A grid array antenna working around 145 GHz is proposed in this paper. The antenna is built on liquid crystal polymer (LCP) and designed for the D-band antenna-in-package application. The intrinsic softness of the LCP material is a limiting factor of the antenna's aperture size. A 0.5-mm-thick copper core is used to compensate. By doing this, the rigidness of the antenna is effectively improved, compared with an antenna without the copper core. Wet etching is used to realize the patterns on the top and bottom conductor. Compared with a low-temperature cofired ceramic counterpart, we obtain a considerable cost reduction with acceptable performance. The proposed antenna has an impedance bandwidth of 136-157 GHz, a maximum gain of 14.5 dBi at 146 GHz, and vertical beams in the broadside direction between 141 and 149 GHz. The fabrication procedures of the antennas are introduced, and a parametric study is carried out, which shows the antenna's robustness against fabrication tolerances, such as the not-well-controlled etching rate and the substrate surface roughness. This makes the antenna a promising solution for mass production.