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Although leadframe assemblies have been well developed and plastic encapsulation has been popular and common for semiconductor packaging, lots of related problems still exist in the packages. During molding process, viscous epoxy molding compound (EMC) flows over wires and drags them for a short distance, then induces the well-known wire sweep problem. Similar phenomena can also happen to the leads of a leadframe and make them shift. The lead deflection phenomena were rarely mentioned or discussed before. However, extreme deflection for one lead could be sufficiently severe to cause leads contact and damage the device. A numerical simulation was developed to predict the lead deflection of a leadframe assembly which was encapsulated with EMC after molding process. X-ray detection of the encapsulated leads and optical microscope inspections of the encapsulated leads, were introduced to investigate and verify the lead shifting phenomenon. The experimental results showed satisfactory consistency with the simulation. The deflection of the leads was calculated by applying the distributed pressure from fluid onto the exterior surface of the leads. The distributed pressure of the flowing compound during encapsulation was obtained from a 3-D flow simulation. Experiments were conducted to obtain proper properties of materials for the flow and deflection simulation. Some key parameters such as package thickness, gate shape and down-set distance were studied to evaluate their effects on the deflection of the leads. It was found that the flow effect of the EMC during molding did play an important roll for the lead deflection and should be carefully considered when designing the leadframe. The results also showed that the approach proposed in tins research did provide a useful tool for the leadframe design engineers to avoid the lead deflection problems during encapsulation.