VSS (pin 103) is the main electrical ground connection between nRF9160 and the application board. It is also the main thermal dissipation path from nRF9160 and the main mechanical connection.
The connection between VSS and the application board must be electrically, thermally, and mechanically strong. It is recommended that the application board Printed Circuit Board (PCB)'s ground planes around nRF9160 are as intact and as solid as possible. Use as many ground vias as possible between the top metal layer of the application board and the inner ground layers connected to nRF9160's VSS pins. Typically, metal filled vias provide the best thermal conductivity.