Apparatus for integrated circuit lead-frame punching

A punch for removing material from lead frames has a substantially rectangular body with an array of grooves along one side and a step removed on one end from the side opposite the grooves to provide an array of punch teeth extending from the body of the punch. The grooves provide a means of reworking the punch to provide a new array of teeth, by machining away all or part of the teeth and then extending the step in the direction of the groove to provide substantially the original length for the individual teeth. By providing a punch with groove length several times the tooth length, the punch can be reworked several times before being discarded.

FIELD OF THE INVENTION 
The present invention is in the area of integrated circuit (IC) 
manufacturing, and pertains in particular to apparatus and methods for 
dambar removal and/or flash removal relative to lead frames in processes 
for encapsulating ICs with leads for mounting to electronic circuitry. 
BACKGROUND OF THE INVENTION 
In general, the plastic packaging of ICs is as follows: Typically, ICs in 
die form are attached to mounting areas called islands on strips called 
lead frames. The lead frames are made of a thin, flat, electrically 
conductive material and typically have several individual islands, each 
for supporting an individual IC. In most cases, densely packaged ICs are 
manufactured to maximize connectivity by utilizing all four sides of the 
chip. 
Around the perimeter of each island a lead frame has a pattern of 
individual conductive leads extending toward, but not contacting the 
island. The islands and individual leads are formed by selective removal 
of material in the lead frame, such as by stamping. The number of the 
leads at a frame depends directly on the configuration of the particular 
IC die to be mounted. A typical IC may have over one hundred external 
connections and each frame will have a corresponding number of individual 
leads. The width of each lead and the separation between adjacent leads is 
dependant on the package size of the finished IC. The thickness of each 
lead is the thickness of the lead frame and is predicated on the current 
carrying capacity required. 
A plastic package with external leads for connecting to, for example, a 
printed circuit board, is typically formed by an encapsulation process. 
Mating molds are placed on each side of the lead frame and liquid-phase 
polymer is injected to encapsulate the IC die. The lead frame is designed 
to dam the flow of liquid-phase polymer as it moves to the outer edges of 
each individual mold, stopping at where each mold contacts surfaces of the 
lead frame. To stop the flow of liquid-phase polymer between leads the 
lead frame has a pattern of dam bars between individual leads, so a 
contiguous band of material is formed around the periphery of the island. 
This contiguous band prevents the polymer from flooding the leadframe. 
After the polymer solidifies and the molds are removed, a following 
operation in the manufacturing process removes the excess plastic in the 
region around the mold outline and the dam bars. This is termed de-junking 
in the art. A de-damming process then removes the dam bar between each 
lead, providing electronic integrity for each lead. De-damming is a 
process of removing all or part of each dam bar by use of a punch with a 
pattern of teeth conforming to the pattern of the dam bars in the lead 
frame. Typically, the de-damming and de-junking can be done in a single 
step. In following processing each lead exposed from the edge of the 
plastic package is further treated such as by plating, and the individual 
packages are trimmed from the lead frame strip. Finally, the leads are 
formed, such as for Surface Mount Technology (SMT) applications. 
In state-of-the-art manufacturing, automated machines perform the 
de-junking and de-damming operations. Automatic machines are marketed by 
Iwtani International Corporation of Tokyo, Japan and Fujitsu of Japan, 
among others. In the de-damming operation, typically a hydraulically 
driven, hardened metal punch is used to trim the dam bar from between the 
conductive leads. The punch is critically machined to provide a clean cut 
for each dam bar to insure physical dimensions and electrical integrity. 
In one case, the de-damming punch is designed to cut the dam bar between 
every second pair of leads of an IC package. This is done to minimize the 
manufacturing cost of the punches used for de-damming. In most instances, 
an automated machine will have two opposing dam bar punches working in 
unison on opposite sides the package. In this case, the complete 
de-damming operation may take up to four stages to trim all dam bars from 
each package. 
The dam bar punches are produced uniquely for specific IC packages since 
many ICs have different lead counts, lead pitches and package sizes. The 
punches must be manufactured to maintain functional integrity over many 
cycles. As pitch sizes get smaller so do the de-damming punch's individual 
teeth that clear the dam bar between individual leads. When a tooth 
breaks, the entire punch must be replaced. The broken punch is typically 
discarded because it is more economical to buy a replacement than to try 
to repair/re-machine a new set of teeth on the same punch. 
As described above, de-damming punches are typically made to punch every 
second dam bar. This is done to control manufacturing costs for the 
punches, as it allows wider spacing between individual teeth, and punches 
with wider spacing are less expensive to produce. There is a disadvantage, 
however. As IC lead aspect ratios decrease because of the higher pin 
counts and smaller packages, shearing forces increase and tend to twist 
the alternately punched leads. Then, when the lead is formed for a SMT 
application, the lead pad positioning may be offset, decreasing production 
yields. 
What is needed is a de-damming punch compatible with existing 
state-of-the-art automated machinery that can be cost effectively reworked 
rather than discarded. Such a punch should preferably punch every dam bar 
on a side in one action, assuring coplanarity in pad positioning. This 
would save money by eliminating the need to provide a new punch every time 
a punch fails in use. 
SUMMARY OF THE INVENTION 
A punch for removing portions of material from a lead frame in a process 
for encapsulating integrated circuit (IC) chips with electrical leads is 
provided in an embodiment of the invention, comprising a mounting portion 
configured to attach the punch to a translating drive in a processing 
machine, and a shank portion with a substantially rectangular shape of 
length L1, Width W1, and thickness T1, extending from the mounting 
portion. The shank portion has a plurality of grooves on a first side 
beginning at the end opposite the mounting portion, spaced apart across 
width W1, extending in the direction of L1 for a length L2, and having a 
common depth T2 &lt; T1. The grooves provide a spaced-apart array of ribs on 
the one side. A step formed across the end of the shank portion at the 
opposite end from the mounting portion, the step extending across width W1 
on a second side opposite the first side to a depth T3 &gt; (T1-T2) for a 
length L3 &lt;&lt; L2, causes the ribs to extend from the end of the shank 
portion as an array of separate punch teeth. 
In a preferred embodiment the punch is made of a tungsten carbide or 
equivalent material such as a through-hardened steel, and the grooves are 
several times longer than the extended teeth. The grooved construction 
allows a worn or damaged punch to be reworked by machining off a length of 
the separate punch teeth past the area of wear or damage, and extending 
the step by an equal amount to provide punch teeth of the original length. 
With conventional punches without extended grooves, reworking a punch is 
equivalent to machining a new punch, requiring forming new teeth. With 
smaller and smaller spacing as a result of further miniaturization of ICs 
and packages, machining between the teeth is a more and more expensive 
process. As a result, a worn or broken conventional punch is best 
discarded in favor of a new punch. With the grooved punch according to 
embodiments of the present invention, reworking is a relatively simple and 
inexpensive process accomplished on a surface grinder, requiring only 
grinding off all or a portion of the length of the worn or damaged teeth, 
and extending the step on the ungrooved side to expose more tooth length. 
In different aspects of the invention, a punch is provided, a punch station 
using the punch is provided, and an automated machine is provided having a 
station using a punch according to an embodiment of the invention. The 
novel and unique means of making and using a punch according to 
embodiments of the invention provides for considerable savings in 
operations for removal of dambars and other regions of material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is an isometric view of a conventional QFP IC package 11. Typically, 
body 13 of IC package 11 is formed of plastic material by a method of 
transfer molding. Die 43 is inside, and completely encapsulated by the 
plastic molded body. The die contains the circuitry that defines the 
electrical functions of a particular IC. The circuitry of the IC is 
connected to the outside environment through individual conductive leads 
15. The leads, as show by this example, are formed into J-bends for SMT 
application in another process. The leads are typically constructed of a 
highly conductive material that is receptive to bending and forming while 
maintaining structural integrity. The package protects the sensitive and 
fragile circuitry on the IC die and fixes the arrangement of individual 
leads. 
FIG. 2 is an idealized plan view of one frame 22 of a typical lead frame 
strip 21 before the process of die placement and encapsulation. Strip 21 
comprises several identical lead frames 22 where individual IC packages 
are constructed. The layout of FIG. 2 is intentionally simplified to 
illustrate the principles involved. Lead frame strips can vary in size and 
design according to different IC packages and the automated machinery that 
produces them. They are made of a conductive material, typically metal, 
and formed in thin sheets. The sheet thickness of lead frame strip 21 is 
the thickness of the leads. 
In the center of lead frame 22, leads 15 are provided approaching, but not 
contacting, island 23. Gap 34 serves to electrically island 23 from each 
lead. Island 23 is supported in this example by legs 25 that are 
contiguous to lead frame strip 21, typically formed to attach at the 
corners of the island. Lead frame 22 also defines the outer edges of the 
plastic encapsulation by means of structures between leads 15 to stop the 
flow of the liquid-phase polymer in the encapsulation process. These 
structures comprise dam bars 30, and their placement between leads 
provides a contiguous strip of material around each island, illustrated by 
broken line 7. The necessity for dam bar 30 means that at this stage all 
leads 15 surrounding island 23 are electrically connected. Embodiments of 
the present invention address the removal of the dam bars in the IC 
packaging process, providing electrical isolation for each lead. 
FIG. 3 is a cross-sectional view of lead frame 21 taken at section line 
3--3 of FIG. 2 after die placement, die bonding, and encapsulation, with 
the encapsulation mold in place. Before encapsulation, an IC die 43 is 
placed on island 23 and wires 45 are bonded between each contact pad in 
the die and its respective lead. Wires 45 span gap 34 between each 
mounting pad on the IC die and its respective lead. Lead frame strip 21 is 
then positioned between two opposing, typically symmetric molds 41A and 
41B. Within the body of the molds, at each island region, there is 
typically a passage for entry of injected liquid-phase polymer and one or 
more passages for bleeding off displaced air. These passages are not shown 
in FIG. 3, but are typically located at the corners of the molds. 
Molds 41A and 41B are positioned and centered on each die 43, and 
liquid-phase polymer is injected and flows until it fills the cavity. When 
the polymer has solidified, molds 41A and 41B are removed and lead frame 
strip 21, with the encapsulated and bonded die 43, is ready for trimming 
to produce individual IC packages. 
FIG. 4 is a top plan view of lead frame strip 21 at individual frame 22 
after encapsulation, ready for subsequent processing to separate all 
individual leads 15 and remove tabs 31 at the perimeter of the IC package. 
This process is typically performed in automated machines as described in 
the Background section above. In machining of this sort, lead frame strips 
21 are typically loaded in a magazine and individually fed to tooling 
positions in the automatic equipment. 
Dam bars 30 to be trimmed can vary in width D1 according to the particular 
design of lead frame 22. Typically, removal of the dam bar is done by a 
punch that fits into a holder on the automated machine. An automatic 
machine is designed to use punches of different size and configuration to 
be able to process lead frames of different size and configuration. 
Mechanisms on the machine can be adjusted to accommodate different 
standard lead frame strips and different punches. 
FIG. 5 is an isometric view of a punch 51 according to an embodiment of the 
invention. FIGS. 6A, 6B and 6C are alternate 2-dimensional views of the 
punch of FIG. 5. In this embodiment punch 51 is made of a hardened 
material with grooves 55 formed to a depth D2 and width D3 typically 
equally spaced along one flat side. The depth of the grooves is somewhat 
greater than the width of one section of dam bar to be removed by the 
punch, and the width of the groove is determined by the lead spacing for 
the particular lead frame to be trimmed. Width D4 of the punch corresponds 
to the width of a side of the IC package to be processed. The number of 
grooves corresponds generally to the number of leads along one side of the 
package although there may be other teeth as well for trimming island 
supports and the like. 
Individual separate teeth 59 are provided on punch 51 by machining away a 
volume 60 across the end of the punch to a depth D6 and a length D9. This 
step removes part of the solid material of the body of the punch, and the 
ridges of material between the grooves 55 for length D9 becomes a set of 
protruding teeth 59 of length D9. The length D9 of the teeth is sufficient 
to punch through lead frame 21, (FIG. 4) removing all material cleanly 
from the surrounding metal strip. Thickness D7 is substantiality equal to 
groove depth D2 plus the machining depth D6. Punch overall thickness D7 
can vary in alternative embodiments of the invention. Region 53 of the 
punch is configured to fit in a standard attachment mechanisms on 
automated machines, and may take various forms depending on the 
requirements of the particular machine. The grooves may extend the full 
length of the body of the punch, as shown, or they may stop short of the 
end of region 53. 
In a unique aspect of the present invention, when a tooth 59 breaks in use, 
punch 51, by virtue of the lengthwise grooves, can be inexpensively 
renewed by a standardized grinding procedure. The process of renewal 
comprises two operations. In one step the punch is shortened at tooth 59 
end past the point of failure. This is typically a surface grinding 
operation. This effectively leaves a uniform symmetric surface for a new 
tooth end. As an example, if one or more teeth were to break off entirely, 
the punch might be shortened to line 62, removing all remaining teeth. In 
a second step material in volume 58 is ground away to a depth of D6 over a 
length D10 so new teeth are exposed. This is typically a simple surface 
grinding operation and the existence of grooves 55 provide for the new 
teeth. The new length D11 of the punch can then be adjusted for in the 
standard fixturing of the existing automated machines. 
Length D8 for punch 51 is determined by structural parameters and 
requirements for universal adaptation in exiting automated machines. 
Typically, length D8 provides for a number of instances of renewing the 
punch. 
FIG. 7 is somewhat idealized cross-sectional view of an operation in an 
automated machine according to an embodiment of the invention. Punch 51 is 
shown advanced, having passed through and removed sections of the dam bar. 
Punch 51 is guided by stripper elements 65 and a mating punch die 67 helps 
to support and the lead frame and provides a shearing interface with the 
punch. In alternative embodiments, punch 51 can be used as a de-junking 
tool, lead frame trimming tool, or as a punch in other sheet punching 
machine operations, both singly and in combination. In most cases, the 
punch is used to simultaneously to cut the dambar, removing a portion of 
it, and to remove plastic flash between the leads near the encapsulated 
package. 
It will be apparent to one with the skill in the art that there are many 
changes that might be made without departing from the spirit and scope of 
the invention. There are, for example, a broad range of materials suitable 
for the punch in various embodiments. Tungsten carbide is a preferred 
material, but punches could also be made from some hard, non-metallic 
materials, and from hard-coated softer materials. In some embodiments the 
grooved potions of the punch might be made to attach with fasteners to the 
mounting end. Also there is very wide variation in dimensions suitable for 
such punches, and it is not required that the grooves extend the full 
length of the punch body as shown in the embodiments described above. 
There are similarly many other alterations in detail that might be made 
without departing from the spirit and scope of the invention.