A packaged integrated circuit (IC) device having a heatsink mounted onto an IC die, itself mounted onto a die pad, is assembled using a lead frame having tie bars that deflect during an encapsulation phase of the device assembly, which enables the die pad, the die, and the heatsink to move relative to the lead frame support structure when compressive force is applied by the molding tool. This movement results in negligible relative displacement between the heatsink and the die during encapsulation, which reduces the probability of physical damage to the die. Each tie bar has a number of differently angled sections that enable it to deflect when compressive force is applied to it.