So-called pre-fabrication lead frames have been proposed to realize high density mounting of semiconductor chips in the form of COL (Chip on Lead), LOC (Lead on Chip), etc. This kind of a lead frame is provided with an insulating film punched to a predetermined configuration and adhesively fixed on an inner lead portion of the frame, wherein the insulation film is composed of a polyimide-based film having a high heat-proof property and thermoplastic or thermosetting adhesive layers on both surfaces (or one surface) of the polyimide-based film.
In providing the insulating film, an apparatus comprising punches and a die is used to punch an insulating film placed on the die by lowering the punches, wherein a clearance between each of the punches and the die is adjusted to suppress the occurrence of chips at the time of punching the insulating film and burrs at edges of a punched insulating film.
In the conventional insulating film to be used for the COL lead frame, the LOG lead frame, etc., however, there is a disadvantage in that the occurrence of chips and burrs can not be suppressed sufficiently, even if a clearance between a punch and a die is adjusted, because mechanical characteristics of an insulating film to be punched are varied, especially, in regard to an edge tearing resistance. Consequently, the dimensional precision of the punched insulating film is lowered by concealing an area for bonding wires on the lead frame by burrs and chips. This means that it is impossible for bonding wires to be bonded between the lead frame and a semiconductor chip mounted thereon.
There are further disadvantages in the conventional insulating film in that burrs and chips tend to be accumulated between the punch and the die, so that the positional precision of fixing a punched insulating film on a lead frame is lowered, and marks of burrs and chips are impressed on a punched insulating film to be thereby classified into a faulty. This means that cleaning of the punch and die must be done often.