Patent Publication Number: US-2011067841-A1

Title: Heat sink systems and devices

Description:
TECHNICAL FIELD 
     The present invention generally relates to heat sinks. More specifically the subject matter disclosed herein relates to structures and devices for increasing thermal efficiency in a heat sink. 
     BACKGROUND OF THE INVENTION 
     One concern in the design of electronic systems is the possibility of high heat loads created by concentrating a large number of circuits onto a single chip and more chips onto a single circuit board thus possibly reducing the life expectancy of such devices. Without efficient cooling systems, today&#39;s sophisticated electronics would fail far sooner than their design life expectancy. This is especially true when such devices must operate in high heat environments. 
     Typically, coolant systems rely on forced air cooling commonly found in electronic devices such as personal computers, or liquid fluid flow heat exchangers that are commonly found in vehicles and in industrial environments. Liquid flow systems are known to be more efficient that forced air systems, particularly in environments where a source of relatively cold air is not readily available. 
     It is generally known that the amount of heat absorbed by a heat sink is proportional to the surface area a heat exchanger and the temperature differential between the environmental temperature and the coolant temperature in the heat exchanger. Newton&#39;s first law of cooling, states that the rate of heat transfer to a body is proportional to the difference in temperatures between the body and its surroundings. A general formulation of heat transfer may be stated as: 
         dQ/dt=h·A ( T   env   −T ( f   f )) 
     where,
 
Q=Thermal energy in joules
 
h=Heat transfer coefficient
 
A=Surface area of the heat exchanger
 
T(f f )=Temperature of the coolant as a function of flow rate and utilization efficiency.
 
T env =Temperature of the heat environment.
 
     As such, the Newton&#39;s first law states that given a specific heat transfer coefficient, which is fixed by the material used, the rate of heat transfer is proportional to both the surface area of the heat exchanger and the difference between the coolant temperature and the ambient temperature, all else being equal. However, inefficiencies in the coolant flow within the heat exchanger are factors that reduce the effective flow and therefore increase effective temperature of the. Thus larger than necessary heat exchangers may be required. 
     Accordingly, it is desirable to have more efficient heat exchangers. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     SUMMARY OF THE INVENTION 
     It should be appreciated that this Summary is provided to introduce a selection of non-limiting concepts. The embodiments disclosed herein are exemplary as the combinations and permutations of various features of the subject matter disclosed herein are voluminous. The discussion herein is limited for the sake of clarity and brevity. 
     The disclosure herein provides for a novel heat sink. The heat sink comprises a thermally conductive cover including a first inside surface portion and a first plurality of pin fins integral to and depending from the first inside surface portion. The heat sink also comprises a thermally conductive body portion having a second inside surface portion and a second plurality of pin fins integral to and extending from the second inside surface portion in a direction towards the first inside surface portion. 
     In another embodiment, the heat sink comprises a thermally conductive cover including a first inside surface portion and a first plurality of pin fins integral to and depending from the first inside surface portion. The heat sink also includes a thermally conductive body portion having a second inside surface and a first boss protruding from the second inside surface portion, running substantially parallel to at least one of the first plurality of pin fins. 
     In another embodiment, the heat sink comprises a thermally conductive cover including a first inside surface portion and a first plurality of pin fins integral to and depending from the first inside surface portion. The heat sink also includes a thermally conductive body portion having a second inside surface portion and a inside bottom surface portion with a first boss protruding from the second inside surface portion, running substantially parallel to at least one of the first plurality of pin fins and extending from the first inside surface portion to the inside bottom surface portion. In addition, the hat sink provides for a second plurality of pin fins integral to and extending from the bottom surface portion in a direction towards the first inside surface portion. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements. 
         FIG. 1   a  is a cross-sectional view of a heat exchanger in accordance with the prior art; 
         FIG. 1   b  is a cross-sectional view of an exemplary heat sink in accordance with the subject matter disclosed herein; 
         FIG. 1   c  is a cross-sectional view of an alternative exemplary heat sink in accordance with the subject matter disclosed herein; 
         FIG. 2  is an isometric view of an exemplary coolant cavity; 
         FIG. 3  is an plan view of an exemplary embodiment showing thermal bosses and chassis pins fins and their configuration as disclosed herein; 
         FIG. 4  is a broader cross-sectional view of an exemplary coolant cavity with abutting heat sources; 
         FIG. 5  is an exploded isometric view of an exemplary heat sink according to the subject matter as disclosed herein; and 
         FIG. 6  is an isometric view of an alternative embodiment of a heat sink as disclosed herein. 
     
    
    
     DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
     The following detailed description is merely exemplary in nature and is not intended to limit the subject matter or the application and uses of the subject matter described herein below. Furthermore, there is no intention to be bound by any theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     The subject matter disclosed herein relates to a novel heat sink or a system also known as a “heat sink”. A heat sink absorbs heat received from a heat source and dissipates the heat to a mass existing at a cooler temperature. The heat sink may be in thermodynamic contact with a heat source by physically abutting a heat source (e.g. an electronic circuit) such that heat is received by conduction, it may abut an intervening component that is indirectly receiving heat from a heat source or it may receive heat directly over an intervening distance by convection or radiation. A heat sink may absorb and dissipate heat from multiple heat sources. 
     A heat sink may be of any shape, and may be designed to match the shape and/or size of a heat source. As a non-limiting example, the subject matter disclosed herein will refer to a heat sink shaped as a parallelepiped for simplicity. As such, a heat source may be easily attached to the top and to the bottom of the heat sink as well as to one of more sides of the heat sink. Further, those of ordinary skill in the art will recognize that the subject matter disclosed hereinafter may be applied to forced gaseous systems as well as to forced liquid systems. However, only forced liquid heat sink systems will be addressed herein in the interest of brevity and clarity. 
       FIG. 1   a  is a cross-sectional view of a portion of one coolant cavity  105  of a forced fluid heat exchanger  5  (i.e. a heat sink) that is known in the art. The coolant cavity  105  comprises a body portion or chassis  100  and a cover  110 . The cover  110  includes a plurality of uniformly dispersed structures or heat sink pin fins  120  that depend from the underside of the top cover. The heat sink pin fins  120  may be of uniform conical or uniform truncated conical shape and may or may not come in contact with the chassis  100 . 
     The purpose of a heat sink pin fin  120  is to increase the surface area of the heat sink  5  that is in contact with the coolant flow  600  (see  FIG. 6 ). As liquid coolant  600  passes the heat sink pin fin  120 , some of the coolant impinges on the heat sink pin fin resulting in the transfer of heat from the heat sink pin fin to the coolant. As the coolant  600  impinges on the heat sink pin fin  120 , the fluid is slowed by friction resulting in an incremental pressure drop across the heat sink  5 . 
     A gasket or an o-ring  101  is used at the junction of the cover  110  and the chassis  100  to prevent coolant leakage therethrough. The Cover  110  is secured to the chassis  100  by fasteners  103  (See  FIG. 5 ) that may be made of a heat conducting material or a heat insulating material. The fasteners  103  may be any type of suitable fastener and may include bolts, clips, and the like. 
     Those of ordinary skill in the art will appreciate that the coolant inlet pressure, coolant flow rate, pressure drop across the heat sink and coolant inlet temperature are all pertinent variables in determining the amount of heat that a heat sink  5  will absorb. A thermodynamic analysis of any particular heat sink embodiment is beyond the scope of the disclosure and will be omitted in favor of brevity and clarity. 
     However, it should be noted that the prior art heat sink depicted in  FIG. 1   a  creates non-uniform coolant channels, or coolant bypasses  115 ′, between the heat sink pin fins  120  and the sides of the chassis  100 . The heat sink of  FIG. 1   a  also creates non-uniform coolant bypasses between adjacent heat sink pin fins  120 . The term “non-uniform” is being defined herein as a varying width of a coolant channel  115 ′. A non-uniform coolant channel  115 ′ allows some laminar coolant flow between components thereby allowing some coolant to avoid significant thermodynamic contact with a heat transferring component of the heat sink  10 , such as heat sink pin fin  120 . 
       FIG. 1   b  is a cross-sectional view of an embodiment of a heat sink  10  described in accordance with the subject matter being disclosed herein. The heat sink  10  includes a plurality of thermal bosses  150  as integral components of the chassis  100  and are preferably cast therewith. The thermal bosses  150  may be spaced regularly along the internal wall of the chassis  100  of the heat sink and protrude substantially perpendicular to the coolant flow. (See  FIG. 2 ). 
     The thermal boss  150  is a coolant bypass elimination feature which eliminates a dead zone where the coolant flow therein resembles laminar flow with a slowly moving boundary layer. Slowly moving boundary layers tend to act like thermal insulators. By inserting a thermal boss  150 , the coolant flow at the location of the thermal boss  150  is converted from laminar flow to turbulent flow by redirecting the coolant flow along the wall of the coolant bypass device towards a nearby heat sink pin fin  120 . The added surface area of the thermal boss and the additional turbulent flow impinging against the heat sink pin fin  120  further increases heat transfer with the heat sink pin fin. 
     The thermal boss  150  has a draft or a slope extending from the o-ring  101  to the floor of the chassis  100  that matches a taper of the heat sink pin fin  120 . The matching draft and taper create a uniform coolant bypass channel  115  between the thermal boss  150  and the nearest heat sink pin fin  120 . 
     The surface of the thermal boss  150  is a smooth, curvilinear surface that minimizes fluid friction across its surface thereby minimizing its incremental contribution to the pressure drop across the entire heat sink  10 . Non-limiting exemplary shapes of the thermal boss  150  may include a half cone, a tapered wave shape (i.e. sinusoidal), or other shape that may be found to both minimize fluid friction and maintain a coolant channel with a uniform spacing between the thermal boss and a proximate heat sink pin fin  120 . 
       FIG. 1   b  also illustrates a complimentary feature comprising one or more chassis pin fins  160  which are depicted herein as being attached to the floor of the chassis  100 . The chassis pin fin(s)  160  may be cast as part of the floor portion of the chassis  100  or may be added after casting by means known in the art such as by welding or sintering. The chassis pin fin(s)  160  may be of any height and can be used to control the pressure drop across the entire heat sink  10 . One of ordinary skill in the art will appreciate that there is a trade off between heat transfer (i.e. pin fin height/surface area) and the pressure drop across the coolant cavity. 
     Similar to the thermal boss  150 , the chassis pin fin  160  may also be designed such that the draft, or taper, of the chassis pin fin is the same as the taper of a proximate heat sink pin fin  120  so that the width of the coolant channel(s)  115  between a heat sink pin fin  120  and a proximate chassis pin fin  160  is uniform along the length of the chassis pin fin  160 . The uniformity in the width of the coolant channel  115 , when applied across the entire coolant cavity  105 , allows the spacing between the heat sink pins fins  120  and the chassis pin fins  160  and between heat sink pin fins and the thermal bosses  150  to be used as an adjustable manufacturing parameter. The spacing may be used to fine tune the fluid flow through and the pressure drop across the heat sink  10 . 
     Since the heat sink  10  may be constructed to include multiple coolant cavities  105  (See  FIGS. 5 and 6 ), each coolant cavity may include chassis pin fins  160  of a different height than the chassis pin fins of another coolant cavity in the same heat sink  10 . This capability may be useful in controlling the pressure drop and heat transfer rate in one coolant cavity differently as compared to a second coolant cavity. 
     For example, a circuit board A attached to a coolant cavity A may generate a heat load that is greater than a circuit board B attached to second coolant cavity B that may be connected in series. Therefore it may be desirable to include chassis pin fins  160  of a greater height to increase the surface area of the coolant cavity  105  and increase the time that the coolant remains in the coolant cavity A (resulting in a high pressure drop) and include smaller chassis pin fins in coolant cavity B (resulting in a small pressure drop) because the heat load is lower. However, the total pressure drop may remain at a constant designated pressure drop across both coolant cavities. 
       FIG. 1   c  is an alternative embodiment. However, the pin fin(s)  120  in  FIG. 1   c  actually make contact with the floor of the chassis  100  whereas the embodiments of  FIG. 1   b  do not. Actual contact with the chassis  100  prevents coolant flow (i.e. laminar flow) under the pin fin  120 . Eliminating the interstitial space between the heat sink pin fin  120  and the chassis  100  forces the coolant into the turbulent flow which increases the heat absorption efficiency of the coolant  600 . The direct contact also allows for heat transfer directly between the cover  110  and the chassis  100 , if so desired. 
     Direct contact of the heat sink pin fin  120  with the chassis  100  may be desirable in some situations and not in others. For example, in embodiments that include a heat source A (See,  FIG. 4 ) attached to only the chassis  100  or cover  110 , a direct contact may be desirable to eliminate a coolant bypass and to more efficiently dissipate heat to the additional mass of the chassis  100  making the coolant  600  flow more efficient. In other embodiments where there may be a relatively high temperature heat source A abutting the cover  110  and a lower temperature heat source B abutting the chassis  100 , direct contact of the heat sink pin fin  120  with the chassis  100  may cause some undesired heat to be transferred from heat source A (high temp) to heat source B (lower temp). Therefore, a designable interstitial gap between the tip of the heat sink pin fin  120  and the chassis  100  may be found useful in some embodiments. 
       FIG. 2  is an isomeric view of a chassis  100  partially defining the coolant cavity  105  showing several exemplary thermal bosses  150  regularly spaced along a side of the chassis  100 . In this particular embodiment there are illustrated two rows of chassis pin fins  160  between heat sink pin fins  120 . However, in some embodiments there may as fewer than two rows of chassis pin fins  160 . There may be three or more rows in other embodiments. 
     When the cover  110  is installed on top of the chassis  100  with O-ring  101  positioned therebetween, the thermal pin fin(s)  120  that depend from the cover are positioned between the pairs of chassis pin fins  160  with substantially uniform spacing between each of the heat sink pin fins  120  and each of the chassis pin fins along their proximate surfaces. (See also  FIG. 3 ). 
       FIG. 4  is a cross-sectional view of the portion of a coolant cavity  105  depicted in the plan view of  FIG. 3  as viewed from line  4 - 4 . Heat sink pin fins  120   a - d  depend from cover  100  upon which a heat source A may be fixedly attached. Chassis pin fins  160   w - z  extend upward from the floor of the chassis  100  upon which a heat source B may be fixedly attached. The width of the channels between heat sink pin fins a-e  120  and the chassis pin fins u-z  160  is essentially uniform. Exemplary spacing between heat sink pin fins  120  and the chassis pin fins  160  is shown in  FIGS. 3 and 4 . (e.g., See spacing (u-a), (a-v), (v-b), (b-w), (w-c), (c-x), (x-d), (d-y), (y-e) and (e-z)) 
       FIG. 5  presents an exemplary embodiment of a single pass heat exchanger manifold  510  comprising three coolant cavities  105 . Each coolant cavity  105  comprises a plurality of thermal bosses  150  and a cover  110  or base plate from which depends the heat sink pin fins  120 . Each cover  110  is thermodynamically attached to a heat source (A, B, C) which may be an electronic power module or other heat source. The heat exchanger manifold  510  also comprises a second cover  100  wherein is configured three sets of chassis pin fins  160  that mesh with a corresponding set of heat sink pin fins  120  thereby creating uniform coolant channels therebetween when assembled. In this particular embodiment, coolant  600  enters the coolant inlet port  512 , passes through each coolant cavity  105  in succession, and exits the heat exchanger manifold  510  through coolant outlet port  514 . While flowing through each coolant cavity  105 , the coolant  600  is evenly dispersed in a turbulent manner amongst the heat sink pin fins  120  where heat transfer takes place. The turbulence is maximized by the presence of the thermal bosses  150  and the chassis pin fins  160  thereby allowing all of the coolant  600  to impinge upon the plurality of heat sink pin fins in each coolant cavity. One of ordinary skill in the art will recognize the chassis pin fins  160  and the thermal bosses  150  also transfer heat to the coolant. 
       FIG. 6  presents an exemplary embodiment of a double pass heat exchanger manifold  510  comprising two manifolds including six coolant cavities  105 . Each coolant cavity  105  comprises a plurality of thermal bosses  150  and a cover  110  or base plate from which depends the heat sink pin fins  120 . Each cover  110 , opposite the heat sink pin fins  120 , is thermally connected to a heat source (A, B, C) which may be an electronic power module or other electronic circuit board. Each heat exchanger manifold  510  also comprises a second cover  100  wherein is configured three sets of chassis pin fins  160  that mesh with the heat sink pin fins  120  depending from each cover  110 . 
     In this exemplary embodiment coolant  600  enters the coolant inlet port  512 , passes through each coolant cavity  105  in succession and exits the heat exchanger manifold  510  through coolant outlet port  514 . While flowing through each coolant cavity  105 , the coolant  600  is evenly dispersed in a turbulent manner amongst the heat sink pin fins  120  where heat transfer takes place. The turbulence is maximized by the presence of the thermal bosses  150  and the chassis pin fins  160  thereby allowing all of the coolant  600  to impinge upon the plurality of heat sink pin fins  120  in each coolant cavity. 
     While  FIGS. 5 and 6  illustrate two exemplary embodiments of a heat exchanger, it will be appreciated that any number of manifolds may be connected in series, in parallel, or in a combination of series and parallel configurations and fall within the intended scope of the disclosure herein. It will further be appreciated that any number of coolant cavities  105  may comprise a heat exchange manifold  510 , and the coolant cavities may be of any desired shape or configuration as may be required. 
     Because the heat exchangers disclosed herein are positive pressure systems (i.e. pump operated), the heat exchangers may operate in any physical orientation (e.g. vertically, horizontally or upside down). The heat exchangers may also operate in a vacuum and in high vibration environments and are therefore suitable for space flight and for general aviation. Further, the subject matter disclosed herein may operate in systems open to the atmosphere or in closed systems where any atmospheric gasses are vacated from the system. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment or exemplary embodiments. It should be understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.