There are a couple of different methods of increasing the flexibility of a flex circuit board. The most common method is to reduce the overall thickness of the flexible dielectric material because it’s thickness is directly related to the flexibility of the circuit board.
The second most common method is to reduce the copper thickness of the traces and more over the thickness of the plane layer. One way of reducing copper, on a plane layer, is by cross hatching the plane. Typically we recommend .015” wide signals with .025” spacing for the cross hatched plane layers.
Ground and power planes are usually cross hatched in flexible printed circuit boards in order to maintain or increase the flexibility of a circuit board. A ground or power plane that is completely flooded doesn’t bend.
A cross hatch plane can be characterized by the ratio of cross hatch conductor width to the cross hatch pitch. The lesser the ratio, the greater the percentage of copper being removed. A 50% copper removal would be achieved if the ratio was about 0.293. For a pitch of 30 mils, this would bean a cross hatch conductor width of about 9 mils.