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36 The PCB Design Magazine • June 2014 Shielding films have a very thin metal coat- ing (approximately 0.1 micron), and an electri- cally conductive anisotropic (conducts in z axis) adhesive. They provide an extra layer of shield- ing without the expense and thickness of an extra copper layer. Some are black, some silver, and are applied after cover film lamination and require cover film openings that expose copper connected to ground. Ninja Flex Design Time I recently designed a flex with shielding film, a simple ZIF jumper (Figure 3). The circuit features two copper layers: Signal and ground plane. The customer wanted complete shield- ing, so rather than adding another copper layer, we specified shielding film. Along each edge of the flex is a wide copper trace tied with vias to the ground plane below. Round cover film openings exposed these traces so the anisotro- pic conductive film could make the electrical connection. In Figure 4 you can see the depres- sions where the lamination process pushed the shielding film down into the cover film open- ings. This particular shielding film is only 8 mi- crons thick, so it really conforms to the surface of the flex. Figure 4 also demonstrates how the shielding film completely hides the trace layout. It's possible to create a fully-shielded flex cir- cuit with only one copper layer by using shield- ing film. Just apply an oversized piece of shield- ing film to both sides of a single-layer flex after the circuit is cut from the panel. The shielding films will bond to one another in the area past the flex circuit's outline. Trim back the bonded shielding film close to the flex outline and you have shielding all the way around! Many companies are currently making shielding film: Tatsuta, Toyo Ink, Doosan, and Innox are major suppliers. So, if your flex application has special re- quirements, look to black cover film, antistatic cover film, or shielding film to give it some nin- ja powers! flexdude NINJA FLEx CIRCUITS continues Figure 3: a ziF flex circuit with two copper layers and a shielding film.

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