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

Ever-increasing levels of heat in today's power electronics necessitate constant efforts to increase heat sink efficiency by expanding the number of extended surfaces (fins) for cooling. Due to the trend toward more densely packed components in smaller packages, this increase in fin count must not add to the volume allotted for system cooling.

Heat sinks cool by aiding the transfer of heat from a components heat input area or mounting surface to the cooler ambient air. The more surface area the heat sink presents to the cooling air, the more heat it is capable of removing. Extended surface area and airflow, expressed as heat transfer coefficient, provide heat removal based on convective heat transfer. Using a flat plate for heat dissipation would increase the airflow, and hence heat transfer, but the surface area would probably need to be larger than could fit in the space available. The addition of fins to the base plate greatly increase the amount of surface in contact with air and, therefore, the amount of cooling, without an increase in the footprint the heat sink occupies. This applies to both forced air and natural convection.

Typically, the fins are formed as part of the aluminum base as it is extruded, or, alternatively, they are later bonded into grooves formed in an extruded base. This bonding technique allows higher fin counts and taller fins than can be achieved with conventional extrusions.

Extruded-fin heat sinks are easy and inexpensive to manufacture. Lengths of aluminum extrusion are produced and then cut and machined to the required size. The strength of the tool steel used in the extrusion tooling and the shaping die in the manufacturing process, however, place limitations on the finished product. Heated aluminum is forced through the steel die under extreme pressure creating the two dimensional shape of the required extrusion. The fin height-to-gap aspect ratio, minimum fin thickness-to-height, and maximum base-to-fin thickness limit the flexibility of the heat sink design. Thin wall sections or tall, thin fins have a tendency to shift during the extrusion process, causing unacceptable results. A typical extrusion fin height-to-gap aspect ratio is 6:1. Some smaller extrusions can achieve ratios as high as 12:1, with the use of small, highly controllable extrusion presses.

EPOXY-BONDED FIN

The bonded-fin process reduces the limits on the fin ratios by separating the base extrusion from the fin extrusion. Bonding fins to the base after extrusion allows almost limitless fin heights and decreased center-to-center spacing for increased cooling. Until recently, the most effective means of attaching the fins to the base was with an epoxy filled joint, as shown in Figure 1. The thermally conductive epoxy conforms to and fills any gaps between the fin and the base, creating a strong joint with no spaces to impede the flow of heat from the component. High conductivity epoxy and very thin bond line joint thicknesses (0.005 in., nominally) result in thermal resistances through the joint that are virtually undetectable in most applications. This bonding process increases the achievable fin height-to-gap aspect ratio to as high as 40:1. Because this bonding is a separate assembly process requiring surface preparation and hand assembly, it adds significantly to manufacturing time. The manual assembly time, as well as the material cost of the epoxy itself, increases the cost of the finished heat sink making it more expensive than conventional aluminum extrusions and commercial market pricing models.

Efforts to eliminate this epoxy interface led to an all-metal joint, in which the fins were cold formed into the grooves in the extruded base. Until recently, tests comparing the epoxy-bonded fins and metal-to-metal bonded fins showed epoxy-bonded fins to be superior in strength and flexibility, and equal in thermal conductivity.

New developments allow the creation of both the heat-sink fins and base from extruded 6063-T5 aluminum. A proprietary cold-forming process securely attaches the fins into matching grooves extruded into the heat-spreading base. The result of the assembly process is a bond joint with high structural integrity, as well as robust heat-carrying capabilities. No joints or bond lines impede the flow of heat through the base. The new all-metal heat sinks show a greater than 12% increase in pull-out strength over a popular epoxy-bonded heat sink. Pull strength is the tensile pull capability of one linear inch of fin-to-base joint. This interface integrity approaches the strength of a solid aluminum extrusion, which results in a strain deformation at approximately 1000 pounds at the point of metal necking at separation. This strength contributes not only to long term reliability of the assembly, but to the thermal conductivity of each extended surface and, therefore, to the products overall thermal resistance.

BETTER THERMAL CONDUCTIVITY

Elimination of air entrapment in the fin-to-base joint was a primary driver in the development of this new process. Air in the joint can cause a minor barrier to heat flow. The size and thickness of this air layer determine the magnitude of the increased thermal resistance. High heat concentration in a small area coupled with high-speed airflow over the extended cooling surfaces will magnify the problem of air in the joint. Higher heat flux driving the total heat to be dissipated through a smaller cross sectional area results in additional temperature difference. High air velocities, typically over 700 linear feet per minute, cause the desired increase in cooling but decrease fin efficiencies. Higher air speeds decrease fin efficiencies, because the heat is pulled from the heat generator to the cool side of the heat sink at a high rate. Any thermal impedance between the hot side and the cooling air will be amplified in the presence of an air gap in the bond joint. A high heat-flux density could cause as much as an additional 2° to 4°C heat sink temperature in a high air speed, high heat flux cooling regime.

Air in the joint also prevents the assembly from being anodized. During the anodizing process, a dc current must pass through all portions of the assembled heat sink to form the anodic layer on the parts surface. Any air in the part during that process will inhibit the current flow and cause a non-uniform anodized fin or produce individual fins with no anodizing, due to poor or no electrical contact. The new metal-to-metal joint allows parts to be anodized.

FIN INTERFACE GEOMETRY

One of the most important facets of the new metal-to-metal joint development process is the exact geometry of the interface between the bottom of the fin extrusion and the inside of the fin groove. Many experiments with various geometries resulted in what appears to be an optimum set of interface surfaces that bond and form together to drive out air and increase interface contact area during assembly and forming. The existing epoxy-bonded parts use a conventional rectangular interface geometry (Figure 1). This joint counts on the strength of the epoxy and its ability to adhere the fins to the base groove detail. The epoxy also replaces virtually all of the air that would be entrapped in a solid metal joint.

The fins used on earlier metal-to-metal, bonded-fin products flare out at the base and are cut off flat on the bottom (Figure 2). When the fins are installed in the groove, this flat bottom geometry can cause an air pocket to form from the rolling process used for the swedging operation. This flat bottomed fin, combined with a flat groove detail in the base interfaces, can produce a mismatch during the rolling assembly operation used to secure the fins in place. In addition, the flat bottom detail creates a bond joint that is physically unstable in certain situations. With this geometry, subjecting the fin to a sharp impact could cause the fin to separate from the base.

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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.