DE20016316U1 - Heatsinks for cooling electronic components in particular - Google Patents

Description

Heat sink and method for its manufacture

The invention relates to a heat sink for cooling electronic components in particular. The heat sink consists of several elements which are soldered together using tin, similar alloys or tin/powder mixtures.

Description 10

Heat sinks for cooling electronic components are available in various designs. The most common design is extruded aluminum profiles. These are easy and inexpensive to manufacture, but they have a poor cooling surface to volume ratio (CSR). Depending on the design, this is between approximately 0.5 and 9 cm ² /cm ³ . In addition, such extruded profiles are rigid and can only be adapted to the length of the element to be cooled.

Heat sinks in which a large number of metal sheets or profiles are attached to a base plate with grooves or tenons have a better cooling surface to construction volume ratio (CSR). The attachment can be done by pressing, for example. Such fastening types are described, for example, in the invention descriptions 22.01.1975 - Hangs - DE-OS 25 02 472 // 31.03.1998 - Masatz - DE-OS 198 14 368 // 04.05.1995 - Alusuisse - DE-GM 295 07 286 // 04.05.1993 - Bock - DE-OS 43 14 663 // 22.05.85 - Alu Singen - DE-OS 35 18 310 // 30.06.1998 - Jacoby - US 5,771,966 // 14.12.1999 Gönner - US 6,000,462 // 04.01.2000 - Gönner - US 6,009,937 // 10.01.97 - Schell - DE-OS 197 00 432 // 31.08.1996 - Dehmel - DE-OS 196 35 468 // 06.08.1996 - Serizawa - US 5,542,176 // 14.05.1991 - Hess - US 5,014,776 and 04.10.1991 - Bock - EP 0 483 058. The heat sinks that can be produced in this way have a KBV of up to 12 cm ² /cm ^3. The manufacturing effort is correspondingly high for the various work steps.

A further proposal is contained in the documents 02.06.1987 - Seidler US 4,669,535 // 06.11.1992 - Hoogovens - DE-GM 92 15 145 //

12.03.1997 - Puska - DE-OS 197 10 225 // 02.06.1998 - Chrysler US 5,758,418 and 20.06.2000 - Kuo - US 6,076,594. According to this, individual rib profiles in the base are connected to one another by screwing or soldering, for example. This leads to considerable heat transfer resistances between the individual ribs. In practice, heat sinks with a KBV of up to 15 cm ² ZCm ³ can be produced using this process. However, the size of the heat sink is very limited, so that the heat transport to the individual ribs in the base plate is severely restricted by the above resistances. In addition, the base area is usually very thick, which means a corresponding increase in weight and size. The assembly effort is correspondingly high.

In 08.08.2000 - Lee - US 6,098,279 the suggestion is made to add additional fins to various standard heat sinks (as described previously). This increases the cooling surface at low cost. However, this method is limited by the tool parameters for punching the mounting holes. The maximum KBV will be around 8 to 10 cm ² /cm ^3. In addition, the cooling fins used have corresponding contact resistances.

In 05.08.1997 - Porter - US 5,653,280 the proposal is made to manufacture the base plate from an elastic material which is filled with heat-conducting powders. In order to give the cooling fins the required stability in the elastic base plate, they are designed in a T-shape which engages in corresponding T-grooves in the base plate. Due to the T-shape, this results in a maximum KBV of approx. 6 to 8 cm ² /cm ³ . In addition, the horizontal heat flow from one fin to the next is very limited, since the elastic material between the fins represents a not insignificant thermal resistance.

In the documents 18.07.2000 - Lee - US 6,088,917 // 12.10.1999 Huang - US 5,964,285 // 05.10.1999 - Chen - US 5,960,871 // 10.02.1996 - Alusuisse - DE-GM 296 02 366 // 13.08.1991 - Jordan US 5,038,858 and 27.04.1982 - Reed - US 4,326,383

Heat sinks with clamp and adhesive connections are proposed. These processes can be used to produce heat sinks with a KBV of up to approx. cm ² /cm ^3. However, they have the disadvantage of high manual production costs and, above all, they have very high contact resistances because the connections are neither materially bonded nor subject to high pressure. These heat sinks are not suitable for high-performance cooling applications in particular.

From the documents 04/25/1989 - Arnold - US 4,823,869 // 12/05/1989 - Hinshaw - US 4,884,331 // 05/28/1996 - Kojima - US 5,519,938 and 08/17/1999 - Steiner - US 5,937,518, heat sinks are known which are milled or sawn from a solid block. These are very good from a thermal point of view, but due to the dimensions of the tools used and the mechanically necessary rib thicknesses, a maximum KBV of 8 cm ² /cm ³ is possible with this process. Due to the high processing performance, production using this process is also very expensive.

Another known method is soldering individual fins onto a base plate (Fig. 1). This enables a KBV of approx. 20 cm ² /cm ^3. However, the disadvantage is that the connection between the base plate and the individual fins is made using a suitable solder (e.g. solder foil). These solders generally have a thermal conductivity of only approx. 16.5% of the thermal conductivity of copper. This results in a corresponding contact resistance. In addition, a lot of processing is required on the connecting edge of the fins. If this does not lie exactly flat (e.g. due to chips, burrs, etc.), the soldered connection is not perfect and the contact resistance would be increased further. The mechanical strength of the soldered connection, especially against bending of the fins, is also not very high.

In the writings June 2, 1997 - Belady - DE-OS 197 23 085 // February 19, 1998 - Damsohn - DE-OS 198 06 978 // June 2, 1981 - Nakamura US 4,270,604 // January 6, 1998 - Yeh - US 5,706,169 // 09/24/1996 Ito - US 5,558,155 and 12/17/1996 - Wright - US 5,584,183

some of the previously mentioned problems when soldering heat sinks are eliminated. Soldering folded sheets is proposed. The resulting larger surface area soldered to the base plate largely eliminates the problems of mechanical load-bearing capacity and contact resistance. However, these heat sinks can only be flowed through with air in one direction (horizontally), since the folded structure would block 50% of the fin surface if the ventilation were vertical. In 27.02.1996 - Morosas - US 5,494,098 and 03.04.1997 Wyler - WO 98/44554 it is proposed to solve this problem by making cutouts on the top of the folded fins. However, this solution also results in a maximum KBV of only approx. 10 cm ² /cm ³ due to the tool parameters of the folding tools.

According to the document 13.01.1988 - Prokopp - EP 0 278 240, a heat sink is constructed from metal sheets. These are connected to one another in the base by soldering and then attached as a block to a base plate by soldering. With this method, a KBV of approx. 20 cm ² /cm ³ is possible. However, this construction has the disadvantage that the material expenditure is correspondingly high due to the soldering foils and intermediate sheets used and left over. Using copper, for example, as the material for the sheets and base body results in a corresponding additional weight. Since the entire area of the intermediate sheets is not surrounded by a cooling medium, the construction height must also be increased by this area of the base.

In the document dated 14.12.1998 - Baxmann - DE-GM 298 22 241 it is proposed to use ribs on a base plate using powder as a binding agent. All components should be made of the same material, e.g. copper. This results in a material-tight connection without contact resistance. Due to the parameters of the sintering molds, a maximum KBV of 12 cm ² /cm ³ is possible with this process.

Another disadvantage is that the sintering process takes place at very high temperatures, just below the melting point of the material used. In this case, the material loses all its inner

Tension and the ribs can then be bent by the slightest mechanical influence.

In the document dated 11.07.2000 - Mashiko - US 6,085,830 it is proposed to connect ribs at the base by pressing in a liquid metal. Due to the mold parameters, in particular the demoldability, a maximum KBV of approx. 10 cm ² /cm ³ is possible with this method. In particular, the thickness of the ribs can only be reduced to a certain minimum (approx. 0.5 mm), since otherwise they would bend when the liquid metal is pressed in. In addition, the costs for such a tool are comparatively high. A similar approach is used on 14.08.1997 - Smalen - WO 99/08821. However, it uses extruded aluminum cooling fins, which are pressed together several times to achieve the necessary mechanical stability. This has the disadvantages that, firstly, the KBV will be a maximum of 6 cm ² /cm ³ due to the necessary side connections of the individual cooling fins, and secondly, due to the press connection on the upper edge of the cooling fins, the air can only flow through them in one direction.

In order to eliminate these disadvantages, the invention is based on the object of creating a finned heat sink which has a KBV of at least 15 cm ² /cm ³ , whose connection between the base plate and the fins has both high mechanical stability and low contact resistance, which can be flowed through in at least two different directions and which can be manufactured at low cost.

According to the invention, the object is achieved by attaching individual cooling fins to a base plate using solder. The thickness of the solder layer is at least 2.5 times as large as the thickness of the cooling fins, and the cooling fins pass through the entire solder layer and rest on the base plate. This is necessary on the one hand to achieve sufficient mechanical stability, and on the other hand to reduce the thermal resistance.

reduce. The fins are ideally made of copper. This has a thermal conductivity of 372 W/mK. The thermal conductivity of standard copper solder (e.g. SnCu3) is, in contrast, around 62 W/mK. This is 1/6 of the thermal conductivity of copper. Since the thickness of the solder layer is at least 2.5 times the thickness of the cooling fins, the contact area between the cooling fins and the solder is 6 times (2.5 times on each side, 1 time on the bottom edge) as large as the thickness of the cooling fins. This adequately compensates for the poorer thermal conductivity of the solder. Thermally speaking, a distance between two fins that is 5 times the thickness of the cooling fins would be optimal. In an example design with cooling fins 0.1 mm thick, the distance between the fins would be 0.5 mm. This would correspond to a grid dimension (distance from one cooling fin center to the next cooling fin center) of 0.6 mm and a KBV of approx. 32 cm ² ZCm ³ .

In an advantageous embodiment of the invention, the solder is mixed with a powder of a material that conducts heat well. This is advantageously copper powder. Care must be taken to ensure that alloys or oxidation do not occur when mixing solder and powder. The thermal conductivity of the mixture increases according to the powder proportion in the solder/powder mixture. Solder/powder mixing ratios of between 90:10 and 60:40 have proven to be particularly suitable. Due to the increased thermal conductivity, the thickness of the solder layer can now be reduced and the distance between the cooling fins, or the grid dimension, can be reduced. In an exemplary embodiment with cooling fins 0.1 mm thick, the grid dimension can in practice be reduced to 0.3 mm. This results in a KBV of approx. 65 cm ² /cm ³ .

In another advantageous embodiment, a flat heat pipe base plate is used instead of a solid base plate. A heat pipe is a well-known system for heat transport, which is based on the principle of evaporation and recondensation of liquids (mostly water under negative pressure). This has the advantage that the heat from

concentrated heat points are distributed very quickly and evenly across all fins. In addition, a heat pipe base is much lighter than a solid base plate with sufficiently comparable performance.
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In another advantageous embodiment, individual or multiple heat pipes or other heat pipe elements are attached to a solid base plate. This has a similar effect to the heat pipe base. In contrast to this, with a heat pipe the heat is only distributed in the direction and extent of the pipe across the solid base plate. In contrast, the heat pipe base distributes the heat evenly across the entire floor area. However, the costs for a heat pipe base are significantly higher, so that it can be replaced by the cheaper heat pipes or heat pipe elements for medium-power hot spots.

In a further advantageous embodiment, the cooling fins are provided with structures that increase the cooling surface. Applied, rolled or punched structures also reduce laminar effects when flowing through the heat sink, so that heat dissipation is improved and the performance of the heat sink is increased.

In another advantageous embodiment, the cooling surfaces of the heat sink are colored dark or black. This increases heat radiation and the performance of the heat sink increases accordingly. The increase in performance can be as much as 15%.

Examples of embodiments are shown in the drawings and are described in more detail below. They show:

Fig. 1: A heat sink with soldered cooling fins.

Fig. 2: Detail enlargement A of the solder layer of the heat sink from Fig. 1 with a soldering of the previous type.

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Fig. 3: Detail enlargement B of the solder layer of the heat sink from

Fig. 1 with a new type of soldering with standard solder. Fig. 4: Detail enlargement C of the solder layer of the heat sink from Fig. 1 with a new type of soldering with a solder / powder

Mixture.
Fig. 5: Schematic representation of the heat distribution of a

heat point on a solid base plate. Fig. 6: Schematic representation of the heat distribution of a

Heat point on a heat pipe floor. Fig. 7: Schematic representation of the heat distribution of a heat point on a solid floor plate with a heat pipe.

Figure 1 shows a heat sink consisting of a base plate (2) and soldered cooling fins (3) .

Figure 2 shows an enlarged detail A of the heat sink with a conventional type of soldering. In this process, a solder layer (4) is applied to a base plate (2). This is usually done using appropriate solder foils with a small thickness (d2). The cooling fins (3) are then applied to the solder layer (4). The cooling fins (3), the solder layer (4) and the base plate (2) are then connected to one another in a thermal process. The cooling fins (3) only have contact with the solder layer (4) at the lower edge (6). In an optimal design, narrow transition areas (5) are formed on the sides (7) of the cooling fins (3) by using appropriate fluxes. The heat flow from the base plate (2) through the solder layer (4) into the cooling fins (3) thus only takes place via the contact areas (5) and (6). Ideally, the cooling fins (3) lie directly on the base plate (2). However, it has been shown in practice that, for example, due to a ridge or angle of the cooling fins (3), a certain distance to the base plate (2) always remains.

Figure 3 shows a detail enlargement B of the heat sink with a new type of soldering. In this type, a solder layer (8) is applied to the base plate (2). The thickness (d3) of the

Solder layer (8) at least 2.5 times as thick as the thickness (dl) of the cooling fins (3). The cooling fins (3) are applied as directly as possible to the base plate (2). This considerably increases the contact area between the cooling fins (3) and the solder layer (8) on the sides (7) of the cooling fins (3). The heat can now flow from the base plate (2) through the solder layer (8) on the one hand through the lower edges (6) of the cooling fins (3), and in particular over the side surfaces (7) of the cooling fins (3). The heat flow from the base plate (2) into the side surfaces (7) is shown by the arrows (9). The distance (d.4) between two cooling fins (3) should be at least 4 times as large as the thickness (dl) of the cooling fins (3) .

Figure 4 shows an enlarged detail C of the heat sink with a new type of soldering. However, the solder layer is composed of a powder (10) and solder (11) that conducts heat well. By adding the powder (10) to the solder (11), the heat conduction (9) from the base plate (2) to the side surfaces (7) of the cooling fins (3) is sometimes considerably increased. This depends on the materials used for the powder (10) and the mixing ratios between powder (10) and solder (11). The better heat conduction means that the thickness (d5) of the solder layer can be reduced and the distance (d6) between two cooling fins (3) can be reduced. This allows the KBV and thus the performance to be increased accordingly.

Figure 5 shows the schematic representation of the heat distribution of a heat point (20) on a solid base plate. The temperatures indicated in the illustration are randomly selected values to clarify the scheme. A round heat point (20) is installed in the middle of the base plate as an example. The temperature zones (21 to 25) are drawn in a circle around the heat point (20). The heat point (20) should have a temperature of 80 ⁰ C. The further a surrounding heat zone (21 to 25) is from the heat point (20), the colder its temperature, or the temperature delta to the heat point (20) is greater. According to this scheme, it is clear that soldered cooling fins in the outer

Temperature zones (25) have a significantly lower heat flow than, for example, cooling fins that are installed directly above the hot spot (20). Due to the lower heat flow, their Δ΄ to the ambient air is lower and they can dissipate less heat to the environment. This reduces the overall performance of the heat sink. It should also be noted that this is a very simplified schematic representation, since such a uniform radial heat distribution does not normally occur in practice, and the contours of the temperature zones are irregular.

Figure 6 shows a schematic representation of the heat distribution of a heat point (30) on a base plate which is designed as a heat pipe. It is assumed that the heat point (30) is generated with the same heat output as the heat point (20) in Fig. 5, and that the heat sinks are otherwise identical except for the base plate. The heat pipe base has the property of distributing heat very quickly and evenly. This can be seen from the fact that the temperature difference between the heat point (30) and the outer zone (33) is only about 1.5°K. This means that all cooling fins receive approximately the same heat flow. This allows the cooling fins to give off the maximum heat via the cooling fins. This is expressed by the fact that the temperature of the heat point (30) is correspondingly lower than the temperature of the heat point (20) in Fig. 5.

Figure 7 shows the schematic representation of the heat distribution of a heat point (40) on a base plate, which is connected to a heat pipe (41). It is assumed that the heat point (40) is generated with the same heat output as the heat points (20) and (30) from Fig. 5 and 6, and that the heat sinks are otherwise identical except for the base plate. The heat pipe (41) absorbs a large part of the heat from the heat point (40) and distributes it evenly across the width of the heat pipe (41). On both sides of the heat pipe (41) corresponding temperature zones (42 to 44) are formed, which become colder the further they are from the heat pipe (41).

Overall, however, the temperature delta between the
hot spot (40) and the coldest temperature zone (44) is lower than with a solid base plate alone (Fig. 5). Therefore, the heat sink can dissipate more power overall than with a solid base plate alone. The hot spot (40) is

accordingly colder than the heat point (20) in Fig. 5. However, the heat distribution is not as good as with a heat pipe base plate,
which is why the heat point (40) is correspondingly warmer than the heat point (30) in Fig. 6.
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Claims (10)

1. A heat sink consisting of a base plate and individual soldered cooling fins, characterized in that the thickness (d3) of the connecting solder layer ( 8 ) is at least 2.5 times as large as the thickness (d1) of the cooling fins ( 3 ) and the cooling fins ( 3 ) rest on the base plate ( 2 ) and the cooling fins ( 3 ) are integrally connected to the base plate ( 2 ). 2. A heat sink according to claim 1, characterized in that the connecting solder layer ( 8 ) consists of a mixture between a solder ( 11 ) and a powder ( 10 ) of a material with good thermal conductivity, whereby no alloying occurs between the solder ( 10 ) and the powder ( 11 ). 3. A heat sink according to claim 2, characterized in that the mixing ratio between the solder ( 11 ) and the powder ( 10 ) is between 90:10 and 60:40. 4. A heat sink according to one of claims 2 or 3, characterized in that the powder ( 10 ) consists in each case pure or mixed from the materials copper, aluminum, silver, gold or diamond. 5. A heat sink according to one of claims 1 to 4, characterized in that the base plate ( 2 ) consists of aluminum or copper. 6. A heat sink according to one of claims 1 to 4, characterized in that the base plate ( 2 ) is designed as a heat pipe. 7. A heat sink according to one of claims 1 to 5, characterized in that heat pipe tubes () or heat pipe elements are attached to the base plate ( 2 ). 8. A heat sink according to one of claims 1 to 7, characterized in that the cooling fins ( 3 ) consist of aluminum or copper. 9. A heat sink according to one of claims 1 to 8, characterized in that the cooling fins ( 3 ) have structures which increase the cooling surface and/or reduce laminar effects in the flow through the heat sink ( 1 ). 10. A heat sink according to one of claims 1 to 9, characterized in that the surface of the heat sink is colored dark or black to improve heat dissipation.