Patent ID: 8008580

Claim:
A flexible printed circuit board (FPCB), comprising: a signal layer comprising a differential pair, the differential pair comprising two transmission lines to transmit a pair of differential signals; two dielectric layers respectively located on and under the signal layer to sandwich the signal layer, wherein dielectric coefficients of the two dielectric layers are different; and an upper ground layer and a lower ground layer, wherein the upper ground layer is attached to the dielectric layer on the signal layer, opposite to the signal layer; wherein the lower ground layer is attached to the dielectric layer under the signal layer, opposite to the signal layer; wherein each of the upper and lower ground layers defines a void aligning with the differential pair, wherein there is a first horizontal distance between each edge of the void of the upper ground layer and a nearest transmission line of the transmission lines, there is a second horizontal distance between each edge of the void of the lower ground layer and a nearest transmission line of the transmission lines, the first and second horizontal distances are different.