Journal of SMT Article


Authors: HyunJung Lee, YeonSeop Yu, HyoJung Kim, Hee-Soo Kim
Company: Samsung Electro-Mechanics Co., Ltd.
Date Published: 1/31/2009   Volume: 22-1

Abstract: We investigated the growth of intermetallic compound (IMC) in lead-free Sn (tin) surface finish and its effect on solderability in flip chip chip-scale package (FC-CSP) application. The tin electroplated on Cu substrate is annealed at 85 - 175°C under atmospheric environment. The IMC thickness and the microstructure have been studied in order to understand the growth mechanism of Cu-Sn IMC. The growth rates of IMC and the activation energy of inter-diffusion are measured in pure tin surface finish. We have found that there exist two kinds of IMC, Cu6Sn5 and Cu3Sn at the Cu-Sn interface on the annealing temperature. The growth kinetics of IMC layer in electrolytic Sn is described by inter-diffusion of elements in Cu and Sn layer. The time exponents, n are 0.35 ± 0.11, 0.44 ± 0.06, and 0.38 ± 0.06 for the Cu6Sn5 layer, the Cu3Sn, and the total IMC layer, respectively. The activation energies, Q are 23.8kJ/mol, 29.3 kJ/mol and 27.7 kJ/mol for Cu6Sn5, the Cu3Sn, and the total IMC layer, respectively. The computer simulation is also performed to investigate the effect of the microstructure on formation and growth of IMC layers. In the growth of total IMC, The diffusion growth model in one dimensional simulation performed with microstructure of Sn agrees well with the experimental data. Ball pull test is employed to characterize solderability, and the brittle failure mode appears only after annealing of 30 days at 150°C which results in IMC layers with a thickness of 7.4µm.

Key words: Intermetallic compound, electroplated tin, surface finish, solderability, FC-CSP

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