This article describes the complete design of high performances Ka-Band small size Spatial Power Combiner (SPC) Amplifier. Several multiple physics aspects are treated in the proposed study as electromagnetic behavior and thermo-mechanical features. The combiner consists of quadruple Fin lines to microstrip (FLuS) transitions inserted into a WR28 waveguide T-junction. In this structure, 16 Monolithic Microwave Integrated Circuit (MMIC) Solid State Power Amplifiers (SSPA’s) are integrated. In order to drive the active devices at full power, thermal exposition has been controlled by an opportune heat-sinker subjected to a cooling air flow. A main design has been followed by FEM simulation using Ansys-Ansoft HFSS and Comsol Multiphysics. Scattering parameters, stresses and strains have been computed together with the temperature and airflow distributions. A mean insertion loss of 2 dB is achieved with a return loss better the 10 dB in the 31–37 GHz bandwidth while operating at maximum power. In such condition, the transistors present a maximum displacement of 28.7 µm caused by the thermal expansion of the material due to a channel temperature of 125◦ C, and special techniques have to been applied to avoid the MMIC’s breakage.