1194 Chada, Laub, Fournelle, and Shangguan 1194 Special Issue Paper Journal of ELECTRONIC MATERIALS, Vol. 28, No. 11, 1999 (Received March 3, 1999; accepted May 6, 1999) INTRODUCTION In an earlier paper 1 a numerical method for predict- ing the thickness of the intermetallic layer developed during reflow soldering of Sn containing solders on Cu substrates was presented. This method required as input the temperature-time profile of the solder while it is in the liquid state on the Cu substrate and an experimentally determined equation or equations describing the isothermal growth of the intermetallic layer. The method basically involved dividing up the temperature-time profile into a large number of equal time intervals and then treating the growth in each interval as isothermal. The total layer thickness at any time during the reflow cycle was taken to be the sum of the individual increments of growth during these isothermal intervals. This method underesti- mated somewhat the thickness of intermetallic layers developed during the reflow of Sn-Pb-Ag solder on Cu. One reason for this may be that the method did not An Improved Numerical Method for Predicting Intermetallic Layer Thickness Developed during the Formation of Solder Joints on Cu Substrates S. CHADA, 1,2 W. LAUB, 1,3 R.A. FOURNELLE, 1 and D. SHANGGUAN 4 1.—Materials Science and Engineering Program, Marquette University, Milwaukee, WI 53201. 2.—Presently at Solectron Corporation, 727 Gibraltar Drive, Milpitas, CA 95035. 3.—Presently at Solectron GmbH, Solectronstr. 2, 71083 Herrenberg, Germany. 4.—Visteon Automotive Systems, Ford Motor Company, 17000 Rotunda Drive, Dearborn, MI 48121 An improved numerical method has been developed for calculating the thickness of intermetallic layers formed between Cu substrates and solders during the soldering process. The improved method takes into account intermetallic disso- lution during heating and intermetallic precipitation during cooling and re- quires as input (1) the temperature-time profile for the soldering process, (2) the experimentally determined isothermal growth parameters for the growth of the intermetallic layer into Cu saturated molten solder, (3) the experimentally determined Nernst-Brunner parameters for the dissolution of Cu into molten solder, (4) the experimentally determined solubility of Cu in molten solder and (5) assumptions about the thickness of the boundary layer in the liquid ahead of the growing intermetallic. Calculations show that the improved method predicts intermetallic growth between Cu substrates and 96.5Sn-3.5Ag solder during reflow soldering better than a previously developed method, which did not take into account dissolution during heating and precipitation during cooling. Calcu- lations further show that dissolution has a significant effect on growth, while precipitation does not. Key words: Cu-Sn intermetallics, Cu substrate, intermetallic compound growth, Sn-Ag solder take into account dissolution of intermetallic during the heating part of the reflow cycle and precipitation of intermetallic during the cooling part. Schaefer et al. 2 have clearly shown that isothermal growth of intermetallic between molten Sn-Pb-Ag solder and a Cu substrate is affected by dissolution. It is the objective of this paper to improve on the numerical technique by taking into account (1) the intermetallic dissolution occurring simultaneously with interme- tallic growth during the heating and cooling parts of the reflow cycle up to saturation of the solder with Cu and (2) intermetallic precipitation occurring simulta- neously with growth during the cooling part of the cycle after saturation. NUMERICAL METHOD Like the numerical method presented previously 1 the method presented here requires as input the relevant temperature-time profile (Fig. 1) of the sol- dering process and an equation for the isothermal thickening of the intermetallic layer as a function of time. However, the improved method also requires