Patent Publication Number: US-2010108292-A1

Title: Heat sink system with fin structure

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to heat fins, and particularly to heat sinks having a plurality of fins. 
     2. Description of the Related Art 
     Integrated circuits and printed circuit boards used in electronics increasingly require high-performance heat sinks that have a heat conducting base plate in thermal communication with a fin structure. Air introduced to the resulting system, often times using a forced-air cooling assembly such as a fan, accomplishes the thermal transfer by carrying away heat energy from the heat sink and fins. The air is directed either at the base plate (in an impinging air flow configuration) or over the base plate (in a parallel flow configuration) and channeled through the fins to carry away the heat. Typically, the fins are metallic and may employ a combination of different lengths and widths to optimize such thermal transfer characteristics. 
     Unfortunately, such fin and base-plate arrangements typically suffer from high pressure drop challenges resulting in increased fan size and increased fan input power requirements. In addition, the power dissipation requirements of electronic devices are increasing at a rapid pace, while their sizes continue to shrink to meet consumer demand. Conventional air-cooling methods are currently limited to thermal transfer power dissipation densities of 5-10 Watts/cm 2 , while liquid cooling techniques that would allow greater power dissipation are expensive and may lower system reliability. 
     A need still exists, therefore, for an air-cooled heat sink with reduced pressure drop and increased thermal transfer characteristics. 
     SUMMARY OF THE INVENTION 
     A heat sink apparatus is disclosed for use with integrated circuits, printed circuit boards and other heat sources. In one embodiment, the heat sink includes a heat conductive base plate and a plurality of fins in thermal communication with the heat conductive base plate. The plurality of fins is configured to form a plurality of curved and branching channels extending radially on the base plate. At least two of the plurality of fins are configured with a gap between them to trip a gas boundary layer formed on a first one of the at least two fins, when a gas boundary layer is present. 
     In one embodiment of a method of cooling a heat conductive base plate, the method includes conducting heat from a heat conductive base plate to a plurality of curved fins, blowing air onto a face of the heat conductive base plate, directing the air through first-tier channels established by the plurality of curved fins to induce primary and vortex flow patterns, directing the air through second-tier branching channels formed by the plurality of curved fins to reduce buildup of back pressure as the air moves across the heat conductive base plate adjacent to the curved fins and passing the air across gaps formed in the plurality of curved fins to trip a developing thermal boundary layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a perspective view of a heat sink that has, in one embodiment, a fin structure forming curved and branching channels extending radially on a heat conductive base plate; 
         FIG. 2  is a top plan view of one embodiment of a fin structure for use in a heat sink. 
         FIG. 3  is a perspective view of the embodiment shown in  FIG. 1  that has a forced-air fan positioned on top of the fin structure; 
         FIG. 4  is a cross-sectional view of the embodiment shown in  FIG. 2  along the line  4 - 4  illustrating primary and secondary gas flow forming a vortex pattern; 
         FIGS. 5   a  and  5   b  are perspective and enlarged views, respectively, of a heat sink that has fins comprising a plurality of pin fins; 
         FIG. 6  is a perspective view of, in one embodiment, a heat sink that has a plurality of fins arranged in a parallel flow configuration; 
         FIG. 7  is a perspective view of the embodiment shown in  FIG. 5 , illustrating a metering port positioned between the plurality of fins and a forced-air fan; 
         FIG. 8  illustrates, in one embodiment, dimples formed on a heat conductive base plate to trip thermal and velocity boundary layers on a heat sink that uses a fin structure. 
         FIG. 9  is a flow diagram illustrating one embodiment of a heat sink method that uses branching and curved fins primary and vortex flow patterns while using gaps in such fins to trip a developing thermal boundary layer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A heat sink, in accordance with one embodiment, includes a plurality of heat conducting fins coupled to a heat conductive base plate, with the fins arranged to form curved and branching channels that extend radially on the base plate. The fins are curved to produce primary and secondary flow characteristics when air is forced through them to provide advantageous thermal transfer characteristics between the air, fins and base plate. Each transition to a new tier of branching channels also results in development of a new thermal boundary layer to provide advantageous thermal transfer characteristics. Each subsequent tier of branching channel has a reduced cross section area for a reduced gas flow rate through such channel. The number of tiers of branching channels is preferably four or higher, but may be designed based on heat load requirement and available fan size. In the preferred embodiment, the adjoining and discontinuous fins are also configured with a gap between them to trip flow and thermal boundary layers to further improve thermal transfer characteristics. Although the preferred embodiment is described for use with gas, it is contemplated that the apparatus may also be used in a liquid environment to form a liquid-cooled heat exchanger. The curved fins, multi-scale branching channels and fin gaps provide a heat sink with reduced back pressure and increased thermal transfer characteristics verses previous heat sink designs allowing the use of smaller fans (in an air-cooled format) for the same heat transfer coefficient and improving overall system reliability. 
       FIG. 1  illustrates a heat sink  100 , preferably an air-cooled heat sink, designed to work with a forced-air fan that blows air in an impinging manner onto its face. Although the following system is described for use with air, it is understood that the term “air” encompasses any number of gases including atmospheric air, nitrogen, helium or any of the noble gases. A plurality of fins  105  are arranged in a radially extending formation on a heat conducting base plate  107 . The heat conducting base plate  107  is preferably solid and formed from metal such as copper, although other metals or composite materials may be used, such as aluminum, stainless steel or nickel. The fins  105  are preferably metal that are either cast on the heat conductive base plate  107  for good thermal connection, or manufactured separately and coupled to the base plate  107  using a soldering or brazing technique for good thermal connection with the base plate  107 . Similar to the base plate  107 , the material of fins  105  is preferably copper, although other metals or composite materials may be used such as aluminum, stainless steel or nickel. Alternatively, the base plate  107  and fins  105  may each be formed from other heat conducting materials, such as silicon, and provided with a suitable thermal coupling to the heat conducting base plate  107 . Although the fins are preferably formed from the same material, different materials may be used to suitably modify thermal conductivity and consequent uniformity of base plate temperature. Spacing between adjacent fins  105  is preferably between 1 mm and 10 mm. The height H of each fin is preferably constant and is dictated by the spacing (b) between adjacent fins  105 . The aspect ratio, defined as H/b, is preferably 1-5, but may approach 10 before advantageous secondary air flow characteristics (see below) are disrupted. In an alternative embodiment, fin height H increases radially outward from the center of the heat sink to promote greater air flow over the base plate  107  for more uniformity of base plate temperature and to lower the pressure drop. In one heat sink designed to receive approximately 2-23 Kg/hour of blown air, the base plate is preferably square and has a length L of approximately 5-25 cm and a width W of approximately 5-25 cm. The thickness of each fin is approximately 1-2 mm. In an alternative embodiment, the fins may be miniaturized to make form a micro-scale heat sink. 
       FIG. 2  illustrates a top plan view of one embodiment of a fin structure for use with a heat sink  200 . In the embodiments illustrated in  FIGS. 1 and 2 , the fins  105  are splayed radially outward from an interior region  202  of the base plate. The fins  105  are configured in a repeating and multi-tiered branching pattern, one example of such illustrated in region  204 . First and second curved fins ( 206 ,  208 ) form a first channel inlet  210  at proximal ends abutting the interior region  202  to receive a flow of air. This first channel is referred to herein as a first-tier branch channel  211 . A third curved fin  212  is positioned between the first and second curved fins ( 206 ,  208 ) with a proximal end  214  of the third curved fin  212  set back from the first channel inlet  210  to establish a second channel inlet  216  leading to a second-tier branch channel  217 . This second-tier branch  217  receives a portion of the air flowing through the first-tier branch channel  211 . Preferably, a fourth curved fin  218  is positioned between the first and third curved fins ( 206 ,  212 ), with a fourth curved fin proximal end  220  set back from the second channel inlet  216  to establish a third channel inlet  222  leading to a third-tier branch channel  223 . Additional fins may be provided (indicated as dash-lined fins) between pairs of each curved fin ( 206 ,  208 ,  212 ,  219 ) to provide additional-tiered branch channels. Each curved fin ( 206 ,  208 ,  212 ,  218 ) is preferably configured with one or more gaps  224  along their lengths to trip respective developing thermal and velocity boundary layers to improve thermal transfer between the respective fins and air flow (or fluid, if used in a fluid system) moving across their surfaces. The gaps preferably extend up and perpendicular from the base plate with a length equal to the height H of each fin. In an alternative embodiment, the gaps extend only a portion of the height H of each fin or along a different vertical portion of each fin. An increase in the width and/or length of the gap would increase tripping effects on the respective boundary layers, while a reduction in the width and/or length of the gap would reduce tripping effects of the respective boundary layers. 
     The described repeating and multi-tiered branching pattern  204  preferably repeats about the periphery of the heat sink  200  to provide a heat sink with reduced back pressure and increased thermal transfer characteristics verses previous heat sink designs. In an alternative embodiment, the relative lengths and relative positions of the curved fins in other sectors of the heat sink are changed to modify flow characteristics according to the fins&#39; use in a fan or fluid system to properly distribute heat energy absorbed from the heat conducting base plate  107 . 
     During operation, the heat conductive base plate  107  is in thermal communication with a heat source such as an operating integrated circuit or printed circuit board to absorb dissipated heat energy. Dissipated heat energy is conducted from the conductive base plate to the plurality of curved fins  105  positioned radially across the base plate  107  to improve the heat removal capacity of the heat sink system. Air is blown directly onto the face of the conductive base plate (impinging air flow configuration). Air is directed in a primary flow pattern through channels ( 211 ,  217 ,  223 ) formed by the curved fins, with the curvature of the fins inducing a secondary flow pattern. Branching channels formed by the curved fins reduce buildup of back pressure as the air moves across the heat conductive base plate adjacent to the curved fins. Developing thermal and velocity boundary layers are repeatedly tripped by passing the air across gaps formed between the plurality of curved fins. 
       FIG. 3  illustrates a heat sink that has a forced-air fan assembly positioned on top of the fin structure, first illustrated in  FIG. 1 , to establish an impinging air flow heat sink structure. An air flow directing port  300  is positioned between a forced-air fan assembly  302  and the fins  105  to direct air drawn through the fan  302  to impinge directly on the base plate  107  and through the fins  105 . The fan  302  may be connected to the directing port  300  and fin/base plate ( 105 ,  107 ) by any suitable means, such as with a threaded bolts or an adhesive or soldering coupling (not shown). The fan  302  and directing port  300  are configured to maximize the flow of air through the fins  105  to achieve improved heat transfer between heat conductive base plate  107 , fins  105  and flow of air provided by the fan  302 . 
       FIG. 4  is a cross-sectional view along the line  4 - 4  in  FIG. 2  illustrating primary and secondary air flow patterns. Impinging air flow  400  is directed from the forced-air fan (See  FIG. 3 ) to impinge upon the base plate  107 . A substantial portion of the air flow  400  is directed between first and second curved fins ( 206 ,  208 ). As the air flow  400  is directed by the curved fins ( 206 ,  208 ), secondary flow develops in a vortex pattern  402  to increase thermal transport between the air  400 , base plate  107  and curved fins ( 206 ,  208 ). Each gap  224  also serves to trip developing thermal and velocity boundary layers adjacent the curved fins ( 206 ,  208 ) and base plate  107 . Although curved fins ( 206 ,  208 ) are illustrated having a uniform height H, as described above for  FIG. 2 , they may each be of increasing heights to lower the pressure drop and to better insure the primary and secondary flow remains between the curved fins ( 206 ,  208 ). 
       FIGS. 5   a  and  5   b  illustrate one embodiment of a heat sink  500  that has fins comprised of pin fins. In this embodiment of an air-cooled heat sink, designed to work with a forced-air fan for direct impinging of air on base plate  107 , a plurality of fins  502  are arranged in a radially extending and branching formation on the heat conducting base plate  107 . Each individual curved fin  502  is comprised of individual pin fins  504  to provide a greater surface area to volume ratio for better system thermal performance. The pin fin configuration also allows more stagnation area of the impinging air flow on the heat sink base plate  107  for a higher local heat transfer coefficient. In the embodiment illustrated in  FIG. 5a , primary flow (not shown) is induced between adjacent fins  502  and is induced between adjacent pin fins  504 . The flow of air past the pin fins  504  also results in a constantly developing thermal boundary layer as the boundary layer is repeatedly tripped by the pin fins  504 . The pin fins  504  are preferably formed from metal such as copper, although other materials may be used, such as aluminum, stainless steel or nickel. As in the embodiment illustrated in  FIG. 1 , the pin fins  504  are cast on the heat conductive base plate  107  for good thermal connection, or may be manufactured separately and coupled to the base plate  107  using a soldering or brazing technique for good thermal connection with the base plate  107 . The height H of each pin fin  504  is preferably constant, although pin height may vary by increasing radially outward from the center of the heat sink to provide more uniformity of base plate temperature. 
       FIG. 6  illustrates one embodiment of a heat sink that has a radially-extending and branching fin structure configured for a parallel flow configuration across a base plate face  601 . The plurality of fins  105  are arranged and splayed in a repeating and multi-tiered branching pattern, one example of such illustrated in region  602 . Unlike in  FIGS. 1 and 2 , the plurality of fins  105  are configured in generally concentric configurations on the heat conducting base plate  107  to receive air directed at a leading edge  603  of the base plate  107 , with at least one focus at one corner  604  of the heat conductive base plate  107 . First and second channel inlets ( 605 ,  606 ) are formed by the fins  105  adjacent the leading edge  603 , leading to respective first-tier branch channels  610 ,  612 . In the example region  602 , successive tiers of branch channels are defined by fins formed in concentric circles about the focus  604  and set back from the first and second channel inlets ( 605 ,  606 ). Each curved fin  105  is preferably configured with one or more gaps  224  along their lengths to trip respective developing thermal and velocity boundary layers formed when air is introduced across them to promote advantageous thermal transfer characteristics. The gaps  224  preferably extend up and perpendicular from the base plate with a length equal to the height H of each fin or may extend along only a portion of the height H of each fin or along a different vertical portion of each fin. 
       FIG. 7  illustrates a heat sink that has a forced-air fan assembly positioned adjacent the fin structure along a leading edge of the heat conductive base plate  107 , to establish a parallel flow heat sink structure. An air flow metering port  700  is positioned between a forced-air fan assembly  702  and the fins  105  to induce air drawn through the fan  702  to flow along the upper surface  601  of the base plate  107 . The fan assembly  702  may be connected to the metering port  700  and fin/base plate ( 105 ,  700 ) with threaded bolts (not shown), or by any suitable means. The fan assembly  702  and metering port  700  are configured in complementary opposition to maximize the flow of air over the face  601  of the base plate and through the fins  105 . 
     In  FIG. 8 , one embodiment of a base plate  800  has concave dimples formed on its upper surface  802  for generating vortices in between the fins. Concave dimples  804  are spaced apart and positioned on a surface  802  of base plate  800  between fins  105 . The concave dimples  804  preferably have an approximate depth of 100 μm-1 mm and width of and 100 μm-1 mm and are preferably spaced apart by approximately 5-20 mm. During operation, as primary air flow  400  is introduced over the surface of the conductive base plate  800 , vortex  806  is generated, thereby promoting advantageous thermal transfer between the base plate  800  and air flow. In an alternative embodiment, the dimples  804  are formed either solely on fins  105  or on both the fins  105  and base plate face  802 . 
       FIG. 9  illustrates operation of one embodiment of a heat sink that uses branching and curved fins with gaps in such fins that are used to trip a developing thermal boundary layer. Air is blown onto a face of the base plate  900  and heat is conducted from the base plate to a plurality of curved fins.  905 . The blown air is directed through at least one first-tier channel established between the curved fins  910  that are themselves sized and configured to induce primary and secondary flow patterns for advantageous thermal transfer characteristics between the blown air and fins. A portion of the air is directed through a plurality of second-tier branching channels established by the plurality of curved fins  915  to reduce the build-up of back pressure as the air moves across the heat conductive base plate adjacent to the curved fins. The air flowing through the curved fins is passed across gaps formed in the curved fins to trip a developing thermal boundary layer  920 . In an alternative embodiment, the air is also passed across concave dimples either formed on a surface of the heat conductive base plate  925  or on at least one of the plurality of curved fins  930  (or both) to further trip any developing thermal boundary layer. 
     While various implementations of the application have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention.