XPS Studies of the Interface Properties of Conductive Adhesive/Eutectic Solder
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Y.C. KOO, M.C. JENNINGS, R.D. DAVIDSON AND N.S. McINTYRE Surface Science Western, Western Science Centre, The University of Western Ontario, London, Ontario N6A 5B7 Canada
ABSTRACT The continuing shrinkage of feature size and environmental concerns regarding the use of lead containing solder are major driving forces in the search for alternative technologies for metal interconnects in electronic packaging. Electrically conductive adhesives are considered a potential substitute for the lead-tin solder. This study concentrated on interfacial properties of eutectic solder and silver-filled epoxy adhesives. A freeze fracture technique was employed in order to preserve interfacial information. The chemical composition of the fractured faces varied; the adhesive side was rich in tin, whereas the eutectic solder side showed lead-rich characteristics.
INTRODUCTION Conductive adhesives are one of several candidate materials for replacement of the lead containing solder widely used in electronic applications. Conductive adhesives have several advantages: they are inherently fluxless; the process temperatures are relatively low compared to most solders; and the adhesives may be capable of making fine pitch (< 0.020 in.) interconnects [1-3]. According to a literature survey 14], none of the North American isotropic conductive adhesives showed adequate performance to replace solder, due to their poor mechanical properties and inadequate adhesion to the metal leads. The mechanical strength of the joints is governed by (1) the strength of the adhesive material, (2) the interfacial properties of the adhesive/metal, including (a) chemistry and interaction of the two joining materials, and (b) the interface perfection (e.g. limited voids formation), and (3) mechanical interlock action provided by rough or porous substrate surfaces. This study concentrated on understanding the bonding mechanism of the conductive adhesive/tin-lead eutectic solder (Pb/Sn = 37/63) interface. X-ray photoelectron spectroscopy (XPS), Auger spectroscopy (Auger) and FT-Raman spectroscopy (Raman) were used to study the chemical properties of the silver- (Ag) filled epoxy adhesive and Pb/Sn eutectic solder (Pb/Sn = 37/63) interface.
EXPERIMENTAL The XPS analyses were performed using a modified Surface Science Laboratory Model, SSX-100 X-ray photoelectron spectrometer. Monochromatized Al Ka X-rays (1486.6 eV) with a spot size of 600 pm or 300 pm were used for the analyses, and a constant pass energy of 150 eV for the survey scans or 25 eV for the high resolution scan were used. The gold 219 Mat. Res. Soc. Symp. Proc. Vol. 390 ©1995 Materials Research Society
4f,,2 peak was used for calibration. The full width at half-maximum (FWHM) of Au 4f7, 2 was 0.83 eV using a 300 prm spot size, 25 eV pass energy and 8 scans. The Auger analyses were performed using a Perkin-Elmer PHI model 600 scanning Auger microprobe with a 3 kV electron beam (30 nA beam current). Sputtering was carried out using a 3 kV Ar+ ion beam rastered over a 2 mm x 2 mm area; under these condi
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