Handling Hot Components Print E-mail
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Written by Chris Dobrowolski   
Saturday, 28 February 2009 19:00
Chinese

Ways to beat the heat range from innovative materials to third-party converters.

Packing more hardware onto printed circuit boards and fitting additional components into shrinking chasses may lead to better performance, but at the risk of generating greater amounts of heat. Dissipation of this heat, accomplished by thermal management techniques, is necessary to keep device temperatures within safe operating limits. Effective thermal management prevents damage to temperature-sensitive internal components, as well as premature shutdown and even system failure. In addition, it can improve performance by enabling electronics assemblies to run at higher speeds.

Thermal management concerns the process of transferring heat from electronic components to a heat sink and then to the ambient environment. To attach an electronic package to a heat sink, the surfaces of the two components must be brought into intimate contact with one another. The surface roughness of the device and heat sink – measured in microns – must be accounted for in order for the contact area to be free of air gaps, which are highly resistant to heat flow.

Therefore, manufacturers use materials to bridge gaps between the two surfaces, forcing air out of the gaps and improving the pathways for heat to travel. One common material that performs these functions is thermal grease, which is inexpensive and effective in eliminating air barriers. The downsides are that grease application is messy and never uniform. Grease is also less stable than solid materials and dissipates over time, reducing its effectiveness.

A better option is engineered thermal transfer materials. These flexible materials offer high thermal conductivity and low thermal impedance, and are more stable and longer lasting than grease. In addition, solid thermal transfer materials provide the means to handle new designs requiring wider gap contact areas. Thickness consistency and elimination of the messy process of applying grease are added benefits.

Thermal Material Options

Fig. 1

A number of different thermal management materials are available today (Figure 1). Material selection depends on the devices to be cooled, the assembly design configuration, and the need to maintain a low coefficient of expansion (CTE), while ensuring against CTE mismatches. Such materials include:

Phase-change materials. These materials offer the thermal performance of grease, but start as solid sheets usually no thicker than 0.007", permitting easy application. The materials typically are employed during board assembly between the electrical component – such as a high-performance microprocessor – and a heat sink.

Once in place and exposed to heat, phase-change materials begin to melt. With slight pressure applied to hold the component and heat sink together, the material then flows into the gaps between surfaces, filling them, and thereby improving thermal pathways. Because the component surfaces come into contact, however, phase-change materials are not used where electrical insulation is required.

Insulating pads. These pads provide thermal conductivity and electrical insulation between high heat load power devices and heat sinks. In installing an insulating pad during board assembly, clamping of the pad to the heat sink is necessary to ensure sufficient contact between the surfaces.

Insulating pads often consist of a silicone binder with glass mesh reinforcement, which combines thermal conductivity and high dielectric strength, along with tear resistance. Reinforcement prevents debris, such as solder balls present on many boards, from puncturing pads, which can result in rapid thermal failure or electrical shorting. In some cases, insulating pads are composites, rather than single-layer structures. As such, they consist of a film dielectric barrier with thermal management material on both sides.

Gap fillers. These elastomers, typically with ceramic particles as fillers, are used to conduct heat across gaps that are relatively large and can vary significantly in width. They are often the choice during assembly where space prohibits the use of a heat sink and where convection air may be insufficient. For such requirements, the elastomer is employed to fill the gap between the hot component and a heat rail or spreader, or even the chassis itself.

With soft silicone gel binders, the materials are flexible enough to fill large gaps with wide tolerance ranges without overstressing components. Gap fillers, however, are less effective than other materials in terms of thermal conductivity, and are limited to applications requiring low to moderate heat dissipation.

Converted materials. A large network of well-known suppliers, such as 3M, Chomerics, Saint Gobain, Laird Technologies, Von Roll, Bergquist, and DuPont, provide a variety of thermally conductive materials for a range of applications. How do electronics manufacturers choose the best thermal management solution for an application? Help in making this crucial decision can be provided by converters, which have in-house teams to analyze the product, available space and heat dissipation requirements (Figure 2).

Fig. 2

Early in the design process, converters team with a customer’s device engineers to identify a material that can handle the thermal challenges posed by a particular product. To prevent puncturing of the thermal management layer, for example, a converter might recommend a glass-reinforced material and then test several different types to find the one best suited to meet the application. Some thermal transfer processes are effective, but do not lend themselves to close tolerance die cutting. Leading converters have the engineering resources to spot such problems and suggest alternatives that will still meet the thermal management requirements.

In addition to helping with new products, converters can recommend thermal management solutions to problems experienced by products already in the field. For example, by assessing whether a particular failure mode was caused by improper material selection or installation/assembly problems, the converter might recommend a material that addresses these issues. The contract manufacturer can then take this recommendation to the product designers for consideration.

Custom conversion tasks, such as lamination and close-tolerance die cutting, are then performed to produce the required thermal management solution. For example, thermally conductive materials may be die cut into a variety of shapes, ranging from gasket-type configurations to scored die-cut parts that can be folded up to fit into an enclosure.

In some cases, the best solution may be a combination of materials – for instance, a composite consisting of a piece of thermal management material and a dielectric film. Laminating the two together produces a single sheet of material to meet heat-dissipation and electrical-insulation needs, thereby reducing handling requirements and increasing manufacturing efficiency.

Multi-step processes are also frequently required to create more complex products that provide both thermal conductivity and dielectric properties. Consider a product that permits positioning of electrically isolated copper interface pads only where power devices will be soldered to heat sinks, thereby minimizing material use and costs for device attachment. One such product includes a layer of polyimide film coated with polyimide adhesive on both sides.

In such cases, the die-cut copper pads are positioned on the composite and attached with polyimide carrier tape. These templates are then laminated to the heat sink to provide a solderable site for the discrete device.

Chris Dobrowolski is market specialist, Northeast, at Fabrico (fabrico.com); ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it ).

 

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