Patent Application
20070285892
Referring now to the drawings in which like reference numbers represent corresponding parts throughout:
1. Overview
As previously mentioned, embodiments of the invention are directed to an encapsulated inert non-conductive fluid used to transfer heat directly from an electrical circuit to an external heat-sink. The top of a chip die is enclosed with a metallic cover. The cover is sealed to an outer frame, which in turn is sealed to metallization on the top of the chip through a flexure, minimizing mechanical load into the chip. This forms a hermetic cavity in which the circuit of the chip resides. This cavity is filled with an inert non conductive fluid, which vaporizes when heated. Condensation occurs on the metal cover where the heat is transported into the outer frame for rejection into the spacecraft. Embodiments of the invention provide more efficient cooling than conventional heat-sinks. In addition, embodiments of the invention may be readily implemented with known chip types and retrofitted to existing circuits.
Embodiments of the invention generally require a smaller footprint on a printed wiring board than conventional circuit cooling solutions. This allows for more electrical connections and thus an increase in the overall efficiency of a given circuit layout on a printed wiring board. In addition, embodiments of the invention may be used to increase the current ASIC dissipation capability from approximately 10 W per ASIC to up to 50 W per ASIC. This reduces the need for high performance heat sinks at the printed wiring board level.
Collected data indicates approximately 0.59° C./W cooling achievable with approximately 40 watts applied to an example test chip. With the example test chip employing an embodiment using a direct impingement spray at approximately 50° C., temperature sensors of the test chip reported approximately 74° C. With minor refinements and optimization, the performance it may be possible to obtain double this performance, i.e., approximately 0.2 to 0.3° C./W. Accordingly, embodiments of the invention may enable approximately 2.5 to 3 times the DSP processing capability (as measured in thermal dissipation). Any circuit manufacturer including application specific integrated circuit (ASIC) developers and/or heat-sink manufacturer may use embodiments of the invention for implementation in space based and/or terrestrial electronic circuit cooling.
2. Encapsulated Multi-Phase Fluid for Cooling Electronics
Referring back to
Embodiments of the invention may also utilize an outer frame 120 fixed to the PWB 122 surrounding the electronic circuit 104 and supporting the metallic cover 102. The surrounding outer frame 120 further supports versatility in adapting and/or retrofitting with the invention. In this case, the metallic cover 102 comprises a flexure 118 that is sealed to metallization on top of the electronic circuit and sealed to the outer frame 120. Thus, this flexure 118 can be used to complete the hermetic cavity 106 directly onto the circuit 104. The flexure 118 is thin in order to reduce any mechanical load that might be imparted to the electronic circuit. This minimizes additional fatigue which would otherwise be applied to the electrical interconnects. In addition, the flexure 118 is thin enough to choke off any significant heat transfer that would otherwise find a return path to the circuit 104. Heat is removed from the electronic circuit 104 being transferred in sequence from the non-conductive fluid 108 to the metallic cover 102 to the outer frame 120.
The heat from the electronic circuit 104 transferred to the metallic cover 102 with the non-conductive fluid 108 may be dissipated through one or more techniques depending upon the particular application. For example, in space applications operating in a vacuum, heat may be conducted away from the operating circuit through the PWB. In addition, some heat may be dissipated radiatively from the outer surface of the metallic cover 102. In other applications operating in air, the heat may be dissipated through convention (forced convection and/or free convention) from the outer surface of the metallic cover 102. In either case, some surface heat dissipation 124 (convective and/or radiative) may be achieved directly from the outer surface of the metallic cover 102.
The non-conductive fluid 108 employed in any of the embodiments of the invention may be one of newer type a dielectric fluid that may be used for electronic circuit cooling. For example, perfluorocarbons and segregated hydrofluoroethers are examples of such fluids. The 3M Corporation has developed a family of fully-fluorinated compounds known as perfluorocarbons (PFCs) identified by the trade name Fluorinert. These fluids are dielectric and may be selected with different boiling points depending upon the particular thermal design. In addition, 3M has also developed a family of segregated hydrofluoroethers identified by the trade name Novec. The segregated hydrofluoroethers are also dielectric and may be selected with different boiling points depending upon the particular thermal design. In addition, segregated hydrofluoroethers have low global warming potential, whereas perfluorocarbons have been designated as having a high global warming potential. In one example, the non-conductive fluid 108 may be implemented in an embodiment of the invention as the segregated hydrofluoroethers designated HFE-7100 from the 3M Corporation.
As illustrated by the graph 300, heat pipes appear to be a worse performing design alternative as shown by the Thermacore heat pipes plot 304. Increasing the power to the test chip with the Thermacore heat pipe resulted in diminished thermal performance—approximately 2.5° C./W. Each of the three tested power levels showed increasing thermal resistance with increasing power using the Thermacore heat pipe. Similarly, the Noren heat pipe showed inferior performance compared with an embodiment of the invention as indicated by the Noren heat pipe plot 308. However, the Noren heat pipe exhibited a trend opposite that of the Thermacore heat pipe with increasing power. Increasing the power to the test chip resulted in increased thermal performance using the Noren heat pipe, approximately 1.8° C./W. Here also, this trend was confirmed across the three tested power levels. The TC1050 strap exhibited a plot 306 that appeared independent of heat flux and showed substantially consistent thermal resistance at all power levels comparable to Noren heat pipe at approximately 1.8° C./W.
In addition, a variation of liquid cooling was also compared, direct impingement. The direct spray impingement plot 310 shown in the graph 300 reveals improving thermal resistance at higher power levels, as well as increased performance over the other tested methods. However, when applied to embodiments of the invention, nucleate boiling (i.e. a stagnate fluid with no spray) is expected to have similar or better than some heat-pipe configurations (including those tested). Embodiments of the invention place the electronics within the cooling fluid (in a manner analogous to placement inside a heat-pipe), reducing the high resistances associated with external die to heat pipe attachments.
3. Method of Cooling with an Encapsulated Multi-Phase Fluid
Embodiments of the invention also encompass a method of cooling an electronic circuit consistent with the foregoing apparatus. The initial object is to form a hermetic cavity over the electronic circuit to be cooled and partially fill it with an appropriate non-conductive fluid. However, the use of the outer frame and flexure significantly improve the technique.
In optional operation 408, the metallic cover is supported by an outer frame fixed to a printed wiring board (PWB) surrounding the electronic circuit. In optional operation 410, a flexure of the metallic cover is sealed to metallization on top of the electronic circuit and sealed to the outer frame. The flexure reduces mechanical load imparted to the electronic circuit and heat is removed from the electronic circuit being transferred in sequence from the non-conductive fluid to the metallic cover to the outer frame.
Note that the order of operations of forming and filling the hermetic cavity may be altered as necessary to achieve the sealed hermetic cavity with the non-conductive fluid partially filing the cavity. In one example the cavity may be sealed to the electronic circuit with the fluid already partially filling the cavity. In another example, the heretic cavity may be sealed over the electronic circuit and then the fluid is filled through a temporary opening (e.g. in the top surface of the metallic cover) which is then resealed. Those skilled in the art will recognize manufacturing alternatives in order to achieve the desired goal of a partially filled hermetic cavity sealed over the electronic circuit.
This concludes the description including the preferred embodiments of the present invention. The foregoing description including the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible within the scope of the foregoing teachings. Additional variations of the present invention may be devised without departing from the inventive concept as set forth in the following claims.
