Leadframe for flash-free insert molding and method therefor

Improved leadframe and method for the insert molding of semiconductor components or the like. A plurality of leads separated by zero-flash tab means for controlling the flash during molding are formed in a single sheet of material. The zero-flash tab means are stamped from the stock sheet and then reinserted into the area between adjacent leads. The zero-flash tab means desirably comprise a ridge section for strength and facilitating removal after the molding operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to the plastic molding of objects having a 
plurality of protruding metallic inserts, and more specifically, to a 
leadframe suitable for the plastic encapsulation of semiconductor devices. 
2. Description of the Prior Art 
During the process of encapsulating in plastic, a semiconductor device (or 
integrated circuit) is assembled on a metallic leadframe which facilitates 
the making of external electrical connections. The device and the 
leadframe are then inserted into a mold cavity while the leads extend 
outside of the cavity. The mold is heated and the plastic is injected into 
the mold in liquid or semi-liquid form under very high pressure. Due to 
its fluidity, the plastic material runs out of the mold through any 
crevices where the sealing is imperfect. In the past, to avoid leakage 
between the leads and the consequent flash, the mold has been castellated. 
This, however, increases the costs of the mold and leads to further 
sealing problems where the thickness of the leadframe stock is variable. 
Another technique used to reduce the flash caused by leakage between the 
metallic leads is to employ dam-bars which are incorporated into the 
leadframe. Dam-bars comprise a portion of the leadframe extending between 
leads so that mold closure may be effected without castellations. 
Subsequent to the molding operation, the dam-bars are punched out. This 
punching out process dictates that the dam-bars must be sufficiently far 
from the package to allow their removal without damaging the package. 
Since the mold compound is present at the dam-bar and the space between 
the leads is small, the punches used to remove the dam-bars are fragile 
and have a limited liftetime. Two other problems inherent to the punching 
out process are that the leads can be damaged by the punches and that mold 
compound is left on the leads which may foul or damage any further 
equipment which handles the device. Technologic progress has led to ever 
increasing lead counts and smaller packages which in turn has led to 
decreasing space between the leads. Therefore, smaller and more fragile 
dam-bar punches are required and the possibility of lead damage and 
unremoved flash is increased. 
Thus a need exists for an encapsulation process, including an improved 
leadframe, which will reduce or eliminate the flash that results from the 
molding operation, and, which will mitigate the requirements of the trim 
and form operation subsequent to the molding process. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an improved leadframe for the 
insert molding of small objects. 
It is another object of this invention to provide an improved leadframe for 
the insert molding of semiconductor electronic devices and integrated 
circuits. 
It is a further object of this invention to provide a leadframe which 
susbstantially prevents flash between the leads of a plastic-encapsulated 
electronic package. 
It is yet another object of this invention to provide a leadframe 
incorporated portions for the preclusion of interlead flash, which 
portions may be easily punched out upon the completion of the molding 
operation. 
It is a further object of this invention to prevent flash between leads 
right up to the body of semiconductor electronics package. 
It is a still further object of this invention to provide an improved 
leadframe which is utilized in an improved encapsulation process. 
In accordance with a preferred embodiment of this invention, a leadframe 
portion, which is not attached to the leads in the vicinity of the edge of 
the package body, replaces the conventional leadframe dam-bars. This 
detached portion of the leadframe is called a zero-flash tab. 
In accordance with another embodiment of this invention, the zero-flash tab 
extends right up to the edge of the package body.

DETAILED DESCRIPTION 
Referring now to FIG. 1, the invention is illustrated by a top view of a 
portion of an improved electronics package comprising a plastic body 
portion 1 and a plurality of leads 10 extending at generally right angles 
from the plastic body portion 1. Both the leads 10 and the tabs 12 are 
integral with and extend inwardly from the outer extremity of metallic 
leadframe 2. The leads 10 extend into the region of the plastic body 1 in 
order to support or connect to an integrated circuit housed in the plastic 
body. As shown in FIG. 3, which is a cross-section of the leadframe where 
mold closure is effected near the plastic body 1, the leads 10 and the 
zero-flash tabs 12 form a substantially continuous member having a uniform 
cross-section. The mold can be positively closed at this point without the 
necessity for castellations to fit between adjacent leads. Leads 10 and 
zero-flash tabs 12 are separated by part lines 16 which are the result of 
stamping the tabs 12 from the sheet stock and then reinserting them into 
the leadframe. 
Referring now to FIG. 4, there is illustrated in cross-section a portion of 
the leadframe exterior to the situs of leadframe closure. From this figure 
and from FIG. 1, it may be seen that the zero-flash tabs 12 hvae a ridge 
section 13 which serves two desirable purposes. First, the ridge or bow 13 
in the zero-flash tabs 12 pulls each tab away from the adjacent leads to 
reduce lead splay and reinsertion forces during stamping. Second, the 
ridge section 13 stiffens each zero-flash tab 12 to aid its removal during 
the trim and form process and to prevent mold compound blowout during the 
molding operation. Referring again to FIG. 1, a series of corner relief 
holes 14 are shown, which relief holes allow the leadframe strip to be 
formed without inducing excessive stress which would distort the 
leadframe. 
FIG. 2 is a cross-section of the leadframe interior near the termination of 
the zero-flash tabs 12 at the extremity of the reinsertion area. As 
further shown in FIG. 1, a small space 11 may be left between the end of 
each zero-flash tab 12 and the plastic body 1 of the electronics package 
in order to facilitate the trimming of the zero-flash tabs from the rest 
of the leadframe. Alternatively, the zero-flash tabs 12 may extend right 
up the plastic body 1 thus eliminating the spaces 11. 
While the invention has been particularly shown and described with respect 
to preferred embodiments thereof, it will be apparent to those skilled in 
the art that changes in form and detail may be made without departing from 
the spirit and scope of the invention.