Heat sink and electronic assembly including same

Details Heat sink and electronic assembly including same Heat sink and electronic assembly including same

2001-07-17

2002-11-05

Tolin, Gerald (Department: 2831)

Electricity: electrical systems and devices

Housing or mounting assemblies with diverse electrical...

For electronic systems and devices

C029S890030, C165S080300, C361S703000, C454S184000

Reexamination Certificate

active

06477053

ABSTRACT:

TECHNICAL FIELD
The present invention relates to heat sinks and more particularly to a heat sink with one or more printed wiring boards mounted thereto.
BACKGROUND INFORMATION
Electronic equipment, such as that used in the telecommunications industry, generates a significant amount of thermal energy or heat. One example of such electronic equipment is an optical amplifier used in a fiber optic telecommunication system. Advances in technology have resulted in smaller and more powerful electronic components, which generate more heat in a given smaller area. Excessive heat in electronic equipment can degrade performance and reliability of electronic components or cause a complete malfunction. In an optical amplifier module, for example, heat is generated by localized heat sources, such as laser pumps and other electronic components on a printed wiring board (PWB). The PWB is enclosed within a box, housing or chassis. One type of laser pump limits the maximum surface temperature rise to below 15° C. over a maximum ambient of 50° C.
The heat generated by the electronic equipment should be dissipated to prevent damage to and failure of the electronic equipment. Various approaches have been used to dissipate heat generated by electronic components. One approach is to use a heat sink to spread the heat in combination with fan units to provide forced convection cooling. One drawback of a forced convection system is the requirement of scheduled maintenance (e.g., a filter change), failures (e.g., fan motor malfunction and infrastructure to support the fan unit monitoring/alarms. The heat sinks used with electronic equipment employing forced convection cooling systems are typically not able to efficiently spread and dissipate heat from electronic equipment without fan units, for example, using natural convection and radiation cooling methods. Because of practical limitations on physical size, the majority of the space in the equipment is usually allocated to electronics rather than to relatively large passive cooling elements.
According to the existing approaches to heat dissipation using heat sinks, the heat sinks are typically mounted to the printed wiring boards (PWBs) using spacers or standoffs. The standoffs or spacers provide individual fastening points, but not continuous support. Between the spacers, the PWB is thus prone to deflection or bowing, making it difficult to maintain the thermal contact between the heat sink and the various surfaces of the electronic components having varying heights. In addition, the space or gap between the spacers is open and air stagnates in an uncontrolled fashion. In some cases, heat sinks are also electrically floating and/or do not span the full surface area of the PWB to provide an effective EMI shield and insure that there is no safety hazard with floating (ungrounded metal).
Accordingly, there is a need for a passive heat sink, capable of spreading and dissipating heat from electronic equipment (i.e., without having to use active fan units). There is also a need for a heat sink that provides mechanical and/or electrical functionality in addition to the improved thermal management.
SUMMARY
In accordance with one aspect of the present invention, a heat sink comprises a base portion having first and second sides and ventilation apertures extending through an approximately central section of the base portion. Mounting ridges extend from the first side of the base portion for defining at least one channel on the first side. Each channel is fluidly connected to the ventilation apertures. The mounting ridges include mounting surfaces for mounting to at least one electronic component board, which forms the other side of the channel(s). Extended heat dissipating surfaces extend from the second side of the base portion. The apertures are located between the extended heat dissipating surfaces. The ventilation apertures create a “chimney effect” and the channel(s) enhance(s) the chimney effect by channeling air beneath the heat sink to the ventilation apertures thus eliminating the presence of stagnating hot air.
In one preferred embodiment of the heat sink, the mounting ridges are formed as one piece with the base portion. At least one of the ventilation apertures is preferably located between each of the extended heat dissipating surfaces, such as fins.
In accordance with another aspect of the present invention, a heat sink comprises a base portion having first and second sides and at least first and second levels of mounting surfaces on the first side. The first level mounting surfaces are located at a first level for supporting a first electronic component board generally at the first level, and the second level mounting surfaces are located at a second level for supporting a second electronic component board generally at the second level. At least some of the first level mounting surfaces are formed on mounting ridges defining, along with the first board, at least one channel on the first side of the base portion. Ventilation apertures extend through an approximately central section of the base portion in fluid communication with the channel (s). Most preferably, extended heat dissipating surfaces extend from the second side of the base portion.
In accordance with a further aspect of the present invention, an electronic component board and heat sink subassembly comprises a heat sink including a base portion having first and second levels of mounting surfaces, a first electronic component board mounted to the first level mounting surfaces generally at a first level, and a second electronic component board mounted to the second level mounting surfaces generally at a second level. Mounting the electronic component boards directly to the heat sink allows the relatively flexible boards to conform to the unevenness of the heat sink both statically and dynamically, including thermal expansion and contraction. This enhances the surface overlap or contact between the heat sink base portion and the electronic components requiring cooling.
In one preferred embodiment of this subassembly, the first level mounting surfaces are formed on mounting ridges, which define at least one channel on the first side of the base portion. Ventilation apertures are preferably located between the extended heat dissipating surfaces and extend through an approximately central section of the base portion in fluid communication with the channel(s). Substantially flat electronic component support pads are preferably located on the first side of the base portion in contact with respective electronic components on one or both of the electronic component boards.
In accordance with a further aspect of the present invention, an electronic assembly comprises a heat sink having a base portion with mounting surfaces and extended heat dissipating surfaces. At least one electronic component board is mounted to the mounting surfaces, and a chassis contains the heat sink and the electronic component board. At least one isolator bushing is mounted to one of the extended heat dissipating surfaces, and at least one pin extends from the chassis into the isolator bushing for mechanically securing and isolating the heat sink. The heat sink is, in addition, grounded to the electronic component board via the mounting screws mounting the board to the heat sink and is electrically isolated from the chassis.
According to one preferred embodiment of the electronic assembly, insulated fasteners are secured to the first side of the base portion, preferably within counterbores. A tray is secured to the insulated fasteners such that the insulated fasteners electrically isolate the heat sink from the tray.
According to yet another aspect of the present invention, a method of assembling an electronic assembly comprises mounting the first electronic component board to the first level of mounting surfaces on the heat sink and securing insulated fasteners to the first side of the heat sink. The second electronic component board is mounted to the second level of mounting surfaces on the heat sink with holes in the

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