Gels, special sinks shed IC heat
By Rick DeMeis -- Design News, May 17, 1998
Menlo Park, CA--Nooks and crannies may be great for English muffins but even the smallest of imperfections over the surface of a heat sink degrades heat transfer. Electronics packages are becoming smaller and smaller, and, just as in dieting, every calorie shed becomes that much more important. Now Raychem has combined solid and liquid material properties in its HeatPath(TM) family of thermally conductive gels as an alternative to elastomeric pads, greases, and adhesives in attaching heat sinks to electronic components.
The combined polymer and extender of the gel produces a conformable characteristic enabling it to easily "flow" and fill all gaps between a component and heat sink--compressibility is more than five times that of elastomers. The low compressive force needed for mating lessens the possibility of damage to delicate parts. Thus thermal resistance is lowered while obviating the need for clamping systems, tight tolerance control, and heating or curing during assembly. Even lower-cost heat sinks having rough surfaces can be used for efficient heat dissipation. "In addition to providing such design flexibility and simple installation, HeatPath allows easy re-entry for board work or repairs," says Greg Bischak marketing manager for Raychem Electronics OEM Components. The cohesive, cross-linked gel does not flow away over time, permitting such reuses.
Bischak notes, "The conformable, soft, and 'tacky' gel is replacing one company's use of thermally conductive grease to 'clean up' the manufacturing process--making it more consistent and reliable with less contamination."
HeatPath comes in three series. The 1000 Series is a thin thermal gel on a woven fiberglass carrier 0.25- to 1.0-mm thick--geared for pc-board use in mating heat sinks with surface-mount devices (SMDs). An open-cell reticulate carrier, 1.5- to 4-mm thick, the 2000 Series fills large air gaps, removes hot spots, and handles heat transients in laptop computers and other electronic modules. Finally, the 3000 Series is cast into sheets or custom profiles in large thicknesses from 5 to 15 mm.
And in the specific area of heat sinks, innovative designs are available for new electronics configurations being used by design engineers. For instance, ball grid arrays (BGAs), with their high-density, under-chip input/output contacts, are seeing wide acceptance for SMD integrated-circuit (IC) packaging. For compact BGA configurations where height and thus airflow are restricted--such as PCI, network routing and switching, and automotive GPS applications--Aavid Thermal Products (Concord, NH) has released its OptiPin(TM) line of heat sinks. These feature cross cuts, forming thin pins, rather than conventional fin surfaces, that allow less restrictive airflow in any direction for improved heat dissipation.
The combination of pin size and spacing is the key, according to Christopher Chapman, Aavid computer industry manager. If conventional heat-sink fin spacing becomes tight, the air bypasses and flows around the sink, he notes. But in going to a pin design, "Adding pins tightens spacing, producing high surface area, but restricting airflow. You can thin down the pins, giving the proper spacing for good flow." A unique manufacturing process allows the company to precision-form the pins in high volume production, keeping costs down.
First product in the OptiPin family is for applications using Intel's i960(reg) RP/RD microprocessor. "It is Intel-recommended as a reference design," says Chapman. The heat-sink maker says it outperforms a conventional sink of the same volume (length, width, and height) by 19%. At 8W, for 150 ft/min (cfm/duct area) of airflow, the Aavid OptiPin produces an 11-degree (Celsius) temperature reduction in a 0.35-inch height space. The sink is attached on top of the Intel processor by double-sided conductive tape, adhesive, or epoxy. Other applications can use special brass pins that maintain proper contact force and prevent warping, giving designers flexibility in placement of components.
In concluding, Chapman says "In a non-critical, non-optimal installation, you probably won't see any difference between heat sinks. But in a compact application, with say 6 to 7 mm of headroom, this is the technology that will keep boards cool."
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