Lead frame

A lead frame is a thin metal skeleton that becomes the legs of a chip. In the middle is a small flat pad where the die sits, and around it are fingers that end in the leads, or legs. Here 'lead' rhymes with 'seed': it means a connecting wire, not the metal lead.

Lead frames are made from strips of copper alloy, often about 0.1 to 0.25 mm thick. The pattern is either punched out by a stamping press or etched with chemicals through a mask, with many frames side by side on one strip. Spots where wires will attach are often plated with silver for a good bond. The die is glued to the pad, bond wires connect it to the fingers, and the center is encased in mold compound. Then the outer rails of the strip are cut away, separating the legs, which are bent into shape. The exposed legs are plated with tin so they take solder easily.

In a power transistor, a thick copper tab draws heat out of the silicon. Lead frames also form the legs of integrated circuits, diodes, the infrared receiver and infrared LEDs, and the base of white LEDs.