Solderability mechanisms and solder materials for thin solder coating for SMT printed circuit boards (PCBs) were studied by a meniscograph testing, auger electron spectroscopy, and X-ray microanalysis. Cu-Sn intermetallic compounds grow at the interface of the solder coating and the Cu by Cu diffusion from the substrate. Supersaturated Cu oxidizes in the Cu-Sn compounds with air-exposed surfaces. This results in low solderability of the thin solder-coated surface. Adding 0.3 wt% Cu to the solder-bath inhibits the growth of Cu-Sn compounds by reducing Cu diffusion. Adding more than 0.1 wt% Indium (In) forms a rich In layer in the soldercoated surface, and this prevents Cu-Sn compounds from oxidizing. Growth of Cu-Sn compounds is, therefore, inhibited, and, even if there are Cu-Sn compounds exposed to the solder-coated surface, it is possible to obtain good solderability by adding both Cu and In to the solder.

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The thermal fatigue lifetime of a flip-chip COB sample has been experimentally studied using various sized chips (3 mm^_??_-15mm^_??_) bonded onto PWB. We found that in the sample without resin insertion, these lifetimes are short, which indicates that they cannot be put to practical use. Also, when a resin with a TEC (Thermal Expansion Coefficient) close to that of the solder is inserted into the gap between the chip and the substrate, the lifetime of these samples is greatly extended, which indicates that they can be put to practical use. We ran failure analysis tests on the samples without resin which failed, and clarified the mode of failure. The reliability of a 4 M-mask-ROM-chip bonded on PWB (0.25mm^t) using a solder bump was also evaluated. There were no problems in the temperature cycling test, the humidity storage test and the high temperature storage test.

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