Electroplating and Characterization of Invar Fe–Ni Alloy Films from Plating Baths Containing Organic Acids

Abstract

In Fe–Ni alloys, coefficients of thermal expansion (CTEs) of Fe–Ni alloys with 30 to 45 mass% Ni are small compared to that of Fe (CTE = 12 ppm/K) or Ni (CTE = 13 ppm/K) at near room temperature. In particular, the Fe–Ni alloy with 36 mass% Ni exhibits the lowest CTE (approximately 1 ppm/K) among the Fe–Ni alloys. These Fe-Ni alloys with 30 to 45 mass% Ni are called “Invar alloys.” The Fe–Ni alloy electroplating can allow lower cost of Invar alloy production compared with the conventional rolling process. Furthermore, the electroforming process can provide precise micrometer-sized three-dimensional structures of Invar alloys with excellent thermal dimensional stability, e.g., MEMS (microelectromechanical systems). An acidic Fe–Ni alloy electroplating bath usually contains an organic acid, which acts as a masking agent for Fe3+ generated via electrochemical and air oxidation and pH buffer agent. However, it is known that an organic additive addition often effects on properties, e.g. internal stress in the Fe–Ni alloy electroplated films. For MEMS applications in particular, internal stress should be minimized. So far, the internal stresses in the electroplated Fe–Ni alloy films in the Invar composition range obtained from the alloy plating baths containing various organic acids have not been studied in detail. In this study, we investigated the influence of the type of organic acid added to the alloy plating bath on composition, current efficiency and internal stress in the Fe–Ni alloy films electroplated from various Fe2+ concentration baths. Electroplating of the Fe–Ni alloy was carried out using sulfate/chloride bath (modified Watts-type Ni plating bath) with additives at a current density of 40 mA/cm2. Bath compositions were as follows: FeSO4 (0 to 0.4 mol/L), NiSO4 (0.95 mol/L), NiCl2 (0.17 mol/L), boric acid (0.49 mol/L), sodium saccharin (0.008 mol/L) and organic acid (0.05 mol/L). Organic acids were selected based on the stability constant (log KMA) of the complex with Fe3+: tartaric acid (log KMA = 6.49), malonic acid (log KMA = 7.52), or citric acid (log KMA = 11.5). The bath temperature was maintained at 50 °C and the bath pH was adjusted to pH 2.5. Pure Ni sheets were used as anodes. Internal stress in the films was estimated by the bending strip method using a Cu–Be foil as a substrate, and the value of internal stress was calculated based on Stoney’s equation. The composition of the electroplated Fe–Ni alloy film was determined by the fundamental parameter method. Regardless of the presence or absence of an organic acid, the Fe content in the electroplated Fe–Ni alloy film increased with the increase in the concentration of Fe2+ in the plating bath, and at the Fe2+ concentration of 0.3 to 0.4 mol/L, Fe–Ni alloy electroplated film with a Ni content of 30 to 45 mass%, in the Invar composition range, was obtained. Without addition of an organic acid, current efficiency showed high efficiency of 95 % or more at any Fe2+ concentration. In contrast, the current efficiency decreased to approximately 90 % due to the addition of the organic acid. Citric acid added bath exhibited lower current efficiency than those form baths with tartaric acid or malonic acid at the Fe2+ concentration of 0.3 to 0.4 mol/L. The decrease in the current efficiency in the Fe–Ni alloy plating reaction was related to hydrogen evolution as a side reaction. Organic acids promoted hydrogen evolution during Fe–Ni alloy plating reaction. Citric acid most promoted hydrogen evolution at 0.3 to 0.4 mol/L of Fe2+ to obtain an Invar Fe–Ni alloy electroplated film. Without addition of saccharin, micro cracks occurred in the Invar Fe–Ni alloy electroplated films because the films generated very large internal stresses (tensile). Addition of tartaric acid or malonic acid to Fe–Ni alloy plating bath including saccharin kept the internal stress in Invar Fe–Ni alloy electroplated film relatively low (tensile, approximately 200 MPa), and furthermore, may prevent a decrease in toughness of the film by masking Fe3+. Consequently, these optimized Invar plating baths can be used for MEMS electroforming. In contrast, due to the addition of citric acid to form a more stable complex with Fe3+, internal stresses in the Invar Fe-Ni alloy electroplated films became very high (tensile, 300 MPa or more) and micro cracks occurred in the films; it was difficult to provide the obtained films as MEMS materials. It is expected that the internal stress behavior in the Invar electroplated films depending on the type of organic acid added to bath will be related to hydrogen evolution and impurity incorporation to the film during the Invar electroplating.