Abstract:
An integrated circuit package includes: a substrate; an electronic circuit located on the substrate, the electronic circuit comprising a topography of at least one level; a cooling device located over the electronic circuit; a compliant interface disposed between the electronic circuit and the cooling device, wherein the compliant interface comprises a first surface and a second surface and wherein the first surface is in thermal contact with the electronic circuit, and wherein the compliant interface is preformed from a compliant material such that the first surface substantially conforms to the topography of the electronic circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application relates to technology similar to that discussed in U.S. patent application Ser. No. 11/151,905, “Cooling Structure Using Rigid Movable Elements;” U.S. patent application Ser. No. 11/151,830, “Compliant Thermal Interface Structure Utilizing Spring Elements;” U.S. patent application Ser. No. 11/151,843, “Compliant Thermal Interface Structure Utilizing Spring Elements and Fins;” and U.S. patent application Ser. No. 11/151,831, “Compliant Thermal Interface Structure with Vapor Chamber;” all of which are herein incorporated by reference. 
   STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
   Not Applicable. 
   FIELD OF THE INVENTION 
   The invention disclosed broadly relates to the field of electronic devices and more particularly relates to the field of compliant interfaces for cooling electronic devices. 
   BACKGROUND OF THE INVENTION 
   During the normal operation of a computer, integrated circuit devices (ICs) generate significant amounts of heat. This heat must be continuously removed, or the integrated circuit device may overheat, resulting in damage to the device and/or a reduction in operating performance. Cooling devices, such as heat sinks, have been used in conjunction with ICs in order to avoid such overheating. Generally, a passive heat sink in combination with a system fan has provided a relatively cost-effective cooling solution. In recent years, however, the power of ICs has increased exponentially, resulting in a significant increase in the amount of heat generated by these devices, thereby making it extremely difficult to extract heat from these devices in order to cool them. 
   Heat is typically extracted by coupling a heat spreader and a thermal cap to the electronic device as a heat sink. Heat sinks operate by conducting heat from a processor to the heat sink and then radiating it into the ambient air. The better the transfer of heat between the two surfaces (the processor and the heat sink metal) the better the cooling. Some processors come with heat sinks attached to them directly, or interfaced through a thin and soft layer of thermal paste, ensuring a good transfer of heat between the processor and the heat sink. The thermal paste serves not only to transfer heat but to provide some degree of mechanical compliance to compensate for dimensional changes driven by the high operating temperatures of the devices. However, the paste is a weak link in the thermal path. Attempts to thin this layer have resulted in failure of the layer when it is exposed to dimensional changes. There are some known mechanically compliant solutions but these solutions still rely on paste film somewhere in the path. 
   Printed circuit boards are constructed of various components, some of which have varying coefficients of thermal expansion (CTE). When the components are heated, this can produce degradation such as cracking in some of the components. This is a common problem with solder balls. In addition to the problem of varying CTEs, the different components are usually of varying heights. This adds additional air gaps between current interfaces and circuits, in addition to the air gaps that exist between the components. Heat conduction across air gaps is generally poor, thereby lessening the effectiveness of the heat sink. 
   In some applications (silicon carrier, multichip modules), it would be desirable to cool several adjacent chips of different thicknesses and perhaps interspersed with capacitors and other components. 
   Thus, there is a need for a solution that overcomes these shortcomings. 
   SUMMARY OF THE INVENTION 
   Briefly, according to an embodiment of the present invention, an integrated circuit package includes: a substrate; an electronic circuit located on the substrate, the electronic circuit comprising a multi-level topography; a cooling device located over the electronic circuit; a compliant interface disposed between the electronic circuit and the cooling device; the compliant interface comprising a surface making contact with the electronic circuit, and preformed from a compliant material such that the surface substantially conforms to multi-level topography. 
   In yet another embodiment of the present invention, a method for cooling an electronic device comprises steps of placing an electronic circuit on a substrate, the electronic circuit comprising a multilevel topography; preforming a compliant interface from a compliant material such that a surface conforms to multi-level topography comprising a surface making contact with the electronic circuit; and loading the compliant interface over the electronic circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and also the advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings. Additionally, the left-most digit of a reference number identifies the drawing in which the reference number first appears. 
       FIG. 1  shows a cross section of an integrated circuit patent with a compliant interface according to an embodiment of the invention; 
       FIG. 2  is a flowchart illustrating a method according to an alternate embodiment of the invention; 
       FIG. 3  is a flowchart of the steps of preforming the compliant interface from step  202  of  FIG. 2 , according to an embodiment of the present invention; 
       FIG. 4  is a flowchart of the steps for performing the compliant interface from step  202  of  FIG. 2 , according to another embodiment of the present invention; 
       FIG. 5   a  shows flexible supports embedded in a temporary support material, according to an embodiment of the present invention; 
       FIG. 5   b  shows the shaped flexible supports, according to an embodiment of the present invention; 
       FIG. 5   c  shows a compliant membrane attached to the surface, according to an embodiment of the present invention; 
       FIG. 5   d  shows the addition of a permanent bottom and temporary supports, according to an embodiment of the present invention; 
       FIG. 6   a  shows the flexible supports of another embodiment of the present invention; 
       FIG. 6   b  shows a baseplate constructed to match the contours of a topography, according to an embodiment of the present invention; 
       FIG. 6   c  shows the temporary support plastically forced to conform to the baseplate, according to an embodiment of the present invention; 
       FIG. 6   d  shows the compliant membrane, according to an embodiment of the present invention; and 
       FIG. 6   e  shows the interface with the temporary support material removed, according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   We describe a preformed, compliant thermal interface for cooling an electronic device to achieve low thermal resistance and high mechanical compliance. This interface is constructed with a material flexible enough to be shaped, yet robust enough to last many cycles without a degradation in performance. The compliant properties of the interface reduce mechanical stresses brought about by differences in the coefficient of thermal expansion (CTE) of different materials used in the construction of chips and cooling devices. This compliance also addresses the problem of cracked solder balls. 
   A first surface of the interface is preformed to match the contours of the printed circuit board to which it will be affixed. Preforming the interface to match the topography of the circuit board creates an effective thermal seal over any circuit shape, ranging from flat to multi-level. This reduces air gaps, thereby increasing heat conduction away from the chip or chips. This interface may be constructed to fit over more than one chip and other structures as well. 
   According to another embodiment of the present invention, a second surface of the interface can also be preformed. In this embodiment, the second surface is preformed to match the contours of a heat sink. The interface may be preformed on only one surface, or on both the first and second surfaces. 
     FIG. 1  shows a cross sectional side view of a cooling structure  100  for an electronic circuit  102 . The electronic circuit  102  is deposited on a substrate  104 . The electronic circuit  102  comprises one or more chips and various components forming a multilevel topology. A heat sink  106  is used to cool the electronic circuit  102 . A compliant interface  108  is disposed between the circuit  102  and the heat sink  106 . The compliant interface  108  is preformed to the topography of the circuit  102  so that it interlocks with the circuit  102 , matching the contours of the circuit  102 , when it is placed over the circuit  102 . 
     FIG. 2  is a flowchart illustrating a method  200  according to an embodiment of the invention. The method begins with step  202 : placing the electronic circuit on a substrate. The electronic circuit  102  has a topography that can range from flat to multi-level. 
   Step  204  preforms a compliant interface from a compliant material such that a surface of the material conforms to the multi-level topography of the electronic circuit. The steps for performing the interface  108  will be discussed in detail with respect to the discussion of the flow charts of  FIGS. 3 and 4 . Preforming the compliant surface to match the chip topography is feasible for many cooling manufacturing approaches, specifically where the fins, wires or springs that support and carry heat away from the interface are initially potted (encapsulated) in a material that is removed later, such as plastic, and the interface is deposited by plating. For such a structure the surface could be machined to match the expected topography, with tapers introduced to avoid load peaks, and then plated. The tapers are used for tapering the thickness change. 
   As illustrated in  FIGS. 5 and 6 , the solid plastic is removed after plating, leaving a relatively thin metal interface supported by the fins, wires or springs. Alternatively for malleable plastics, a baseplate matching the topography could be made and the entire sheet of plastic with its internal metal structure pressed to fit the baseplate. 
   Step  206  loads the compliant interface  108  over the electronic circuit  102 . A thermal paste or thermal grease may be used in conjunction with this interface  108 . The interface  108  may be affixed to the heat sink by conventional means or it may be part of the structure of the heat sink. 
   As stated in step  204 , one embodiment for preforming the compliant interface  108  with a preformed multilevel topography as shown in  FIG. 1  is detailed in the flowchart of  FIG. 3  and illustrated in  FIGS. 5   a - d.    
   In step  302 , flexible supports  502  for the compliant interface  108  are embedded in a temporary support material such as a plastic or metal  501 . This is shown in  FIG. 5   a  which shows the preformed surface (the surface in contact with the device). 
   Next, in step  304  (as shown in  FIG. 5   b ), the support material and flexible supports  502  are cut or machined to conform to the shape of the desired multilevel topography. Once cut, in step  306 , a compliant membrane  503  is attached or plated onto the surface of the temporary support material. The compliant membrane  503  is important for contact surface area as described above. A thermally conducting metal is the preferred material for this membrane  503 . In step  308  a permanent bottom  505  and surrounding supports  504  are attached or plated as shown in  FIG. 5   c.    
   In step  310  (see  FIG. 5   d ) the structure is completed by removal of the temporary support material. This is done by either melting or chemical methods and in step  312  the addition of structures  506  to complete a heatsink, water manifold or vapor chamber structure is attached depending on the compliant membrane device type. Materials for this compliant membrane  503  are preferably metal, such as copper, but could be partly or mostly nickel, gold, silver, or other metals with good thermal conductivity and malleability. Carbon film may also be used. 
   Referring to  FIG. 4  there is shown a flow chart for another embodiment of a method for construction of a multilevel compliant interface device. The process begins with step  402  wherein flexible supports  602  for the compliant interface are embedded in a temporary support material such as a plastic or metal  601 , as shown in  FIG. 6   d  (which shows both surfaces preformed). Next, in step  404 , a baseplate  604  is made conforming to the desired multilevel topography as shown in  FIG. 6   b.    
   Following this, in step  406  a mandrel (a device for the formation of cavities)  603  matching the desired topography for a compliant material is used to plastically force the temporary support to conform to the baseplate  604  as shown in  FIG. 6   c.    
   In step  408  (see  FIG. 6   d ) the compliant membrane  605  is plated or attached along with a bottom support which may also be a plated membrane attached to a baseplate  607 . This baseplate  607  may or may not be the same as the baseplate  604 . Next in step  410  the temporary support material is removed by melting or chemical methods (see  FIG. 6   e ). In step  412  the structure is completed by the addition of structure(s)  606  to complete a heatsink, water manifold or vapor chamber structure attached depending on the compliant membrane device type, also shown in  FIG. 6   e.    
   Therefore, while there have been described what are presently considered to be the preferred embodiments, it will be understood by those skilled in the art that other modifications can be made within the spirit of the invention.