Patent Publication Number: US-2011073292-A1

Title: Fabrication of high surface area, high aspect ratio mini-channels and their application in liquid cooling systems

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of heat exchangers. More particularly, this invention relates to a method of fabricating heat exchangers having high surface area, high aspect ratio minichannels and/or high aspect ratio microchannels, and their application in fluid cooling systems. 
     BACKGROUND OF THE INVENTION 
     Effective heat transfer in a fluid cooling system has a flowing fluid in contact with as much surface area as possible of the material that is thermally coupled to extract heat from the device to be cooled. Fabrication of a reliable and efficient High Surface to Volume Ratio Material (HSVRM) structure is therefore extremely critical for developing an effective heat exchanger. 
     The use of silicon microchannels is one heat collector structure in fluid cooling systems previously proposed by the assignee of the present invention. For example, see U.S. Pat. No. 7,017,654, which issued on Mar. 28, 2006 and entitled “APPARATUS AND METHOD OF FORMING CHANNELS IN A HEAT-EXCHANGING DEVICE”, which is hereby incorporated in its entirety by reference. 
     High aspect ratio channels are fabricated by anisotropic etching of silicon, which has found widespread use in micromachining and MEMS. However, silicon has a low thermal conductivity relative to many other materials, and especially relative to true metals. 
     Methods for fabrication and designs for micro-heat exchangers from higher conductivity materials exist in the prior art, but either use expensive fabrication technologies or involve complicated structures without specifying economically feasible fabrication methods. 
     SUMMARY OF THE INVENTION 
     The present invention provides methods and apparatuses which achieve high heat transfer in a fluid cooling system, and which do so with a relatively small pressure drop across the system. 
     The present invention discloses high aspect ratio, high surface area structures applicable in micro-heat-exchangers for fluid cooling systems and cost effective methods for manufacturing the same. 
     In some embodiments of the present invention, fins used to construct mini-channels are fabricated with self-aligning features. The self-aligning features allow the fins to be stacked within a heat exchanger cannister without bonding each fin, such that the cannister only needs to be heated once to bond the entire heat exchanger. 
     In some embodiments of the present invention, methods of fabricating fins are utilized which are especially commercially practical. In some embodiments, fins are fabricated with wall features to mix fluid passing through a mini-channel. In other embodiments, fins are fabricated with one or more passages, conduits or vents passing therethrough to reduce pressure drop in a heat exchanger. In yet other embodiments, fins are fabricated having both wall features and passages therethrough. 
     In some embodiments of the present invention, methods are employed to reduce pressure drop in a heat exchanger. In some embodiments, a unique geometry is provided to divert fluid flow paths in order to reduce pressure drop. In other embodiments, a manifold layer is used to divert fluid flow paths in order to minimize pressure drop. 
     It is an object of the present invention to provide a heat exchanger which effectively transfers heat from the heat exchanger to a fluid, which subsequently cools the fluid and which reuses the cool fluid in a closed loop system. It is also an object of the present invention to fabricate a commercially feasible heat exchanger capable of doing the same. 
     In some aspects of the present invention, the coupling of the microchannel fins to the spacers is provided by the use of a brazing material. The brazing material is placed in contact with the microchannel fins and the structure and heated to above the melting temperature of the brazing material. In another aspect of the present invention, the step of coupling the microchannel fins to the structure is provided by thermal fusing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a schematic view of a fluid cooling system utilizing the heat exchanger with mini-channels. 
         FIG. 1B . illustrates a schematic isometric view of a partially assembled heat exchanger according to some embodiments of the present invention. 
         FIG. 2A  illustrates a schematic view of a high aspect ratio plate with a mask for etching according to some embodiments of the present invention. 
         FIG. 2B  illustrates a schematic view of an I-Beam fin fabricated through etching according to some embodiments of the present invention. 
         FIG. 2C  illustrates a schematic view of a stack of I-Beam fins to be used in a heat-exchanger according to some embodiments of the present invention. 
         FIG. 2D  illustrates a schematic view of a T-Beam fin fabricated through etching according to some embodiments of the present invention. 
         FIG. 2E  illustrates a schematic view of a stack of T-Beam fins to be used in a heat-exchanger according to some embodiments of the present invention. 
         FIG. 3A  is an exploded schematic view illustrating the parts which comprise the heat exchanger according to some embodiments of the present invention. 
         FIG. 3B  is a partially exploded schematic view illustrating a partially assembled cannister and lid according to some embodiments of the present invention. 
         FIG. 3C  illustrates a schematic view of a fully assembled heat exchanger positioned above a heat-producing surface according to some embodiments of the present invention. 
         FIG. 4  illustrates an exemplary process for fabricating patterned fins by photochemical etching. 
         FIG. 5A  illustrates a side view of a fin treated with a mask in preparation for the step of forming wall features on the fin. 
         FIG. 5B  illustrates a close-up side view of the surface of a fin treated with a fluid etchant, forming wall features on the fin. 
         FIG. 5C  illustrates a side view of a fin with wall features formed from etching. 
         FIG. 6A  illustrates an isometric view of an individual fin with rectangular wall features. 
         FIG. 6B  illustrates an isometric view of an individual fin with triangular wall features. 
         FIG. 6C  illustrates an isometric view of an individual fin with rounded wall features. 
         FIG. 7A  illustrates a schematic view of an example of a fin having angled wall features according to some embodiments of the present invention. 
         FIG. 7B  illustrates a schematic view of an example of a fin having angled wall features and straight wall features according to some embodiments of the present invention. 
         FIG. 7C  illustrates a schematic view of an example of a fin having angled wall features and an empty center according to some embodiments of the present invention. 
         FIG. 7D  illustrates a schematic view of an example of a fin having zig-zag wall features according to some embodiments of the present invention. 
         FIG. 7E  illustrates a schematic view of an example of a fin having sinusoidal wall features according to some embodiments of the present invention. 
         FIG. 7F  illustrates a schematic view of an example of a fin having crosshatch wall features according to some embodiments of the present invention. 
         FIG. 7G  illustrates a schematic view of an example of adjacent complimentary fins having complimentary wall features according to some embodiments of the present invention. 
         FIG. 7H  illustrates a schematic view of an example of adjacent complimentary fins having complimentary wall features according to some embodiments of the present invention. 
         FIG. 8A  illustrates a schematic view of an example of a fin having of pin wall features according to some embodiments of the present invention. 
         FIG. 8B  is a schematic side view of a heat exchanger with fins having pin wall features forming a structured pseudo foam according to some embodiments of the present invention. 
         FIG. 9A  illustrates a schematic side view of a high aspect ratio, high surface area heat exchanger using mini-channels and a metal mesh between the mini-channels according to some embodiments of the present invention. 
         FIG. 9B  illustrates a schematic side view of a high surface area heat exchanger using a stack of metal mesh layers according to some embodiments of the present invention. 
         FIG. 9C  illustrates a schematic side view of a high surface area heat exchanger using an open-cell metal foam insert according to some embodiments of the present invention. 
         FIG. 10A  illustrates a schematic side view of a fin having pin wall features and vents passing therethrough. 
         FIG. 10B  illustrates a schematic side view of a stack of fins having pin wall features and vents passing therethrough. 
         FIG. 10C  illustrates a schematic isometric view of a heat exchanger with a stack of fins having pin wall features and vents passing therethrough. 
         FIG. 11A  illustrates a schematic isometric view of fins having conduits and a fin without a conduit used in heat exchangers according to some embodiments of the present invention. 
         FIG. 11B  illustrates a schematic isometric view of a heat exchanger with fins having apertures for reducing the path length of the fluid. 
         FIG. 12  illustrates a schematic top view of a heat exchanger with a spine divider for reducing the path length of fluid. 
         FIG. 13  illustrates a schematic top view of a heat exchanger with a spine divider and four quadrants for cooling multi-core integrated chips. 
         FIG. 14  illustrates a schematic isometric view of a heat exchanger with a manifold layer for dividing the fluid for separate fluid paths. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Those of ordinary skill in the art will realize that the following detailed description of the present invention is illustrative only and is not intended to limit the claimed invention. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. It will be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer&#39;s specific goals. Reference will now be made in detail to implementations of the present invention as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts. 
       FIG. 1A  illustrates a schematic view of a fluid cooling system  199  according to some embodiments of the present invention. The fluid cooling system  199  utilizes a heat exchanger  100  with internal mini-channels  150 . As shown by directional arrows, fluid is pumped through the heat exchanger  100  and to a heat rejecter  140  by a pump  110 , which is controlled by control module  120 . The heat exchanger  100  with high aspect ratio fins  150  transfers heat from a surface (not shown) to the fluid pumped through the fins of the heat exchanger. Heat exchange in such a fluid cooling system is improved by configuring the flowing fluid to be in contact with as much surface area as possible of the material that is designed to extract the heat from the surface. The fabrication of a heat exchanger with high surface area structures is therefore advantageous for developing an effective heat-exchanger. However, it is desired that the fabrication process be low cost in order to be competitive in consumer electronics markets. Therefore, it is an object of the invention to provide a low-cost fabrication process for producing heat exchangers which effectively cools a surface. For the purpose of this disclosure the term heat exchanger and the term cannister are synonymous and may be used interchangeably. 
     In some embodiments of the present invention the heat exchanger is comprised of copper. In other embodiments of the present invention, the heat exchanger is comprised of aluminum. Furthermore, although specific examples of suitable construction materials are given, it will readily apparent to those having ordinary skill in the art that a number of materials are suitable for use in constructing the heat exchanger. 
       FIG. 1B  illustrates a schematic isometric view of a partially assembled heat exchanger  100  according to some embodiments of the present invention. The heat exchanger  100  comprises a cannister  101 , a thermal interface section  102 , a block of mini-channels  105  and conduits  103  and  104 . The heat exchanger  100  is positioned on a surface (not shown) such that the interface section  102  is positioned directly on top of a heat-producing portion of the surface. The heat exchanger  100  is thermally coupled to the heat-producing portion of the surface in order to transfer heat to the fluid flowing through the heat exchanger  100 . A Thermal Interface Material (TIM) is used to couple the heat exchanger  100  to the surface. For example, thermal grease may be used to couple the heat exchanger  100  to the surface. 
     In some embodiments of the present invention the block of channels  105  are positioned lengthwise in the cannister  101 . In some embodiments of the present invention, the individual fins  150  comprising the block of channels  105  are spaced very close together, but do not touch one another. The size of the channels are preferably on the order of millimeters or micrometers. Some methods of producing closely spaced stacks of metal fins are known, but are not economically feasible. The present invention provides inexpensive methods of making high aspect ratio mini-channels. 
     A first method of making high aspect ratio mini-channels involves stacking individual high aspect ratio fins  150  having self-aligning features to form channels between successively stacked fins  150 .  FIGS. 2A-2C  illustrates the process of creating a block of mini-channels from individual high aspect ratio fins  150 . First, high aspect ratio plates  149  are formed into high aspect ratio fins. In some embodiments of the present invention, separator patterns are built into high aspect ratio plates  149  through wet-etching or by mechanical means. The separator features serve as self-aligning features. According to the wet-etching embodiment, masks  148  are placed on high aspect ratio plates  149  and etched to create desired patterns.  FIG. 2A  illustrates a high aspect ratio plate  149  with a mask  148 . The high aspect ratio plate  149  undergoes wet-etching to remove material from the plate. The end result of the etching process is a fin  150  with channels  151  and spacer elements  152 . As shown in  FIG. 2B , the fin  150  is the shape of an I-Beam. The spacer elements  152  allow a number of fins  150  to be stacked together without the danger that the stack will collapse. Furthermore, since the depth of the channels is known based on the etching parameters, the spacing between successively stacked fins  150  is uniform. This offers a manufacturer of mini-channel heat exchangers the ability to precisely control the width of the mini-channels depending on the desired application.  FIG. 2C  illustrates a stack of fins  150  to be used in a heat-exchanger. 
     Any method of producing the fins  150  may be used, however, etching the fins  150  has distinct advantages over machining a work piece to the same parameters. First, the etching process results in work pieces with extremely straight, clean surfaces. Any machining process will have the problems of deformation of the pieces and contamination of the pieces with dirt, oil, grease, cutting fluid, etc. Additionally, etching the work pieces is much less expensive than machine processes. Furthermore, the etching process allows the mini-channels to be produced with extremely fine features. 
       FIGS. 2D and 2E  illustrate another embodiment of the present invention which utilizes a fin  150  in a T-shape with full length spacers  152  on the upper part of the fin and footers  153  at the lower corners of the fin  150 . In  FIG. 2E , the fin  150  is stacked in the same manner as in  FIG. 2C , except that fluid present in the channels in  FIG. 2E  are in direct contact with the bottom surface of a heat exchanger (not shown). Although in preferred embodiments of the present invention, the fins  150  are constructed with a conductive material, the embodiment described in  FIG. 2E  having minimum thickness of the bottom plate in contact with the heat producing source provides minimum resistance to heat transfer. Therefore, the channels shown in  FIG. 2E  are more effective than the channels shown in  FIG. 2C  in transferring heat from a heat producing source (not shown) to a fluid medium in a fluid cooling system. 
     In some embodiments of the present invention, a brazing process is utilized to individually bond fins  150  and other pieces together to construct a heat exchanger. Exemplary brazing processes include, but are not limited to, vacuum brazing, inert atmosphere brazing, and reducing atmosphere brazing. However, it is desirable to provide a method for the fabrication of a heat exchanger in which the parts only need to be heated once in order to braze all the parts. By eliminating multiple brazing steps, the process becomes less expensive and less time-consuming. Therefore, it is desirable to use self-aligning fins which are able to stay in place while preparing the rest of the parts for heating. 
       FIG. 3A  illustrates an exploded view of the parts which comprise the heat exchanger  100  according to some embodiments of the present invention. The bottom part of a cannister  320  is selected to be placed on a heat producing surface (not shown). The bottom part of the cannister  320  includes a thermal interface section  335  comprising a section of the floor of the bottom part of the cannister  320  which has a high thermal conductivity. Preferably, a layer of a brazing substance  330  is positioned within the bottom part of the cannister  320  to thermally couple a stack of fins  351  to the thermal interface section  335 . In some embodiments of the present invention, CuSil is used as a brazing substance  330 . In other embodiments, the brazing substance has a portion of copper, a portion of nickle, a portion of tin, and a portion of phosphorous. An example of a brazing substance that includes copper, nickel, tin, and phosphorous is CuproBraze™ which has approximately 67% copper, approximately 7% nickel, approximately 9% tin, and approximately 7% phosphorous. In some embodiments, the brazing material is in the form of a paste, a foil, or a wire. Next, individual fins  350  are stacked up on top of the brazing substance  330 , along the width of the bottom part of the cannister  320 , forming mini-channels. In some embodiments of the present invention, a second brazing substance  360  is lined on the top edge of the bottom part of the cannister  320  for brazing the lid  370  to the bottom part of the cannister  320 . In some embodiments of the present invention, CuSil is used as a second brazing substance  360 . In other embodiments, the second brazing substance has a portion of copper, a portion of nickle, a portion of tin, and a portion of phosphorous, such as CuproBraze™. In some embodiments, the second brazing material is in the form of a paste, a foil, or a wire. Finally, the lid  370  is coupled to the top of the bottom part of the cannister  320 . 
       FIG. 3B  illustrates a partially assembled cannister  380  comprising a bottom part of a cannister  321  and lid  370 . As shown, the fins  350  are positioned in the bottom part of a cannister  321  forming a block of mini-channels  390 . After the lid  370  is attached to the bottom part of the cannister  321 , the pieces are subjected to heat to bond the parts. 
       FIG. 3C  illustrates a fully assembled heat exchanger  300  according to some embodiments of the present invention. Again, for the purpose of this disclosure the term heat exchanger and the term cannister are synonymous and may be used interchangeably. The heat exchanger  300  is positioned over a heat producing surface  319 . As shown, the heat producing surface  319  is an integrated chip. However, the heat exchanger  300  according to the present invention can be used to cool any heat-producing surface  319 . In some embodiments of the present invention, a Thermal Interface Material (TIM)  330  such as thermal grease is placed between the heat-exchanger  300  and the heat-producing surface  319 . The embodiments illustrated in  FIGS. 3A-3C  are fabricated such that the heat exchanger is only heated once to braze all the pieces together. 
     The above methods of fabricating heat exchanger mini-channels offer economically feasible solutions over machining mini-channels mechanically. Utilizing high aspect ratio mini-channels increases the heat transfer rate in fluid cooling heat exchangers. It is also an object of the present invention to provide plates with wall features to further enhance the heat transfer rates in these systems. 
     In some embodiments of the present invention, fins or plates with wall features increase the overall surface area of the mini-channel which allows more fluid to interact with the thermally conductive material. By increasing the liquid-to-plate interaction, more fluid is heated by the plates and the fluid is heated more evenly. The wall features also provide a means to mix the fluid, resulting in an even more homogeneously heated fluid. Obtaining more homogeneously heated fluid results in better overall performance of the heat exchanger. In some embodiments of the present invention, the wall features allow laminar flow mixing of the cooling fluid. In other embodiments of the present invention, the wall features cause turbulent flow therethrough. 
     The wall features on the fins are created by a variety of mechanical methods including, but not limited to cold rolling, laser cutting, stamping, etc, or by photochemical etching. Preferably, the wall features are fabricated using a wet etching process, thus achieving economic feasibility.  FIG. 4  illustrates an exemplary process for fabricating patterned fins by photochemical etching. At the step  400 , a metal sheet is cleaned to remove grease and other surface contaminants. At the step  402 , photoresist is applied to both sides of the cleaned metal sheet. At the step  404 , the metal sheet with photoresist is exposed and patterned such that the photoresist forms a series of tabbed fins with desired patterns. At the step  406 , the metal sheet patterned with photoresist is exposed to an etchant, thereby forming an etched metal sheet including the series of tabbed fins with desired patterns. Each patterned fin is separated from an adjacent fin on the etched metal sheet by one or more etched tabs in the etched metal sheet. At the step  408  the etched metal sheet is rinsed and dried. At the step  410 , individual patterned fins are detached from the etched metal sheet by breaking the tabs. 
       FIG. 5A  illustrates a side view of a fin  550  prepared to be etched with wall features (not shown) according to some embodiments of the present invention. The fin  550  is masked with masks  560 . Once masked, the fin  550  is exposed to an etchant.  FIG. 5B  illustrates a side view close-up of the etching process. As the surface  551  of the fin  550  is exposed to an etchant, fin material is removed in multiple directions (as indicated by the directional arrows). Finally,  FIG. 5C  illustrates the fin  550  after being exposed to the etchant with the masks  560  removed. 
     Furthermore, depending on the desired effect and the method used to form wall features on the fins, the cross section of the fin&#39;s groove will range in shape and will react differently to fluid flowing over its surface.  FIGS. 6A-6C  illustrate isometric views of fins  650 ,  660 ,  670 , all with wall features according to some embodiments of the present invention.  FIG. 6A  illustrates a isometric view of a fin  650  with a substantially rectangularly-shaped grooves  651  as a wall feature. In some embodiments of the present invention, the grooves  651  are disposed on both sides of the fin  650 .  FIG. 6B  illustrates a isometric view of a fin  660  with substantially triangularly-shaped grooves  661 . The grooves  661  shown in  FIG. 6B  are disposed on both sides of fin  660 .  FIG. 6C  illustrates a isometric view of a fin  670  with substantially rounded grooves  671 . Furthermore, the grooves  671  shown in  FIG. 6C  are disposed on both sides of fin  670 . Although the grooves  651 ,  661  and  671  are shown as straight uni-directional grooves, it will be clear to those having ordinary skill in the relevant art, that a number of different configurations are possible for the orientation of the groove, depending on a number of design and implementation goals. 
       FIGS. 7A-7F  illustrate examples of the wall features on the fins according to some embodiments of the present invention. The wall features in  FIGS. 7A-7F  are channels formed into the fins  751 - 760 . Preferably, a wet-etching technique is used to create the wall features, although any other process can equally be used. Further, it is clear to those skilled in the art that, although channels are illustrated, the wall features can be protrusions.  FIG. 7A  illustrates an example of a fin  751  having diagonal wall features according to some embodiments of the present invention.  FIG. 7B  illustrates an example of a fin  752  having angled wall features and straight wall features according to some embodiments of the present invention.  FIG. 7C  illustrates an example of a fin  754  having angled wall features and a channel-less center according to some embodiments of the present invention.  FIG. 7D  illustrates an example of a fin  756  having zig-zag wall features according to some embodiments of the present invention.  FIG. 7E  illustrates an example of a fin  758  having sinusoidal wall features according to some embodiments of the present invention.  FIG. 7F  illustrates an example of a fin  760  having crosshatch wall features according to some embodiments of the present invention. 
       FIGS. 7G and 7H  illustrate adjacent fins  770  and  780  having complementary wall features according to some embodiments of the present invention.  FIG. 7G  illustrates an isometric view of fin  770  and fin  780  laid down on its side to show detail. As shown, fin  770  as diagonal wall features  771  that slope from the upper left side of the fin  770  to the bottom right side of the fin  770  (decreasing gradient diagonal configuration). Fin  780  has diagonal wall features  781  that, when the fin  780  is stood upright, slope from the lower left side of the fin  780  to the upper right side of the fin  780  (increasing gradient diagonal configuration).  FIG. 7H  illustrates an isometric view of fins  770  and  780  orientated such that a channel  775  is formed between them. The slope of the wall features  771  and  781  crisscross to encourage turbulent flow within the channel  775  as the channel  775  is flooded with a fluid (not shown). 
     In some embodiments of the present invention, fins with pin protrusions are utilized.  FIGS. 8A and 8B  illustrate an example of pin wall features according to some embodiments of the present invention. In some embodiments, the fins with pin protrusions have vent features. These vent features will be described more thoroughly in the discussion of  FIGS. 10A-10C  below. 
       FIG. 8A  illustrates an example of a fin  850  having pin protrusion wall features according to some embodiments of the present invention. As shown, the fin  850  has a number of right face protrusions  860  and left face protrusions  865 . In some embodiments, the right face protrusions and the left face protrusions are slightly staggered, so that when two fins  850  are pushed together they are self-aligning and stack much like the fins with built in separators as described above.  FIG. 8B  illustrates a heat exchanger  801  according to some embodiments of the present invention with fins  850 . As shown, a layer of brazing material  830  is laid on the bottom surface of the cannister  800 . In some embodiments, the brazing material  830  is CuSil. In other embodiments, the brazing material  830  has a portion of copper, a portion of nickle, a portion of tin, and a portion of phosphorous, such as CuproBraze™. In some embodiments, the brazing material  830  is in the form of a paste, a foil, or a wire. The fins  850  with wall features  860  and  865  are then stacked to create a series of structured pseudo-foam conduits  870 . Next, a brazing material  880  is placed around the top of cannister  800  and a lid  890  is placed over the cannister  800 . In some embodiments, the brazing material  880  is CuSil. In other embodiments, the brazing material  880  has a portion of copper, a portion of nickle, a portion of tin, and a portion of phosphorous, such as CuproBraze™. In some embodiments, the brazing material  880  is in the form of a paste, a foil, or a wire. Once constructed, the heat exchanger  801  is heated in a furnace to braze the pieces together. As explained above, it is desirable to braze the heat exchanger only once in order to conserve time and money. 
     The fins and heat exchangers illustrated in  FIGS. 7A-8B  provide an efficient way to provide a large surface area for heat transfer in a mini-channel heat exchanger. Another method of providing a greater surface area is through the use of porous structures between or in the place of mini-channels.  FIG. 9A  illustrates a side view of a high aspect ratio, high surface area heat exchanger  900  using mini-channels  950  and a metal mesh  960  between the mini-channels  950 .  FIG. 9B  illustrates a side view of a high surface area heat exchanger  902  using a stack of metal mesh layers  960 .  FIG. 9C  illustrates a side view of a high surface area heat exchanger  904  using an open-cell metal foam insert  980 . Preferably, the pore diameter of the open-cell metal foam insert  980  ranges from one micron to one millimeter. 
     In some cases, the use of high surface area, high aspect ratio mini-channels in the heat exchanger causes a large pressure drop between the inlet conduit and the outlet conduit of the heat exchanger. This high pressure drop results in additional technical challenges for the other components within the system, including the pumps, other heat exchangers, and the heat rejector. 
     It is an object of this invention to decrease the pressure drop across the heat exchanger. Methods of decreasing pressure drop in heat exchanger apparatuses have previously been disclosed by the applicant in U.S. Pat. No. 6,988,534 B2, which issued on Jan. 24, 2006 and entitled “Method and Apparatus for Flexible Fluid Delivery for Cooling Desired Hot Spots in a Heat-Producing Device”, U.S. Pat. No. 6,986,382, which issued on Jan. 17, 2006 and entitled “Interwoven Manifolds for Pressure Drop Reduction in Heat Exchangers”, U.S. Pat. No. 7,000,684, which issued on Feb. 21, 2006 and entitled “Method and Apparatus for Effective Vertical Fluid Delivery for Cooling a Heat Producing Device”, and Co-Pending U.S. patent application Ser. No. 10/698,180, filed on Oct. 30, 2003 and entitled “Optimal Spreader System, Device and Method for Fluid Cooled Micro-scaled Heat Exchange”, which are all incorporated herein in their entirety. Other novel means for the reduction of pressure drop are disclosed below. 
       FIGS. 10A-14  illustrate novel methods and apparatuses for reducing pressure drop in the heat exchangers described herein according to some embodiments of the present invention. In all of the following examples, a reduction in pressure drop is achieved through dividing the fluid by providing alternate paths of fluid flow. 
       FIGS. 10A-10C  illustrate a pin-vent fin wall structure for dividing fluid flow in a heat exchanger according to some embodiments of the present invention.  FIG. 10A  illustrates a side view of a single fin  1050  with pin protrusions  1060  along its surface. The fin  1050  also has vents  1070  which completely pass through the surface of the fin  1050 . Preferably, the pin protrusions  1060  and the vents  1070  are formed on the fin  1050  through a wet-etching process. 
       FIG. 10B  illustrates an end view of a stack of fins  1050  with pin protrusions  1060  and vents  1070  (indicated with dashed lines) passing therethrough. As shown, the fins  1050  are self-aligning in a similar way to the fins illustrated above. Therefore, a heat exchanger (not shown) can be fabricated using fins  1050  without the requirement that the fins  1050  be bonded to the cannister (not shown) individually, thus saving cost by eliminating steps in the fabrication process. 
     The narrow passages created between the fins  1050  when they are stacked together can result in a pressure drop over the length of the fin  1050 . Including the vents  1070  in the fins  1050  gives the fluid an alternate path to flow, thereby reducing the pressure drop across the system. 
       FIG. 10C  illustrates a isometric view of the stack of fins  1050  having pin protrusions  1060  and vents  1070 . Fluid is pumped between the fins  1050  and the fins  1050  absorb heat from the heating source. Fluid is mixed by the pin protrusions  1060  to achieve a more homogeneously mixed fluid. Furthermore, fluid traverses between rows of fins  1050  through the vents  1070  to further mix fluid and to alleviate the pressure in the heat exchanger. 
       FIGS. 11A and 11B  illustrates another embodiment of the present invention used to alleviate pressure drop in a heat exchanger  1100  by diverting fluid through holes in mini-channels.  FIG. 11A  illustrates a schematic isometric view of a plurality of fins  1150  and a fin  1152  used in heat exchangers according to some embodiments of the present invention. The fins  1150  have apertures (indicated with dashed lines  1151 ) to divide the fluid flow and one fin  1152  does not include an aperture and is used to block the passage of fluid. The fins  1150  are included in a heat exchanger ( FIG. 11B , element  1100 ) and form a series of channels  1153 . Preferably the fins  1150  are made of a material with a high thermal conductivity so that when fluid flows through the channels  1153 , effective heat exchange occurs. 
       FIG. 11B  illustrates a schematic isometric view of a heat exchanger  1100  utilizing the fins with conduits  1150  (indicated with dashed lines  1151 ) and the fin  1152 . Each fin  1150  and fin  1152  extend substantially across the heat exchanger  1100  in the X-direction. However, some amount of space exists between the walls of the heat exchanger  1100  and the ends of the fins  1150  and fin  1152  so that fluid exits the channels in the X-direction. 
     Fluid is pumped into a reservoir  1115  in the heat exchanger  1100  through conduit  1105  where it encounters the first of a series of fins  1150  with an aperture (not labeled). A portion of the fluid is forced through the aperture and some portion of fluid is pushed along the face of the fin  1150  towards each wall of the heat exchanger  1100 , effectively dividing the fluid flow path by some amount. As such the pressure drop is reduced because the fluid only needs to be pushed along half the length of the fins  1150 . Furthermore, since the system pressure is used to push the fluid in two directions, the velocity of fluid traveling through the channels  1153  is reduced. Therefore, the fluid moves at a slower pace through a shorter fluid path causing a more effective heat exchange between the fluid and the channel walls. 
     As fluid progresses through the series of fins  1150 , the channels  1153  formed by the fins  1150  become at least partially flooded and effectuate heat exchange with the fluid. Heated fluid is forced out of the channels  1153  and forced into a reservoir  1120 , and out of a conduit  1110 . 
     In some embodiments, the fins  1150  can be stacked with wall features of the types shown in  FIGS. 7A-7E . One or more apertures are introduced between the wall features. In some embodiments, only one aperture exists on the fins  1150 . In other embodiments, multiple apertures exist along the fin  1150 . In some embodiments having multiple apertures, the number of apertures on each fin vary. In other embodiment having multiple apertures, each fin has the same number of apertures. As shown, the apertures are circular, however, the shape of the apertures can be selected from any shape. As shown, the apertures are lined up, each centered on the fin  1150 . In other embodiments, the apertures are staggered on the fins  1150 . In alternate embodiments, the conduits  1105  and  1110  are situated either on the sides of the heat exchanger  1100 , on the bottom of the heat exchanger  1100 , or in a combination of the top, bottom or sides. 
       FIG. 12  illustrates a top view of an alternative configuration for reducing the path length that fluid travels in a mini-channel heat exchanger  1200 , thereby reducing pressure drop. The heat exchanger  1200  includes an intake conduit  1205  leading to reservoir  1215 , an output conduit  1210  drawing from reservoir  1220 , walls  1252 , fins  1250 , and a vertical spine  1251 . Fluid is pumped into the heat exchanger  1200  via the input conduit  1205  into the reservoir  1215 . The fluid is split by the spine  1251 . The spine  1251  also effectuates heat transfer from the heat source (not shown) to the fluid. In some embodiments, the spine  1251  can be configured with wall features. The spine  1251  forces the fluid into mini-channels  1253  formed by the fins  1250 . The walls of the mini-channels  1253  transfer heat from the heat source (not shown) to the fluid. The heated fluid is then forced out of the channels  1253 , into the reservoir  1220  and out of the output conduit  1210 . 
       FIG. 13  illustrates an alternative embodiment of a heat exchanger with a spine  1351  and four quadrants I, II, III, and IV of heat exchange. Fluid is pumped into reservoir  1315  via input conduit  1305 . The spine  1351  divides the fluid into the four quadrants I, II, III, and IV. Each quadrant is separated with walls  1352  and contains mini-channels  1353  formed by fins  1350 . Heat exchange occurs in the mini-channels  1353  and the heated fluid recombines in the reservoir  1320  and is pumped out of the output conduit  1310 . In some embodiments, each quadrant I, II, III, and IV is positioned above a separate heat source (not shown). Alternatively, each quadrant I, II, III and IV is positioned above a specific zone of a single heat source (not shown). Preferably, the heat exchanger  1300  is used to cool the multiple heat zones associated with multi-core integrated chips. 
     The heat exchangers illustrated in  FIGS. 11-13  all divide the fluid path internally, within the heat exchanger itself. In other embodiments, a manifold layer is positioned on top of the thermal interface section of the heat exchanger and is used to divide the fluid into separate fluid paths. 
       FIG. 14  illustrates a cut-out isometric view of a heat exchanger  1400  with a manifold layer  1470  and an interface layer  1460 . The interface layer  1460  includes thermally conductive mini-channels  1465 . The manifold layer  1470  sits on top of the interface layer  1460  and supplies the interface layer  1460  with fluid for fluid cooling. As shown, fluid (not shown) is pumped into the manifold layer  1470  of the heat exchanger  1400  via inlet conduit  1405 . A wall  1415  is preferentially included to impede the fluid flow and cause fluid to pool in the manifold layer  1470 . The pooled fluid drains through a narrow slit  1420  and into the interface layer  1460 . Draining fluid contacts the interface layer  1460  and is forced out both sides of the mini-channels  1465 . As such, the fluid only interfaces with one-half the length of a mini-channel  1465 , effectively reducing pressure drop in the heat exchanger  1400 . Although a single slit  1420  is shown as the conduit between the manifold level  1470  and the interface level  1460 , it will be readily apparent to those ordinarily skilled in the art that multiple slits or openings in multiple locations and configurations are equally conceived. 
     The heat exchanger of the present invention effectively transfers heat from a surface through a conductive cannister, through mini-channel walls and into a fluid flowing therethrough. The present invention also discloses providing the fins used in the mini-channels with wall features to mix fluid and provide alternative fluid paths to reduce pressure drop. The present invention also discloses alternative methods of reducing pressure drop including providing unique geometries to divert fluid flow and providing the heat exchanger with a manifold layer. The present invention also discloses cost-effective methods of fabricating the heat exchanger, mini-channels, fins with wall features and manifolds. 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications can be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention. Specifically, it will be apparent to one of ordinary skill in the art that the device and method of the present invention could be implemented in several different ways and have several different appearances.