Semiconductor device having contoured package body profile

An overmolded semiconductor device (30, 50) has a contoured package body profile instead of a conventional flat package body surface for uniform filling during the molding process. A cross-section of the semiconductor device reveals a substantially uniform thickness of plastic (36 and 38) covering the carrier substrate (14) and overmolding the semiconductor die (12). Alternatively, a thicker layer of plastic (58) overmolds the semiconductor die (12') than the layer of plastic (56) covering the carrier substrate (14'). The contoured package body profile is designed to allow a uniform flow front progression of molding compound during the molding process to eliminate voids in the package body by providing the same resistive pressure to the molding compound flow front during filling.

FIELD OF THE INVENTION 
The present invention relates generally to a semiconductor device, and more 
specifically to a semiconductor device having a contoured package body and 
a method for its fabrication. 
BACKGROUND OF THE INVENTION 
A semiconductor die is a microchip having intricate circuitry on its active 
surface which performs various functions, such as controlling a computer, 
storing data, processing signals for a pager, and the like. In order for a 
semiconductor die to operate as intended, it is often necessary to package 
the die to provide it with external electrical connections which can be 
connected to a printed circuit board. A packaged semiconductor device 
typically has a package body to protect the semiconductor die and the 
internal electrical connections from handling damage and from being 
contaminated by the environment. One common type of package body is made 
from plastic. 
In the semiconductor device manufacturing area, plastic package bodies are 
composed of a polymeric molding compound, typically a thermosetting 
epoxy-resin based material, which is molded with a transfer molding 
process to form the package body. One such molded device 10 is illustrated 
in FIG. 1, wherein a semiconductor die 12 is mounted on a carrier 
substrate 14 and wire bonded thereto. The package body 16 is formed by 
transfer molding a molding compound through a system of runners and gates 
in a mold. A portion of a runner 18 is shown to be still attached to the 
package body 16 in FIG. 1. Although this process sounds relatively simple, 
problems exist in trying to mold a void-free package body. Voids, or 
holes, in the package body, are not desired because they may allow 
contaminants to become trapped in the package body which increases the 
chance of contaminants reaching the semiconductor die. Additionally, voids 
are cosmetic defects that the end user or customer does not want to see, 
nor does a manufacturer want to send such a device to the customer. 
FIGS. 2-5 illustrate, in top views, the typical flow front 20 of a molding 
compound during the package body molding process. The molding compound 
enters through the gate (not shown) at the bottom left corner as depicted 
in FIG. 2. The compound continues to fill the cavity by flowing across the 
substrate and semiconductor die toward the opposing corner, as shown in 
FIG. 3. The molding compound hardens or gels as it is filling the mold 
cavity. As the molding cycle progresses, as illustrated in FIG. 4, the 
molding compound advances non-uniformly with the mold front 20 advancing 
faster in the thicker cross section at the edge of the die 12. In the last 
stage of filling, the compound flow fronts 20 join (knit together) and 
close off the vent (not shown, but would be located at the top right 
corner) leaving a portion of the cavity unfilled as depicted in FIG. 5. As 
a result of non-uniform molding compound front progression, a void 22 (or 
voids) in the package body are often formed at a corner of the 
semiconductor die which can sometime even expose a portion of the die 
surface. 
Thus, a need exists for a semiconductor device that allows uniform molding 
compound front progression to eliminate the package body voiding problem 
associated with transfer molding.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Generally, the invention provides a semiconductor device having a 
semiconductor die mounted upon a carrier substrate. The die is 
electrically connected to the substrate either by wire bonds or by 
equivalent means. A plastic package body is overmolded over the 
semiconductor die to protect it. However, instead of a conventional flat 
package body surface, the package body of the present invention is 
contoured such that, in one embodiment, a cross-section of the 
semiconductor device reveals a substantially uniform thickness of plastic 
covering the carrier substrate and overmolding the semiconductor die. In 
another embodiment, a cross-section of the contoured package body reveals 
that a thicker layer of plastic overmolds the semiconductor die than the 
layer of plastic covering the carrier substrate. In both embodiments, the 
contoured package body is designed to allow a uniform flow front 
progression of molding compound during the transfer molding process to 
eliminate voids in the package body. 
These and other features, and advantages, will be more clearly understood 
from the following detailed description taken in conjunction with the 
accompanying drawings. It is important to point out that the illustrations 
may not necessarily be drawn to scale, and that there may be other 
embodiments of the present invention which are not specifically 
illustrated. 
FIGS. 6-8 illustrate a semiconductor device 30 having a contoured package 
body in a first embodiment of the invention. In FIG. 6, a cross-section 
view of the device 30 is shown, while FIG. 7 shows a top view of the same 
device, and FIG. 8 illustrates an enlarged portion of FIG. 6. The 
semiconductor die 12 is mounted upon carrier substrate 14, which is 
typically composed of an epoxy-glass cloth material such as those used to 
form printed circuit boards. The die 12 may be attached using any type of 
suitable die attach adhesive, such as silver-filled epoxy resin materials, 
silver-filled silicones, or may be a thermoplastic adhesive. Although not 
specifically illustrated, carrier substrate 14 would have conductive 
traces on its surface for electrical connections and routing. 
Additionally, carrier substrate 14 may have conductive traces on its 
bottom surface as well as solder pads for the addition of solder balls, 
solder columns, pins, or some other equivalent type of external electrical 
connections to enable the device 30 to be mounted to a next level 
interconnect. 
As shown in FIG. 6, the semiconductor die 12 is electrically connected to 
the carrier substrate through a plurality of wire bonds 32. Methods of 
wire bonding are known in the art, and can include low loop wire bonding. 
However, the invention is in no way limited to wire bonding as a means of 
establishing electrical connections between the semiconductor die and the 
carrier substrate. Other methods of forming electrical contact can be 
used, such as flip-chip bonding, inverted TAB bonding, or using a Z-axis 
conductive die attach material. The type of semiconductor die that would 
most likely benefit from the present invention include, but are not 
limited to, a microcontroller unit (MCU), a microprocessor (MPU), a 
memory, a digital signal processor (DSP), a reduced instruction set chip 
(RISC), and an application specific integrated circuit (ASIC). 
The package body 34 is illustrated in FIG. 6 to be contoured, wherein one 
top surface 40 over the carrier substrate 14 and the other top surface 42 
over the semiconductor die 12 are on different planes. An enlarged drawing 
of this portion of the device is depicted in FIG. 8. The contoured design 
of the present invention is deliberate so that the thickness 36 of the 
package body, that portion of the package body covering the carrier 
substrate 14, and the thickness 38 of the package body, that portion 
covering the semiconductor die 12, are substantially the same. This can be 
accomplished by forming a step in the upper mold platen of the transfer 
molding equipment to create a uniform thickness across the package body. 
Other aspects of the transfer molding equipment do not require 
modification and are known in the art. The curved portion 44 of the 
package body which provides the transition between the two top surfaces 40 
and 42 is designed to allow a smoother flow path for the molding compound 
during the filling of the die cavity. Additionally, the curvature 
eliminates stress points and allows easy removal of the package body from 
the mold. Alternatively, other non-orthogonal surfaces, such as chamfers 
or other contours, may be used as the connecting surface between the two 
top surfaces 40 and 42. The radius 46 from the edge of the die 12 to the 
contoured surface 44 is the same as the cross-sectional thickness 36 or 38 
By holding the cross-section constant, the cavity presents the same 
resistive pressure to the molding compound material flow to allow uniform 
filling. Additionally, a radius 48 may be desirable on the outside of the 
package body to allow easy removal of the molded device from the mold 
cavity. For example, the radius 48 may be designed at 0.5 mm. 
Other than the modification to the upper mold platen as described above, 
the transfer molding process to form this package body does not require 
significant re-engineering. The process parameters for the molding would 
have to be optimized for a particular molding compound used, but such a 
step is also required with devices of the prior art design, which a person 
of ordinary skill in this art would possess the skill to do. Molding 
compounds that can be used for the package body include, but are not 
limited to polymeric transfer molding compounds, injection molding 
materials, or liquid systems that could be used for encapsulation. 
Moreover, practicing this invention can be performed on, but not limited 
to, conventional or multiplunger molding systems. Additionally, the 
invention can also be applied to plate molds, which have their material 
gate in the top edge of the mold. 
FIG. 9 illustrates, in cross-section, a semiconductor device 50 having a 
different contoured package body 54 in a second embodiment of the 
invention. In this embodiment, the semiconductor die 12' is depicted to be 
flip-chip bonded to the carrier substrate 14' using a plurality of 
interconnect bumps 52. Again, for ease of illustration, the conductive 
traces on carrier substrate 14' have not been explicitly illustrated but 
are understood to be inherently present. It should be understood that 
although flip-chip bonding is illustrated, a person practicing this 
embodiment of the invention is not limited to flip-chip methods but can 
also use wire bonding, flip-TAB, or Z-axis conductive die attach. In this 
embodiment, the package body is contoured such that the thickness 56, that 
portion covering the carrier substrate, is thinner than the thickness 58, 
that portion covering the semiconductor die. This can be accomplished by 
putting the appropriate size step in the upper mold platen. The intent is 
to force uniform flow of molding compound across the semiconductor die 
surface so as to eliminate the formation of voids at the corner of the 
die. The ratio between the two thicknesses should be determined either 
empirically or through modeling based upon specific geometry as well as 
the type of internal electrical connections used, because the presence of 
wire bonds is known to retard the compound front progression. 
Nevertheless, it is expected that the ratio of the first thickness 56 to 
the second thickness 58 would be in the range of 0.3 to 0.8. 
The foregoing description and illustrations contained herein demonstrate 
many of the advantages associated with the present invention. In 
particular, it has been revealed that a semiconductor device having a 
contoured package body allows a uniform flow progression of molding 
compound during the transfer molding process to eliminate the formation of 
voids at the corner of the semiconductor die opposite from the gate area. 
The contouring can be tailored for different die sizes such that a 
cross-sectional thickness would show either a same amount of plastic over 
the carrier substrate as over the semiconductor die, or that it would show 
a thinner amount of plastic over the carrier substrate than over the 
semiconductor die, depending on what is required to provide uniform flow 
across the semiconductor die. 
Thus it is apparent that there has been provided, in accordance with the 
invention, a semiconductor device having a contoured package body and a 
method for its fabrication that fully meet the need and advantages set 
forth previously. Although the invention has been described and 
illustrated with reference to specific embodiments thereof, it is not 
intended that the invention be limited to these illustrative embodiments. 
Those skilled in the art will recognize that modifications and variations 
can be made without departing from the spirit of the invention. For 
example, the described embodiments can utilize different gate locations, 
molding methods, or molding materials. Furthermore, other package types 
using substrates may derive benefit from practicing this invention. 
Therefore, it is intended that this invention encompasses all such 
variations and modifications falling within the scope of the appended 
claims.