Transient three dimensional simulation of mold filling and wire sweep in an overmold BGA package

The transient three-dimensional filling of a overmold BGA package with 173 wirebonds was simulated using a commercial CFD software. The geometry of the flow front of the molten molding compound and the three-dimensional velocity distribution were obtained as a function of time. At each time-step, the distribution of flow-induced forces acting along each wirebond was computed and employed in a finite element structural analysis program to determine the three-dimensional deformation of the wirebond. Thus the wire sweep pattern of the 173 wirebonds during the filling process was obtained. The analysis was done for two different positions of the gate - one at the middle of a side and the other at a comer. The pattern of wire sweep was different for the two different gate locations. In the finite element simulation of the structural deformation of the wirebonds, the wirebonds were assumed to have no initial stress before the action of the flow-induced forces. In reality there is some residual stresses in the wirebond due to the wire looping process, especially at the neck of the wirebond adjacent to the ball bond. To determine the effect of such residual stresses, the wire looping process was first simulated. With the resultant residual stresses in the wirebond, the flow-induced forces on the wirebond are applied and the deflection of the wirebond computed. Then the residual stresses in the wirebond are removed and the same flow-induced forces applied.