Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2000-08-11
2002-02-26
Picard, Leo P. (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S689000, C361S690000, C361S709000, C361S717000, C361S718000, C257S712000, C257S713000, C257S714000, C257S717000, C257S721000, C174S015100, C174S016100, C174S016300, C165S080300, C165S080400
Reexamination Certificate
active
06351384
ABSTRACT:
This application is related to and claims priority from Japanese Patent Application No.
11-227211
, filed Aug. 11, 1999, the entire disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to large scale integrated circuit chips (herein sometimes termed LSI chips) having semiconductor elements that provide high speed, high integration, high heat-generation density, high power dissipation, and large size. In particular, the invention relates to a cooling technique for removing, with low thermal resistance, the heat generated by a multi-chip module (MCM) in which a large number of LSI chips are mounted at high density.
As computers such as large mainframe and server computers, and super computers, gain higher computing speed and larger storage or memory capacity, LSI chips for use in them have become higher speed, highly integrated, larger and resulting in much greater heat-generation. To transmit signals at high speed among the LSI chips, the electric wiring among them should them as short as possible. Accordingly, such computers now often include MCM's each having a number of LSI chips mounted densely on a multi-layer circuit substrate. To avoid delay in signal transmission among the LSI chips, it is important for the MCM to be provided with suitable cooling to improve LSI chip operation by reducing the temperature and equalizing the distribution of temperature among the chips.
One conventional cooling device for removing heat generated by LSI chips mounted in an MCM includes a water-cooling jacket over the top of the MCM. Cooling water at a predetermined temperature flows in the jacket. For example, Japanese patent laid-open publication No. 08-279578 discloses a cooling device for an MCM. This cooling device includes a water cooling jacket having cooling channels formed in it. Cooling fins extend in parallel in the cooling channels. The cooling channels terminate in headers for reversing the direction of flow of the cooling water. The cooling water flowing in the jacket flows successively through the cooling channels, reversing in the direction of flow, to remove heat generated by the LSI chips in the MCM.
A common method for improving the cooling performance of a water cooling jacket involves deepening and/or narrowing the cooling channels in the jacket. For example, the deepened cooling channels make it possible to increase the heating surface areas, and to improve the cooling performance. If the same pump is used for cooling-water circulation, however, the cooling performance worsens because the total sectional area of the cooling channels increases, reducing the average flow velocity of the cooling fluid and the fin efficiency, as the channels deepen, although the decrease in pressure loss of the cooling fluid increases the total circulating flow rate of the cooling water. Accordingly, both of the foregoing actions or effects restrict the improvement of the cooling performance of a water cooling jacket.
If the total circulating flow rate is constant, the average flow velocity of the cooling fluid increases, thereby improving the cooling performance, but the pressure loss of the fluid increases quickly or suddenly, as the cooling channels narrow. If the same pump is used for cooling-water circulation, the increase in pressure loss reduces the total circulating flow rate, thereby restricting the improvement of the cooling performance of a water cooling jacket.
As the cooling channels narrow, the fluid flow through them transits or changes to laminar flow. This can be explained with a Reynolds number, a characteristic number representing the flow condition through cooling channels. The Reynolds number is found by multiplying the hydraulic equivalent diameter of cooling channels by the average flow velocity of the cooling fluid flowing through the channels, and dividing the resulting product by the kinematic viscosity of the fluid. The flow of cooling fluid through cooling channels is said to be turbulent if the Reynolds number is larger than a range between about 2,300 and about 3,000, and to be laminar if the Reynolds number is smaller than this range. As the cooling channels narrow, their hydraulic equivalent diameter decreases nearly in proportion to the channel width. On the other hand, the average flow velocity of the cooling fluid flowing through the cooling channels increases in inverse proportion to the channel width, but the total circulating flow rate decreases, and consequently the increase in the average flow velocity of the fluid is small. As a result, the Reynolds number decreases monotonically. Accordingly, as the cooling channels narrow, the flow of the cooling fluid changes to laminar flow. The foregoing description has been made on the assumption that the number of cooling channels is constant. If the cooling channels are narrower and the fins are thinner, however, the number of cooling channels is larger and accordingly the total sectional area of the channels is larger. This reduces the average flow velocity of the cooling fluid, making the Reynolds number even smaller. It is known that, in general, as the flow through cooling channels changes from turbulent flow to laminar flow, the Reynolds number decreases and the heat transfer performance of the channels decreases. Therefore, the improvement in cooling performance of a water cooling jacket is limited if its cooling channels are merely deepened and/or narrowed.
To remedy the foregoing disadvantages of methods for improving the cooling performance of general or common water cooling jackets, Japanese patent laid-open publication No. 7-70852 discloses prior art for improving the cooling performance of a water cooling jacket by locally reducing the sectional area of the cooling channel without changing the size of the entire channel.
FIG. 7
is a cross section showing the principle of the improvement in cooling performance of a conventional water cooling jacket.
With reference to
FIG. 7
, a cooling device includes a cooling plate
41
having a flow channel
44
. The cooling plate
41
is positioned in contact with an electronic part
42
at a place
43
. An inner wall of the flow channel
44
has a protrusion
46
formed over the place
43
. The protrusion
46
locally accelerates the flow of the cooling fluid through the flow channel
44
, improving the heat transfer performance of that portion of the channel wall which is contacted by the accelerated flow. Immediately in front of and behind the protrusion
46
, however, a separating phenomenon occurs in the fluid flow, decreasing the heat transfer performance. This phenomenon is described in Yoshiro Kato's “Dennetsugaku Tokuron” published by Yokendo on Oct. 5, 1984, page 211. Thus, the protrusion
46
and the electronic part
42
are in the same position on inner or outer both sides of an inner wall
45
of the cooling plate
41
. If the protrusion
46
is smaller than the area of the place
43
where the electronic part
42
is in contact with the wall
45
, the average cooling performance of the contact place
43
is such that the decrease in heat transfer performance in front of and behind the protrusion
46
and the increase in heat transfer performance just under the protrusion
46
cancel each other. This restricts the improvement in cooling performance of the cooling plate
41
. If the protrusion
46
is larger than the area of the contact place
43
, the region where the cooling fluid accelerates is longer, increasing the pressure loss of the fluid. Consequently, in either case, the improvement in cooling performance of the cooling plate
41
is limited.
Japanese patent laid-open publication No. 2-257664 discloses a device for cooling an MCM, which includes a large number of LSI chips mounted on a multi-layer circuit substrate. This cooling device includes a one-piece or integral water cooling jacket, which has cooling micro channels formed therein. The back sides of the LSI chips are soldered to the back side of the water cooling jacket. The cooling water flowing in the jack
Daikoku Takahiro
Ichikawa Junri
Kasai Kenichi
Nishihara Atsuo
Chervinsky Boris L.
Hitachi , Ltd.
Picard Leo P.
Townsend and Townsend / and Crew LLP
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