Characterization of Copper Surfaces Used in Electronic Circuit Boards by Reflectance FT-IR

Abstract

ABSTRACTOrganic and organo-metallic coatings are presently being applied over bare copper as an approach to improve the co-planarity of circuit boards. Conformal organic solderability preservative coatings (OSP) are environmentally and economically advantageous over the more commonly used lead based coatings [1-2]. Problems arise in assessing the solderability of the bare copper and the integrity of the organic coating.Specular reflectance Fourier transform infrared spectroscopy (FT-IR) was utilized to monitor and evaluate the formation of Cu oxides occurring on copper substrates used in the manufacturing of electronic circuit boards. Previous studies reported the utility of this technique[3,4]. By measuring the oxide and protective coating characteristics of these surfaces, their solderability performance can rapidly be evaluated in a manufacturing environment. OSP coated test specimens were subjected to hot-dry and hot-wet environmental conditions using MIL-STD-202F [5] and MIL-STD-883E [6] as guides.The resultant FT-IR spectra provided clear evidence for the formation of various Cu oxides at the Cu/OSP interface over exposure time, for the samples subjected to the hot-dry environment. IR spectral bands consistent with O-Cu-O and Cu202 formation appear, while very minimal deterioration to the OSP coating was observed. The appearance of the Cu oxide layers grew steadily with increased environmental exposure. Specimens subjected to the hot-wet conditions showed no significant signs of deterioration. The IR data can be directly correlated to solderability performance as evaluated by wet balance testing.