Method for manufacturing fine pitch lead frame

A pitch lead frame and a method for manufacturing the same is disclosed herein. A preferred embodiment of a fine pitch lead frame is generally comprised of a plurality of fine pitch leads, a die pad area, and die pad area support arms. The leads are comprised of a base lead portion formed in an unetched region and a fine pitch lead tip portion formed in an etched region on an electrically conductive material. The base lead portions are substantially the same thickness as the conductive substrate and the lead tip portions are of a smaller thickness. Etching one or more region(s) on an electrically conductive substrate of a substantial uniform thickness to a fraction of the thickness of the unetched regions allows for the formation of fine pitch lead tips in the etched region(s). Some possible methods for forming the leads includes an additional etching, stamping out the leads with a finely tapered stamp tool punch or using conventional stamp tool punches in conjunction with finer stamp tool punches to create the fine pitch lead tips. There is also disclosed examples of various methods for forming the fine pitch lead frame in different configurations.

BACKGROUND OF THE INVENTION 
This invention relates generally to lead frames for use in integrated 
circuit packaging and more particularly, to a fine pitch lead frame where 
the pitch (i.e., the distance between the centers of adjacent leads) is 
very small. 
Referring to FIG. 1, a lead frame 10 is generally comprised of a die pad 
area 17, die pad support arms 15 and leads 11. In the manufacture of 
conventional lead frames the die pad area 17, support arms 15 and leads 11 
are formed in one step. The lead forming step may be either an etching 
process or a stamping process. In these processes the areas between the 
leads 11 and between the lead tips 12 are removed through respectively 
stamping or etching. The area between the lead tips are usually referred 
to as slots 14. In the etching process, a mask of the pattern of leads, 
die pad area and support arms is laid over a metal strip. The exposed 
areas are then etched away creating the slots 14 between the leads. The 
stamping process usually consists of stamping out the slots 14 between the 
leads, die pad area 17 and support arms 15. Multiple stamp tool punches, 
shaped in the form of the respective slots 14, punch out the slots 14. 
The leads 11 are formed around the die pad area 17. As the leads approach 
the die pad area they become narrower and narrower and the distance 
between the leads become smaller. This results in a finer pitch between 
the centers of the lead tips 12 as the leads get smaller and smaller. The 
lead tips 12 are formed to have the smallest pitch possible so the maximum 
number of leads can approach the die pad area 17 and to allow the leads to 
get as close as possible to the die pad area 17. 
The support arms 15 extend out from the die pad area supporting the die pad 
area during handling. An integrated circuit is placed on the die pad area 
17, in the center of the lead 
The support arms 15 extend out from the die pad area supporting the die pad 
area during handling. An integrated circuit is placed on the die pad area 
17, in the center of the lead frame 10. After the leads 11, die pad area 
17 and support arms 15 are formed, the lead frame 10 may be annealed to 
strengthen and relieve stress on the leads and support arms. This may be 
followed by plating the lead tips 12 with an electrically conductive 
material. Plating also allows for better bonding to the bonding wires when 
the leads are connected to the integrated circuit. 
The leads may then be taped with an adhesive strip to keep the leads from 
moving during handling. The adhesive strip may be a single picture frame 
style strip that is placed across the leads. An alternative is the 
application of separate strips placed across a set of leads. The former 
process allows for better tolerances when the tape either shrinks or 
expands causing the leads to move. Then, the lead frame 10 may be trimmed 
to free the lead tips 12 from the die pad area 17. 
After trimming, the lead frame is ready to be packaged. An integrated 
circuit is placed on the die pad area 17 and bonded to the lead tips 12 
with bonding wires. The inner portion of the lead frame 20 is then 
encapsulated with an encapsulating material. The excess metal that 
supported the entire lead frame 22 is then trimmed away to free the leads 
11 from each other. The end product is a packaged semiconductor device. 
Semiconductor manufacturers are creating integrated circuits with smaller 
and smaller die sizes and at the same time increasing the number/of 11in, 
s they require. For example, integrated circuit manufacturers are 
developing dies for 208 pin quad flat packages (QFP) that have a bonding 
pad pitch of 4 mils and requiring a 13 mil space at the corners. 
Therefore, a die with 208 bonding pads (i.e., 52 bonding pads per side) is 
approximately 0.234 by 0.234 inches in size. Semiconductor assemblers 
require a 0.010 inch margin around the die. Therefore, the minimum die pad 
size is 0.254 inches square for a 208 pin die. As the die sizes of 
integrated circuits get smaller the lead tips on the lead frames need to 
get finer and finer in pitch in order to approach the ever shrinking 
integrated circuit. Current die pads are approximately 0.433 inches square 
since conventional leads, limited by pitch, cannot approach a smaller die 
pad. Long bonding wires can connect currently available lead frames with 
the integrated circuits of a smaller die size. Although, interference and 
crosstalk occurs in long bonding wires making this method undesirable. 
This creates pressure on manufacturers of lead frames for integrated 
circuit packaging to obtain smaller and smaller pitches in their lead 
frames. A solution to the problem utilizes a finer pitch lead frame. 
The limitations of stamping and etching technologies currently constrain 
the process for making lead frames. Both stamping and etching can only 
obtain a slot width (the width between adjacent lead tips) of about 4 mils 
in a 6 mil thick metal strip. This results in a lead tip 4 mils wide and a 
slot 4 mils wide which gives the lead tips an 8 mil pitch between the 
centers of two adjacent lead tips. The 8 mil pitch corresponds to the 
presently obtainable ratio of stamping slot width versus strip thickness 
of approximately 4:6 in a 6 mil. Therefore, if a thinner metal strip is 
used, such as a 3 mil strip, a pitch of approximately 5.5 mils is 
obtainable, which can and has been accomplished with current stamping 
technologies. 
Stamping technology is limited by the ability to punch through a given 
thickness of metal or other electrically conductive material. A stamp tool 
punch smaller than the 4:6 ratio is subject to breakage. For example, a 
stamp tool punch smaller than 3.5 mils thick would not be able to punch 
through a 6 mil thick metal strip without repeatedly breaking. Etching 
technology is similarly constrained due to the ability to control the 
etching process. Due to the diffusion of an etchant during exposure the 
slots become more irregular through a thicker material. The diffusive 
nature of etching limits its ability to etch out fine pitch lead frames. 
As discussed above, the use of thinner material will allow the manufacture 
of finer pitch lead frames. 
Although, a thinner metal strip provides a finer pitch lead frame another 
constraint is the requirement of using strips of a particular thickness 
from chip package manufacturers. Presently, 6 mils is the most common 
standard used in the art. The standard exists due to present manufacturing 
and molding technologies that rely upon the 6 mil standard as well as the 
customer's insistence upon the usage of 6 mil thick lead frames. 
Therefore, in such systems the problem becomes obtaining a finer than 8 
mil pitch using a 6 mil thick strip. 
One technique for accommodating smaller integrated circuit dies is the 
method of interposing. Interposing consists of using a conventional lead 
frame with a conventional pitch and bonding a miniature printed circuit 
board on top of the lead tips. The printed circuit consists of fine pitch 
traces from the lead tips to the integrated circuit die which is bonded 
onto the printed circuit board. While this process leads to smaller pitch 
lead frames it is prohibitively expensive. 
Another technique used is to extend every other lead tip. The extended lead 
tips are shaped to maintain a 4 mil space between the extended tips and 
the adjacent unextended tips. The ends of the extended tips end in a 
rectangular pad large enough for bonding. The extended tips allow the 
centers of the untapered tips and the centers of the tapered tips to 
approach closer to each other giving a smaller pitch. This technique is 
only capable through etching and gives only an incrementally smaller 
pitch, as for example, approximately 7 mils in 6 mil thick lead frames. 
The present invention provides a method for obtaining significantly finer 
pitches from a given strip, although it is not limited to a specific 
thickness and can be applied to other thicknesses. The presently disclosed 
method lends itself especially to the manufacture of lead flames for QFP 
devices, although not limited to such packages. Further, the invention is 
not prohibitively expensive and is possible with current technology. 
SUMMARY OF THE INVENTION 
These and other advantages of the present invention will become apparent to 
those skilled in the art upon a reading of the following description of a 
fine pitch lead frame and method for manufacturing the same and a study of 
the several figures of the drawings. In a preferred embodiment a fine 
pitch lead frame is generally comprised of fine pitch leads, a die pad 
area, and die pad support arms. The leads each have a base lead portion 
formed in an unetched region and a fine pitch lead tip portion formed in 
an etched region on an electrically conductive material. The base lead 
portions are substantially the same thickness as the conductive substrate 
and the lead tip portions are of a smaller thickness. Etching one or more 
region(s) on an electrically conductive substrate of a substantial uniform 
thickness to a fraction of the thickness of the unetched regions allows 
for the formation of fine pitch lead tips in the etched region(s). Some 
possible methods for forming the leads includes an additional etching, 
stamping out the leads with a finely tapered stamp tool punch or using 
conventional stamp tool punches in conjunction with finer stamp tool 
punches to create the fine pitch lead tips. The combination of an etch and 
a lead forming step allows for the manufacture of a fine pitch lead frame. 
The first step includes the etching of a metal strip to create a region on 
the metal strip that has been etched to a fraction of the original 
thickness. The etched region therefore has a smaller thickness than the 
rest of the strip. The second step forms the leads on the metal strip such 
that the lead tips are formed in the etched regions and the lead bases are 
formed in the unetched regions. The lead tips can be of a finer pitch 
since the thickness of the strip in the etched region(s) is smaller than 
the thickness in the unetched region. 
The lead forming step can be accomplished in several ways. One method 
utilizes multiple stamp tool punches that have substantially tapered tips. 
The metal strip is stamped with the tapered punches in such a way that the 
tapered tips would punch out fine pitch slots in the etched regions. The 
untapered part of the punches stamps out the slots in the unetched 
regions. Thus the tapered punches cream the slots in between the base 
leads and fine pitch lead tips in one stamping. 
An alternate approach is to use two sets of stamp tool punches. The first 
set of punches are ordinary stamp tool punches. These punches stamp out 
slots in the unetched regions. A second set of fine pitch stamp tool 
punches then extends the slots in the etched regions, but with a finer 
pitch. 
A third approach is to mask the etched region and do a second etch. Once 
again because the etched region is of a smaller thickness the second etch 
can create fine pitch leads. The second mask would be applied to the 
unetched and etched regions, with the mask in the etched region being of a 
finer pitch. After exposure the base leads and the fine pitch lead tips in 
the etched regions will have been formed. 
In one embodiment the thickness of the etched region is about half the 
thickness of the remainder of the strip. In another embodiment a fine 
pitch lead frame is formed from a 6 mil thick metal strip. Although, the 
method applies to various thicknesses as well as different types of 
electrically conductive strips. The first etching step allows for the 
creation of fine pitch lead frames with smaller pitches than 7.5 mils in 
current 6 mil thick strips. For example, if half the thickness was etched 
away the etched region would be about 3 mils thick. A 3 mil thick area 
would readily allow for a pitch of 5.5 mils or smaller. This can be 
achieved since thinner stamping tools of a thickness less than 4 mils can 
be utilized in a thinner etched region. This holds true as well for 
etching since there is a lesser amount of dispersion through a thinner 
etched region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring initially to FIGS. 2a, 2b and 2c, one embodiment of the present 
invention will be described in more detail. A region 30 is etched from an 
electrically conductive substrate 32. The etched region 30 can be a 
singular etched region as depicted in FIG. 2a which encompasses an etched 
die pad area 117. Alternatively, the etched region 30 can be a plurality 
of etched regions 30' surrounding an unetched die pad area 217 and 
bordered by unetched die pad area support arms 15 as illustrated in FIG. 
2c. 
Generally, a lead frame includes a die pad area, die pad area support arms, 
and leads although in some embodiments the die pad area and/or the die pad 
support arms may be eliminated. In an embodiment of the present invention 
of a fine pitch lead frame the leads include a thicker base portion 112 
and a thinner lead tip portion 42. By way of example, when a 6 mil thick 
metal strip is used pitches less than 7.5 mils, which today is generally 
considered about the minimum attainable pitch using mass production 
technologies, can be obtained. Pitches of less than 6 mils are readily 
achievable as evidenced by such pitches when conventionally processing a 
uniform 3 rail thick metal strip. Further, pitches of less than 5 mil can 
be obtained in a 6 mil thick material as necessary for various pin counts 
and package types. Of course, the thickness of the metal strips used in 
the process may be widely varied in accordance with the needs of a 
particular system. 
One of the advantages of the present invention is illustrated with 
reference to FIGS. 3a and 3b specifically the creation of an etched region 
30 allows for the creation of fine pitch lead tips 42 that can approach a 
smaller die pad area 72. Conventional lead tips 40 can only approach 
conventionally sized die pad areas 70. FIG. 3a depicts the die pad area 70 
of a conventional 208 pin lead frame approximately 0.433 by 0.433 inches 
in size and the surrounding lead tips 40. Superimposed therein is a 
smaller die pad area 72 that can be utilized in a 208 pin fine pitch lead 
frame of approximately 0.300 by 0.300 inches. A smaller die can be used 
since finer lead tips 42 can approach a smaller die pad area 72. As shown 
in FIG. 3b the fine pitch lead tips 42 have a smaller pitch 52 than the 
conventional pitch 50 of the conventional lead tips 40. 
In another embodiment the etched region 30 can be repeated on a metal strip 
38 such that a number of fine pitch lead frames can be made from a single 
metal strip 38 as shown in FIGS. 4a and 4b. Creating a number of fine 
pitch lead frames on a single metal strip increases the efficiency of the 
manufacturing process. In order to align the strip through the etching and 
the later lead forming processes location holes 35 can be formed on the 
metal strip 38. The location holes may be created during the etching of 
the etched region 30 or 30' or in a prior or subsequent stamping process. 
Once the etched region is formed the fine pitch lead tips 42 and the 
thicker base leads can be formed. As shown in FIGS. 5a and 5b the fine 
pitch lead tips 42 and the thicker base leads 112 can be made by utilizing 
tapered fine pitch stamp tool punches 180. In one embodiment each of the 
fine pitch stamp tool punches 180 is a singular piece. Each punch 180 is 
used to punch out a corresponding fine pitch slot 114. The punch 180 is 
positioned such that the finely tapered portion of the punch 82 punches 
out the part of the slot 114 in the etched region 30 and the base portion 
of the punch 80 punches out the part of the slot 114 in the unetched 
region. As can be seen in FIG. 5b the results are a base lead portion 112 
in the unetched regions and fine pitch lead tip 42 created between the 
slots 114 extending within a conventional die pad area 70 and approaching 
a smaller die pad area 72. 
In another embodiment each stamp tool punch 180' can be formed from a pair 
of stamp tool punches. That is, a fine pitch tapered stamp tool punch 82' 
and a base stamp tool punch 80'. The two punches 80' and 82' acting 
together serve the same function as a single piece stamp tool punch 180 
described earlier. The fine pitch stamp tool punch 82' punches out an area 
in the etched region and the base stamp tool punch 80' punches out an area 
in the unetched region. The punched out areas intersect such that they 
form a fine pitch slot 114 in both the etched and unetched regions. The 
two punch process also results in the creation of base lead portions 112 
in the unetched region and fine pitch lead tips 42 in the etched region 30 
approaching a smaller die pad area 72. Using multiple punches 180 or 180' 
a fine pitch lead frame can be formed from an etched metal strip 38. 
A second etching may also be employed to create the fine pitch lead tips 
42. By placing a mask in the shape of a fine pitch lead frame over the 
etched metal strip 38 and etching away the slots 114. Fine pitch lead tips 
42 can thus be created in the same form as the earlier embodiments. As 
with stamping the second etching can achieve a finer pitch than 
conventional lead frames due to the smaller thickness of the etched area. 
After the lead forming step the lead frame can be configured in any 
suitable manner, a few of which are depicted in FIGS. 6a-6d in a cross 
sectional view. Plating, annealing and taping the fine pitch lead frames 
are some possible additional steps in manufacturing a fine pitch lead 
frame. Plating the fine pitch lead tips 42 and the die pad area 117 or 217 
with a metallic coating facilitates bonding the fine pitch lead tips 42 
and the integrated circuit with bonding wires. Taping the leads across the 
base lead portions 112 provides support to the leads during shipping and 
handling. FIGS. 6a-6c illustrate some possible configurations where the 
etched region 30 is a singular etched region including an etched die pad 
area 117 the lead frame can be configured in many ways. FIG. 6a displays a 
cavity up lead frame with the etched region 30 face up and the plating 91 
placed within the etched region 30. Tape 93 is placed on the top surface 
of the unetched region. FIG. 6b alternatively shows a cavity down lead 
frame after the lead forming step with the etched region 30 faced down and 
the plating 91 and tape 93 placed on the unetched side of the lead frame. 
The etched die pad area 117 can be downset, as shown in FIG. 6c, to place 
the die pad area 117 below the plane of the lead frame to facilitate 
bonding. The etched die pad support arms 115 are bent to lower the die pad 
area 117. FIG. 6d relates to a lead frame where the die pad area is 
unetched 217. The plating 91 and the tape 93 are placed on the unetched 
side of the lead frame. The unetched die pad area 217 is supported by 
unetched die pad support arms (not shown). This configuration can be down 
set as well, similar to the configuration shown in FIG. 6c. 
The configurations shown in FIGS. 6a, b and d can also be manufactured 
without die pad area support arms. By taping the entire fine pitch lead 
frame after the leads have been formed, the die pad area 117 or 217 can be 
kept in place while applying the plating 91 and tape 93. 
A method of forming a fine pitch lead frame in accordance with one 
embodiment of the present invention is generally outlined in FIG. 7. An 
etching step 700 etches out a region 30 or regions 30' where fine pitch 
lead tips 42 are to be formed. A lead forming step 702 can be accomplished 
by stamping or etching out the slots 114 between the fine pitch lead tips 
42 and base leads 112. Stamping can be accomplished by a single stamp tool 
punch 180 with a fine pitch tapered tip 82 and a base stamp tool portion 
80. In another embodiment a set of stamp tool punches comprised of a fine 
pitch tapered stamp tool punch 82' and a base stamp tool punch 80'. The 
lead forming step 702 is then followed by an annealing step 705 to 
strengthen and relieve stress throughout the lead frame. A plating step 
707 then applies plating 91 on the die pad area 117 or 217 and the ends of 
the fine pitch lead tips 42 to facilitate bonding. Tape 93 is then applied 
in the taping step 709 across the base leads 112 to support the lead frame 
during shipping and handling. Finally, the fine pitch lead tips 42 are 
trimmed so than there is no contact between the lead tips 42 and the die 
pad support area 117 or 217. The trimming step 710 also removes other 
unneeded portions of the lead frame which may have provided support to the 
lead frame structure during processing. The steps in the above described 
embodiment can be varied as to chronology. In producing a fine pitch lead 
frame with a down set, a down setting step can be included in the 
described embodiment. Additionally, some of the steps, other than an 
etching 700 and a lead forming step 702 may be omitted. 
The present application has discussed methods for manufacturing fine pitch 
lead flames utilizing a single level etch in the etching step 700. 
Although, the invention is not limited to a single level etch. Multiple 
etches can be utilized to obtain a tiered etched region that can provide 
finely pitched lead tips while also providing better strength. With the 
refinement of etching technologies a gradually sloping etched region, 
resembling the inside of a shallow bowl, may be achieved to create a more 
uniform lead thickness transition in a fine pitch lead frame. Also, the 
etching need not only be applied to one side of an electrically conductive 
substrate. Etching a smaller thickness on both sides of an electrically 
conductive strip rather than etching only one side with a greater 
thickness is possible while obtaining the same results. 
Although, several embodiments of the present invention have been described 
in detail, it should be understood that the present invention may be 
embodied in many other specific forms without departing from the spirit or 
scope of the invention. Particularly, the lead forming step can be 
performed by other methods other than by etching or stamping. Also, future 
developments such as laser etching or improved techniques in lead forming 
technology can be readily incorporated in the advantages of the present 
invention. The metal strip can be any type of electrically conductive 
material not necessarily a metallic element. The present invention applies 
to all types of semiconductor device packages such as, but not limited to, 
dual in line pin, lead chip carrier, quad flat pack and pin grid array 
packages utilizing a variety of encapsulating materials ranging from 
plastic and ceramic to metal. Therefore, the present examples are to be 
considered as illustrative and not restrictive, and the invention is not 
to be limited to the details given herein, but may be modified within the 
scope of appended claims.