The High-Granularity Timing Detector (HGTD) will improve the performance of the ATLAS
detector for the high luminosity upgrade of the LHC (HL-LHC) by providing precise timing
information with a resolution of about 30 ps per track. The basic unit of the detector consists of a
hybrid module of a 2×4 cm$^2$ Low Gain Avalanche Detector (LGAD) bump-bonded to two ASICs
and wire-bonded to a Flexible Printed Circuit (FPC; FLEX cable). The latter transmits high-speed
signals (1.28 Gb/s) for data readout while providing power and bias voltage to the module. Its
design must fulfil the HGTD requirements both mechanically and electrically, combining different
signal types. The design and the test plan of the initial prototype are presented.