Lead frame wire bonding by preheating

A method for welding lead wire or bonding wire held in the capillary bonding tool of a bonding machine to a metal lead frame finger where one end of the lead wire has already been bonded to a circuit chip on the lead frame comprising:

• melting a portion of the surface of said lead frame finger thereby forming a molten pool in said surface while the lead frame is substantially isolated from electrical and thermal coupling between the lead frame finger and circuit chip, said melting achieved by positioning an electrode over the lead frame finger and applying electrical energy to the lead frame finger through the electrode thereby preheating the lead frame finger and forming the molten pool;
• retracting the electrode and terminating application of electrical energy to said lead frame finger through the electrode prior to contact of the wire with said lead frame finger;
• immersing a section of the lead wire in said molten pool;
• allowing the pool to harden;
• and serving the lead wire adjacent the surface of the lead frame finger.

A method for bonding lead wire or bonding wire held in the capillary bonding tool of a bonding machine to a metal lead frame finger where one end of the lead wire has already been bonded to a circuit chip mounted on the lead frame comprising:

• preheating said metal lead frame finger to a temperature below the melting point of the lead frame finger metal but above the temperature at which damage might occur to the circuit chip while the lead frame finger is substantially isolated from electrical and thermal coupling between the lead frame finger and circuit chip, said preheating achieved by positioning a preheating electrode over the lead frame finger and applying electrical energy to the lead frame finger through the preheating electrode;
• retracting the preheating electrode and terminating application of electrical energy to said lead frame finger through the preheating electrode prior to contact of the lead wire with said lead frame finger;
• contact bonding a section of the lead wire to the surface of the preheated lead frame finger;
• cooling said lead frame finger and bond;
• and serving said lead wire adjacent said bond and lead frame finger.

Apparatus for welding lead wire or bonding wire from a circuit chip mounted on a lead frame to a metal lead frame finger by preheating of the lead frame finger comprising:

• inert gas enclosure means including a base and side walls said inert gas enclosure means forming at least a partial housing for said apparatus and defining a welding and bonding zone;
• first inert gas delivery means comprising gas distribution means for flooding the space within said enclosure with inert gas;
• lead frame strip feeding means for feeding a strip of lead frames through said enclosure means;
• electrode means and electrode mounting means for positioning said electrode means over a selected lead frame and lead frame finger;
• capillary wire holding tool mounting means for mounting a capillary wire holding tool in said enclosure over a selected lead frame;
• second inert gas delivery means for delivering a directed stream of inert gas for cooling and purging in the vicinity of the selected lead frame and lead frame finger;
• circuit means comprising said electrode means and selected lead frame finger;
• and pulse generator means for generating an electrical pulse train of controlled duration, said generator means coupled to said circuit means for discharging of said pulses between the electrode means and lead frame finger means for delivering a controlled amount of energy to the lead frame finger to achieve a desired temperature.