Integral extended surface cooling of power modules

The semiconductor wafer of the power module is mounted directly, separated only by thermal expansion control layers and insulator layers, on the metal base which has fins or pins integrally formed thereon. This module is mounted on a coolant fluid flow chamber so that the module mounting is not a thermal interface. In this way, maximum power module cooling is achieved with minimum fluid flow.

FIELD OF THE INVENTION 
This invention is directed to the structure of an electronic power module 
which has its wafer mounted directly (except for thermal expansion and 
insulating layers) on the base, and the base has extended cooling surface 
thereon. The module is directly mounted on a coolant fluid flow chamber 
with the extended surface in the chamber to minimize thermal resistance 
between the wafer and the coolant fluid. 
BACKGROUND OF THE INVENTION 
Semiconductor electronic devices have internal losses which generate heat 
in use. When the electronic device is configured for a substantial amount 
of power, cooling is necessary in order to limit the semiconductor device 
to a sufficiently low temperature to provide a reasonable life. Power 
modules containing this type of semiconductor component are traditionally 
attached to a cold plate. The cold plate may have fins thereon with 
circulation of fluid therepast. The fluid is usually liquid when greater 
amounts of heat must be dissipated. As long as there is sufficient power 
to provide more flow of liquid, such a cooling system is satisfactory. 
A commonly used current method for cooling high-density hybrid power 
modules is by clamping them onto a typical internally finned flow-through 
cold plate. Characteristics of the thermal interface between the module 
and the cold plate are dependent on numerous variables such as surface 
finish and roughness, bolt torque, pressure uniformity, interface material 
conductivity and consistency, and the type of interface material such as 
gasket, grease or foil. In addition, thermal expansions and contractions, 
vibration, shock and handling will often vary the initial parameters and 
cause performance degradation. Typical finned flowthrough cold plates are 
constructed of aluminum material for manufacturing reasons. The mismatch 
in thermal expansion rates between the aluminum cold plate and the module 
base plate creates additional degradation at the thermal interface. 
The dissipated power density of present-day power hybrid modules has 
reached a nominal 500 watts per square inch at the semiconductor chip. 
Current package power modules are clamped or bolted with the module base 
plate against the cold plate. The cold plate may have internal passages 
for fluid flow and internal fins for increased cold plate-liquid coolant 
interface area. This structure requires the cold plate of minimum thermal 
resistance and an interface between the power module base plate and the 
cold plate also of minimum thermal resistance. Achieving flatness is 
difficult, and tight bolting causes distortion. Thus, there is need for an 
improved structure by which the semiconductor devices can be cooled with a 
minimum amount of coolant power. 
SUMMARY OF THE INVENTION 
In order to aid in the understanding of this invention, it can be stated in 
essentially summary form that it is directed to an electronic power module 
which has extended cooling surface on its base plate, and there is a 
minimum number of layers between the semiconductor and wafer to minimize 
thermal resistance. 
It is thus a purpose and advantage of this invention to provide a power 
module which has a metallic base plate therein with an electronic 
semiconductor wafer mounted on the base plate on a minimum number of 
interface layers, and the base plate carries integrally formed fins or 
pins which are immersed in liquid in a cold plate. 
It is another purpose and advantage of this invention to provide an 
integral fin-cooling system for power modules which incorporates cooling 
fins integral to the base plate of the power module so that the fins or 
pins can be bathed in cooling fluid to cool the semiconductor wafer. 
It is a further purpose and advantage of this invention to provide a power 
module having integral fins or pins thereon so that thermal resistance is 
minimized and liquid coolant can cool the fins or pins with a minimum 
expenditure of pumping power. 
It is another purpose and advantage of this invention to provide direct 
contact between fins or pins on the base plate of a power module with the 
liquid in a cold plate so that the cold plate need not be made of low 
thermal resistance materials or with high accuracy at the power module 
mounting face. 
Other purposes and advantages of this invention will become apparent from a 
study of the following portion of the specification, the claims and the 
attached drawings wherein:

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A power module configured in accordance with this invention is generally 
indicated at 10 in FIGS. 1, 2 and 3. In FIG. 3, the near side of the cover 
12 is broken away to schematically illustrate the equipment therein. The 
power module 10 comprises a base 14 which has an upper mounting surface 16 
and which has a plurality of fins integrally formed therewith and 
extending out of the base on the side opposite the mounting surface. Fins 
18 and 20 are specifically identified in FIG. 3, but it is seen that the 
fins are positioned under the active area of the upper mounting surface. 
The base 14 is sufficiently wide so that it can serve as an overlap 
mounting flange against the cold plate, as is described hereafter. The 
cover 12 is preferably of a dielectric material or has dielectric 
feedthroughs therethrough. The cover is secured to the outer edges of the 
base 14 at the sides, as seen in FIGS. 1 and 2, but is short of the ends 
of the base to expose a securing flange, as seen in FIGS. 1 and 2. If 
desired, the cover could be narrower, in the direction seen in FIG. 3, to 
leave an exposed securing flange all the way around. 
Mounted on top of the active area of the upper surface is the electronic 
stack indicated at 22 in FIG. 3. As previously indicated, the lowermost 
layer is the base 14. This is preferably a copper layer to achieve high 
thermal conductivity in a relatively inexpensive metal. Appropriate 
intermediate layers are provided between the semiconductor wafer and the 
copper base layer 14 in order to provide security of mounting, maximize 
heat transfer, minimize stresses in the wafer caused by differential 
expansion of the materials, and provide electrical insulation where 
necessary. The wafer in the stack has semiconductor electronic components 
therein, and particularly high-power electronics together with control 
components for controlling the main power flow. Principal electronic 
connections 26 and 28 are shown as the power connections to the power 
junctions in the module. These electrical connections are shown as 
extending out of the top of the cover for external connection. In 
addition, the control connections are brought out of the cover in a 
similar way, but with suitably smaller conductors. The semiconductor 
devices in the silicon wafer generate the heat which must be extracted to 
maintain the silicon wafer at a sufficiently low temperature that it 
remains operative over a long life. The heat is conducted downward through 
the stacks to the base layer 14. The base layer 14 has the plurality of 
fins thereon under the active area of the upper mounting surface. 
Cold plate 40, see FIGS. 1, 2 and 3, has a fluid inlet 42 and a fluid 
outlet 44, see FIG. 1. The inlet and outlet are respectively connected to 
inlet passage 46 and outlet passage 48. There are one or more channels in 
the cold plate depending on how many power modules 10 are to be mounted on 
each cold plate. As illustrated in FIG. 1, there are two channels in the 
cold plate 40. The first is channel 50 on which the module 10 is mounted, 
and there is also a channel 52 on which no power module is mounted for 
illustration purposes. In use, all channels would be covered. Furthermore, 
FIG. 1 is shown as being broken away so that the cold plate 40 may be 
longer in the upper right direction so as to provide the desired number of 
channels to accommodate the desired number of power modules. Since the 
cold plate 40 has no need for providing low resistance thermal 
connections, flatness is not a problem, and it can be made out of any 
convenient material, such as die-cast aluminum or molded synthetic polymer 
composition material. The modules are mounted on the cold plate with a 
gasket 54 therebetween to avoid coolant leakage out of the face of the 
cold plate. There is no need for the extra special flat surfaces required 
for thermal interface, but the surfaces need only be flat enough for 
sealing with the gasket. The power modules are held in place on the face 
of the cold plate by any convenient means. Machine screws 56 and 58 are 
illustrated in FIG. 1, and spring clamps 60 and 62 are illustrated in FIG. 
2 as additional and/or alternate means for securing the power module on 
the cold plate. 
The fins and channels are sized so that the fins reach all the way to the 
bottom of the channel, as seen in FIGS. 2 and 3. The fins may be attached 
by any convenient means, such as adhesive 64, if desired. When not 
adhesively attached, the modules are more easily replaced. When they are 
attached, then the back wall 66 is supported better and fluid flow beyond 
the ends of the fins is illuminated. 
FIG. 4 illustrates the same structure except that pins are used instead of 
fins. Pins are employed as an alternate way of extending the surface. The 
parts are the same as in FIG. 3, except for the presence of fins instead 
of pins. Pins 68 and 70 are particularly identified. The pins are 
preferably integrally formed with the base 14 for maximum thermal 
transfer, but may be separate pins pressed into holes in the base. The 
pins are simply a special configuration of the fins. When a power module 
is attached to each channel and secured and gasketed in place, coolant 
liquid delivered into fluid inlet 42 travels through inlet passage 46 and 
thence up the channels 50 and 52 between the fins or pins integrally 
formed on base 14. The spent coolant liquid is collected in outlet passage 
48 and discharged at fluid outlet 44. The coolant fluid is preferably 
air-cooled and pumped back into inlet 42. The liquid flow is controlled so 
that it all passes through the channels 50 and 52 and related channels. 
The fluid in each channel directly cools the base plate of the associated 
power module. A minimum thermal resistance is achieved, and this minimizes 
the coolant fluid pumping requirement. 
This invention has been described in its presently contemplated best mode, 
and it is clear that it is susceptible to numerous modifications, modes 
and embodiments within the ability of those skilled in the art and without 
the exercise of the inventive faculty. Accordingly, the scope of this 
invention is defined by the scope of the following claims.