Today's electronic equipment dissipates more power with every new design. Despite the "increased power - decreased size" scenario that has existed for decades, the ability to make smaller electronic components has mandated smaller cooling pieces.

There are other problems besides the total power level. The issue of heat density at semiconductors, or Watts per square centimeter, is also growing. Denser electronics and higher clock frequencies are responsible for this. The main problem for thermal management is the combination of high heat flux densities with high power levels. For a heat sink to be successful, it needs to have an appropriate surface area and airflow.

Air-Cooling in the Past

Air has traditionally been used to cool electronics. The thermal design was traditionally only considered after lab tests failed or a component turned a PC board brown. The following limitations apply to stand as a heat transfer medium:

• Heat carrying capacity is low due to low density and specific heat
• Thermal conductivity is low

Potential Enhancements in Air-Cooled Heat Sinks

• The heat transfer coefficient, h, should be increased.
• Increase the amount of exposed surface.

Watts are dissipated per square meter per degree C of temperature rise above the cooling medium. According to Newton's law of cooling, heat dissipated equals the heat transfer coefficient (h) times the area times the difference in temperature between the heat sink and the cooling air or:

q = h � A � (Theat sink – Tair)(1)

Designers can control only "A," the number of square centimeters of fin surface area, and "h," the effective heat removal from each square centimeter of surface. Cost and marketability limit both of these parameters.

The heat transfer coefficient "h" can be increased by several methods:

• Airflow past the heat sink fins generally increases "h." Additionally, it reduces the overall temperature of the exit air by increasing the mass flow rate. Backpressure and acoustic noise increase significantly when air speeds exceed 8 to 10 m/s.
• This method has the disadvantage of resulting in an additional pressure drop. When airflow is unpredictable, cross-cutting is most effective.
• A fin is augmented by adding a twist to its leading and trailing edge, similar to cross-cutting. The curvature of the metal in the airflow reduces the "h" by scrubbing the "dead" air molecules away. For augmenting to work, air velocity must be at least three m/s.
• A high-speed airflow cools heat sinks using impingement (jet) cooling. This cooling layout must be designed with an eye toward the system and tested in situ.

They are leading to increased cooling air velocity. The price of increased heat removal is increased pressure drop. In a conjugate analysis of the heat sink, fan, and system, we must determine if these techniques produce lower thermal resistance than simply increasing surface area through thin, flat fins. The ability to control contamination in the air stream is a fundamental limitation in this area. Closely spaced fins can clog and reduce thermal performance.

Increase Exposed Cooling Surface Area – Inexpensively

The amount of heat removed by convective heat transfer depends on the cooling surface area available. Therefore, increasing the number of tall, thin, flat fins often provides better performance and less pressure drop than modifying the fin shape. In terms of airflow resistance, flat fins maintain the lowest pressure drop, resulting in the most significant air mass flow.

Aluminum Extrusions

Aluminum is an excellent heat sink material due to its many characteristics. An extrusion tool can produce top fins without failing based on the extrusion aspect ratio.

There has long been a belief that serrations on extruded fins increase surface area and heat transfer. However, most of the time, they do not add any thermal effect. Although serrations can increase heat removal, they are typically not tall enough to provide additional cooling over flat fins.

Impact Extrusions

As pin fins or elliptical fins, impact extrusions use the same high-conductivity alloy. As a result, surfaces a result, surface area per unit volume increases with extrusion or draw ratios exceeding linear extrusions.

Bonded Fin Heat Sinks

Heat sinks with bonded fins consist of separate bases and connected fins. These parts have offered an alternative to extrusion's limitations for over a decade.

• Epoxy bonding of fins into a heat-spreading base
• Brazed assemblies
• Cold-formed or swaged
• Welded – ultrasonic or resistance
• Stacked fins (fin and base extruded individually and created together)

Each of these styles has its advantages. Diffusion bonding and other new techniques will increase the number of alternatives available. The lowest cost per square inch of the exposed cooling surface will be the ultimate criterion.

Folded Fin

They provide a large surface area with minimal weight in the military and aerospace industries. However, from personal computers to radio transmitters, the commercial cooling market has adopted this method of making cooling surface area (Figure 4). These folded lengths of metal are bonded, brazed, or soldered to heat-spreading surfaces. Therefore, we can manufacture fins with heights up to 4.0 inches and densities of up to 10 fins per inch. The most common type of fin is flat.

Castings

Low-cost heat sinks can be integrated into enclosures using castings. However, due to cost, castings are limited to high-volume production programs that employ tens of thousands of parts. In most cases, castings produce near-net-shaped parts that require little or no machining before assembly. In addition, aluminum casting alloys have traditionally had a lower thermal conductivity than extruded alloys.

Future Developments

Air-cooled heat removal alternatives are becoming more advanced. However, the market has yet to see sintered heat sinks, metal injection molding, 5X performance fans, aluminum extrusions at a 25:1 ratio, and self-fanning piezoelectric fin heat sinks. Nevertheless, several companies are developing these next-generation products.

Improving Thermal Conductivity

In addition to heat spreading in the base, fin efficiency plays a role in thermal conduction. Through intimate contact between molecules, conduction transfers heat within a material.

• Copper as a base material
• Heat pipes and thermosiphons
• The increased conductivity of heat spreaders
• Copper or carbon-based materials
• Vapor chambers

These materials or systems will increase heat conduction compared to aluminum extrusion. They will, however, all be more expensive and heavier than aluminum.

Heat Pipes

Heat pipes are closed-loop, phase-change systems that do not dissipate heat. However, with a very low-temperature rise, it does move heat from one point to another. The equivalent thermal conductivity of an average heat pipe is up to 1000 W/m-K. Although it is not strictly correct to quote the thermal conductivity of a heat pipe, it is used here as a comparison to solid materials.

The heat transport mechanism of a heat pipe is latent heat transport via vapor flow and not thermal conduction. The thermal conductivity of a heat pipe is usually equal to the thermal conductivity needed for a solid conductor of the same cross-sectional area and length to conduct the same amount of heat. Adding heat pipes to an aluminum heat spreader can improve conductivity and reduce spreading resistance with a minimum weight increase.

The best defense is the education of electrical engineers, while still in college, about the nature and consequences of thermal problems and their solution sets.