In this study, we have fabricated and characterized an In0:6Ga0:4As metamorphic high-electron-mobility transistor (mHEMT) device packaged using flip-chip-on-board (FCOB) technology. A low-cost polymer substrate was adopted as the carrier for cost-effective purposes. The impact of bonding temperature on the device performance was also experimentally investigated. While the DC performance was not as sensitive, serious degradation in RF performance was observed at high bonding temperature. Such degradation was mainly due to the thermal-mechanical stress resulting from the mismatch in the coefficient of thermal expansion (CTE) between the GaAs chip and the polymer substrate. Quantitative assessment was also performed through equivalent circuit extraction from S-parameter measurements.