Date of Award

Winter 2004

Project Type

Dissertation

Program or Major

Chemical Engineering

Degree Name

Doctor of Philosophy

First Advisor

Dale Barkey

Abstract

Electrodeposition has played an essential role in electronics packaging because it gives superior thickness control for chip interconnects. Pure tin, tin-silver and tin-silver-based solders have been identified as leading candidates for lead-free solders due to the desirable melting point, good creep resistance and thermo-mechanical behavior.

The initial stage of electrodeposition, or the formation of small clusters of atoms (nuclei), is significant in the overall electrodeposition kinetics, as well as the appearance, structure, and properties of the resulting coatings. In this work, nucleation and growth kinetics of tin and tin-silver electrodeposition on copper and nickel substrates were investigated by electrochemical methods and surface analysis techniques (SEM and AFM). The nucleation rate, number density of active sites, critical nucleus size and free energy of critical nucleus formation were extracted under various electrolyte solutions.

Based on chronoamperometric studies, in acid tin solution without additives, tin electrodeposition is instantaneous nucleation with three-dimensional diffusion controlled growth. Addition of thiourea to the solution changes the nucleation process from instantaneous to progressive, which was confirmed by SEM and AFM analysis. In the acid tin solution containing thiourea-silver, the process of nucleation is unclassified. In all cases, the number density of active sites increases exponentially as the deposition potential changes to more negative values, and it is the highest in the solution with thiourea-silver.

According to classical and atomistic theories, in all cases, tin electrodeposition proceeds with three-dimensional phase formation and growth. The number of atoms forming the critical nucleus (Ncrit) consists of 0--2 atoms. The critical Gibbs free energy (DeltaGcrit) follows the sequence Cu-S > Ni-P > Cu-P, which is consistent with the number density of active sites on the substrate surface.

The number density of nuclei in the AFM images was measured by AFM. In comparison with calculated nucleus densities, the measured values are much higher. At -550 mV, the measured density is about 2 times as great as the calculated value in acid tin solution without additive. In the presence of thiourea-silver, the measured density is about 6 times as great as the calculated one.

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