Corrosion Protection of Copper Using Organic Solderability Preservatives

Abstract

The emergence of new interconnection technologies involving double‐sided surface mounted components has put stronger restrictions on the method of preserving the solderable finish on printed circuit (PC) boards. The popular Sn/Pb coatings have come under strong scrutiny due to environmental hazards of lead and also because they do not provide flat, planar surfaces for SM assembly. Organic solderability preservative coatings (OSP) are emerging as strong contenders for replacing Sn/Pb surface finishes. Benzotriazole based organic coatings have been successfully used in the past by several electronics manufacturers. However, assembly technologies involving multiple thermal operations have necessitated a fundamental understanding of the thermal stabilities and the mechanism of corrosion protection provided by the OSPs. This paper reports the results of an investigation of the thermal stabilities of two organic corrosion protection coatings. Although both are organic azole based, they operate in two distinct regimes: one forming thin films (∼100 Å) and the other forming thick films (∼5000 Å). The mechanism of surface protection has been studied using direct surface analytical techniques such as X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), scanning transmission electron microscopy (SEM/TEM) and Fourier transform infrared spectroscopy (FT‐IR). The solderability of the copper was measured by wetting balance techniques and correlated to the amount of copper oxidation. The results indicate that, although the thin films provide excellent protection for storage and handling operations, they decompose under heat, thereby causing oxidation of the copper. The thick films appear to withstand multiple thermal cycling. However, the underlying copper substrate can still be oxidised by oxygen diffusion through pores or cracks, or the film may undergo chemical changes that render the copper unsolderable.

Citation

Artaki, I., Ray, U., Gordon, H.M. and Opila, R.L. (1993), "Corrosion Protection of Copper Using Organic Solderability Preservatives", Circuit World, Vol. 19 No. 3, pp. 40-45. https://doi.org/10.1108/eb046211