 220, and 240 °C, respectively.