Heat Pipes Protect Against Thermal Failure in Embedded Electronics

Embedded computing systems are important in the military of today. A given mission uses the critical functionality for data processing and significant computing power. The systems’ electronics are extremely expensive. Often the thermal management solution is overlooked. Electronics intensify thermal challenges. Trends lean toward packaging with less volume and weight and higher heat fluxes.

Multiple electronics cards that are inserted into a chassis or rack are embedded into computing systems. They are sealed to assure no small particles or liquid comes in contact with the electronics. Mechanical retainer clamps hold the cards in the chassis. Heat is conducted to the edge of the card, through the clamp, along the chassis until it is rejected through pumped liquid or convection.

The most economical and easiest approach to improving thermal performance is the reduction of thermal gradients caused by conduction. Embedded heat pipes are the best method to achieve the reduction. Heatpipes offer passive, efficient heat transfer.

Utilizing heat pipes is a two-phase process of transferring heat. The vaporization of a fluid’s latent heat is used to an advantage. The closed loop system is made up of a small amount of liquid, an internal wick structure, and a sealed envelope or tube. During the process, there is no air in the tubes, which allows heatpipes to maintain vapor and liquid phases over a wide range of temperatures. 

Waste heat enters the heat pipes near the evaporator, which is a heat generating component. As the heat vaporizes the liquids, an internal pressure gradient is formed. Vapor is forced to the condenser, by the internal pressure gradient. Here the fluid loses its heat, condenses, and is returned to the evaporator through the wick structure’s capillary force.

Most common heatpipes use copper envelopes for applications involving terrestrial electronic cooling. The heat pipes also have a copper wick and the working fluid used, is water. The system provides the ultimate in power capacity within the temperature range of typical electronic operation.

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Selecting Suitable Heat Sinks

Computer designers keep products cool by following some fundamental equations for designing heat sinks that use forced air or natural convection cooling. High current diodes, numerous power-supply regulator assemblies, and power modules and transistors generate more heat than the mass can dissipate safely. To effectively increase the surface area and mass of heat-dissipating junctions, suitable heatsinks must be securely fastened to power devices.

Heat sinks are available in a variety of surface finishes, colors, sizes, and shapes for general and specific semiconductor applications. The amount of heat generated by a power module or semiconductor depends on the applied signal shape, signal amplitude, bias level, and operation mode. The devices need heatsinks to dissipate the generated internal power.

High current diodes and silicon power transistors need heat sinks that allow maximum junction temperature. Heat-generating devices that are not connected to heatsinks that provide greater surface area and mass will barely approach the voltage and junction current specifications, before exceeding the maximum temperature of the collector junction. That is why device specification sheets list characteristics with devices mounted on heat sinks of considerable size, usually referred to as ‘infinite’ heatsinks.

The surrounding air temperature of the heat-generating devices must be considerably less than the allowable temperature of the junction for devices to cool. Heat sinks attempt to reduce the temperature of the junction to that of surrounding mediums. To be thermally perfect the heatsinks would allow the transistor junction temperature to reach the temperature of the lower ambient. 

Reaching that temperature is impossible in practice. The connections are thermally imperfect and consequently, produce temperature differentials. The variable between ambient and collector junctions are kept to a minimum with heat sinks.

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Bonded Grid Arrays, BGA Heat Sinks, Solve Configuration Problems

The level of heat in today’s power electronics is ever increasing. Efforts to expand the number of fins for cooling have become necessary. Densely compact components are making the cooling system packages smaller. The increased fin count cannot add to the volume of the systems.

Heat sinks cool as heat input from a component, or mounting surface is transferred to the cooler ambient air. More heat can be removed with more heat surface area. A flat plate would increase airflow needed to dissipate heat, but the surface area needed, would be too large to fit in the available space. Adding fins to the base plate increases the amount of surface coming in contact with the air. The amount of cooling is increased without increasing the footprint occupied by the heat sink. The application is used for both natural convection and forced air.

The fins can be part of the aluminum base. As an alternative, bonded grid arrays are used. Bonded grid arrays allow a higher fin count. Taller fins than those of conventional extrusions are used.

Extruded heat seats are inexpensive and easy to manufacture. Aluminum extrusion lengths are produced, cut, and machined to the size requirement. The shaping die and extrusion tooling used in the process of manufacturing places limits on the finished product. The heat sink’s flexibility is limited by the maximum base-to-fin thickness, the minimum thickness-to-height, and the height-to-gap ratio aspect when heated aluminum is forced through the die of steel and creates the required two-dimensional shape.

Bonded grid arrays reduce the limits of fin ratios by separating base and fin extrusions. BGA heat sinks allow nearly limitless fan heights and increased cooling due to decreased center-to-center spacing. Before the recent use of BGA heat sinks, the most effective way to attach fins was the use of epoxy filled joints. The thin bond line and high conductivity of the epoxy result in a bit of thermal resistance. BGA heat sinks increase the achievable fin height-to-gap ratio to as much as 40:1

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Advantages of Extruded Aluminum Heatsinks

Using extruded aluminum heatsinks provides a greater range of convection solutions needed for high power systems and components. By forcing raw aluminum through extrusion dies, complex fin profiles have increased surface area that allows greater heat dissipation. The time and cost of an equivalent shape using block aluminum is eliminated.

The benefits of extruded aluminum heatsinks include:

•    More efficiency

•    Lower costs

•    Availability of many sizes and shapes

•    Easy customization for any application

•    Weight advantage over copper

•    Mounting tools and hardware are eliminated

The latest technology is used to test and prototype extruded aluminum heatsinks to provide the most effective thermal products. Extruded aluminum heat sinks are designed for forced air cooling and natural convection. Plating options include chromatic and anodization. Multiple extruded aluminum heatsinks can be produced. Shapes include tube, solid, semi-hollow, rod, bar, profile, and hollow. Customers can also specify unique custom shapes.

The compact designs of today require one-piece aluminum heat sink extrusions. That design prevents premature failure and limits temperature rise. Heat sink extrusions satisfy a wide range of needs for semiconductor cooling. Heat sink extrusions are customized to perform optimally for any application.

Aluminum heat sink extrusions have eliminated labor-intensive, bonded-fin heat sinks. The overall size has been reduced. The original heat sink prototype was overdesigned. A thermal analysis that used maximum operating conditions, environmental constraints and a power component layout allowed a high-ration extrusion that met the thermal requirements to be developed. 

That is the process used to find solutions to heat sink extrusion challenges. Often,  heat sink prototypes that are too expensive, too heavy, or too large can be replaced by extruded aluminum heatsinks.

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When to Use Copper and LED Heatsinks

Understanding the physics behind functionality of heatsinks helps determine the engineering and design elements that govern the quality of a heatsink. Materials affect the efficiency of heatsinks. Air has low thermal conductivity. That explains why air cannot be blown past a central processing unit (CPU) for performance-grade cooling achievement. 

Copper makes an excellent heatsink material in many situations. It has the best potential for conductive heat transfer. Copper allows heat to transfer quickly. For that reason, applications that require quick heat transfer utilize the metal. An example of such an application is found in copper heatsinks.

Searching for copper heatsinks, that utilize copper fins and copper heat pipe structures is commendable. Copper fins are not a necessity, but copper heat pipes are highly recommended. Some manufacturers use aesthetic materials such as nickel plating to cover up copper. Appearance does not always define the material used in copper heatsinks.

LEDs are used in many applications such as automotive, industrial, and household lighting. LED applications present thermal challenges because of cost restrictions, complex environment, dissipation, and small size. The small size allows LEDs to be clustered together to produce more or brighter light. Transferring heat from LEDs into heatsinks is extremely important.

LED heatsinks are manufactured to be compatible with specific LED components. The material used in LED heatsinks is light weight aluminum. Radial-fin geometry optimizes natural convection. LED heatsinks are easily mounted with standard hardware. The level of performance and size are matched to the specific requirements of the purchaser.

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Benefits of the Technology of Heat Pipes

Enclosures, having no moving parts other than air circulating fans, are efficiently and effectively cooled by the technology of heatpipes. Applications requiring sealed enclosures, but having limited power supplies, are highly suitable for heat pipes.

Heatpipes consist of sealed tubes fitted with external fins made of aluminum to improve the transfer of heat. After evacuating air from heat pipes, small amounts of a refrigerant fill the heatpipes. The refrigerant exhibits characteristics that are suitable at an ambient temperature.

The refrigerant begins as a liquid near the bottom of the tube. Hot air, coming from the enclosure, blows over the section at the bottom. As the liquid evaporates, it absorbs heat and rises to the section near the top of the tube. The air that surrounds the bottom is cooled. The section of tube at the top has cooler outside air exposure that causes the vapor to condense and return to liquid form. Vapor emits heat that warms the section of the tube near the top. The aluminum fins transfer the heat to the air outside the tube. Condensation causes the liquid to flow to the bottom where the process is repeated.

Heatpipes use no power. The hot air in the enclosure provides energy for operation. Overall efficiency is improved when small air circulation fans draw warm air from the enclosure, pass it over finned tubes, and return cool air to the enclosure. Ambient air is blown over the end of the heat pipes that are hot, by external fans to increase the heat removal rate. Less than 60 watts of power are required to allow heat exchange operation, having a 10ᵒ F differential, that can remove 400 watts of enclosure heat.

Because there are no moving parts that will fail, the heat exchange is entirely static. Only small circulating fans produce any noise. That characteristic makes heatpipe technology excellent for noise sensitive areas.

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How a Heat Sink Is Selected

The reduction in form factors and increasing heat dissipation makes thermal management more important that every in the design of electronic devices. Life expectancy and performance reliability are inversely related to the temperature of equipment components. There is an exponential increase in life expectancy and reliability of a typical silicon semi-conductor as operating temperature is reduced. That is to say, reliable performance and long life are effectively achieved by controlling the operating temperature of a device, with the limits set by the engineers who designed it.

Heatsinks enhance heat dissipation from a surface that is hot. That surface is usually the heat generating component. It is dissipated to a cooler ambient, typically air. The lead efficient is the heat transfer that crosses the interface between the surface and the coolant. A heat sink lowers the barrier by increasing the surface area that comes into direct contact with a coolant. Lowering the barrier either lowers the operating temperature, allows more heat dissipation, or both.

When selecting the appropriate heat sink, various parameters that affect the heatsink performance, as well as, the system’s overall performance must be considered. The choice of heat sink depends on external conditions that surround it and the thermal budget it allows. Air flow must be classified as high forced convection, low force mixed, or natural. The required volume must be determined. The performance of a typical heat sinks is proportional to its width in the perpendicular direction of the flow. It is beneficial to increase width, rather than the length of a heat sink.