Mechanical Deformation of Leadframe Assemblies in Plastic Packages During Molding

Abstract

We evaluate the maximum stresses and deflections in electric leads subjected to flow induced forces during transfer molding of plastic packages. It is found that lead deflection is proportional to the fifth power of the lead length and the inverse third power of lead thickness. These dependencies explain why it is more difficult to mold high pin count packages which have significantly longer and thinner leads to accommodate the higher interconnection density. In addition we find that the maximum value of the elastic lead deformation is about 0.9 mm (35 mils) for large state-of-the-art packages; exceeding this value causes permanent (plastic) lead distortion that compromises the symmetry of the package. We find also that minimizing the difference in flow front locations through careful gate design, and lowering the velocity over the leads by using balanced mold filling can make a significant difference in lead deformation. The results obtained are useful in establishing the appropriate geometry of the leads, as well as in choosing the materials and process parameters, so that excessive stresses and lead deflections can be avoided.