Leadframe designs for minimum molding-induced warpage

The current packaging trend toward thinner packages has pushed the leadframe manufacturing technology to the limits. For instance, with Ultra Thin Small Outline Packages (UTSOPs), the total form factor is less than 0.5 mm thick, which is about twice the thickness of the typical leadframe used in current standard IC packages. Thus, the leadframe must be scaled down appropriately with the rest of the package components. The thinner leadframe stock brings a host of handling difficulties. Most important of all is the bending and twisting of the leadframes caused by molding induced stresses. The resulting deformation affects the subsequent post-molding steps such as trim and form, and may seriously increase the production yield loss due to lead non-coplanarity. This study addresses the manufacturing and design issues involved with minimizing these molding-induced stresses. Through a combination of good control on the process parameters (e.g., post-molding handling, cooling, deflash procedures, etc.) and leadframe designs (e.g., package relative positioning within the leadframe strips, relief holes and slots, strengthened support strips, etc.), leadframe warpage from thermomechanical stresses can be reduced. Warpage predictions from finite element simulation will be presented for various leadframe designs in this paper