A novel process eliminates copper plating, potentially increasing board flexibility and reliability.
A new flexible printed circuit uses circuit layers connected by conductive paste. The technology is based on metal nanoparticle technology developed by Sumitomo.
The method, called paste via method, consists of forming a circuit, drilling holes in the insulating film, and filling the holes with conductive paste. Since the paste via method does not use copper plating, it reportedly reduces the thickness of circuit boards by nearly 30% compared with that of traditional boards, Sumitomo Electric Industries says.
The newly developed FPC is expected to improve the wiring design flexibility of mobile terminals and other electronics devices, and to save space.
A flex circuit is a flexible wiring material consisting of an insulating film on which an electric circuit (conductor layer) is formed using copper foil. The FPC is thin, light and features superior heat resistance, while enabling various parts to be mounted on it. FPCs include single-sided boards (single-layer circuits), double-sided boards (two layers of circuits with insulating film in between), or multilayer boards (three or more layers of circuits) (Figure 1).
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Conventionally, panel plating (Figure 2) has been widely used to connect the circuits on a double-sided or multilayer board. Panel plating consists of 1) drilling holes in the insulating film, 2) adding two types of conductive copper plating, and 3) forming a circuit. The method is seen as limited in forming precise circuit patterns due to the thickness of the plated copper layers.
In contrast, the paste via method for connecting circuits with conductive paste consists of 1) forming a circuit, 2) drilling holes in the insulating film, and 3) filling the holes with conductive paste. Use of conductive paste and the development of a patent-pending technology that can accurately and evenly fill the paste into the vias enhance circuit connection reliability and boost product quality. Since the paste via method does not use copper plating, it reduces the thickness of circuit boards by nearly 30% compared with that of traditional boards, thereby improving the flexibility of the circuit boards and permitting the formation of more precise circuits. Compared with panel plating, paste via method has simplified flex circuit production and produces higher quality boards.
In April, the firm started full-scale operation of its mass-production line at Minakuchi Works, and has recently started to ship the double-sided FPCs.