Time-accurate, 3-D computation of wire sweep during plastic encapsulation of IC components

Wire sweep is one of the major defects encountered in plastic encapsulation of electronic components by transfer molding, and the need to understand and control this phenomenon is becoming greater as lead counts increase and package dimensions decrease. This paper reports on the development of a comprehensive computational technique for accurate prediction of wire sweep which advances the current state-of-the-art by modeling the time-dependence of the flow and wire deformations and stresses, including the effect of the propagation of the melt-front. The technique is based on the use of a coupled code consisting of a Computational Fluid Dynamics code (CFD-ACE-U) that computes the transient flowfield during molding, and a Structural Dynamics code (FEM-STRESS) that computes the transient deformations and stresses in wires due to the hydrodynamic loads imposed on them by the flowfield. The coupling between the two codes is based on accurate analytical and empirical relations between the loads on obstacles in a viscous flowfield and the local flow conditions. Since it accounts for the detailed time-dependent material, geometric, and loading variations in wires, the technique presented here can describe how wire sweep evolves with time throughout a package, helping a designer identify corrective modifications to the mold design or the processing conditions