Abstract:
A series of hierarchical channels are formed in a first member surface of a first member using a continuous-feed manufacturing process. The channels are configured to control particle stacking. The first member is pressed to a second member with a layer of particle-filled viscous material between the first member surface and a second member surface of the second member. An inventive assembly includes mating surfaces with at least one surface formed with a series of parallel hierarchical channels configured to control stacking of the particles during pressing together of the surfaces. The surface is substantially free of any other hierarchical channels formed thereon.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to the electrical and electronic arts and, more particularly, to formation of thermal (and other) interfaces for packaging of electronic circuits and/or components, and the like. 
     BACKGROUND OF THE INVENTION 
     Modern electronic circuits and components may have quite high power densities. The interface between an electronic circuit or component, and a heat sink, should exhibit low thermal resistance so that heat can be conducted away from the circuit or component. One technique for achieving such low thermal resistance interfaces is the use of particle-filled adhesives or greases. Such adhesives and greases are also employed in other applications, for example, as electrical interfaces when the particles are electrically conductive or as mechanical bonds that may be enhanced by particles or fibers added to an adhesive matrix. 
     Co-assigned US Patent Publication 2005-0263879 of Michel et al., (now U.S. Pat. No. 7,748,440), discloses a patterned structure for a thermal interface. The Michel et al. invention provides a thermal interface with a first and a second face that are in contact to each other by a thermal conducting material. A first face includes grooves that are at least partly filled with the thermal conducting material, wherein at least two types of grooves are arranged, namely first grooves having a larger width than second grooves. The first face comprises an array with protrusions that are confined by the second grooves, the array being divided by the first grooves into sub-arrays. 
     SUMMARY OF THE INVENTION 
     Principles of the present invention provide techniques for one-dimensional hierarchical nested channel designs for continuous feed manufacturing processes. An exemplary embodiment of a method for forming an interface, according to one aspect of the present invention, includes the steps of forming a series of hierarchical channels in a first member surface of a first member using a continuous-feed manufacturing process, the channels being configured to control particle stacking; and pressing the first member to a second member with a layer of particle-filled viscous material between the first member surface and a second member surface of the second member. 
     In another aspect, an inventive assembly includes a first member having a first member surface; a second member having a second member surface; and a layer of particle-filled viscous material pressed between the first member surface and the second member surface. At least one of the surfaces is formed with a series of hierarchical channels configured to control stacking of the particles during pressing together of the surfaces, and the hierarchical channels are substantially parallel. The at least one of the surfaces is substantially free of any other hierarchical channels formed thereon. 
     One or more embodiments of the invention advantageously provide a one-dimensional channel nesting configuration that accommodates localized high heat flux or hot-spot zones and can be manufactured using extrusion, rolling or other continuous feed systems. 
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a simplified stacking pattern for an infinite aspect ratio interface (length&gt;&gt;width); 
         FIG. 2  shows an exemplary one-dimensional hierarchical channel design, according to an aspect of the invention; 
         FIG. 3  shows an exemplary optional secondary finishing technique, according to another aspect of the invention; 
         FIG. 4  presents an exemplary series of views illustrating transition of bifurcation zones through sequential filling of each channel hierarchy, according to yet another aspect of the invention; 
         FIG. 5  shows final stack positions for a channel design intended to produce higher concentrations of particles in specific locations, according to still another aspect of the invention; 
         FIG. 6  presents non-limiting examples of different types of channels that may be employed in one or more inventive embodiments; 
         FIG. 7  depicts an embodiment with orthogonal channels on mating surfaces, according to a further aspect of the invention; and 
         FIG. 8  depicts an embodiment with hierarchical channels on a roller, according to still a further embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     One or more embodiments of the invention provide a design methodology for hierarchical nested channels, which is compatible with continuous feed manufacturing systems, while still producing a significant benefit in terms of controlled particle stacking and assembly pressure. The improvement of thermal interfaces is one of the simplest and lowest cost ways to extend the limits of current thermal management solutions and to improve the performance of electrical interfaces made from particle filled materials. In one or more embodiments of the invention, the interface of the cooling device, chip or substrate is patterned with an inventive layout. 
     Alternatively, the 1-D hierarchy channel pattern can also be built into the surface of a roller used for roll-to-roll high volume manufacturing. When the channels are integrated onto the surface of the roller, they allow the deposition of high particle concentration lines on the target surface, for applications such as electrical current collection on flexible solar panels manufactured on thin polymer layers. This will be discussed further below with regard to  FIG. 8 . 
     Inventive techniques can be employed, for example, in conjunction with particle filled thermal and electrical pastes and adhesives. Further, one or more embodiments of the invention substantially reduce or eliminate the problem of interface aging, as long-term voiding is substantially reduced or eliminated, by reducing the pressure drop of the interface material during thermal cycling of the interface system. 
     Continuous feed manufacturing systems, such as extrusion and rolling, typically result in the lowest cost parts due to the high rate of production and reduced need for secondary manufacturing steps. Such manufacturing systems only allow one to define a cross-section profile without three-dimensional features. One or more hierarchical channel designs, in accordance with the invention, are compatible with the reduced geometrical complexity of continuous-feed systems, while still providing a benefit in terms of particle stacking control and reduced assembly pressure. The particle stacking pattern resulting from an interface with a length many times longer than its width is also simplified, as compared to more “square-like” interfaces, leaving a particle stack equally spaced between the neighboring channels or interface edges. 
       FIG. 1  shows a simplified stacking pattern for an infinite aspect ratio interface (length&gt;&gt;width). A material  102 , such as adhesive or grease, includes particles  104  (which may be, for example, thermally and/or electrically conductive). The material  102  is “squeezed” as first and second plates  106 ,  108  are forced together (as symbolized by arrows  110 ). The plates mentioned herein can, in general, be any types of structures or components; by way of example and not limitation, cold plates, heat pipe evaporators, or other heat sinks; electronic circuits or components or packages for such components or circuits, or the surface of a roller (for example, interfacing with a workpiece) and so on. While the terminology “plate(s)” is employed for the examples in the specification, in general, techniques can be applied to first and second members having surfaces; the members can be, for example, any of the aforementioned structures or components. The material  102  is substantially incompressible, and accordingly “spreads out” as plates  106 ,  108  are forced together; by symmetry, the flow will bifurcate at the stacking line  112  (as best seen in the detail at the left of  FIG. 1 ). The flow bifurcation is symbolized by arrows  114 . Due to the bifurcation of flow to the parallel edges, in contrast to the “X” pattern associated with square or near 1:1 aspect ratio interfaces, the aforementioned simplified particle stacking pattern is obtained. 
     In order to provide a channel design that is compatible with an extrusion profile or master roller, and which provides good control of particle stacking and pressure reduction, while maintaining a high solid fill factor, an inventive channel pattern rule is defined based on the limited geometry of the system. One exemplary approach, depicted in  FIG. 2 , begins by subdividing the interface  202  into a number of equal lengths, L, with the first level of hierarchy or largest channels  204 , then dividing these regions in half with a smaller second level hierarchy (channel  206 ), and in half again with a third level channel  208  with the smallest dimensions. This subdivision of additional hierarchies can continue depending on the need for reduced assembly pressure and particle stacking. Thus, it will be appreciated that in general terms, more or fewer subdivisions can be employed, and further, that  FIG. 2  depicts only one repeating portion of what is typically a large array of channels. Further, note that in one or more embodiments, the hierarchical channels are substantially parallel and there are no other hierarchical channels formed in the surface with the hierarchical channels, that is, two-dimensional hierarchical patterns are not employed, such that hierarchical channels are advantageously formed with the aforementioned continuous-feed manufacturing process. 
     The (local or global) pitch, L, of first level channels  204  can, in some instances, be chosen so as not to block regions of higher heat or electrical current flux near concentrated sources. That is, near such sources, channels would not be located, such that only the relatively thin layer of material  102  would separate the plates  106 ,  108 , thereby resulting in less temperature drop or voltage drop, as compared to the case where heat or current had to flow around the channel to reach the surface or through a greater thickness of material due to the presence of the channel. Note that the bondline thickness  102  should not be confused with the thickness of material between the bottom of a channel and the opposing surface ( 102 +channel depth). 
     For the case of substantially uniformly distributed thermal and/or electrical sources, the first level channels  204  can be chosen so as to maintain reservoirs of interface material at key locations that are preferential for reliability or mechanical reasons, such as the center of the total interface or along regions of higher mechanical stress. 
     Attention should now be given to  FIG. 3 . In order to increase the performance of the channeled interface, additional finishing steps can optionally be performed, such as orthogonal channel crosscutting with an array of saw blades, stamping, rolling, and so on, after the pieces have been extruded. The orthogonal channels need not be hierarchical. These finishing steps allow the particle filled interface material to flow between the extruded or rolled channel hierarchies via a network of intersecting channels added through the finishing steps. In  FIG. 3 , the lower left-hand corner presents a top plan view of a plate  302  having a plurality of first level channels  304 , similar to channels  204  of  FIG. 2 , which may be formed by a continuous feed process, such as extruding or rolling. At the top of the figure, a side elevation view of plate  302  is presented. As shown, a plurality of orthogonal channels  350  may be formed, for example, by “imprinting” with a roller wheel  352  (as noted just above, other techniques, such as gang-sawing, stamping, and so on, could also be employed). In another approach, channels can be formed on both surfaces (e.g., plates  106 ,  108 ), using extrusion, rolling, or other techniques, and can be oriented orthogonal to each other upon assembly (see discussion of  FIG. 7  below). In some instances, one surface could have hierarchical channel sizes as described herein, while the other might have uniform channel sizes. 
     Design of the one-dimensional hierarchy channel width and depth is preferably based on the requirements of the continuous feed parameters, such as draft angles and maximum aspect ratio for extrusion reliability. Channels should be given as high of an aspect ratio (that is, the depth below the surface divided by the width) as possible, so as to have a beneficial impact on the flow of material  102 , without constricting heat flow through the solid surface  202 . As is the case with two-dimensional hierarchical nested channel arrays, the minimum channel depth is preferably determined by the maximum particle size of the particles  104  in the interface material  102  (typically, the minimum channel depth is approximately 2-3 times the maximum particle size). Minimum channel depth applies to channels in the smallest level of hierarchy  208 . 
     Attention should now be given to  FIG. 4 . Due to the placement of the channel hierarchies, the flow bifurcation zones are continuously shifting throughout the assembly of the interface, as smaller channels become filled with interface material.  FIG. 4  shows three successive views,  480 ,  482 , and  484 , as assembly proceeds. Elements  102 ,  110 ,  202 ,  204 ,  206 , and  208  are as described above. Material  102  is squeezed between plates  460  and  202 . In view  480 , assembly has just started, and flow bifurcates substantially midway between the channels  204 ,  206 ,  208 , as shown by arrows  462 . Once the smallest channels  208  have filled, as shown in view  482 , flow now bifurcates over channels  208  as these are approximately halfway between channels  204  and  206 , as shown by arrows  464 . The second level channels  206  serve as a final collection zone for large particles caught in the final bifurcation system. Higher particle concentrations or stacking lines from earlier phases of the assembly are also pushed to the 1 st  level channels  204  at the end of the assembly process, as shown in view  484 , where flow bifurcates over channels  206 , as shown by arrows  466 . 
     By way of summary and conclusion,  FIG. 4  shows transition of bifurcation zones (horizontal arrows  462 ,  464 ,  466 ) through sequential filling of each channel hierarchy. Spots  1402  between arrows  462 ,  464 ,  466  indicate potential stacking regions, with spots  1404  in views  482 ,  484  indicating a stacking region that has been pushed to a new location or diffused as the assembly progress and flow directions change. 
     In order to control the transitions between bifurcation zones, one or more embodiments scale the channel depth and width between the hierarchies—for example, the 2 nd  level channels  206  are about one-half the cross sectional area of the first level channels  204  (the drawings are not necessarily to any scale or proportion). Dimensioning of the channels begins by defining the minimum bondline thickness based on maximum particle size. The channels do not effectively disrupt the flow pattern of the adhesive or grease until the bondline becomes considerably smaller than the channel depth; thus, the smallest channels should not be designed with a depth value smaller than the maximum particle size, unless the channel is intended to absorb the final particle stack at the end of the assembly process. Hierarchy levels with larger channels can then be scaled up from the minimum channel dimension. Channel dimensions can also be chosen such that the channel becomes filled with material at a given bondline thickness by calculating the displaced volume of grease or adhesive from an initial thickness and defining channels with a similar volume. 
     In some instances of the invention, the channel design can also be modified (enhanced or optimized) so as to intentionally create regions of higher particle concentration or stacking at specific locations, instead of seeking to avoid such regions.  FIG. 5  depicts final stack positions for a channel design intended to produce higher concentrations of particles in specific locations. As the bifurcation is controlled by the channel dimensions and initial material thickness, a stacking line can be deposited in specific locations if desired. The higher concentrations in  FIG. 5  are in the middle between the two smallest channels  508 , since this is a flow bifurcation point at all stages of assembly. The spots  593  are the final particle stack locations, while spots  591  are intermediate bifurcation points. The channels in plate  502  are designated  504 ,  506 ,  508 , from largest to smallest, while the upper plate is designated as  560 . Material  102  and arrows  110  are as described before. 
     Typically, high volume manufacturing processes allow geometrical optimization in a two-dimensional plane. This plane could, for example, be the cap-to-cold plate interface (etching, coining, rolling, and the like) or a cross-section of the part orthogonal to the interface (extrusion). Optimized paste evacuation geometries with minimal heat flux constriction include semi-buried channels, as opposed to channels with a large surface opening and narrowing cross section. Attention should now be given to  FIG. 6 . Due to the low thermal conductivity of the grease or adhesive material  102 , as compared to the material of the adjacent plate, heat flux peaks in the adhesive or grease layers should preferably be reduce or minimized. Heat flux mal-distribution in the cap (referred to generically elsewhere herein as a plate) is not as great a concern, since its thermal conductivity is significantly greater, for example, up to 100 times larger, than that of the interface material  102 . 
       FIG. 6  shows two cases of upper and lower plates  602 ,  604 . In the left-hand case, the channel  606  is of substantially uniform width d h  over its entire depth. The right-hand case shows a channel with a narrow opening and a substantially circular cross-section with diameter d h  recessed below the surface of plate  604 . It will be appreciated, based on the discussion in the previous paragraph, that the nominal width of the channels should be preferably offset below the surface of the cap or heat sink respectively (represented by generic plate  604  in the figure), so as to preserve heat transfer area at the interface surface. Ideally, the minimum channel openings would be equal to the size of the largest expected particle  104  in the interface material  102 . This technique permits the contact surface fill-factor to be increased for a given channel hydraulic diameter d h , and the temperature profile is flattened (compare profile  674  to profile  676 ). The profiles  674 ,  676  show the temperature plotted against transverse position X; the peak of  674  is substantially higher than that of  676  because more interface surface area in the left-hand case is taken up by relatively poorly-conducting material  102 . 
     Surface patterning processes are only able to form open or narrowing channel geometries. However, extrusion processes allow the optimization of channel geometry in the cross-section plane. Semi-buried channels, as shown on the right-hand side of  FIG. 6 , are able to substantially improve or optimize the system performance, in terms of minimal peak temperature for a given hydraulic diameter of the channels. 
     Channel cross sections for extrusion profiles can also be enhanced or optimized based on a trade-off between mechanical, thermal, and extrusion process needs, that may result in triangular or diamond shaped profiles being preferred. A one-dimensional hierarchy of channels can also be integrated so as to control bifurcation and particle stacking patterns during assembly of the interface, as discussed above. 
     As some parts may require channels on several sides of an extruded piece in order to improve a multiplicity of thermal (or other) interfaces, channels can be added as needed to an extrusion profile or roller surfaces. Chip packaging lids often require an optimized thermal interface with the chip on the inside of the package (TIM 1 ) and a second interface (TIM 2 ) between the package and cooler on the outside of the package. For such a component, aspects of the invention can be employed wherein an extrusion cross section is used that has channel features on both top and bottom surfaces. 
     One or more exemplary embodiments of the invention have been presented herein. For illustrative purposes, applications pertaining to thermal control applications have been discussed. However, aspects of the invention are also valuable for the general case of improved bonding with adhesives, as one or more embodiments allow creation of thinner bondlines for non-thermal as well as thermal applications. 
     Attention should now be given to  FIG. 7 . A plate  202  as described above can be mated to another plate  702  with a series of channels  704  which need not be hierarchical. Grease or adhesive is omitted from  FIG. 7  for clarity. Upon assembly, channels  704  can be oriented substantially orthogonally to hierarchical channels  204 ,  206 ,  208 . Section A-A is taken along cutting plane line A-A. 
     As noted above, an inventive 1-D hierarchy channel pattern can also be built into the surface of a roller used for roll-to-roll high volume manufacturing. As shown in  FIG. 8 , channels  804 ,  806 ,  808  are integrated onto the surface of the roller  802 , and they allow the deposition of high particle concentration lines of material  812  on the target surface of workpiece  810 , for applications such as electrical current collection on flexible solar panels manufactured on thin polymer layers. “Parallel” hierarchical channels, in the context of a roller surface, mean that the channel centerlines define substantially parallel planes when viewed end-on as at the right hand side of  FIG. 8 . The hierarchical channels in a cylindrical roller would not necessarily be formed by a continuous manufacturing or rolling process. Because this is a master part used for extended periods, it could be formed with more expensive or time consuming approaches. 
     In yet another aspect of the invention, one or more techniques set forth herein can be used to obtain mechanical benefits of controlled particle or “fiber” stacking in adhesive and cured structural materials (i.e., composite glues). This aspect is beneficial, for example, for composite adhesives filled with carbon fibers or carbon nanotubes, as the fibers improve mechanical properties, but too high of a density of fibers results in poor mechanical performance. 
     The techniques set forth herein can be used, for example, to package and cool circuits realized on an integrated circuit chip. The chip design can be created, for example, in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer may transmit the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design can then be converted into an appropriate format such as, for example, Graphic Design System II (GDSII), for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks can be utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
     Resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die or in a packaged form. In the latter case, the chip can be mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a mother board or other higher level carrier) or in a multi-chip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case, the chip may then be integrated with other chips, discrete circuit elements and/or other signal processing devices as part of either (a) an intermediate product, such as a mother board, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end or consumer electronic applications to advanced computer products. The techniques set forth herein can be used, by way of example and not limitation, for thermal, mechanical, and/or electrical interfaces for a variety of electronic packages. 
     It will be appreciated and should be understood that the exemplary embodiments of the invention described above can be implemented in a number of different fashions. Given the teachings of the invention provided herein, one of ordinary skill in the related art will be able to contemplate other implementations of the invention. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made by one skilled in the art without departing from the scope or spirit of the invention.