Lead frame with deformable buffer portions

A lead frame has a central portion and groups of lead terminals distributed around the four peripheral sides of the central portion, along with main buffer portions coupling the lead terminals together in corresponding groups to prevent warping of the lead frame as a result of contraction of an injection-molded resin body during hardening. In addition, slits are provided outside the buffer portions which extend farther at one end of the slits than the corresponding side of the resin body, and auxiliary buffer portions located at both ends of the slits support the main buffer portions.

BACKGROUND OF THE INVENTION 
This invention relates to lead frames for rectangular flat packages having 
lead terminals which extend outwardly through the four peripheral sides of 
a resin body. 
In a conventional lead frame 1 for a rectangular flat package shown in FIG. 
3, the lead frame includes a central portion 3 on which a semiconductor 
element 2 is to be mounted and a plurality of lead terminals 4 distributed 
around the central portion 3 and extending toward the four peripheral 
sides of the frame. The inner ends of the lead terminals 4 terminate 
adjacent to the central portion 3 which is supported by two central 
portion leads 5. The lead terminals 4 and the central portion leads 5 are 
coupled together partway along their length by dam bars 6 and they are 
divided into four groups corresponding to the four peripheral sides of the 
frame 1. The lead terminals 4 and the central portion leads 5 are also 
joined at their outer ends to peripheral buffer portions 7 separated from 
the rest of the frame by slits 8. Two side rails 10 extend along the edges 
of the frame 1 and have guide holes 9 at preselected intervals so that the 
guide holes can be used, for example, to position the frame. In the 
arrangement shown in FIG. 3, the lead frame 1 is coupled to adjacent lead 
frames in a strip by section bars 11. 
A rectangular flat package is made from a lead frame 1 as described 
hereinafter. The semiconductor element 2 is first bonded onto the central 
portion 3 of the frame 1 and the terminals of the element 2 are then 
connected by wires to the lead terminals 4 of the frame. An epoxy resin or 
similar material is molded, for example, by transfer molding, around the 
lead frame 1 so as to make a resin body 12 in an oblong area inside the 
dam bars 6 as shown by a dot-dash outline in FIG. 3. The semiconductor 
element 2 is thus encapsulated by the resin body 12. Thereafter, the dam 
bars 6 are punched out and the lead terminals 4 are cut off and bent. 
When the epoxy resin or similar material thermally hardens during molding 
to make the resin body 12, the molded material contracts. Since the lead 
frame 1 has a lower coefficient of thermal expansion than the molded 
material and is very thin, the frame is likely to undergo warp or 
deformation as the resin body contracts. However, because of the presence 
of the slits 8, the buffer portions 7 are deformed to allow the resin body 
12 to contract in the longitudinal and transverse directions without 
causing the lead frame 1 to undergo much warp or deformation as a whole. 
Nevertheless, the conventional lead frame 1 has a problem as described 
hereinafter. The resin body 12 of the rectangular flat package contracts 
toward its center in all directions, but the deformation of the buffer 
portions 7 of the lead frame does not prevent the contraction of the resin 
body in the diagonal directions from causing the lead frame to undergo 
warp or deformation. For that reason, the frame 1 cannot smoothly be 
conveyed during the punching and bending operation after the manufacturing 
of the resin body 12. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a lead 
frame which overcomes the disadvantages of the prior art. 
Another object of the invention is to provide a lead frame which is 
prevented from undergoing warp or deformation in any direction when a 
resin body contracts. 
These and other objects of the invention are attained by providing a lead 
frame which includes groups of lead terminals arranged to extend outwardly 
through the four peripheral sides of the resin body on the frame and main 
buffer portions coupling the lead terminals together at the outer ends 
thereof in each of the groups, respectively, along with slits extending 
outside the buffer portions with each of the slits extending farther at at 
least one end thereof than the corresponding side of the resin body. In 
addition, the main buffer portions are coupled at both ends to section 
bars or side rails by auxiliary buffer portions located at both ends of 
the slits and arranged to support the main buffer portions. 
In a lead frame arranged in accordance with the present invention, the main 
buffer portions are deformed during contraction of the resin body so as to 
enable the frame to permit the contraction primarily in the longitudinal 
and transverse directions of the resin body, and the auxiliary buffer 
portions located at both ends of the slits and supporting the main buffer 
portions are also deformed during the contraction so as to enable the 
frame to permit contraction in the diagonal directions of the resin body. 
For that reason, the lead frame is inhibited from undergoing any warp or 
deformation as a whole due to the contraction of the molded portion.

DESCRIPTION OF PREFERRED EMBODIMENT 
In the typical embodiment of the invention shown in FIG. 1, a lead frame 20 
includes a central portion 3 on which a semiconductor element 2 is to be 
mounted and lead terminals 4 distributed around the four peripheral sides 
of the central portion. In addition, central portion leads 5 support the 
central portion and dam bars 6 connect all of the lead terminals 4 and the 
central portion support leads 5. The lead terminals are connected to main 
buffer portions 7 which join the lead terminals 4 and the central portion 
leads 5 together at the outer ends thereof and side rails 10 having guide 
holes 9 extend along the edges of the lead frame while section bars 11 
separate the lead frame 20 from adjacent lead frames in the strip and 
L-shaped auxiliary buffer portions 22 are provided at the ends of the main 
buffer portions 7. The lead terminals 4 are coupled to the central portion 
leads 5 by the dam bars 6 and the main buffer portions 7. The guide holes 
9 are provided at prescribed intervals in the side rails 10. 
The lead frame 20 also has slits 21 disposed outside the main buffer 
portions 7 and, as shown in FIG. 1, each of the slits 21 extends farther 
at one end than the corresponding side of the resin body 12 of a 
rectangular flat package which incorporates the lead frame 20 in the same 
manner as the conventional rectangular flat package described above. Since 
a gate, which is not shown in the drawings, for injecting an epoxy resin 
into a cavity between molding dies to manufacture the resin body 12 must 
be provided at a location corresponding to a corner of the resin body, the 
slits 21 cannot extend farther than the corresponding side of the resin 
body 12 at their other ends. However, each of the slits 21 should 
preferably extend farther at both ends than the corresponding side of the 
resin body 12 in order to more effectively prevent the lead frame 20 from 
undergoing deformation or warp as a whole. Each of the four groups of the 
lead terminals 4 extending outward from the four sides of the central 
portion 3 are coupled together at the outer ends of the terminals by the 
corresponding main buffer portion 7 extending along the slit 21. The main 
buffer portions 7 are coupled at both ends thereof to the side rails 10 or 
to the section bars 11 by the auxiliary buffer portions 22 extending at 
both ends of the slits 21 and supporting the main buffer portions. 
When the epoxy resin thermally hardens during the molding of the resin body 
12 during the molding process, the resin contracts in all directions as 
shown by the arrows in FIG. 2. At that time, the main buffer portions 7 
extending along the slits 21 are deformed to enable the lead frame 20 to 
permit the contraction of the molded portion 12 chiefly in the 
longitudinal and transverse directions thereof, and the auxiliary buffer 
portions 22 extending at both ends of the slits are deformed to enable the 
frame to permit the contraction of the resin body in the diagonal 
directions thereof. For that reason, the lead frame 20 is prevented from 
being deformed or warped as a whole due to contraction of the resin body. 
Thus, the lead frame provided in accordance with the present invention has 
slits along main buffer portions coupling lead terminals together in 
groups which extend farther at least at one end of the slit than the 
corresponding side of the resin body of the rectangular flat package, and 
has main buffer portions which are coupled at both ends to section bars or 
side rails by auxiliary buffer portions located at both ends of the slits 
to support the main buffer portions. For that reason, when the resin body 
contracts during the thermal hardening of an epoxy resin which has been 
injected to make the body, the main and auxiliary buffer portions are both 
deformed to enable the lead frame to permit contraction of the resin body 
in the longitudinal, transverse and diagonal directions so that the frame 
is prevented from undergoing warping or deformation as a whole. As a 
result, the lead frame can be easily conveyed during cut-off of dam bars 
and bending of the lead terminals after the molding has been completed. 
Although the invention has been described herein with reference to a 
specific embodiment, many modifications and variations therein will 
readily occur to those skilled in the art. Accordingly, all such 
variations and modifications are included within the intended scope of the 
invention.