Heat exchange – Heat transmitter
Reexamination Certificate
2000-03-08
2001-05-01
Yeung, James C. (Department: 3763)
Heat exchange
Heat transmitter
C165S080300, C361S697000, C361S703000, C361S708000
Reexamination Certificate
active
06223815
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-074897, filed Mar. 19, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a cooling unit for facilitating the radiation of heat from a heat-generating component, e.g., a semiconductor package, and also relates to an electronic apparatus incorporating the cooling unit.
Electronic apparatuses, e.g., portable computers and workstations, have a CPU each. The CPU is designed to process multimedia information, such as characters, speech, sound, and images. The data-processing speed and the number of functions of the CPU have continuously increased. The higher the speed the CPU processes data and the more functions it performs, the more electric power it consumes. The amount of heat the MPU generates while operating increases in proportion to the power it consumes.
In order to guarantee a stable operation of the CPU, a heat-radiating, cooling module such as a heat sink is indispensable. This is because the cooling module can cool the CPU with high efficiency.
Conventional heat sinks have a heat-receiving section and a heat radiating section each. The heat-receiving section receives the heat generated by a CPU. The heat-radiating section radiates the heat transmitted to the heat-receiving section. Such a heat sink is secured by screws to the housing of an electronic apparatus or to the circuit board provided in the housing and having a CPU mounted thereon. Thus, the heat-receiving section of the heat sink is thermally connected to the CPU.
A semiconductor package for use as a CPU in portable computers is a BGA (Ball Grid Array) package in most cases. The BGA package has a wiring substrate made of synthetic resin and an IC chip mounted on the wring substrate and connected thereto by flip-chip method. When the BGA package is mounted on a circuit board, its height may vary by 0.25 mm at most. Since the heat sink is an injection molding made of aluminum, i.e., a metal excelling in thermal conductivity, its parts, including the heat-receiving section, need to have dimensional tolerances.
In view of this, a heat-conducting sheet is interposed between the IC chip and the heat-receiving section of the heat sink. The sheet is made of rubber that has high thermal conductivity. The heat-conducting sheet is clamped between the IC chip and the heat-receiving section and elastically deformed when the heat sink is secured to the circuit board or housing of the electronic apparatus. Thanks to the deformation of the sheet, the change in the height of the BGA package and the change in size of the heat sink are compensated for. As a result, the heat-receiving section and the IC chip can remain in a stable thermal connection.
This conventional thermal connection between the heat sink and the BGA package is, however, disadvantageous. When the heat sink is secured to the circuit board or housing of the electronic apparatus, its heat-receiving section is pressed directly onto the heat-conducting sheet and, hence, indirectly onto the IC chip of the BGA package. In other words, the force the screws apply, fastening the heat sink to the circuit board or housing of the electronic apparatus, act as a direct stress on the BGA package. If the BGA package is strong enough to overcome this stress, no problems will arise at all. In fact, the BGA package can hardly be said to withstand the stress since the IC chip is exposed outside the package and the wiring substrate supporting the IC chip is made of soft synthetic resin.
Thus, the stress concentrates on the IC chip once after the heat-receiving section of the heat sink is thermally connected to the IC chip of the BGA package. The IC chip may therefore be broken. Moreover, a load is imposed, pushing the IC chip to the wiring substrate. This load acts a bending stress on the wiring substrate, deflecting or warping the wiring substrate. Consequently, a stress keeps acting on the junction between the IC chip and the wiring substrate. This may results in an inadequate electrical connection between the IC chip and the wiring substrate.
Hence, the load that can be applied to the heat sink to thermally connect the heat-receiving section to a semiconductor package such as a BGA package is limited. That is, the heat-receiving section of the heat sink cannot be pressed with a large force onto the semiconductor package. High thermal resistance is likely to develop at the junction between the heat sink and the semiconductor package. Heat cannot be efficiently transmitted from the semiconductor package to the heat sink.
BRIEF SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above. The object of the invention is to provide a cooling unit that can efficiently cool a heat-generating component by effectively transmitting heat from the component to two heat sinks, without applying a large stress on the heat-generating component.
To attain the first object, a cooling unit according to the first aspect of the invention comprises: a first heat sink overlapping a heat-generating component and thermally connected thereto, the first heat sink having a plurality of heat-conducting sections; and a second heat sink covering the first heat sink, having a plurality of heat-receiving sections with which the heat-conducting sections are set in engagement. The second heat sink is so positioned that a first gap is provided between the first heat sink and the second heat sink and a second gap is provided between each heat-receiving section and one heat-conducting section and communicating with the first gap. The first gap and the second gap are filled with a heat-conducting medium. The heat-conducting medium is semi-solid material having viscosity and thermally connecting the first heat sink and the second heat sink.
To achieve the object described above, an electronic apparatus according to the invention comprises: a housing; a circuit board provided in the housing; a heat-generating component mounted on the circuit board; a first heat sink overlapping the heat-generating component and thermally connected thereto and having a plurality of heat-conducting sections; and a second heat sink covering the first heat sink, having a plurality of heat-receiving sections with which the heat-conducting sections are set in engagement. The second heat sink being so positioned that a first gap is provided between the first heat sink and the second heat sink and a second gap is provided between each heat-receiving section and one heat-conducting section and communicating with the first gap. A heat-conducting medium is filled in the first gap and the second gap. The heat-conducting medium is semi-solid material having viscosity and thermally connecting the first heat sink and the second heat sink.
Since the heat-conducting sections of the first heat sink are engaged with the heat-receiving sections of the second heat sink, respectively, a great heat-conducting area is provided at the junction between the first heat sink and the second heat sink. In addition, since the gap between the first and second heat sinks is filled with the heat-conducting medium, the thermal resistance at the junction between the first and second heat sinks is low. As a result, the heat can be conducted from the first heat sink to the second heat sink with high efficiency. This enables the heat-generating component to radiate heat more efficiently.
Since the first and second heat sinks are spaced with the first and second gaps between them, not contacting each other, the first and second gaps compensate for the changes in size of the heat sinks and the heat-generating component. Moreover, even if the first and second gaps change in size and shape due to the change in size of the heat sinks, the heat-conducting medium filled in the gaps flows, keeping the first and second heat sinks spaced from each other. This is because the medium is a soft, semi-solid material and free
Duong Tho
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Yeung James C.
LandOfFree
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