Abstract
Published data augmented by new work suggests that cobalt is a suitable constituent of many under bump metallisations, its function being as the barrier metal. This stems from the combination of cobalt’s high wettability by and low solubility in, molten solders, together with the slow rate of growth and favourable mechanical properties of the interfacial intermetallic compounds formed. Although other metals and alloys exhibit similar characteristics, the range of compatible solders is always limited. Cobalt appears to be suitable for use with low melting point bismuth- and indium-based filler metals, lead–tin eutectic, lead-free and high melting point lead-based alloys, as well as the gold-rich solder family.
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