Investigation of lead cracking in TSSOP during burr-up forming process

Cracking was encountered on stamped TSSOP leads during a burr-up forming process. Most of the cracks were presented at the corner of the dambar and the external lead on the outer-bending surface. A comprehensive investigation including metallographical examination, material elongation testing and on-site modification testing was conducted to identify the causes of the cracking, and to suggest remedies to reduce or eliminate the problem. Results showed that the leadframe dambar radius, burr height, materials ductility, lead forming methods and forming profile are the factors related to the cracking tendency. Among them, the dambar radius, burr height and forming profile are the three essential factors causing lead cracks for the current burr-up forming process. The leadframe design was then modified by increasing the dambar radius and the forming profile was changed by enlarging the form anvil radius. Such modifications resulted in zero lead cracking with the same leadframe material at the same stamping burr height