CORROSION OF LEAD, TIN, AND THEIR ALLOYS

The corrosion resistance of lead, tin, and their alloys was measured in water and mildly acidic electrolytes using electrochemical techniques with foil samples of varying Pb-Sn alloy compositions. In addition, foils and evaporated solder pads on chips were exposed to an environmental temperature and humidity (T/H) chamber with or without gaseous contaminants. Following the corrosion exposure, samples were analyzed by Auger electron spectroscopy (AES), ellipsometry, x-ray diffraction (XRD), and electron microprobe. The results show that lead and the lead-rich alloys are significantly more vulnerable to attack in corrosive environments than are tin and the tin-rich alloys. Data are discussed in terms of the expected corrosion resistance of chips with solder pads in environments relevant to processing and storage. The solderability of Pb-Sn pads varying from 0 to 5 wt% Sn was evaluated by joining to ceramic substrates, measuring the tensile strength of the joint, and characterizing the appearance of the fractured joint. The quality of the joints is shown to be correlated with the corrosion resistance of the lead-tin alloy.