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New Zealand No. 314487 International No PCT/
TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION
Priority dates 28 03 1996,
Complete Specification Filed 26 03 1997
Classification (6) H05K7/20, H05K9/00, H01L23/36.552
Publication date 29 September 1999 Journal No 1444
new zealand patents act 1953
COMPLETE SPECIFICATION
Title of Invention
Heat sink fins with built-in EMI/RFI sheilding ventilation holes
Name, address and nationality of applicant(s) as in international application form
MOTOROLA, INC , a Delaware corporation of 1303 East Algonquin Road, Schaumburg, Illinois 601 96, United States of America
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Patents Form No 5 Our Ref JT208035
Patents Act 1953 COMPLETE SPECIFICATION
HEAT SINK FINS WITH BUILT-IIM EMI/RFI SHEILDING VENTILATION HOLES
We, MOTOROLA, INC , a corporation of the state of Delaware, USA of 1303 East Algonquin Road, Schaumburg, Illinois 60196, United States Of America hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the 1
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PT05A37849
(followed by page 1a)
HEAT SINK FINS WITH BUILT-IN EMI/RFI SHIELDING
VENTILATION HOLES
1. Field of the Invention
This invention relates generally to electromagnetic interference (EMI) and radio frequency interference (RFI) shielding techniques, and more 10 particularly, to an EMI/RFI filter that doubles as a heat sink
2. Background of the Invention
EMI/RFI filters are well known and are used to block unintentional electromagnetic radiation from electronic equipment which can interfere with external equipment such as radio, television, or computer units Additionally, EMI/RFI radiation is capable of interfering 20 with internal circuitry within the unit generating the EMI or RFI In some environments, an electronic module is constructed to double both as an EMI shield as well as a heat sink, designed to transfer heat away from the electronic module 25 A dual function electronic module is described in U S Patent no 5,030,793 to McCarthy The McCarthy module attenuates EMI radiation in the range of frequencies generated by or disruptive to the electronic equipment disposed within a main cavity, and also 30 transfers heat way from the electronic module via air passages provided by upstanding heat sink fins that permit the flow of forced air to pass between the heat sink fins and into the main cavity In this manner, the cooling air removes the heat absorbed by the heat fins,
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as well as removes heat from the electronic equipment disposed within the cavity
Due to the heat sink dimensioning, the air passages act like an electromagnetic waveguide array 5 that filters and attenuates a predetermined range of EMI radiation McCarthy provides the equations for calculating fin waveguide dimensions to best approximate the required operating EMI cutoff frequency and an EMI attenuation/absorption loss level for a 10 particular electronic module In this regard, assuming a known EMI radiation range of frequencies, and a desired EMI cut-off frequency, appropriate heat sink fin dimensions may be calculated, in the manner described Due to space limitations and cost 15 restrictions, it is often times desirable to add both fins and ventilation holes to the module to allow heat transfer via convection from the module and convection via air exchange within the module, as is provided in the partial-section wall 10 of a conventional heat sink 20 shown in FIG 1
When including both fins 11 and holes 12, it is common practice to die cast or machine ventilation holes 12 directly in the base surface area 13 separating adjacent ones of the fins 11 The direction of atr flow 25 through the fins 11 is shown, in the illustration, by the direction of the large arrow pointing out of the page Similarly, the direction of air flow through the ventilation holes 12 is shown by the small, oppositely pointing double arrows 30 The holes 12 in prior art FIG 1 are EMI/RFI-
shielding/ventilation holes and dimensioned accordingly using known waveguide array principles to provide 1 1 °^\
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sufficient levels of EMI attenuation Assuming a knownv
EMI radiation range of frequencies, and thus a desired t i _ „ , ] |
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cut-off frequency, an appropriate opening (circular, rectangular, or the like) thickness dimensioning may be readily calculated per the equations described for example in the reference entitled, "A Handbook Series On Electromagnetic Interference and Compatibility", EMI Control Methods and Techniques, Vol 3, Don White Consultants, Inc , 1973, pp 11-1-119
Generally described, the range of EMI/RFI attenuation levels for particular applications are a function of the thickness or cross-sectional diameter (9) of each hole and the hole's wall depth (d) Defining the area (A) of a circular cylindrical hole as A=32/4*tc, it follows that air flow increases in squared relation relative to an increase in the diametrical cross-sectional width of the hole It should be readily apparent, therefore, that the largest calculated hole width, at a pre-defined cut-off frequency, also corresponds to a width dimension at which maximum possible air flow is available
Additionally, in order to provide adequate EMI/RFI shielding with such a size hole, the depth (d) of the hole must be large enough to provide the desired level of attenuation shielding required for a particular design Varying the depth (d) alters the attenuation provided by the waveguide array which is comprised of the ventilation hole Thus, the greater the hole depth, the greater the effective attenuation effect and the greater the resulting thickness of the heat sink base It follows, therefore, that while it may be preferable in some applications to make the ventilation holes 12 as deep as possible (providing maximum attenuation), thickening the base creates a heavy, costly and over designed part
One alternative to improving heat transfer using ventilation holes of specified maximum thickness so as not to compromise EMI attenuation levels, involves incorporating many small holes into the surface areas 13 5 between the fins 11 to provide adequate air flow and heat dissipation The obvious disadvantage with adding many small holes is that it makes casting of the unit impossible, requiring each small hole 12 to be separately drilled or machined adding significant costs and time to 10 the manufacturing process
Accordingly, a need exists for a dual function electronic module that is economical to manufacture,
easy to cast or machine, compact, and capable of dissipating large amounts of heat efficiently without 15 compromising EMI or RFI allowable attenuation levels
Summary of the Invention
This need and others are substantially met by 20 providing heat sink fins with built-in EMI/RFI shielding ventilation holes, which holes are dimensioned using waveguide principles known in the art
This is achieved by providing an electronic module defining a cavity for receiving therein electronic components, said module including a heat transfer portion for blocking electromagnetic radiation and for transferring heat generated by the electronic components away from the module, said heat transfer portion comprising a base of fixed thickness having a first side and an opposite side, at least one heat transfer element formed integral with said base and extending a predetermined distance outwardly from said first side of said base, travelling away from the cavity, to define a heat radiating surface and a peak portion thereof, and at least one ventilation hole including first and second ends communicating with each other with said first end being at the opposite side of said base and the second end being at the peak portion of said at least one heat transfer element, said ventilation hole
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being operable as a frequency selective waveguide of predetermined cutoff frequency and at predetermined levels of electromagnetic attenuation,
wherein said base and said at least one heat transfer element are all thermally and electrically conductive
FIG 1 illustrates an isometric, fragmentary view of a conventional EMI-shielded electronic module 10 provided with heat sink fins and including ventilation holes located at the base area separating adjacent fins, FIG 2 illustrates an isometric, fragmentary view of an EMI-shielded electronic module constructed in accordance with a first embodiment of the present 15 invention,
FIG 3 illustrates an isometric view of an elongated fin with ventilation holes formed therethrough in accordance with a second embodiment of the present invention, and 20 FIG 4 illustrates an isometric view of parallel disposed pin-shaped fins with ventilation holes formed therethrough in accordance with a third embodiment of the present invention
Detailed Description of the Preferred described with reference to FIG 2, where there is 30 illustrated a fragmentary portion of an EMI-shielded electronic module 20 constructed in accordance with a first embodiment of the present invention constituted in the illustrative embodiment by an inner
Brief Description of the Drawings
Embodiments
The present invention can be more fully
The module 20 has a large inner cavity 21
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wall surface of a heat transfer portion 22 Heat transfer portion 22 defines a base 23 of fixed thickness having on a first side 24 thereof heat transfer elements 25, with the opposite side 26 forming the partially enclosed 5 cavity 21 This cavity 21 is adapted for receiving electronic equipment or components (e g , printed circuit boards) to provide shielding from electromagnetic radiation, in the manner to be described below
In the illustrative embodiment of FIG 2, the 10 heat transfer elements 25 are elongate-shaped fins defining plural heat radiating surfaces 27 which extend no more than a first length substantially perpendicularly to, and outwardly from, the base 23 and which cooperate to form air flow passageways During operation of the 15 electronic module, air (forced or natural) travels through the passageways as heat generated by the electronic components is transferred away from the module 20 via convection The direction of air flow, in the instant case, is shown by the large broken arrow seen pointing to 20 the right
The heat transfer elements 25 further define partial outer circular-cylindrical surfaces 28 and inner circular-cylindrical wall surfaces 29 which cooperate to define ventilation holes 30 The holes 30 are open ended 25 with the respective ends communicating with one another Holes 30 function as through-holes through which air is allowed to flow from/to the outside to/from the cavity 21
Assuming a known EMI radiation range of 30 frequencies, and a desired EMI cut-off frequency,
appropriate fin 25 dimensions can be calculated per the discussion in U S patent no 5,030,793, incorporated herein by reference Because ventilation holes 30 function as electromagnetic waveguide arrays, it is
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equally important to dimension the holes 30 so that the proper radiation attenuation and frequency attenuation is achieved. However, unlike prior art module 10 where the additional air flow holes 12 are provided in the area 13 5 between adjacent fins 11 (see FIG 1), because the holes 30 of the present illustrative embodiment are combined (built-in) with the fins, the depth (d) to cross-sectional diameter ratio is much improved (increased) making it possible to provide larger diameter holes, up to a 10 maximum possible cross-sectional width (as determined by desired cut-off frequency requirements), without the added cost of drilling many small holes, and without compromising the EMI/RFI shielding properties of the module 20
The module 20 may be made by a single casting of thermally conductive material, such as aluminum, zinc, magnesium, brass, or iron The module 20 could also be machined from a single piece of thermally conductive material, however, when 20 manufacturing large quantities this method is significantly more expensive than the casting method Alternatively, the module 20 could be made by fabricating the base 23 and the heat transfer elements 25 separately, and assembling these pieces using a 25 secondary process The base may be fabricated by casting or machining and the heat radiating surfaces may be fabricated using casting, machining, extrusions, stampings, or other available technologies
Referring to FIGS 3 and 4, there are shown 30 alternative fin constructions provided with built-in ventilation/RFI shielding holes More specifically, FIG 3 shows a fin construction comprised of an elongated fin 40 having a smooth outer surface 41 disposed on a module base 42, shown in fragmentary view Similarly,
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FIG 4 shows a pin-shaped fin 50 defined by a substantially circular-cylindrical wall surface 51 disposed on a module base 52, also shown in fragmentary view It should be appreciated that the waveguide array 5 frequency cut-off principles described above in connection with the fins 25 shown in FIG 2 are equally applicable to the similarly constructed fin designs of FIGS 3 and 4
With the aforementioned electronic modules, 10 more effective heat sinking of thermally dissipative devices is made possible in a smaller volume at much lower costs, as two types of convection are provided, i e , through fin passageways 27 as well as through fin ventilation holes 30 15 In summary, an electronic module constructed in accordance with the present invention is economical to manufacture, compact and capable of dissipating heat efficiently Furthermore, excellent shielding is provided as the built-in ventilation holes 30 act as a waveguide 20 with a very high cut-off frequency Larger diameter holes can be cast directly during module 20 fabrication, saving money and labor The diameter (3) squared (A=92/4*7u) nature of the area (A) of a ventilation hole built into a fin allows for significant increases in air 25 flow in comparison to the smaller holes of prior art modules that don't have built-in holes Moreover, well established waveguide array principles make the calculations of hole and fin dimensioning easy to determine and allow for many varied constructions of 30 fins and hole sizes
The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics The described embodiments are to be considered in all respects only as illustrative
9 31448 7
and not restrictive. The scope of the invention is,
therefore, indicated by the appended claims rather than by the foregoing description All changes which come within the meaning and range of equivalency of the 5 claims are to be embraced within their scope