Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame
Reexamination Certificate
2001-01-09
2003-04-22
Potter, Roy (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
C257S670000, C257S676000, C257S787000
Reexamination Certificate
active
06552417
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the design of a semiconductor package; and, in particular, the present invention relates to a semiconductor package designed for high electrical and thermal dissipation performances.
2. Discussion of the Related Art
Semiconductor devices are becoming larger, integrating a larger number of circuits, and operating at increasingly higher clock frequencies. As a result, semiconductor devices are requiring, without compromising reliability, packages of increasingly higher lead count, and higher electrical and thermal performances.
In the prior art, conventional plastic molded packages can dissipate up to 2 watts of power. With some improvements in the lead frame, and by adding a heat spreader or heat sink, a plastic molded package can dissipate up to 4 watts. A further improvement in power dissipation can be achieved by attaching the semiconductor device, also called the semiconductor “die”, onto an integral heat sink. Such a heat sink typically has a surface exposed to the ambient to conduct heat away from the package. An example of such a package, also called a “thermally enhanced” package, is shown in FIG.
1
.
FIG. 1
shows a thermally enhanced package comprising a semiconductor die
105
attached by a layer of thermally conductive epoxy to a metallic heat sink
101
. The input and output terminals of semiconductor die
105
are electrically coupled to connection terminals (“leads”) of a lead frame
103
by wire bonds
104
, which connect the bonding pads of semiconductor die
105
to individual leads in lead frame
103
. Lead frame
103
attaches to heat sink
101
by a layer of dielectric adhesive
107
. The thermally enhanced package is encapsulated in a plastic molding
102
. In package
100
, high thermal dissipation is achieved by attaching semiconductor die
105
directly onto the lower surface of heat sink
101
using a thermally conductive epoxy layer
106
.
Although plastic molded packages are typically of high reliability, the incorporation of a heat sink in a thermally enhance plastic molded package, such as package
100
of
FIG. 1
, leads to failures which are directly related to the design and the material used in the heat sink. For example, heat sink
101
is often made of aluminum. The large difference between the thermal expansion coefficients (TCE) of the silicon die, at 3 ppm/° C., and of aluminum, at 25 ppm/° C. induces significant strain on semiconductor die
105
. Such strain causes die- cracking and thus a package failure. For this reason, in the prior art, semiconductor die sizes are kept well below 10×10 mm to minimize the induced stress. Alternatively, a heat sink material with lower TCE can be chosen to minimize the large mismatch in the heat sink's and the semiconductor die's coefficients of thermal expansion.
A similar mismatch in TCEs exists between heat sink
101
and the plastic molding
102
. Typically, a plastic molding compound has a TCE of about 17 ppm/° C. The thermal cycle package
100
experiences during assembly and normal operations induces high stress at the metal-to-molding interface (i.e. between heat sink
101
and plastic molding
102
) which can lead to delamination, cracking of the molding, and die failures. For this reason, a close matching of the TCEs of heat sink
101
to plastic molding
102
is very desirable.
During the assembly of package
100
, plastic molding
102
shrinks significantly after the molding operation and during post-mold curing, which is typically carried out at or about 175° C. The shrinking molding causes significant stress at the metal-to-molding interface, which can lead to delamination. Delamination is very undesirable and usually causes long-term reliability failures. Delamination can be minimized by including on the heat sink “locking” features, which strengthen mold adhesion, and by choosing a heat sink material with a TCE closer to that of the molding compound.
In the prior art, frequency performance is limited by the electrical parasitic impedances of the lead frame to 50 MHz or less. The lead frame usually consists of a single metal layer without the ability to provide controlled impedance connections. U.S. Pat. No. 4,891,687, entitled “Multilayer Molded Plastic IC Package”, to Mallik et al, filed on January 27, and issued on Jan. 2, 1990, discloses a package achieving a high electrical performance. However, the package disclosed in U.S. Pat. No. 4,891,687 requires two lead frames, and hence, such package is significantly more costly than a conventional plastic molded package.
Furthermore, the prior art's use of long wire bonds between the semiconductor die and the lead frame increases the impedances of ground connections. A high impedance to a ground connection results in “ground bounce”and other electrical noises which further restrict the overall electrical performance of the conventional plastic molded package. In logic semiconductor devices, which usually require high lead counts, about 25% of the leads in each package are used for power and ground connections. The large number of leads devoted to power and ground connections significantly reduces the number of pins available for signal connections, which usually determine the level of available performance.
SUMMARY OF THE INVENTION
In accordance with the present invention, a plastic molded package is provided comprising (i) a heat sink having an upper surface and a lower surface, (ii) a ceramic or dielectric ring attached by an adhesive film to the lower surface of the heat sink; (iii) a semiconductor die attached using a thermally conductive epoxy adhesive to the lower surface of the heat sink through an aperture in the dielectric ring; (iv) a lead frame, which is attached to a surface of the dielectric ring, having a number of leads extending outside of the plastic molded package; and (v) a plastic molding enclosing the ceramic ring, the lead frame, except at the exposed portion of the leads and the semiconductor die.
In accordance with one aspect of the invention, the heat sink comprises a base portion enclosed in the encapsulation and a raised portion protruding above the base portion having a surface exposed to the ambient. In one embodiment, the exposed surface of the raised portion is free of corners (e.g. in the shape of a circle). The base portion of the heat sink includes a number of conical protrusions enclosed in the molding, and a number of through holes filled by the molding. The exposed portion of the raised surface of the heat sink is coated with nickel to provide a good conductive surface for attaching an external heat sink. Suitable materials for the heat sink includes oxygen free high conductivity copper, copper/molybdenum/copper laminate, copper/tungsten/copper laminate and beryllium composites.
In accordance with another aspect of the present invention, the lead frame of the plastic molded package further comprises an interposer ring downset and attached to the heat sink. The interposer ring comprises either a single loop (360°), or a number of electrically isolated sections for independent connections to power and ground terminals. Such electrically isolated sections of the interposer ring can be supported in the encapsulation by tie bars of the lead frame. For an electrically isolated section of the interposer ring, an electrical short to the heat sink allows the heat sink to be used as a ground plane for the semiconductor die. That electrical short can be accomplished by a drop of electrically conductive adhesive. The leads of the lead frame allow the internal power and ground planes in the interposer ring to be connected to power and ground supplies outside of the plastic molded package.
In accordance with another aspect of the present invention, the dielectric ring comprises a material selected from the group consisting ceramic materials, epoxy materials including Ablefilm 564 AKHM, and a dielectric sheet material sold under the trade name of Neoflex. A ceramic dielectric ring provides higher thermal conduct
Asat, Limited
Gallagher Thomas A.
Gallagher & Lathrop
Potter Roy
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