The critical oxide thickness for Pb-free reflow soldering on Cu substrate

• Published in: Thin solid films 2012-06, Vol.520 (16), p.5346-5352
• Main Authors: Chung, C. Key, Chen, Y.J., Li, C.C., Kao, C.R.
• Format: Article
• Language: English
• Subjects: Chemical interdiffusion; diffusion barriers; Condensed matter: structure, mechanical and thermal properties; Copper; COPPER OXIDE; Cross-disciplinary physics: materials science; rheology; Diffusion in solids; Exact sciences and technology; HUMIDITY; Materials science; Metal oxide; Methods of deposition of films and coatings; film growth and epitaxy; MICROSTRUCTURES; Nanostructure; Organic solderable preservative; OXIDATION; OXIDES; Physics; Reflow soldering; REFLOWING; Relative humidity; SCANNING ELECTRON MICROSCOPY; SOLDERING; SOLDERING ALLOYS; Solders; Solid-fluid interfaces; Structure and morphology; thickness; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Theory and models of film growth; Thin film structure and morphology; Thin films; Transport properties of condensed matter (nonelectronic); Wetting
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/j.tsf.2012.03.034

Oxidation is an undesirable effect of reflow soldering. Non-wetting occurs when the oxide layer grows above the critical thickness. Characterizing the critical oxide thickness for soldering is challenging due to oxide's nano-scale thickness and irregular topographic surface. In this paper, the critical copper oxide thickness was characterized by Time-of-Flight Secondary Ion Mass Spectrometry, Scanning Electron Microscopy, Energy-Dispersive X-ray spectroscopy, and Transmission Electron Microscopy. Copper substrates were coated with an Organic-Solderable-Preservative (OSP) layer and baked at 150°C and 85% Relative Humidity for different amounts of time. The onset of the non-wetting phenomenon occurred when the oxide thickness reached 18±5nm. As the oxide grew beyond this critical thickness, the percentage of non-wetting solder joint increased exponentially. The growth of the oxide thickness followed a parabolic rate law. The rate constant of oxidation was 0.6×10−15cm2min−1. Oxidation resulted from interdiffusion of copper and oxygen atoms through the OSP and oxide layers. The oxidation mechanism will be presented and discussed. ► Critical oxide thickness for Pb free solder on Cu substrate is 18±5nm. ► Above the critical oxide, non-wet solder joint increases exponentially. ► A maximum 13-nm oxide thickness is suggested for good solder joint. ► Initial growth of oxide thickness is logarithmic and then parabolic after 12nm. ► Thick oxide (360–560nm) is formed as pores shorten the oxidation path.