A large 9-10 GHz tile-based scalable phased-array system (over 128 elements) is built by flip-chip bonding of elementary antenna-in-package (AiP) modules on a large interposer PCB. Each module has a top radiating surface and a bottom BGA bonding surface. Due to the strict spacing requirement of a phased-array system, rework of any failed modules from the interposer PCB is prone to create more damages to the system and is prohibited. It is necessary to have a reliable and Automatic Test Equipment (ATE)-compatible test procedures to pre-screen qualified elementary modules for bonding. However, conventional ATE-compatible test jig designs can neither receive radiation signals from antenna nor feed test signals into the antenna. In this paper, a waveguide jig for AiP pre-screening is proposed. This jig system consists of a WR-90 adapter, a horn antenna, a Torlon cap and socket with POGO pins, and a PCB to interface with ATE. To test AiP transmitter, controls and test signals are provided through PCB connectors, with radiation signals collected from the WR-90 adaptor. To test the AiP receiver, radiation signals are feeding through the WR-90 adaptor, and the receiver output signals are collected from the PCB connectors. To calibrate waveguide jig return loss, a set of AiP SOL modules are used to remove PCB and test fixture loss from the overall system loss. The waveguide jig achieves a flat simulated -2.4 dB from AiP antenna port to waveguide port from 8 to 12 GHz. This waveguide jig provides a fast and reliable approach to select qualified AiP for phased array system assemblies, and the final system-level over-the-air testing.