Shrink accommodating lead frame

A lead frame design which can be used with a number of different die sizes is described. To customize the lead frame, a punch excises an amount of lead finger material to form a void between the lead fingers for receiving a die. The amount of material removed is greater for larger sized die. A material, such as an adhesive tape, attaches the die to the lead frame. The bond pads on the die are then wire bonded to the lead fingers. The adhesive tape also locks the lead fingers into place, thereby preventing movement which could detach the bond wires from the die or lead frame.

FIELD OF THE INVENTION 
This invention relates to the field of semiconductor manufacture. More 
specifically, a lead frame design which can accommodate a wide variety of 
die sizes while allowing for short bond wires is disclosed. The invention 
also results in immobilized lead fingers, thereby reducing bond wire 
detach problems. 
BACKGROUND OF THE INVENTION 
Various types of semiconductor devices are manufactured in much the same 
way. A starting substrate, usually a thin wafer of silicon or gallium 
arsenide, is masked, etched, and doped through several process steps, the 
steps depending on the type of devices being manufactured. This process 
yields a number of die on each wafer produced. The die are separated with 
a die saw, and then packaged into individual components. 
During the packaging process, several semiconductor die are attached to a 
lead frame, often with a material such as metal, or an epoxy or other 
viscous adhesives. Bond wires couple each of several bond pads on each die 
to conductive leads on the lead frame. The die, the wires, and a portion 
of the leads are encapsulated in plastic or encased in ceramic. These 
leads couple the die with the device into which the component is 
installed, thereby forming a means of I/O between the die and the device. 
One step of semiconductor manufacture that is not without problems is the 
die-lead frame attachment. During the manufacturing process, several die 
are attached to the lead frame, bond wires are connected from the bond 
pads on each of the die to the "fingers" on the lead frame, then the die 
is encapsulated in a protective plastic casing. The plastic packages are 
separated, and the leads are formed into a desired shape. The lead frame, 
part of which will eventually form the conductive leads of the components, 
contains a major surface to which the die is attached, called the 
"paddle." The die is normally bonded to the paddle with epoxy or metal, 
although thermoplastic, tape, or another materials are also used. FIG. 1A 
is a top view, and FIG. 1B is a cross section, showing a conventional lead 
frame 10 having die paddles 12 with die 14 attached. The frame 10 
comprises dam bars 16 which restrict the flow of encapsulating material 
during encapsulation, exterior leads 18 which are unencapsulated, and lead 
fingers 20 which will be encapsulated. Bond wires 22 electrically couple 
bond pads 24 on the die 14 with the lead fingers 20. The adhesive 26 used 
to attach the die 14 to the lead frame 10 is dispensed on the die paddle 
area 12 of the lead frame 10. The die 14 is placed on the uncured epoxy 
(for example) and held at a specific pressure by die attachment equipment 
having a surface contact tool or an edge contact only tool (collet). The 
die is pressed down into the adhesive at a specific pressure, and often at 
a controlled temperature, by the tool and held in place long enough to 
ensure adhesion. X-Y movement (scrub) is sometimes used to increase 
adhesion and to speed the process. The attach process often requires a 
follow-on cure in a separate cure oven. After the attach process, the 
assembly within the dam bars 16 is encapsulated. The paddle 12 of the lead 
frame 10 is usually at a lower plane 28, which allows better control of 
the plastic encapsulation material as it is being injected into the mold. 
This lessens the chance, for example, of the bond wires 22 detaching from 
the lead fingers or bond pads. 
Various problems are associated with the connection of the die to the die 
paddle. Occasionally a corner of the die will crack, thereby making the 
semiconductor inoperable. This can result from stress placed upon the die 
by the adhesive due to an uneven thermal coefficient of expansion between 
the die and the adhesive. After the die is attached to the lead frame and 
oven cured, the assembly is heated at the wire bond step to attach the 
wire to the die pad. If the die and the adhesive expand at different 
rates, undue stresses can be inflicted on the die. Cracks can also occur 
from stress on the die due to shrinkage of the adhesive as it cures, 
although in recent years chemical improvements in adhesive has reduced 
this cause of cracking. 
Occasionally the die and epoxy may come loose from the lead frame, a 
problem referred to as "popping die." Popping die can result from too 
little adhesive under the die, a poor bond between the adhesive and the 
paddle, or from bowing of the die paddle from heat or pressure. This can 
be a serious problem, not only because it results in scrapping the die but 
also because the loose die can damage the molds which are used to 
encapsulate the package. 
Other problems are also associated with the lead frame. Once the die is 
attached to the die paddle, bond wires electrically couple bond pads on 
the die to lead fingers on the lead frame. Before encapsulation the lead 
fingers can move, thereby damaging the bond wire coupling the bond pads 
and lead fingers which results in an unreliable or nonfunctional device. 
Another problem associated with packaging of a semiconductor device is that 
a new lead frame design must be used each time a die size changes. With 
conventional lead frames, the die paddle must be approximately the same 
size as the die, and therefore as the die decreases in size a lead frame 
with a smaller paddle must be used. Also, the lead fingers must be 
sufficiently close to the die to keep the bond wires which couple the lead 
fingers to the bond pad as short as possible. Long bond wires require more 
material (usually gold) than shorter wires and are therefore more 
expensive, and also have a greater likelihood of shorting or breaking. 
When a die is shrunk a new lead frame having a smaller die paddle and 
longer lead fingers must be produced. A die shrink therefore requires a 
retooling of the lead frame stamp, which currently costs more than 
$50,000. In addition, unused stock is discarded or recycled. 
U.S. Pat. No. 4,868,635 to Frechette et al. describes a lead frame which 
can be customized to accommodate semiconductor dies of different sizes. 
This lead frame, however, would have the problem of lead movement, as can 
be determined from studying FIG. 3 of Frenchette et al. 
A lead frame design which solved the problems associated with the die 
paddle and long leads, and those resulting from a die shrink would allow 
for reduced costs and increased yields. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a lead frame having no die paddle, 
thereby eliminating the problems associated with the die paddle. Another 
object of the invention is to provide a lead frame which can be used with 
a number of different die sizes. Yet another object of the invention is to 
provide a lead frame design which increases yields due to its 
immobilization of the lead fingers which prevents damage to the bond wired 
resulting from lead movement. Still another object of the invention is to 
provide a lead frame which lowers the production costs by increasing 
yields and by eliminating the need to retool the lead frame to accommodate 
a die redesigned for size reduction. 
These objects of the invention are realized by a lead frame having an 
inventive design which can accommodate a number of different die sizes 
with minimal customization. The paddle-less lead frame comprises a number 
of leads which converge on the center of the die placement area. Prior to 
attaching the die and the lead frame, the leads are trimmed, the amount 
trimmed material depending on the die size. A smaller die requires less 
lead finger material to be trimmed. In any case, the leads are excised to 
provide a void to receive the die. To attach the die to the lead frame, a 
tape material having an adhesive on one side is attached to the leads, and 
the die is held in place by the tape. The tape also serves to immobilize 
the leads so that lead movement is greatly reduced or eliminated. This is 
especially important after attachment of the bond wires, as lead movement 
is known to damage the bond wires or their attachment to the die or lead 
frame, thereby producing a nonfunctional device. 
Other possible configurations of the tape, lead frame, and die are possible 
.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 2 shows a lead frame assembly comprising the invention. The lead frame 
30 comprises lead fingers 32 which extend to the center of the die 
placement region 34. The lead fingers can be connected for support in this 
area as shown, or in other configurations, as this area will be removed 
before assembly of the die. It may also be possible to leave the leads 
unjoined. Depending on the size of the die which will be attached to the 
lead frame 30, a portion of the lead fingers is trimmed away, thereby 
leaving a void for receiving the die. The trimming of the lead fingers 32 
establishes an inset of the lead fingers 32, which is a changes in the 
size of the void for receiving the die. The leads fingers should be 
trimmed as little as possible to allow for short bond wires. FIG. 3A shows 
an excise punch 36 which could trim the lead fingers 32. FIG. 3B shows a 
similar excise punch 38 which would trim the lead fingers 32 in 
preparation to receive a die having a smaller size than the punch of FIG. 
3A. 
After the lead fingers 34 are trimmed, they are immobilized with a tape 
material 40 having an adhesive 42 on one side, as shown in FIGS. 4A (top 
view) and 4B (cross section). A thermoplastic or thermoset adhesive 42 can 
be used, as well as other workable materials. The adhesive carrier 40 can 
comprise plastic, polyimide, or other suitable materials. Once the leads 
fingers 34 are immobilized, the die 14 is placed onto the tape 40, and the 
assembly goes through a wire bond step to couple the lead fingers 34 and 
the bond pads 24 on the die 14. It may also be possible to adhere the die 
14 to the tape 40, then attach the die-tape assembly to the lead frame 30. 
A major advantage of the invention is that an excise punch which forms the 
lead frame does not need to be retooled each time a die size is changed. 
During a conventional die shrink, to keep the bond wires short, a lead 
frame having a new design with longer lead fingers must be produced to 
allow for short bond wires. Longer bond wires are known to have problems 
of detaching during device encapsulation, and detaching before 
encapsulation due to lead finger movement. Retooling the lead frame 
currently costs more than $50,000, while retooling an excise punch costs 
less than $5,000. In addition, a punch which shapes the tape, which could 
be similar to that shown in FIG. 3, does not need to be retooled each time 
the die changes. The same size tape is workable for each die size. Another 
advantage of the invention is that the tape which serves to hold the die 
in place also prevents lead movement, which can cause problems as 
described above. 
A second embodiment of the invention is shown in FIGS. 5A (top view) and 5B 
(cross section). In this embodiment, a double-sided tape 50 has an amount 
of material 52 removed from the central area. The tape 50 is attached to 
the lead frame 34, and the die 14 is attached to the underneath side of 
the tape 50 to form a "leads over" configuration. 
While this invention has been described with reference to illustrative 
embodiments, this description is not meant to be construed in a limiting 
sense. Various modifications of the illustrative embodiments, as well as 
additional embodiments of the invention, will be apparent to persons 
skilled in the art upon reference to this description. It is therefore 
contemplated that the appended claims will cover any such modifications or 
embodiments as fall within the true scope of the invention.