Properties of thin conductor with lead-free solder plating for flexible flat cables

摘要

Flexible flat cables (FFCs) are increasingly being used to wire the mobile parts of high-performance electronic equipment such as printers because of their bending-fatigue lifetime and excellent cost performance. An FFC is comprised of conductors and insulation film, which are held together by adhesive. Lead-free solders for tin-lead solder plating of conductors (0.05 to 0.03-mm thickness, 0.5 to 0.3-mm width), and halogen-free adhesive and insulation film are desired from an environmental viewpoint. So tin-plating solder is being adopted instead of conventional Sn-5mass%Pb solder. FFCs have come to be used in small electronic equipment such as compact-disc reading only memories (CD-ROMs) and digital video discs (DVDs). Conductors used for FECs are becoming thinner, recently reaching thicknesses of 0.02 mm or less. The quality of plating is considered to have a strong influence on many properties of thin conductors. We studied the influence of plating quality on bending-fatigue lifetime, wearing-out, whisker growth and solderability. We found that these properties of tin-plating conductors depend on the amounts of pure tin layer and intermetallic compound layer, and it is necessary to control these layers to balance these properties. In particular a large amount of pure tin plating layer is needed to obtain an adequate bending lifetime.