Abstract

Typical electro-deposition of conventional metals produces deposits that are polycrystalline in nature, comprised of many crystal grains separated by grain boundaries. Adding grain refiners to a plating solution and employing pulse-plating techniques can reduce the grain size and produce a nanocrystalline deposit. The average grain size of the nanocrystalline copper deposit is about 100 nanometers. This is about 80 times smaller than the conventional deposit average grain size of 2 microns. 

Nanocrystalline copper deposits have negligible porosity and superior physical, mechanical and electrical properties. The hardness, strength and wear resistance of the deposit are greatly enhanced. Stress corrosion cracking is virtually eliminated, while the hydrogen diffusivity and solubility are increased. This paper compares the electrochemical, mechanical and physical properties of nanocrystalline copper deposits with conventional polycrystalline copper deposits on printed wiring boards (PWB). Test boards were evaluated after thermal shock and thermal stress tests. Copper thickness and uniformity are evaluated both by microsection and X-ray fluorescence measurement techniques.

Introduction

The electrodeposition of nanocrystalline metals has attracted considerable interest due to their improved electrochemical, mechanical and physical properties. It has been demonstrated that nanocrystalline deposits produced by pulsed electro deposition (PED) have a higher hardness, lower friction coefficient and lower electrical resistance compared to polycrystalline deposits produced by direct current (DC) plating [1].

The deposition of nanostructure deposits by PED is possible by optimizing the pulse length (time on), the time between two pulses (time off), the peak height (pulse) and the average current density [2]. Pulse electro-deposition permits electrolysis with a high current density during a short period of time [3].

The addition of organic additives, such as complex formers and inhibitors, are also necessary to achieve smaller grains. These additives aid in inhibiting crystallite growth resulting in a finer grained structure.

This paper compares the mechanical and physical properties of PED nanocrystalline copper deposits with DC conventional polycrystalline copper deposits on printed wiring board plated through holes.