Plastic encapsulated integrated circuit package having an embedded thermal dissipator

An integrated-circuit die attached to a thermally conductive substrate having surface variations formed into the surface of the thermally conductive substrate. A lead frame has inwardly-extending fingers, which are attached to the thermally conductive substrate. The integrated circuit die, lead frame, and substrate are enclosed within a mold cavity. The surface variations of the thermally conductive substrate provide for a more balanced flow of plastic material over the top and bottom of the substrate provide a molded package body substantially free of voids.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to integrated-circuit package designs and, 
more specifically, to improved molded-plastic package designs for 
thermally enhanced integrated circuits. 
2. Prior Art 
FIG. 1A shows a conventional quad fiat package assembly 10 for an 
integrated-circuit die 12. The integrated-circuit die 12 is attached to an 
upset die-attach paddle portion 14, which is at the center region of a 
conventional lead frame 16. Various inwardly-extending leads terminate at 
their inner ends in bonding fingers (typically shown as 18 and 20). A 
layer of adhesive, not shown, is commonly used to attach die 12 and leads 
18 and 20 to die-attach paddle 14. The bonding fingers are connected to 
respective bonding pads on the integrated-circuit die 12 using respective 
bonding wires (typically shown as 22), as indicated in the Figure. The 
entire assembly described above is conventionally encapsulated in a molded 
plastic material, which forms a molded-plastic body 26 for the package 
assembly 10. 
FIG. 1B is a plan view of the lead frame 16 for the conventional quad fiat 
package assembly 10. Note that the ends of the bonding fingers do not 
extend all the way to the die-attach paddle and are not connected to the 
die attach paddle portion 14. This provides substantial spaces 30, 32, 34, 
and 36 between the inner ends of the bonding fingers 18 and 20 and the die 
attach paddle 14 for the flow of compound during the encapsulation 
process. The die attach paddle portion 14 is secured in place at the 
center of the lead frame by tie bars 17. The tie bars 17 extend inwardly 
from the corners of the lead frame 16 and, unlike bonding fingers 18 and 
20, are attached to die paddle portion 14. The assembled die 14 and lead 
frame 16 combination is encapsulated in molded plastic material by being 
placed in a cavity formed by the two halves of a mold and by injecting 
plastic material into the top half of the mold at one corner of the lead 
frame. Air vents are provided in the mold at the other three corners of 
the package. Some of the paths for the plastic material to flow into the 
bottom half of the mold from the top half of the mold are provided by the 
spaces 30, 32, 34, and 36 which are present between the inner ends of the 
bonding fingers and the die-attach paddle. Other paths are provided by the 
spaces between the leads of the lead frame 16. These flow paths permit the 
flow of the plastic molding material top be substantially balanced between 
the top half and the bottom half of the of the mold. As the plastic 
material flows through the mold, air is expelled out of the air vents at 
the three corners of the mold by the flowing plastic material so that no 
air remains trapped within the molded-plastic body. If air were to be 
trapped, it would cause voids, blow holes, or pin holes, in the molded 
plastic body 26. 
FIG. 2 shows a package mold 60, which has a top mold-half 62 and a bottom 
mold-half 64, for molding a thermally-enhanced, quad fiat package. A 
thermally-enhanced, electrically-insulated substrate 66, which is formed 
of a material such as, for example, aluminum nitride, has an 
integrated-circuit die 68 mounted thereto. The thermally conductive, 
electrically-insulated substrate 66 replaces a conventional die attach 
paddle (such as the die-attach paddle 14 of FIGS. 1A and 1B) and improves 
the thermal performance of a molded-plastic package. Bonding fingers 
(typically shown as 69 and 70) at the inner ends of the leads of a lead 
frame 72 are attached to the outer margins of the thermally conductive, 
electrically-insulated substrate 66 with an adhesive film 73 formed of a 
material such as R-flex 1000. A layer of adhesive, not shown, is also 
commonly used to attach die 68 to substrate 66. As in the case of a 
conventional quad fiat package assembly, the thermally conductive, 
electrically-insulated substrate 66 and its attached integrated-circuit 
die 68 are placed in the cavity formed between the two halves 62 and 64 of 
the mold 60. Plastic material is injected into the top half of the mold at 
the inlet gate 74. The plastic material enters the top half 62 of the mold 
and flows through the spaces between the bonding fingers of the lead frame 
into the bottom half of the mold. Vents 76 in the mold corners release 
trapped air. 
The arrows shown in FIG. 2 indicate the flow of plastic molding material 
through the top half of the mold and through the bottom half of the mold. 
Note that the thermally conductive, electrically-insulated substrate 66 is 
much greater in thickness and is also much wider than the conventional 
die-attach paddle 14 shown in FIGS. 1A and 1B. The intrusion of the much 
thicker bulk of the thermally conductive, electrically-insulated substrate 
66 disrupts and unbalances the flow of plastic material in the mold in 
several ways. 
One way that flow is disrupted is that the open spaces between the ends of 
the bonding fingers and the edge of the integrated-circuit die are blocked 
by the substrate 66. 
Another way that flow is disrupted is that the bulk of the substrate 
intrudes into the lower half of the mold so that the cross-sectional area 
for the flow of molding material in the lower space of the cavity is 
smaller and the flow resistance is greater for the lower space. This 
results in the flow of the molding material in the upper half of the mold 
being faster than the flow of molding material in the lower half of the 
mold. As a result of the differences in flow, the air at different places 
within the mold halves is expelled at different rates so that, for 
example, some air is trapped within the bottom part of the mold. The 
trapped air creates voids, also called blow holes or pin holes, in the 
body of the package. A typical void 80 is created on the side of the 
package which is opposite the inlet gate 74, as illustrated in FIG. 2. 
FIG. 3 shows a package mold 60, as shown in FIG. 2, having a top mold-half 
62 and a bottom mold-half 64, for molding a thermally-enhanced, quad fiat 
package. However, instead of the thermally-enhanced, 
electrically-insulated substrate 66 of FIG. 2, a slug 82 formed of copper 
or other similar metal is used as the substrate to which 
integrated-circuit die 68 is mounted. A layer 84 of insulating material 
electrically insulates the bottom of the integrated circuit die 68 from 
the top surface of the slug 82. The slug 82 replaces a conventional die 
attach paddle (such as the die-attach paddle 14 of FIGS. 1A and 1B) and 
improves the thermal performance of a molded-plastic package. Bonding 
fingers (typically shown as 69 and 70) at the inner ends of the leads of a 
lead frame 72 are attached to the outer margins of the slug 82 with an 
insulating layer of material disposed between the ends of the bonding 
fingers 69 and 70 and the slug 82 to insure that the slug 82 is 
electrically insulated from the bonding fingers 69 and 70 of the lead 
frame 72. 
However, as indicated in FIG. 3, the bottom portion of the slug 82 may 
extend completely to the bottom of the mold cavity. Thus, after 
encapsulation of the entire assembly described above, the bottom of slug 
82 remains exposed so that heat may be conducted away from 
integrated-circuit die 68. Since slug 82 extends to the bottom of the mold 
cavity, the plastic encapsulating material in the bottom of the mold 
cavity is unable to flow under slug 82 and is only able flow around the 
edges of slug 82. Again, this results in the flow of the molding material 
in the upper half of the mold being greater than the flow of molding 
material in the lower half of the mold, and as a result of the differences 
in flow, air within the mold halves is expelled by plastic material 
flowing at different rates creating voids or blow holes in the body of the 
package. 
Consequently, a need exists for a technique to prevent the formation of 
voids on the body of a thermally-enhanced molded plastic package body 
caused by differences in the flow rate of the encapsulating material 
through the mold. 
SUMMARY OF THE INVENTION 
The present invention provides for a uniform flow of a molding compound 
around a thermally conductive substrate and attached integrated-circuit 
die, to provide for void free encapsulation of such integrated circuits. 
A package for a thermally-enhanced, molded-plastic quad fiat package is 
provided. An integrated-circuit die is attached to a thermally conductive 
substrate, which has a central region to which the integrated-circuit die 
is attached. A lead frame with inwardly extending bonding fingers is 
attached to the outer margins of the thermally conductive substrate. 
Bonding wires are connected between respective bonding pads formed on the 
integrated-circuit die and the inwardly extending bonding fingers of the 
lead frame. A plastic molded package body is formed around the 
integrated-circuit die, the thermally conductive substrate, and the 
bonding fingers. 
The invention provides one or more channels or grooves, which are formed 
across the bottom surface of the thermally conductive substrate extending 
from one corner diagonally across the substrate. The thermally conductive 
substrate and attached integrated-circuit die and lead frame are placed 
into a mold cavity such that one end of the channel is located proximate 
to the inlet gate of the mold cavity. As the plastic molding material is 
injected into the mold cavity, the channels provide a pathway for the 
plastic molding material to flow across the bottom surface of the 
thermally conductive substrate thereby balancing the flow of the plastic 
molding material over the top and bottom of the substrate, to provide a 
molded package body substantially free of voids. Because the channels or 
grooves, are arranged diagonally, the molding material is able to flow 
through the inlet gate of the mold cavity and into the channels or grooves 
without having to drastically change direction. As such, the present 
invention eliminates the significant drag or restriction created by 
causing the flowing mold material to abruptly change direction. 
In another embodiment of the invention, a slot for facilitating uniform 
flow is produced having an opening on the top surface of the thermally 
conductive substrate and extending completely through the substrate 
terminating with a bottom opening on the bottom surface of the substrate. 
The slot is disposed at the corner of the thermally conductive substrate 
and is arranged such that the greater of the lengthwise and widthwise 
dimensions of the slot openings extends from the corner of the substrate 
towards the center of the substrate surfaces. By arranging the slot in 
this manner, the plastic molding material flows from the corner of the 
substrate and through the slot with minimized friction. The slot also 
provides for a more balanced flow of the plastic molding material both 
over and under the substrate. 
In yet another embodiment of the present invention, in addition to forming 
a slot in the corner of the thermally conductive substrate, the tie bars 
of the lead frame, which are used to secure the die paddle in place, are 
removed such that no portion of the lead frame is present in the corner of 
the substrate. By removing the tie bars from the lead frame, the plastic 
molding material is able to flow from the mold inlet gate of the mold 
cavity and into the slot without encountering restriction caused by the 
lead frame. As a result, the amount of drag on the plastic molding 
material is even further reduced. 
In yet another embodiment of the present invention, in addition to forming 
a slot in the corner of the substrate, and removing the tie bars of the 
lead frame, one or more diagonally arranged channels are formed across the 
bottom surface of the thermally conductive substrate as described above. 
The channel is formed having one end proximate to the bottom opening of 
the slot. The thermally conductive substrate and attached 
integrated-circuit die and lead frame are placed into a mold cavity such 
that one end of the channel and the slot are located proximate to the 
inlet gate of the mold cavity. As the plastic mold material is injected 
into the mold cavity, the plastic mold material is able to flow 
unobstructed through the slot and into the channel. Thus, the friction on 
the flow of the plastic molding material is further reduced and a more 
balanced flow of molding material in both the upper and lower halves of 
the mold cavity is achieved resulting in a molded package body 
substantially free of voids.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to the preferred embodiments of the 
invention, examples of which are illustrated in the accompanying drawings. 
While the invention will be described in conjunction with the preferred 
embodiments, it will be understood that they are not intended to limit the 
invention to these embodiments. On the contrary, the invention is intended 
to cover alternatives, modifications and equivalents, which may be 
included within the spirit and scope of the invention as defined by the 
appended claims. 
With reference to FIG. 4, a side view of a modified thermally conductive 
substrate 130, which has channels 132 formed into the bottom surface 
thereof, is shown. Channels 132 are formed diagonally across the bottom 
surface of thermally conductive substrate 130. 
FIG. 5 shows a bottom view of thermally conductive substrate 130 to more 
clearly show the diagonal arrangement of channels 132 across the bottom of 
substrate 130. In this typical embodiment of the invention, the channels 
have a depth of approximately one-half the thickness of the substrate 130. 
The width of the channels may vary as shown. To compensate for the 
progressively longer length of the channels nearest the center of 
substrate 130, the channels nearest the center of substrate 130 have a 
progressively greater width. 
Referring now to FIG. 6, a side sectional view of a molded plastic package 
134 containing a substrate 130 with channels 132 formed across its bottom 
surface is shown. In the embodiment of FIG. 6, substrate 130 is formed of 
an aluminum nitride material, although any of the numerous other similar 
ceramic-type substrate materials well known in the art are suitable. 
Additionally, channels 132 may be formed using any of the numerous etching 
or machining techniques well known in the art. The molding material is, 
for example, a standard molding compound such as provided by the Sumitomo 
Company as 6300 HS or HG molding compound, or as 7320C low viscosity 
molding compound. Additionally, a layer of adhesive, not shown, is 
commonly used to attach die 150 and leads 158 and 160 to substrate 130. 
Several advantages are achieved by forming channels 132 diagonally across 
the bottom surface of substrate 130. By forming channels 132 into the 
bottom surface of thermally conductive substrate 130, the flow 
cross-sectional area in the lower space of the cavity is increased. The 
additional cross-sectional area allows the molding compound to flow under 
thermally conductive substrate 130 with less restriction and at a more 
balanced rate with the flow of the plastic molding material over the top 
of thermally conductive substrate 130. Channels 132 are formed diagonally 
across the bottom of substrate 130 such that when substrate 130 and an 
attached lead frame 154 and integrated circuit die 150 are placed within a 
mold cavity, channels 132 are oriented such that one end of the channels 
132 is located proximate to the inlet gate of the mold cavity. In so 
doing, a straight flow path for the plastic molding material is provided. 
The plastic molding material is able to flow into the mold inlet gate and 
through the channels 132 without significantly altering the flow direction 
of the plastic molding material. As such, the amount of drag or resistance 
on the molding material is substantially reduced. 
FIG. 7 is a top view illustrating the positioning of substrate 130 within 
mold cavity 135. Substrate 130 is positioned such that as plastic molding 
material is injected into mold cavity 135 at inlet gate 136, the mold 
material will enter one end of channels 132 and flow diagonally across the 
bottom surface of substrate 130. Although several channels are shown 
formed into the bottom of substrate 130, the methods of the present 
invention are also well suited to the use of a single channel. 
With reference now to FIG. 8, a cross sectional view of another embodiment 
of the present invention is shown in which a copper slug 138 is used as 
the thermally conductive substrate. Although copper is used in the present 
embodiment, the present invention is also well suited to use any of the 
numerous thermally conductive metals well known in the art. Additionally, 
when an electrically conductive material such as copper is used as the 
substrate 138, and layer of insulating material shown as 140 and 142 must 
be placed between the ends of inwardly-extending bonding fingers 158 and 
160 and integrated circuit die 150, respectively. As shown in FIG. 8 when 
the thermally conductive substrate 138 extends to the bottom of the mold 
cavity channels 132 allow the plastic molding material to flow under 
substrate 138 such that plastic molding material is not limited to flowing 
solely around the edges of substrate 138. As a result a more balanced flow 
of plastic molding material over and under the substrate is attained. 
Furthermore, the additional pathways for the plastic molding material 
provided by channels 132 do not decrease the surface area of the bottom of 
the substrate. As such, no compromise in the ability of the substrate to 
remove heat from the integrated circuit occurs. On the contrary, channels 
132 increase the effective surface area of the bottom of substrate 138 
allowing for increased removal of heat from integrated circuit 150. 
With reference now to FIGS. 9A and 9B, additional embodiments of the 
present invention are shown in which a die attach paddle 152 having an 
integrated circuit 150 mounted thereon is attached directly to the 
thermally conductive substrate. Die attach paddle 152 is bonded to a 
thermally conductive substrate using an adhesive film such as R-flex 1000, 
or epoxy pastes. Additionally, a layer of adhesive, not shown, is used to 
attach leads 158 and 160 to die attach paddle 152. In FIG. 9A, the die 
attach paddle 152 is attached to an aluminum nitride substrate 130 as in 
the embodiment of FIG. 6. In the embodiment set forth in FIG. 9B the die 
attach paddle is attached to a copper slug 138 as in the embodiment of 
FIG. 8. Again, by forming channels 132 into the bottom surface of 
thermally conductive substrate 130, the flow cross-sectional area in the 
lower space of the cavity is increased. The additional cross-sectional 
area allows the molding compound to flow under thermally conductive 
substrates 130 and 138 with less restriction and at a more balanced rate 
with the flow of the plastic molding material over the top of thermally 
conductive substrates 130 and 138. Channels 132 are also formed diagonally 
across the bottom of substrates 130 and 138 such that one end of the 
channels 132 is located proximate to the inlet gate of the mold cavity. 
With reference now to FIG. 10 another embodiment of the present invention 
is shown. As in the previous embodiments, an integrated-circuit die 150 is 
mounted to a central region of a thermally conductive substrate 144 formed 
of aluminum nitride. Although aluminum nitride is used in the preferred 
embodiment, any of the numerous similar materials well known in the art 
would be suitable. Additionally, a layer of adhesive, not shown, is 
commonly used to attach die 150 and the leads to substrate 144. As shown 
in FIG. 10, slots 166 and 168 are formed which extend completely through 
thermally conductive substrate 144. Slots 166 and 168 are located in the 
corner region of thermally conductive substrate 144, such that when 
substrate 144 is placed into the mold cavity, the slots are disposed in 
close proximity to the mold inlet gate. Slots 166 and 168 allow the 
plastic molding material to pass through substrate 144. Therefore, as the 
plastic mold material is introduced into the mold cavity via mold inlet 
gate, not shown, the plastic mold material is able to flow between the 
bonding fingers of the lead frame and pass through slots 166 and 168 to 
allow for a more balanced flow of the molding material both over and under 
substrate 144. Slots 166 and 168 may also be elongated such that the 
openings of slots 166 and 168 extend in a lengthwise and a widthwise 
direction across substrate 144, with the lengthwise dimension of slots 166 
and 168 being greater than the widthwise dimension of slots 166 and 168. 
Furthermore, slots 166 and 168 are aligned such that the lengthwise 
dimension extends from the corner of substrate 144 towards the center of 
substrate 144. In so doing, the plastic mold material is able to flow from 
the inlet gate of the mold cavity and through slots 166 and 168 with 
minimized resistance. Because the greater dimension of the openings of 
slot 166 and 168 is aligned towards the center of substrate 144, the flow 
of the plastic molding material towards the center of substrate 144 is 
facilitated. 
As shown in FIG. 11, slots 166 and 168 may also be angled such that the 
bottom opening of slots 166 and 168 are closer than the top openings to 
the center of substrate 144. 
By angling slots 166 and 168 as set forth in FIG. 11, as the plastic mold 
material molding passes through slot 166 nearest the mold inlet gate, not 
shown, the molding material is directed under and towards the center of 
substrate 144. Air under substrate 144 is pushed up through slot 168 and 
the towards outlet gate of the mold cavity. In so doing, the formation of 
voids in the package body due to trapped air is prevented. Additionally, 
as the plastic molding material reaches the opposite side of substrate 
144, the mold material is able to pass up through slot 168. Once again, a 
more direct flow path is created for the plastic molding material thus 
reducing the amount of friction or drag on the molding material. In so 
doing, a more balanced flow of the plastic molding material both over and 
under substrate 144 is achieved. Since there is a more balanced flow over 
and under the substrate, the plastic material flowing in the top of the 
mold cavity and the bottom of the mold cavity meets near the outlet gate 
of the mold cavity. This eliminates voids, or blowholes, caused by air 
trapped within the mold. 
Referring now to FIG. 12, a top view of the embodiment of FIG. 10 is shown 
in order to more clearly show the positioning of slots 166 and 168. As 
shown in FIG. 12, slots 166 and 168 are disposed at the corner of 
substrate 144 and are arranged such that the greater of the lengthwise and 
widthwise dimensions of the slot openings extends from the corner of 
substrate 144 towards the center of substrate 144. By arranging slots 166 
and 168 in this manner, the plastic molding material flows directly from 
the corner of substrate 144 and through slots 166 with minimized drag or 
resistance. Any air remaining under substrate 144 will be expelled through 
slot 168. As a result, slots 166 and 168 provide a more balanced flow of 
the plastic molding material both over and under substrate 144. The more 
balanced flow in the top and bottom halves of the mold cavity eliminates 
voids, or blowholes, caused by air trapped in the mold. Although two slots 
are used in the present embodiment of the present invention, a single slot 
or additional slots located at each corner of substrate 144 are also well 
suited to the present invention. 
In the embodiment of FIG. 13 slot 166 and 168 are shown formed into a 
copper slug 146 which is used as the thermally conductive substrate. As in 
the embodiment of FIG. 10, slots 166 and 168 are formed extending 
completely through thermally conductive substrate 146. Slots 166 and 168 
allow the plastic molding material to pass through substrate 146. 
Therefore, as the plastic mold material is introduced into the mold cavity 
via mold inlet gate, not shown, the plastic mold material is able to flow 
between the bonding fingers of the lead frame and pass through slots 166 
and 168 to allow for a more balanced flow of the molding material both 
over and under substrate 146. Slots 166 and 168 may also be angled such 
that the bottom opening of slots 166 and 168 are closer than the top 
openings to the center of substrate 146. 
In the embodiment of FIGS. 14A an 14B, die attach paddle 152 is located in 
the central region of a lead frame 154. Lead frame 154 is centrally 
attached to a thermally conductive substrate formed of aluminum nitride 
144 or other similar material as set forth in FIG. 14A, or a slug 146 
formed of copper or other similar material as set forth in FIG. 14B. In 
either case, however, the die attach paddle 152 may be bonded to the 
thermally conductive substrate using an adhesive film such as R-flex 1000, 
not shown. As set forth in previous embodiments, slots 166 and 168 are 
formed extending completely through the thermally conductive substrate 144 
and 146 to allow the plastic molding material to pass through the 
substrates 144 and 146, such that a more balanced flow of the molding 
material both over and under substrates 144 and 146 is achieved. As 
before, slots 166 and 168 may also be angled such that the bottom opening 
of slots 166 and 168 are closer than the top openings to the center of 
substrates 144 and 146. 
FIG. 15 shows another embodiment of the present invention shown in which 
slots 166 and 168 are formed into the thermally conductive substrates 
formed of aluminum nitride or other similar material, or formed into a 
slug of copper or other similar material. The tie bars which are located 
directly above slots 166 and 168 are removed from lead frame 154. In so 
doing, plastic mold material may flow through slots 166 and 168 without 
any friction caused by the presence of tie bars above slots 166 and 168. 
As a result the unimpeded flow of the plastic molding material through 
slots 166 and 168 is possible. Die attach paddle 152 is secured in the 
center of lead frame 154 by remaining tie bars 174. Slots 166 and 168 may 
also be angled as describe above. 
FIG. 16 shows a bottom view of yet another embodiment of the present 
invention. In addition to forming slots through the thermally conductive 
substrates and eliminating the tie bars from the corner of the lead frame, 
channels, as set forth in FIGS. 4-9B, are formed into the bottom of the 
substrates. As shown in FIG. 16, channels 202 can be formed diagonally 
across the bottom of substrate 200 such that slots 204 and 206 reside 
within channels 202. Since slots 204 and 206 are within channels 202, slot 
204 and 206 may be utilized even when substrate 200 extends completely to 
the bottom of the mold cavity. In such instances, the plastic mold 
material flows unimpeded by any tie bars through slots 204 and 206 into 
only the middle channel of channels 202. Substrate 200 is arranged within 
the mold cavity with the inlet gate of the mold cavity located proximate 
to slot 204 to facilitate the flow of the plastic mold material into slot 
204 and through channel 202. Forming channels into the bottom surface of 
substrate 200 along with the formation of slots 204 and 206 and the 
removal of the tie bars from the region over slots 204 and 206 balances 
the flow of plastic material both over and under substrate 200, and 
provides a molded package body substantially free of voids. 
Thus, the present invention provides for a uniform flow of a molding 
compound around a thermally conductive substrate and attached 
integrated-circuit die, to provide for void free encapsulation of such 
integrated circuits. Additionally, although the present invention employs 
a die-up configuration, the methods of the present invention are also 
well-suited for use in die-down configurations. Furthermore, the present 
invention is also well-suited for use with additional substrates such as 
printed circuit boards. 
The foregoing descriptions of specific embodiments of the present invention 
have been presented for the purposes of illustration and description. They 
are not intended to be exhaustive or to limit the invention to the precise 
forms disclosed, and obviously many modifications and variations are 
possible in light of the above teaching. The embodiments were chosen and 
described in order to best explain the principles of the invention and its 
practical application, to thereby enable others skilled in the art to best 
utilize the invention and various embodiments with various modifications 
as are suited to the particular use contemplated. It is intended that the 
scope of the invention be defined by the claims appended hereto and their 
equivalents.