The use of force feedback in an assembly robot can be very useful since it allows to overcome position errors and tolerance effects that cannot be detected otherwise. In order to obtain an efficient and robust force feedback loop a system capable of measuring forces with the required accuracy must be available. In addition, the relation between the forces acting on the different parts and their deformation must be known. The rigidities of the three main components of the modelled assembly system, namely the robot arm, the force sensor and the assembled parts should be known. Unfortunately, this is not always possible. A certain amount of assumptions can be made just by knowing what stiffness is smaller in relation to the others. When force feedback is used in micro-assembly operations factors not relevant in the conventional size domain start to gain importance (and vice-versa) and must therefore be considered in the control loop. In addition a method based on dimensional analysis that allows to chose the most convenient type of force sensor in relation to the resolution, measuring range and even fabrication process is described.