The effect of the cross-section of outlead on lead skew

On high performance package like Quad Flat Package (QFP), lead skew of the outer lead exists in its asymmetrical cross-section. For fine pitch lead frame with the range of 0.3-0.5 mm pitch, the cross-section of outer lead seems to accelerate the skew. The skew value depends on the mismatch of top and bottom patterns of photo-imaging process. Reduction of lead width caused by fine pitch, enhances this asymmetry. The results of the experiment and simulation suggest the following mechanism. In the products with the asymmetrical cross-section of outer lead, the mismatch from top to bottom surface forms the geometric moment of inertia. The moment that is rotational brings about the lead skew at bending which is outer lead forming process. The lead skew of outer lead is determined with the computer simulation by FEM and the allowable cross-section of outer lead is obtained. The cross-section that is from right to left and top to bottom, doesn't contribute any lead skew. A shorter width at the top than the bottom makes the skew value smaller. This guides the design of upside-down chip package.<>