We have developed a thin, high-density device-mounting, multilayer flexible printed circuit (hereinafter flexible printed circuit is referred to as FPC) "SBic" (stands for Solder Bump Interconnection Circuit) based on our original materials and manufacturing technologies of FPCs. The features of this multilayer FPC are attainment of high-density device mounting, high reliability, and super-thin thickness, for example six layers: 0.3mm, eight layers: 0.4mm, etc., with an all-layer IVH (stands for Interstitial Via-Hole) structure. The interconnection consists of a solder bump, which has a copper bump as the core, and a capture pad. The flux activity of Deoxidizable Bonding Film, our proprietary interlayer adhesive, supports the bump and the pad connected by solder. We verified formation of the alloy of tin and copper by cross sectional observation. The reliability of the interconnection passed a temperature cycle test (1,000 cycles at the range of-25°C to 125°C), a thermal shock test (100 cycles at the range of room temperature to 260°C), etc. The conductive resistance of the interconnection kept an almost theoretical value during the temperature cycle test. We adopted the simultaneous interconnecting lamination of all-layer method, and attained shortening of the processes and a high yield ratio. Moreover, halogen-free and lead-free materials were adopted for all the components in consideration of environmental protection.
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