All-Metal Bonded-Fin Heat Sinks Shrink Size, Improve Thermal Characteristics

May 1, 2010 12:00 PM
Chris Soule Aavid Thermalloy, Concord, New Hampshire

The newest generation of all-metal heat sinks uses a tapered bottom joint geometry that coordinates with the complimentary angle of the base mounting-slot detail (Figure 3). The bottom taper provides many advantages before, during, and after assembly. In the pre-forming process, the fins are placed in position and held prior to cold forming. The taper at the bottom of the fin maintains the fins in straight and parallel rows with little additional support. This helps reduce assembly time and labor costs.

During the cold forming process, the tapered bottom fin mates with and is formed exactly to the bottom of the groove, eliminating any air entrapment. The material between the fins is flowed over the top of the fin base detail, which mechanically locks the fins in place and seals the joint from contamination. The resulting parts can be machined, finished, and handled as if they were conventional aluminum extrusions. This innovative assembly process increases fin count and extends surface area with apparent extrusion ratios up to and exceeding 35:1. This exact extrusion ratio can be increased based on the larger fin gaps (greater than 0.12 in.) and increased height (greater than 5 in.) typically associated with natural convection cooling applications. This allows the creation of small, high-density heat sinks ideal for use in reduced volume systems.

The new all-metal heat sinks offer a wide selection of sizes, but, as with conventional extrusions and epoxy-bonded heat sinks, there are limits to the width, height, and fin density per inch. Figure 4 shows the all-metal heat sinks. The primary limitation is the base width. Base sections of up to 24-in. wide are possible, but widths of 18-in. or less are more typical. Close fin spacing may not be possible on some wider parts, due to the limitations of larger extrusion presses. Fins can be made in a wide variety of heights and thicknesses. The standards listed in Table 1 are only indications of conventional sizes. Fin thicknesses (Table 2) that vary with the height of the fin are possible on a custom basis. These designs strive to achieve maximum thermal performance with minimum fin height.

PULL-OUT STRENGTH

Pull-out strength (Table 3) of the average epoxy bonded joint is 582 pounds per linear inch of fin. The new metal-to-metal bonded heat sink strength averages 657 pounds per inch with a much tighter, more controllable, deviation in the process. This compares to a solid extrusion strength of 1050 pounds per linear inch for a fin of equivalent thickness. In other words, the aluminum fin break point under tensile stress is 1050 pounds. The new all-metal part compares favorably to the strength that would be expected from a solid extrusion. Figure 5 shows the test fixture for measuring the pull-out force.

The primary driver for the bonded-fin design was to reduce costs. The decrease of assembly labor and the ability to manufacture standard parts in stick length parts (48 in. and above) realize this objective. The extruded fins incur no added cost for serrations. They ease the manufacture of custom finned parts to increase the exposed surface area and create a measurable thermal performance gain.

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