Wireworking – Crimping
Reexamination Certificate
2000-08-22
2002-06-11
Larson, Lowell A. (Department: 3725)
Wireworking
Crimping
Reexamination Certificate
active
06401765
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for forming exposed pad features during lead frame downset for preventing bleed of encapsulant over an integrated circuit exposed pad or slug for utilization as a heat sink.
BACKGROUND OF THE INVENTION
In the fabrication of integrated circuits, it is often necessary to provide an exposed thermal pad or slug of highly thermally conductive material, such as, for example, copper, which is an integral part of or attached to a lead frame in order to conduct heat away from the semiconductor chip attached to the lead frame. The preferred finish for an exposed pad lead frame is palladium so as to avoid any wet process steps. The purpose of the palladium coating, when used, is set forth in Ser. No. 08/190,729, the contents of which are incorporated herein by reference. The exposed thermal pad is generally defined as the surface of the lead frame opposed to and adjacent to the semiconductor chip. It should be understood that, as an alternative, the lead frame can be made thicker in the region of the thermal pad or with an attached thermal slug. A portion of the processing operation involves encapsulation of the chip and lead frame, this operation requiring that the thermal pad be exposed to the external environment in order to allow for maximum dissipation of heat from the semiconductor chip to the external environment through the thermal pad. During encapsulation, the pad is disposed against the mold surface, preferably under compression against the mold surface, to minimize the likelihood of encapsulant traveling between the mold surface and the thermal pad and thereby over the thermal pad surface.
A problem that arises during the encapsulation as described above is that the compression of the thermal slug against the mold surface is often insufficient to prevent mold bleed wherein the molding material not only encapsulates the lead frame and semiconductor chip, but the molding material also passes over and forms a thin, generally transparent coating over the thermal pad. This coating of encapsulant over the thermal pad reduces the thermal performance of the thermal pad by reducing the exposed pad area. The encapsulant coating over the thermal pad also prevents or reduces the ability to bond or solder to the external thermal sink. In addition, irregular mold bleed makes it difficult to specify manufacturing limits and causes a problem during quality control to judge acceptable and unacceptable criteria. Uncontrolled mold bleed is also a cosmetic defect that may result in unacceptability of an otherwise good product.
In the prior art, this mold bleed has generally been removed by abrading or by chemical action of some type. The abrading or chemical action step involves an economic cost in that the abrading and/or chemical action step must be added. In addition, the abrading and/or chemical action step may damage, contaminate or cause removal of all or part of the palladium coating over the lead frame, thereby minimizing or eliminating the beneficial effects derived from the palladium coating.
Stamp forming tools are used to bend sheet metal to desired shapes. By applying additional force at the bends, it is possible to set the shape. This action is called coining. Coining occurs when pressure is applied to localized areas so as to result in flow of the material. More particularly, this process is used in this application to form lead frame exposed pad features and insure that the pad is flat to the edges in order that the bleed barrier groove is effective.
FIG. 19
illustrates stamp forming tool applied to the creation of deep downset exposed pad for a lead frame. The lead frame is placed between a tool
1906
and an anvil
1908
. The tool
1906
is moved to bend the lead frame
1910
to the desired shape and coins the lead frame against the anvil
1908
. The area of the lead frame
1910
to be coined is controlled by the clearances between the tool
1906
and anvil
1908
. Where less coining is desired, greater clearances are allowed and conversely, where more coining is desired, the clearances are reduced. The lead frame exposed pad
1910
includes a wing
1912
; typically, there will be 2 to 4 wings. The wing
1912
is formed during the downset of the of the exposed pad. However, typically coining in the area indicated by arrow
1902
and
1904
reaches its limit before the bottom of the exposed pad bottoms out on the anvil. This leads to a problem in that the exposed pad edge
1914
is not flat. Flatness of the exposed pad
1910
a
is generally required all the way to the edge
1914
in order to slow the mold bleed sufficiently so the groove
1916
can freeze off the bleed.
As illustrated in
FIG. 19
, the anvil
1908
is a single piece. Since it is difficult to machine the comer
1918
of this single piece without rounding the corner
1918
, it is difficult to achieve a nonrounded or sharp angular corner
1920
of lead frame
1910
through coining of the lead frame.
SUMMARY OF THE INVENTION
In accordance with the present invention, the above described problems of the prior art are minimized if not eliminated.
Briefly, the above is accomplished by providing a barrier to the encapsulant travel over the thermal pad during device encapsulation. This is accomplished in several ways in accordance with the present invention. In accordance with one group of embodiments of the invention, a groove or grooves of various possible shapes are formed in the lead frame itself by stamping, etching or the like. The groove surrounds all or part of the thermal pad so that any encapsulant traveling toward the center of the thermal pad freezes at the entrance to the groove and therefore acts as a block to the travel of encapsulant over the thermal pad. In some cases, the encapsulant may fill the groove and travels no further. As a second group of embodiments, the groove or grooves can be formed in the mold. As a third group of embodiments, ridges can be placed in the mold to prevent the movement of the encapsulant over the exposed surface of the thermal pad. As a fourth group of embodiments, ridges can be formed on the lead frame surface. In the case of the grooves, the encapsulant is either trapped in the grooves or freezes when entering a groove to inhibit further travel of encapsulant over the thermal pad surface. In the case of the ridges, the ridges rest against the surface of the mold, generally but not necessarily under compression, to act as a barrier to the travel of the encapsulant beyond the ridges and over the surface of the thermal pad. The action is the same as the groove but reversed in implementation.
In those instances wherein the features of the grooves are stamped into the lead frame, there can be greater swelling due to displaced material where grooves meet at a corner. This swelling at the corners results in a failure to maintain the lead frame flat with the mold and permits encapsulant to travel to the thermal pad via longer straight line regions. This problem can be minimized for stamped grooves by avoiding the sharp corners and, instead, providing relatively large corner radii. However, large radii are not often practical. The solution for stamped grooves was to pattern ends of grooves short of the corner or groove line segment such that swelling by displaced groove material created an effective swelling equivalent to the swelling of the straight line segments (see
FIGS. 13
,
13
a
,
13
b
and
14
). Etched grooves do not introduce any swelling and can easily be patterned for any shape.
The grooves can take various shapes, examples of these shapes being “V,” check, sawtooth, square and rectangular, as well as a concave cup in the case of etching. The grooves will generally have a depth equal to about half of their width though there is no requirement to utilize this dimension ratio.
As a result of the subject invention, bleed control is provided by design into the lead frame or mold to provide lower cost and higher assembly throughout than is obtained by the prior art chemical deflash, mechanical deflash, liqui
Carter, Jr. Buford H.
Davis Dennis D.
Brady W. James
Larson Lowell A.
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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