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Despite the higher thermal conductivity and the higher lifetime offered by silver sintering technologies, most packaged GaN devices are attached using solders due to technological difficulties in the sintering process. In this work, a silver sintering process for a packaged GaN power transistor on a printed circuit board (PCB) was successfully developed. Different sintering paste types were examined regarding their suitability for this application. Electrical measurements, shear tests, and metallographic cross sections were used for the evaluation. Numerical analyses were used to study the internal stress distribution in the GaN package after sintering depending on the paste structure. In the final sintering process, a shear strength of 20 MPa for sintering at 15 MPa and 240°C, for 300 s with electrical functional devices could be obtained by using nanoscale sintering paste. The authors contribute this to the high initial stiffness of the silver layer, which is obtained much earlier in the sintering process compared with the stiffness of a microscale silver paste. This high initial stiffness counteracts the semiconductor device deflection from the applied sintering pressure and reduces the stresses inside the semiconductor.