Patent ID: 8110747

Claim:
A flexible printed circuit board (FPCB), comprising: a signal layer comprising a differential pair, and two grounded sheets made of a conductive material arranged in the signal layer, the two grounded sheets being located at two opposite sides of the differential pair respectively, the differential pair comprising two transmission lines, wherein the transmission lines are capable of transmitting a pair of differential signals, and wherein a length of each of the two grounded sheets in the signal layer is equal to a length of the differential pair; two dielectric layers located on and under the signal layer respectively, to sandwich the signal layer; and an upper ground layer and a lower ground layer, wherein the upper ground layer is attached to the dielectric layer located on the signal layer, opposite to the signal layer; wherein the lower ground layer is attached to the dielectric layer located under the signal layer, opposite to the signal layer, wherein each of the upper and lower ground layers comprises a grounded sheet made of a conductive material, two voids are defined in each of the upper and lower ground layers and located at opposite sides of the grounded sheet of each of the upper and lower ground layers, wherein distances between a middle line of the grounded sheet of the upper ground layer and middle lines of the two transmission lines are equal, and distances between a middle line of the grounded sheet of the lower ground layer and the middle lines of the two transmission lines are equal, wherein a width of the grounded sheet in the upper ground layer and a width of the grounded sheet in the lower ground layer are different, in response to dielectric coefficients of the two dielectric layers being different.