Characteristics of Bonded Grid Arrays and BGA Heat Sinks

Bonded grid arrays are heat sink assemblies that are highly efficient, cost effective, reliable thermal management solutions for density packed or high-power packaged applications. These characteristics hold true in demanding vibration and shock environments. Bonded grid arrays are used when simple extrusions are impractical. They lower thermal resistance and increase surface area. There are various configurations that can be assembled.

Any industry, employing high-powered electronic circuits can use bonded grid arrays. A partial list of industries includes factory automation, medical equipment, wireless and RF, military, broadcast, telecommunications, and renewable energy. When choosing bonded grid arrays consider the overall dimensions, specified thermal resistance, and the finish. Finishes include gold iridite, clear iridite, gold chromate, or unfinished.

Finding the device that is appropriate for a cooling system to be effective is important to the overall design of a component. BGA heat sinks are available in ceramic and plastic. A major benefit of BGA heat sinks is the increased heat dissipation rate.

The ball grid design is achieved by advance circuitry that allows quick thermal energy conveyance without the levels of resistance, prone to occur with the surface mounting approach of the gull wing. Gull wing leads cannot reach the heatsink pin count capacity of BGA heat sinks.

The most innovative BGA heat sinks aspect is the ability to self-align based on the heat flow from its source. This characteristic has the potential to allow greater thermal energy transfer levels and increase efficiency throughout the heatsink.

There are some drawbacks to consider before making a final decision. Because of the joint solder complexity, standard reworking and inspection methods cannot be achieved. Solder joints cannot be individually adjusted and basic visual inspection is not possible. A proposed solution to this problem is x-ray use to ensure reliable BGA heat sinks.

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Designers Use Copper Heatpipes for Superior Performance

Heatpipes offer highly effective thermal conductivity and energy efficiency. Other positive characteristics are the low cost, light weight, and flexibility that provides many different shape and size options. Heatpipes offer a heat transfer system that is simple and reliable to operate. The system has no moving parts, transports heat over a long distance, and is a quiet, vibration-free operation.

Heatpipes are filled with small quantities of working fluid such as sodium, ammonia, methanol, nitrogen, acetone, or water. The working fluid is vaporized as heat is absorbed. The vapor transports the heat to a condenser region. The heat is released to a cooling medium by condensed vapor. Capillary action is created as the condensed working fluid returns to the evaporator by the heatpipe’s wick structure or gravity. Planar and cylindrical heatpipes have an inner surface that is lined with wicking material capillaries.

Military and commercial designers turn to copper heatpipes for superior power density, regardless of orientation or gravity. Copper heatpipes are designed specifically for applications where gravity or high heat loads present thermal challenges. Long life and reliability are critical. The copper heatpipe operates flawlessly against gravity and is rugged enough to withstand temperatures that range from 55ᵒ below zero to 180ᵒ C, and numerous freeze-thaw cycles.

Copper heatpipes use water as the working fluid. It smoothly moves heat from the source to an area where it can be managed effectively through liquid or air dissipation or radiation to space. These heatpipes are integrated into customized metallic cold plates or heat sinks. The integration improves the efficiency and conductivity. It allows the thermal designer to improve the performance of the overall system. The heatpipes can be integrated into extended surfaces, cold plates, and heat sinks through mechanical interference, solder, or epoxy.

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Uses for LED and Copper Heatsinks

LEDs are a solid-state lighting form. Typical LED lights are made of small semiconductors; optical lenses, integrated to shape the radiation pattern; and LED heatsinks to keep a low operating temperature for the semiconductor and dissipate heat.

The life expectancy and performance of LED lights are directly linked to operating temperatures. The lower the operating temperature is, the longer the operating life and the better it performs. Thermal management is critical in a LED light design aspect. Thermal management increases the lighting quality and durability. LED lights are cooled by LED heatsinks using natural convection.

Honeycomb LED heatsinks have a LED chip located in the center of the heatsinks. Air passes through the hexagon-shaped holes and is carried away from the chip. The cooling performance of the heatsinks is affected by the rib-space ratio, the honeycomb aspect ratio, and the number of cells that surround the heat source.

Each new generation of devices having semiconductors has shrinking packages and a rise in the levels of power dissipation. Because of this trend, copper heatsinks are being used more often in a variety of applications.

A common technology available in copper heatsinks is the pin fin technology. Copper’s superior thermal properties satisfy challenging cooling requirements. When comparing aluminum and copper heatsinks  types of cooling scenarios and the value of copper, copper was recommended.

To understand the impact of copper heatsinks the thermal conductivity of aluminum versus copper must be considered. Applications demanding thermal stability are best served by copper heatsinks. Heatsinks having significant thermal mass are needed when power dissipation has a wide fluctuation, but a constant temperature is needed. In such a situation, copper with its 40 percent higher thermal mass is a better choice than aluminum.

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The Difference between Bonded Grid Arrays and BGA Heat Sinks

Engineers are faced with increased performance demands and miniaturization. Heat dissipation is an issue for both challenges. If heat dissipation were not necessary, all electronic devices would operate faster. Heat dissipation is necessary to keep devices from becoming unreliable, overheating, and failing. To produce reliable devices with long life and acceptable performance, heat dissipation must be introduced. Heat sinks are a method of removing heat.

Heatsinks are devices that dissipate heat from a heat generating component to a medium, usually air that is cooler. Heat sinks placed on hot components improve the transfer of heat by an increase of surface area that has direct air contact. The component’s operating temperature is lowered as heat is dissipated. Heat sinks maintain the temperature of a device below the maximum specified by the manufacturer. 

To choose the appropriate heat sink necessary for thermal performance various parameters are considered and calculations performed. Extruded heatsinks allow two-dimensional profile formations that dissipate large heat loads. BGA heat sinks are simple extrusions. BGA heat sinks typically convert extruded fins into pins. The BGA heat sinks are crosscut to allow more diverse applications.

Bonded grid arrays are built to customer specifications. Very few power electronic applications are the same. A one-size-fits-all theory is not feasible. Bonded grid arrays dissipate as much as three times the amount of heat dissipated by the average extruded heatsink. Two styles of bonded grid arrays are available. They are single and folded fins. Fin density, thickness, and height are used in countless combinations to provide the desired performance.

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Benefits of Extruded Aluminum Heatsinks

Extruded aluminum heatsinks provide a range of convection solutions for high power systems and components. Complex structures force raw aluminum through a die. The complex fin profiles allow heat dissipation through a greater surface area while eliminating the time and cost associated with using block aluminum machining to make an equivalent shape.

There are many benefits to using extruded aluminum heatsinks. Stamped heat sinks are not as efficient as extruded aluminum heatsinks. Heat sink extrusions cost less than fully machined assemblies. Many standard sizes and shapes are available. Heat sink extrusions are easy to customize for any application. A significant weight advantage exists compared to copper. Mounting tools and hardware are eliminated.

Heat sink extrusions combined with friction stir welding technology provide high-performing, cost-effective solutions. Heat sinks that are cooled by friction stir welding provide a product of higher quality and more efficient than pressed or bonded fin products.

Extruded heat sinks are the most common type used for thermal management. Extruded materials start as 30 to 40 sticks that are very soft. The material is grabbed and stretched to produce straight sticks. After being stretched, the material is either over or air aged depending on the hardness required. Final fabrication such as pockets and holes are cut after the aging process.

A finish is usually applied to enhance the thermal performance. A chromate finish provides somewhat of a corrosion protector. It is also used as a primer before a powder coating, or final paint is applied. Extruded shapes are unique to the needed requirements, yet are the most cost-effective solution for cooling. Each shape is designed to achieve the optimal structural and thermal performance. 

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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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