Hsuan Lee

Effects of additive formula and plating current density on the interfacial reactions between Sn and Cu electroplated layer

Electroplating Cu technique becomes more and more important in advanced three-dimensional integrated circuit (3D-IC) packaging because of its advantages in the electrical/thermal conductivity, low cost, and hole-filling performance. Formula of the Cu electroplating solution, especially the additive like suppressor, accelerator, and leveler, plays a crucial role in the electroplating process. Previous studies have indicated that some organic impurities originated from the additives were incorporated in the Cu plated layer during the electroplating process. Moreover, the incorporated organic impurities segregated to the interface between the Cu plated layer and solder in a solder joint and led to formation of voids at the Cu/solder interface. In this study, effects of additive formula and plating current density on the Cu/solder interfacial reactions thermally aged at 150 and 200 °C were further investigated. If the additive formula contained only suppressor, the grain size in the Cu electroplated layer became bigger and the void quantity reduced as the current density reduced. However, when accelerator was added in the plating solution, an opposite trend was observed. The grain became smaller and a larger number of voids formed at the Cu/solder interface as the current density reduced.

Electrodeposition of Ni on Bi2Te3 and Interfacial Reaction Between Sn and Ni-Coated Bi2Te3

Bismuth-telluride (Bi2Te3)-based compounds are common thermoelectric materials used for low-temperature applications, and nickel (Ni) is usually deposited on the Bi2Te3 substrates as a diffusion barrier. Deposition of Ni on the p-type (Sb-doped) and n-type (Se-doped) Bi2Te3 substrates using electroplating and interfacial reactions between Sn and Ni-coated Bi2Te3 substrates are investigated. Electrodeposition of Ni on different Bi2Te3 substrates is characterized based on cyclic voltammetry and Tafel measurements. Microstructural characterizations of the Ni deposition and the Sn/Ni/Bi2Te3 interfacial reactions are performed using scanning electron microscopy. A faster growth rate is observed for the Ni deposition on the n-type Bi2Te3 substrate which is attributed to a lower activation energy of reduction due to a higher density of free electrons in the n-type Bi2Te3 material. The common Ni3Sn4 phase is formed at the Sn/Ni interfaces on both the p-type and n-type Bi2Te3 substrates, while the NiTe phase is formed at a faster rate at the interface between Ni and n-type Bi2Te3 substrates.

Effects of Cu plating formulas on the solder joint reactions

Two types of additive combinations are used in the Cu plating solution to prepare the Cu deposits. The Cu deposit prepared with PEG and Cl− results in the formation of a specular structure composed of voids, Cu-impurity compounds, and intermetallic compounds when it reacts with Sn. The specular structure exhibits a very fast growth rate which is unusual for the Sn/Cu interfacial reaction. Introduction of a Ni diffusion barrier in between Sn and Cu effectively inhibits the formation of the specular structure. A new (Ni, Cu)3Sn4 phase is formed at the Sn/Ni interface and no any voids and Cu-impurity compounds are observed.