Liquid DIMM cooling device

FIELD OF THE INVENTION

The invention relates generally to cooling of electronic

components, and specifically to a liquid cooling device for electronic circuit boards.

BACKGROUND OF THE INVENTION

Microelectronic components such as memory modules contained in computers and electronic devices generate substantial amounts of heat during operation which must be removed in order to ensure reliable performance and long lifetime of the components. In the past, this has been achieved by using air cooling, for example with fans forcing an air flow through the electronic device. However, these air cooling arrangements were found to consume a considerable amount of space and/or fail to adequately cool the microelectronic components. In particular, usage of densely packed powerful microelectronic components such as dual in-line memory modules (DIMMs) requires a powerful cooling system.

A DIMM is a narrow elongated electronic circuit board which holds memory modules on both of its sides. Due to the high density and close vicinity of powerful memory modules on the circuit board, DIMMs need reliable and highly effective cooling which is difficult to implement using air flow cooling.

As a consequence, various alternatives to air flow cooling have been explored: US 7,369,511 B2 describes a cooling assembly for microelectric components which makes use of a low melting alloy composition having exhibiting a phase change at typical operating temperatures of the microelectronic device. This low melting alloy composition is used as thermal interface material between the microelectronic device to be cooled and an adjacent heat sink and effectively bridges gaps between the

microelectronic device and the heat sink. While this kind of thermal interface may be very effective, the low melting alloy is susceptible to oxidation and needs to be sealed which makes an exchange or replacement of the microelectronic device very cumbersome. A considerable amount of mechanical work is required to dismount the cooler, exchange the electronic component and remount the cooler.

In order to increase cooling rates, liquid cooling systems have been developed especially for DIMMs . US 2006/0250772 Al

discloses a liquid cooling system to be used for cooling a plurality of DIMMs arranged parallel to each other on a

motherboard. The system contains a water cooled heat sink base which is situated on the top edges of the DIMMs' circuit boards; cooling fins extending from the heat sink base along the sides of the DIMMs are equipped with thermal interface layers for ensuring good thermal contact between the memory modules of the DIMMs and the fins. The thermal interface layers are made up of a soft material so as to be able to slip the fins between the DIMMs already in place on the motherboard and to provide enough contact pressure to allow for adequate thermal transfer. While this cooling system is easily dismountable and thus allows a quick exchange of DIMMs, its water cooled heat sink base

arranged on top of the DIMMs requires a considerable amount of space. As the cooling system adds to the height of the DIMM arrangement, it cannot be used in Server Blades or thin rack mounted servers where the free space on top of the DIMMs is very limited . US 2008/0251911 Al shows a very compact liquid/vapor cooling assembly for a DIMM with a plurality of memory modules. In this assembly, heat spreaders are disposed along the plurality of the memory modules, and a heat pipe and/or a vapor chamber extends along the heat spreader. The various components of the cooling assembly are held in place by clips. While this cooling assembly consumes very little space, it is quite intricate since it requires a very precise dimensioning of the heat pipe in order to ensure good thermal contact. Moreover, an exchange of a DIMM is quite complicated and involved since it requires dismantling and reassembling the pipe assembly arranged around the DIMM.

Thus, there is a need of a liquid DIMM cooling device which takes up minimal space and permits easy and time-efficient exchange of the DIMM module while providing effective cooling of the DIMM.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a liquid cooling device for a DIMM that requires little space and permits easy mounting and dismounting of the DIMM. Moreover, the cooling device should be cost-effective to produce and to operate.

These objectives are achieved by the features of the independent claim. The other claims and the specification disclose

advantageous embodiments of the invention.

According to the invention, a liquid cooling device for cooling a dual in-line memory module (DIMM) comprising a plurality of memory modules disposed on a circuit board with an edge

connector is provided. The liquid cooling device comprises a heat spreader disposed along the plurality of memory modules, a cold rail block extending along the heat spreader and a thermal adapter interleaved between the cold rail block and the memory modules of the DIMM. The thermal adapter is compressible in a direction perpendicular to the plane of the DIMM' s circuit board so that its thickness can be altered reversibly by exerting pressure on it.

The compressibility of the thermal adapter interleaved between the cold rail and the DIMM allows the components of the cooling device to be moved and adjusted relative to each other in a direction perpendicular to the plane of the DIMM' s circuit boards. The compressible adapter thus acts like a spring which holds the components in place and assures good mechanical and thermal contact. The compressible adapter thus compensates dimensional differences and inaccuracies of the various

components of the liquid cooling device; by manually compressing the adapter, the DIMM may be removed from the cooling device. In a preferred embodiment, selective components within the cooling device possess matching smooth surfaces which permit a low- friction gliding of these components relative to each other. This allows the DIMM to be easily removed from the cooling device and to be replaced without effort and without tooling.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above-mentioned and other objects and advantages may best be understood from the following detailed description of the embodiments, but not restricted to the embodiments, wherein is shown in:

FIG. 1 a schematic view of a computer system with a

motherboard containing heat generating components and a cooling system;

FIG. 2a a perspective side view of a liquid cooling device according to a first embodiment; FIG. 2b a sectional view of the liquid cooling device of

FIG. 2a along plane Ilb-IIb in FIG. 2a;

FIG. 2c a perspective side view of a cold rail block

contained in the liquid cooling device of FIG. 2a;

FIG. 3a a perspective side view of a liquid cooling device according to an alternate embodiment;

FIG. 3b a DIMM with a sheathing assembly forming part of the liquid cooling device of FIG. 3a.

In the drawings, like elements are referred to with equal reference numerals. The drawings are merely schematic

representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 shows a schematic view of an embodiment of a computer system 100 comprising various electronic components and modules that generate heat during operation. Computer system 100

generally comprises a power supply 110, a motherboard 120 and a plurality of storage media 130 such as magnetic and/or optical disk drive 130. The motherboard 120 contains a variety of electronic components such as a central processing unit (CPU) 122, input/output (I/O) interfaces 124 for accessing external devices/resources 140 such as keyboard, mouse, voice recognition system, printer, monitor, facsimile etc. as well as dual in-line memory modules (DIMMs) 2 mounted into sockets 126 in motherboard 120. The standard arrangement of these DIMM sockets 126 results in the DIMM modules to be aligned parallel to each other and perpendicular to the plane of the motherboard 120. FIG. 1 shows three DIMMs 2 mounted on the motherboard 120 as well as one empty slot socket. Computer system 100 is equipped with a liquid cooling system 150 for cooling electronic components of the computer system 100. Liquid cooling system 150 comprises liquid cooling devices 1, 1' designed in such a way that they

accomplish effective cooling of the DIMMs 2 while enabling fast and easy exchange of the DIMMs. Two preferred embodiments of such liquid cooling devices 1, 1' will be explained in detail in conjunction with FIGs 2a - 2c and FIGs 3a and 3b.

FIG. 2a shows a perspective side view and FIG. 2b a schematic sectional view of a liquid cooling device 1 according to a first embodiment of the invention. The cooling device 1 is designed for cooling two DIMMs 2 mounted in adjacent parallel sockets 126 on a motherboard 120 of an electronic device such as a

workstation. Each DIMM 2 comprises a printed circuit board (PCB) 3 holding a plurality of memory modules 4 on each of its sides and an edge connector 30 with a plurality of contact pads 31 which make electrical contact with a multitude of connectors as the DIMM 2 is inserted into socket 126 on motherboard 120.

Each DIMM 2 is sheathed in a heat spreader 5 which is bent into a U-shape and slid over the top of the DIMM 2 in such a way that the inside surfaces of 6 the heat spreader 5 lie flat on the surfaces of the memory modules 4. The heat spreaders 5 may be manufactured from a sheet of a metal with a good thermal

conductance, for example a copper or aluminum sheet of 1 mm thickness. A thermal grease or an elastic thermal pad is applied to the inner surfaces 6 of the heat spreaders 5 in order to ensure good thermal contact between the surfaces of the memory modules 4 and the heat spreader surfaces 6 facing them.

The cooling device 1 comprises a cold rail block 7 located in the gap between the two parallel DIMMs 2, as well as thermal adapters 8 bridging the gaps between the cold rail block 7 and the heat spreaders 5 mounted on the DIMMs 2. The cold rail block 7 comprises two cold rail sleeves 9 sandwiching a tube 10 through which a cooling fluid, e.g. water, oil or a (compressed) gas, is conducted. A detailed view of the cold rail block 4 is shown in FIG. 2c. The cold rail sleeves 9 are clamped to the tube 10 (which is not shown in FIG. 2c) using bracket assemblies 11 located on the far ends of the cold rail sleeves 9. In the embodiment of FIG. 2c, the bracket assemblies 11 comprise bracket bodies 12 which are attached to the cold rail block 7 using hexagonal socket screws 13 as well as bracket top sections in the shape of clamp nuts 14. The cold rail sleeves 9 are pressed onto the tube 10 by fastening set screws 15 in these clamp nuts 14. Thermal grease is spread on the interface 16 between the tube 10 and the cold rail sleeves 9, thus assuring good thermal contact.

Interleaved between the cold rail block 7 and the heat spreaders 5 are thermal adapters 8. The thermal adapters 8 are elastic (reversibly compressible) in the sense that its thickness will decrease (locally) as (local) pressure is exerted on the thermal adapter 18, but will return to its original shape as the

pressure subsides.

Retention brackets 20 span the heat spreaders 5, thermal

adapters 8 and the cold rail block 7 and hold them in position by clamping and compressing them in a direction perpendicular to the plane of the DIMMs 2, thus ensuring good mechanical and thermal contact between the components 5, 7 and 8 of the cooling device 1. The retention brackets 20 are elastic and act as compression springs causing the compressible thermal adapters to be pressed onto the sides of the cold rail block 7, while the heat spreaders 5 covering the DIMMs 2 are pressed onto the sides of the thermal adapters 8.

The thermal adapters 8 have a high thermal conductivity and are compressible in a direction vertical to the plane of the DIMMs' circuit boards 3. Due to this compressibility, the thermal adapters 8 can be squeezed to an adequate thickness by the retention brackets 20 which exert a compression, thus adjusting for positional inaccuracies of the DIMMs 2 as well as the remaining components 5, 7, 8 of the cooling device 1.

In the embodiment of FIGs. 2a and 2b, each thermal adapter 8 is made up of a compressible thermal pad 18 mounted to a rigid shim 17. The thermal pad 18 possesses an inherent compressibility and flexibility which enables it to recede and to compensate for dimensional inaccuracies of the thermal adapter 8 as well as of the DIMMs 2 and/or their heat spreaders 5. The thermal pad's 18 surfaces are sticky, so that the thermal pad 18 is attached to the cold rail block 7 and holds the shim 17 in position relative to the cold rail block 7. In a preferred embodiment, the thermal pad 18 may be made up of Laird Technologies' Tflex™ material, for example Tflex 300, Tflex 600 or Tflex 700. In an alternate preferred embodiment, Bergquist's Gap Pad 3000S30 material may be used; this is a soft gap filling material with a thermal conductivity of 3 W/m-k.

The thickness of the shim 17 depends on the spacing of the sockets 126 of the motherboard 120 and may vary from about 0.1 millimeter to several millimeters. The shim 17 may be made up of any thermally conducting material, such as copper or aluminum. If the shim 17 is very thin, it may also be made out of steel. The shim 17 is supported and held in place by catches 21 on the bottom of the cold rail block brackets bodies 12 (see FIG. 2c) . The shim surface 19 facing the heat spreader 5 is smooth, which allows the heat spreader 5 to be displaced relative to the shim 19 without exerting much effort. In order to reduce friction between the shim surface 19 and the heat spreader 5, a coating of high viscosity silicone oil may be deposited on the shim surfaces 19, thus ensuring good thermal contact as well as facilitating sliding the heat spreader 5 relative to the thermal adapter 8 during mounting and dismounting of the DIMM 2.

The compressibility and flexibility of the thermal pads 18 permits an easy and time-effective exchange of a DIMM 2 that does not require any tooling: When one (or both) of the DIMMs 2 is to be removed, the retention brackets 20 are lifted off the cooling assembly 1, and the DIMM 2 - together with the heat spreader 5 surrounding it - is taken out of its socket 126 on the motherboard 120 while cold rail block 7 and thermal adapters 8 remain in place on the motherboard 120. As the DIMM 2 and the heat spreader 5 are removed from the motherboard 120, the heat spreader 5 glides along the surface 19 of the shim 17, and the shim 17 - being attached to the thermal pad 18 which sticks to the cold rail block 7 - stays in position. Subsequently, a new DIMM 2 - sheathed in a heat spreader 5 - is slid along the outside surface 19 of the shim 17 and plugged into the socket 126 of the motherboard 120. Before introducing the new DIMM 2, a thin film of high viscosity oil and reasonable thermal

conductance is spread on the surface 19 of the shim 17 facing the new DIMM 2, thus ensuring low friction and good thermal conductance. In order to facilitate mounting of the new DIMM 2, the heat spreader 5 exhibits a small wedge on its bottom corner which presses the thermal adapter 8 against the cold rail block 7 during the plugging operation. Once the new DIMM 2 has been plugged into its socket 126 and the retention bracket 20 has been put into place, the thermal path between the memory modules 4 on the DIMM 2 and the cold rail block 7 is closed.

Note that since the cold rail block 7 is located in the gap between adjacent DIMMs 2 and since the heat spreaders 5

sheathing the DIMMs 2 add very little height (only about 1 mm) , the cooling device 1 of FIGs. 2a - 2c uses up very little space above the DIMMs 2. Thus, the cooling device 1 is well-suited for usage in Server Blades or thin rack mounted servers where the free space on top of the DIMMs 2 is very limited.

Note also that while the cooling device 1 of FIGs. 2a - 2c is shown to span two DIMMs 2, the cooling device could easily be extended to cooling three or more DIMMs 2. In this case, a cool rail block 7 is interleaved between each pair of adjacent DIMMs 2, and the retention bracket 20 be dimensioned in such a way that it spans the whole set of DIMMs 2 to be cooled.

FIGs. 3a and 3b show an alternate embodiment of the invention. Whereas the cooling device 1 depicted in FIGs. 2a - 2c is designed to provide cooling to two (or more) DIMMs 2, the cooling device 1' of FIGs. 3a and 3b is designed for cooling only one DIMM 2. As is shown in FIG. 3a, the cooling device 1' comprises two parallel cold rail blocks 7' which are positioned on the motherboard 120 in such a way that the DIMM 2 to be cooled can be introduced between them and is sandwiched between them. The cold rail blocks 1' correspond to the cold rail blocks 7 shown in FIG. 2c; they comprise tubes 10 in which a cooling fluid such as water, water vapour, oil etc. is conducted.

Cooling device 1' comprises two flat heat spreaders 5' attached to each side of the DIMM 2 that is to be cooled. Compressible thermal adapters 8' in the shape of compressible thermal pads 18' are interleaved between the DIMM 2 and the heat spreaders 5' on both sides of the DIMM 2. The thermal pads 18' are sticky and keep the heat spreaders 5' in position relative to the DIMM 2. In order to secure the heat spreaders 5' in place, fastening pins 22 pressed into the far ends of the DIMM' s 2 circuit board 3 are inserted into matching holes 22' in the heat spreaders 5', thus holding the heat spreaders 5' aligned with the sides of the circuit board 3 while allowing movements of the heat spreaders 5' in a direction perpendicular to the plane of the circuit board 3. The cold rail blocks 7' are rigidly mounted to the motherboard 120, forming a gap 23 between them into which a sheathed DIMM assembly 24 comprising DIMM 2, heat spreaders 5' and thermal pads 18' (see FIG. 3b) is to be inserted. The thickness of the thermal pads 18' is dimensioned in such a way that the thermal pads 18' are slightly compressed when this sheathed DIMM

assembly 24 is located in the gap 23 between the cold rail blocks 1' . The cold rail blocks 1' exert pressure on the

outsides of the heat spreaders 5' which in turn compress the thermal pads 18' and press them onto the DIMM modules 4. This ensures a good mechanical and thermal contact of the interfaces between DIMM modules 4, thermal pads 18', heat spreaders 5' and cold rail blocks 1' . Depending on the way in which the cold rail blocks 1' are mounted on the motherboard 120, elastic retention clamps similar to the ones shown in FIG. 2a may be used in order to ensure that the cold rail blocks 1' exert a predetermined pressure on the outsides of the heat spreaders 5' .

If the DIMM 2 needs to be exchanged, the sheathed DIMM assembly 24 comprising DIMM 2, thermal pads 18' and heat spreaders 5' is pulled out of the gap 23 between the cold rail blocks 1' . The pins 22 on the DIMM 2 engaging into holes 22' of the heat spreaders 5' ensure that this assembly 24 can be withdrawn as a whole. In order to effectuate easy withdrawal of sheathed DIMM assembly 24, a film of silicone oil of high viscosity is placed between the cold rail blocks 1' and the heat spreaders 5' . It acts as thermal contact and supports the gliding motion during the insertion or removal of the DIMM 2. In order to put a new DIMM 2 into place on the motherboard 120, the DIMM 2 is equipped with thermal pads 18' and heat spreaders 5', thus forming the sheathed DIMM assembly 24 shown in FIG. 3b. By manually exerting a pressure on the outside faces of the heat spreaders 5' , the thermal pads 18' are compressed thus decreasing the thickness 25 of the sheathed DIMM assembly 24 until it can be inserted into the gap 23 between the cold rail blocks 1' and the edge connector 30 of DIMM 2 can be introduced into its corresponding socket 126 in the motherboard 120.

Note that the cooling system 1' of FIGs. 3a and 3b does not extend beyond the height of the DIMM 2, so that it takes up no extra space on top of the DIMM 2. Thus, it is especially well- suited for use in Server Blades or thin rack mounted servers where the free space on top of the DIMMs is very limited.