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We have developed the ultrathin 4-layer flexible printed circuits (FPCs) with the thickness of 135 urn. This 135urn ultrathin 4-layer FPC is made by the molecular bonding technology, by which the electroless plating (metallizing) can be directly applied to polyimide (PI) by using molecular junction agents. In this study, we show the fabrication procedures, the mechanism of direct copper plating, and the reliability verification of the ultrathin 4-layer FPC prototype.