Characterizing Mixed-Metal Heatsinks
High temperature and heat dissipation are the factors currently limiting electronic system capabilities. Two primary applications require heatsinks: the microprocessor at the pc-board level and power electronics. With the trend of shrinking package size and increasing heat dissipation, heat flux also increases. To reduce the heatsink thermal resistance, designers use materials such as copper (Cu), which has higher thermal conductivity than aluminum (Al). Examples include mixed-metal heatsinks, such as copper fin/aluminum baseplate, aluminum fin/copper baseplate and all-copper sinks, where the use of copper improves heat spreading.
For low-power dissipation and low-heat flux applications, extruded heatsinks are most commonly used because of their cost effectiveness. However, the extrusion process has limitations. When producing high aspect-ratio fins, the extrusion die breaks more readily as the fin thickness and fin spacing decreases. For high-power dissipation and high flux applications, bonded heatsinks with high aspect ratio can be used. For high-volume applications, the die-casting manufacturing technique is a more-likely alternative because of its low averaged cost. Nevertheless, in the case of die-casting, high-porosity and low-purity alloys result in lower thermal conductivity products.
In heatsinks with bonded fins, the base is extruded with slots to allow the insertion of plates or extruded fins. Attaching the fins to the baseplate can be done using thermal epoxy, brazing or “swaging.” Thermal epoxy is the common method used to bond high aspect-ratio heatsinks. Because epoxy possesses a very low thermal conductivity compared to aluminum, epoxy thickness should be minimized to reduce its thermal impedance. Brazing is a form of welding that takes place at temperatures above the liquid state of a filler material (450°C) and below the solid state of the base materials. Capillary action plays a major role in filler flow through the joints.
The heatsinks tested in these experiments were bonded using a metal-displacement process referred to as swaging. The swaging process, shown in Fig. 1, can be described as a cold-forming process, which is used in the fabrication of high fin-density heatsinks. Currently, this process involves the placement of fins with a tapered base into a slotted baseplate followed by the application of a rolling pressure on the opposite sides of each fin. This process results in vertical and lateral pressure of the base unit material, which tends to push the fin toward the bottom of the groove in the baseplate. This secure connection provides very good thermal contact between the fins and base, and also prevents air and moisture from entering the grooves, thereby preventing corrosion and allowing the heat-sink to be anodized.
Fig. 2 shows four heatsink designs that were tested. These sinks included aluminum base/aluminum fin, copper base/aluminum fin, aluminum base/copper fin and copper base/copper fin models. Generally, the heatsink baseplate area, fin height and fin center-to-center distance were the same for all of these heatsinks. The weights of each heatsink and their weights relative to the all-aluminum type are shown in Table 1.
To minimize heat losses (heat not dissipated by the heatsink) and accurately measure heat dissipation from the heatsink, matching pairs of heatsinks were configured in a back-to-back configuration, as shown in Fig. 3. The heatsinks were firmly bolted to heater plates using four countersunk machine screws at an equal distance from the center (Figs. 4 and 5). In one set of tests, the heater plate covers 10% of the surface area of the heatsink baseplate (Fig 4). These experiments simulate a concentrated heat source. In a second set of tests, a distributed source of heat is simulated. In this case, the surface area of each heater plate covers 60% of the baseplate surface area (Fig. 5).
The thermal performances of the four heatsink combinations under forced convective heat-transfer mode were measured for the concentrated heat source. Measurements are depicted graphically in Fig. 6, with results summarized in Table 2. As expected, the all-copper (Cu base-Cu fin) heatsink had the lowest thermal resistance with a 22% average reduction in thermal resistance as compared to the all-aluminum sink. However, the copper heatsink possessed 3.4 times the weight of the aluminum heatsink.
Up to 9% reduction in thermal resistance was achieved by using an aluminum base/copper fin sink with 2.5 times the weight of the aluminum heatsink. The copper base-aluminum fin heatsink had the lowest weight increase, being only twice as heavy as the all-aluminum heatsink with a reduction in thermal resistance of 11.4% compared to the all-aluminum sink. For low source coverage, it can be concluded that the high conductive material is more efficient when it is used as base material rather than using it as fin material.
Fig. 7 shows the thermal performances of the four heatsink combinations for the distributed heat source. The all-copper (Cu base-Cu fin) heatsink had the lowest thermal resistance with 17% average reduction in thermal resistance as compared to the all-aluminum sink. Up to 14% reduction in thermal resistance was achieved by using an aluminum base/copper fin sink. Meanwhile, the relatively light copper base/aluminum fin heatsink provided a marginal reduction in thermal resistance of 3% compared to the all-aluminum sink. For high source coverage, it can be concluded that the high conductive material is more efficient when it is used as fin material rather than using it as base material.
Copper heatsinks improve thermal performance more than 22% for a concentrated heat source and 17% for a distributed heat source. We can conclude that the high conductive material is more efficient when used as fin material rather than using it as base material for both concentrated and distributed heat sources. The thermal performance improves 9% for concentrated source and up to 14% for distributed source.
The high thermally conductive material in the baseplate is a good option for concentrated heat source. In this case, copper helps spread the heat across the base to reduce the temperature rise near the heat source. In weight-sensitive applications, use heat pipes as an alternative to copper for heat spreading in the base.
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