In order to improve the durability of the black oxide layer treated on the surface of inner copper layer for multi layer PWBs, we studied the new chemical reduction technology of the black oxide layer using NaBH₄ as a reducing agent.Two reduction techniques, which provide the uniformity of treatment, the strong adhesion strength to prepreg, and the excellent durability against acid solution, were obtained. One is the two-step reduction method which reduces the black oxide fully by an alkaline HCHO solution after the preliminary reduction by a NaBH₄ solution.The other is the one-step reduction method in which a NaBH₄ solution containing polyoxyethyleneglycol is used as an additive. In the former method, it is possible to use inexpensive HCHO as a reducing agent. The latter method simplifies the reduction process maintaining the uniformity of the treated surface. We expect that these methods will be widespread among the field of the higher density multi-layer PWBs.

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Resin-filled flip-chip packages with a 6.00 millimeter square chip, mounted on a large coefficient of thermal expansion soda-glass substrate using liquid crystal display devices, to test conditions of -30°C to 85°C thermal cycle per hour showed no electrical disconnection even after 3000 cycles. Detailed analysis of this chip-on-board (COB) model, which closely resembles prototype package, gave the following conclusions. In changes from high to low temperature, the bimetallic effect causes a maximum 34 MPa σy tensile stress to operate on glass substrate pedestal's solder bumps boundaries. In changes from low to high temperature, σy's high tensile stress (36MPa) operates on the entire chip boundary. σy is thought to be a major factor in deterioration. However large amounts of stress are not operating on the glass substrate. The maximum equivalent strain on the solder is a low 0.34%, allowing expectations of long life of at least 10⁴ cycles. These results lead us to believe that σy caused by the bimetallic effect is related to deterioration. Designs that balance the large tensile stress of σy on chips with high rupture strength and on glass substrates with low rupture strength will lengthen device life.

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[in Japanese]

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[in Japanese]

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