Patent Publication Number: US-6908170-B2

Title: Devices for dissipating heat in a fluid ejector head and methods for making such devices

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention is directed to apparatus and methods for dissipating heat in fluid ejector heads. 
     2. Description of Related Art 
     A variety of devices and methods are conventionally used to dissipate heat in a thermal fluid ejector head. The thermal fluid ejector heads of fluid ejection devices, such as, for example ink jet printers, generate significant amounts of residual heat as the fluid is ejected by heating the fluid to the point of vaporization. This residual heat will change the performance and ultimately the ejection quality if the heat remains within the fluid ejector head. The ejector performance is usually seen by a change in the drop size, firing frequency, or other ejection metrics. Such ejection metrics are required to stay within a controllable range to have acceptable ejection quality. During lengthy operation or heavy coverage ejection, the temperature of the fluid ejector head can exceed an allowable temperature limit. Once the temperature limit has been exceeded, a slow down or cool down period is required to maintain the ejection quality. 
     Many fluid ejection devices, such as, for example, printers, copiers and the like, improve throughput by improving thermal performance. One technique to improve fluid ejector head performance is to divert excess heat into the fluid being ejected. Once the fluid being ejected has exceeded a predetermined temperature, the hot fluid is ejected from the fluid ejector head. During lengthy operation or during heavy area coverage ejection, this technique is also susceptible to temperatures in the fluid ejector head exceeding the maximum allowable temperature. 
     Another technique is to use a heat sink to store or conduct heat away from the fluid ejector head. Typically, these heat sinks are made from copper, aluminum or other materials having high thermal conductivity to remove heat from the fluid ejector head. 
     SUMMARY OF THE INVENTION 
     When such materials are used, however, the heat sink adds additional weight, size, cost and energy usage to the fluid ejector head, especially for fluid ejector heads that are translated past the receiving medium. Additionally, fluids, such as inks, typically use solvents and/or salts which are likely to corrode aluminum or copper. 
     The heat sinks are typically bonded to a substrate. The substrate materials are often made from a conductive metal, such as aluminum or copper, that conducts heat away from a die module of the fluid ejector head. However, some fluid ejection devices use a plastic substrate that has a relatively low thermal conductivity. When metal heat sinks are used, the bond between the substrate and the die is subjected to significant stress due to temperature changes. The stress is generated from the large mismatch between the coefficients of thermal expansions of the substrate and the die. 
     These stresses create delaminating problems, where the die separates from the substrate, or the layers of the die separate. Also, the stress presents additional fluid ejection quality and reliability issues. 
     This invention provides systems and methods for dissipating heat in a fluid ejector head. 
     This invention separately provides devices and methods for obtaining better thermal conductivity in polymer heat sinks. 
     In various exemplary embodiments of the devices and methods of this invention, a heat sink including a surface area molded from a polymer having thermally conductive filler materials is attached to structure to be cooled. In various exemplary embodiments, the surface area of a heat sink is shaped to dissipate heat. 
     In various exemplary embodiments of the systems and methods of this invention, a die module of a fluid ejector is bonded to a heat sink made of materials having similar coefficients of thermal expansion. 
     In various exemplary embodiments of the devices and methods of this invention, filler materials within a polymer heat sink are oriented in an oriented flow area so that the filler materials extend substantially parallel to the die module of the fluid ejector head. 
     These and other features and advantages of the this invention are described in, or apparent from, the following detailed descriptions of various exemplary embodiments of the systems and methods according to this invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary embodiments of the invention will be described in detail with reference to the following figures, wherein: 
         FIG. 1  is a block diagram illustrating a first exemplary embodiment of a fluid ejector head usable with various exemplary embodiments of the systems and methods according to this invention; 
         FIG. 2  is a block diagram of a first exemplary embodiment of a heat sink formed using the systems and methods according to this invention; 
         FIG. 3  is a block diagram illustrating a second exemplary embodiment of a heat sink formed using the systems and methods according to this invention; 
         FIG. 4  is a block diagram illustrating a third exemplary embodiment of a heat sink formed using the systems and methods according to this invention; 
         FIG. 5  is a block diagram illustrating a fourth exemplary embodiment of a heat sink formed using the systems and methods according to this invention; 
         FIG. 6  is a schematic diagram illustrating a first exemplary embodiment of a technique for molding a heat sink usable according to this invention; 
         FIG. 7  is a schematic diagram illustrating a second exemplary embodiment of a technique for molding a heat sink usable according to this invention; and 
         FIG. 8  is a flowchart outlining one exemplary embodiment of a method for manufacturing a print head having a heat sink according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following detailed description of various exemplary embodiments of the fluid ejection systems according to this invention may refer to and/or illustrate one specific type of fluid ejection system, an ink jet printer, for sake of clarity and familiarity. However, it should be appreciated that the principles of this invention, as outlined and/or discussed below, can be equally applied to any known or later developed fluid ejection systems, beyond the ink jet printer specifically discussed herein. 
     Various exemplary embodiments of the systems and methods according to this invention enable the dissipation of heat from fluid ejector heads, such as, for example, thermal ink jet printers, copiers and/or facsimile machines, by using a polymer mixed with one or more thermally conductive filler materials. In various exemplary embodiments, the device and techniques according to this invention provide polymer heat sinks having one or more filler materials with properties that allow the polymer heat sink to more readily dissipate heat, while the polymer heat sink, as a whole, has a similar coefficient of thermal expansion to the die of the thermal fluid ejector head. 
     In various exemplary embodiments, the heat sink according to this invention is manufactured using a highly conductive polymer. The highly conductive polymer has thermal conductivities in the range of above 10 W/m° C. to about 100 W/m° C. This thermal conductivity is typically about 50-500 times greater than that of standard plastics, which ranges from 0.1-0.3 W/m° C. The highly conductive polymer has a thermal conductivity which is close to the thermal conductivity of aluminum. The thermal conductivity of aluminum is about 100-150 W/m° C. These polymers may also be easily injection molded into shapes that tend to maximize the surface area, and thus the heat dissipation rate, of the heat sink. 
     In general, these highly-thermally-conductive polymer materials can be either electrically conductive or electrically non-conductive. In general, the electrically-conductive highly-thermally-conductive polymer materials are more thermally conductive than the electrically-non-conductive highly-thermally-conductive polymer materials. Because these highly-thermally-conductive polymer materials are being used in close proximity to miniature electrical circuits in the fluid ejector head, using electrically-conductive highly-thermally-conductive polymer materials may cause strange or improper behavior in the electrical circuits. Thus, an insulation layer between the fluid ejector head and the heat sink may be provided when electrically-conductive highly-thermally-conductive polymer materials are used. 
     The heat sink is used to carry heat away from a die of a thermal fluid ejection head, allowing the fluid ejector head to operate for extended periods of time. Operating a fluid ejector head for extended periods of time typically increases the temperatures in the die of the fluid ejector head. Dissipating the heat away from the die allows the fluid ejector head to operate at temperatures cool enough to enable high quality fluid ejection. 
     In various exemplary embodiments according to this invention, the highly conductive polymers used for the heat sink material includes base polymers mixed with a variety of filler materials. For example, one such polymer material is COOL POLY™ made by Cool Polymers Inc. Specifically, the COOL POLY E200™ polymer material is an injection-moldable, a liquid-crystal-polymer-based material having a thermal conductivity of 60 W/m° C. and a coefficient of thermal expansion (parallel to flow) of about 5 μm/m per degree C. 
     Recently, other companies, such as Polyone, LDP Engineering Plastics, RTP Company, GE and Dupont, have developed highly conductive polymers that may also be used with the heat sinks according to this invention. 
     Typical filler materials include graphite fibers and ceramic materials, such as boron nitride and aluminum nitride fibers. In various exemplary embodiments, blends of highly conductive polymers having high thermal conductivity use graphite fibers formed from a petroleum pitch base material. Typical base material for the polymers include liquid crystal polymer (LCP), polyphenylene sulfide and polysulfone. 
     In various exemplary embodiments, the heat sink is bonded to the die of the fluid ejector head. The die of the fluid ejector head is typically made from silicon, which has a coefficient of thermal expansion of about 4.67 μm/m° C. 
     Table 1 lists various properties for some commonly used substrate materials and for an exemplary highly conductive polymer, i.e, COOL POLY E200™ manufactured by Cool Polymers Inc. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Coefficient 
                   
                   
               
               
                   
                   
                 of Thermal 
                 Elastic 
                 Shear Force 
               
               
                   
                   
                 Expansion 
                 Modulus 
                 (Calculated 1 ) 
               
               
                   
                 Material 
                 (μm/m° C.) 
                 (Gpa) 
                 (N) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Aluminum 
                 23 
                 70 
                 2.14 
               
               
                   
                 Copper 
                 11.7 
                 110 
                 1.18 
               
               
                   
                 Noryl 
                 72 
                 2.4 
                 0.32 
               
               
                   
                 CoolPoly E200 
                 5 
                 60 
                 0.033 
               
               
                   
                 (parallel to flow 
               
               
                   
                 direction) 
               
               
                   
                 CoolPoly E200 
                 15 
                 60 
                 1.06 
               
               
                   
                 (perpendicular 
               
               
                   
                 to flow) 
               
               
                   
                   
               
               
                   
                   1 The calculated shear force in Table 1 assumes a 3 mm × 1 mm × 25 mm silicon die bonded to 5 mm thick substrate for a 30° C. temperature change.  
               
            
           
         
       
     
     The calculated shear force F between the die and heat sink substitute is determined as:
 
 F =[(α s −α d )Δ T ]/[(1 /E   s   A   s )+(1 /E   d   A   d )],
 
where:
 
     α s  is the thermal expansion coefficient of the substrate; 
     α d  is the thermal expansion coefficient of the die, which is 4.67 um/m° C. for die mode of silicon; 
     E s  is the elastic modulus of the substrate; 
     E d  is the elastic modulus of the die, which is 70 GPa for dies formed of silicon; 
     A s  is the cross-sectional area of the substrate; and 
     A d  is the cross-sectional area of the die. 
     As shown in Table 1, when the one or more thermally conductive filler materials are oriented parallel to the flow direction in a mold, the coefficient of thermal expansion of the polymer/filler material mixture is 5 μm/m° C. Often, the filler materials are fibers. When fibers are used, the long axis of the fibers becomes aligned with the flow direction, the polymer/filler material mixture has anisotropic coefficient of thermal expansion properties. When the one or more thermally conductive filler materials are oriented in the polymer perpendicular to the flow, the coefficient of thermal expansion of the polymer/filler material mixture is 15 μm/m° C. across the fibers, but is less along the fibers. By orienting the thermally conductive materials parallel to the flow direction, the coefficient of thermal expansion more effectively matches the coefficient of thermal expansion of the material used to make the die module. Thus, a significant reduction in the shear forces is obtained and more effective bonding is achieved. 
       FIG. 1  illustrates a first exemplary embodiment of a thermal fluid ejector assembly  100  including a device that dissipates heat from the thermal fluid ejector head. As shown in  FIG. 1 , the thermal fluid ejector assembly  100  includes a heat sink  110 , a fluid ejector element  120 , a fluid supply manifold  130  and a printed circuit board  140 . 
     It should be appreciated that the fluid supply manifold  130  is optional. Thus, the fluid supply manifold  130  or a device with a similar function and/or operation may or may not be used. 
     The fluid ejector element  120  is attached to the heat sink  110  by epoxy resin, thermal welding or any other appropriate attaching method. The fluid ejector element  120  includes a plurality of apertures  121  through which fluid, such as ink, is injected. The fluid ejector element  120  is connected to a printed circuit board  140 . 
     In various exemplary embodiments, the printed circuit board  140  includes electrically connected traces on a substrate with contact pads  141  at one end, which are connected to the fluid ejector element  120 . The other end of the printed circuit board  140  is shaped to be connected to an electrical connector. The printed circuit board  140  may be shaped in various sizes and shapes to allow a suitable electrical connection. The printed circuit board  140  also includes slots for sandwiching the printed circuit board  140  between the heat sink  110  and the fluid supply manifold  130 . 
     In various exemplary embodiments, the fluid supply manifold  130  includes a fluid chamber  131 , a filter  132  and a face tape  133 . The filter  132  is attached to the top of the fluid chamber  131  and the face tape  133  is attached to the lower portion of the fluid chamber  131 . 
     In various exemplary embodiments, the fluid ejector manifold  130  includes a fluid chamber  131  that may or may not be periodically refilled. The fluid ejector element  120  is attached under the fluid chamber  131 . The fluid chamber also includes mounting posts for attaching the fluid chamber  131  to the heat sink  110 . The printed circuit board  140  is electrically connected to the fluid ejector element  120  and sandwiched between the fluid chamber  131  and the heat sink  110 . In various other exemplary embodiments, rather than the fluid ejector manifold  130  containing the chamber  131 , the fluid chamber  131  is provided integrally with the heat sink  110  and this is formed using the same material as the heat sink  110 . In this case, the fluid manifold  130  conducts the fluid from the fluid chamber  131  to the fluid ejector element  120 . 
       FIG. 2  illustrates a first exemplary embodiment of the heat sink  110  of the thermal fluid ejector head  100  of FIG.  1 . In various exemplary embodiments, the heat sink  110  includes a base  111  from which a plurality of heat transfer surfaces extend outwardly. As shown in  FIG. 2 , in this first exemplary embodiment, the heat transfer surfaces are fins  114 . As shown in  FIG. 2 , the heat sink  110  includes seven fins  114  extending from the base  111 . Although  FIG. 2  illustrates the fins  114  in a specific arrangement, it should be appreciated that any suitable effective number, size and/or orientation of the fins  114  may be used. 
     As shown in  FIG. 2 , the fluid ejector element  120  is attached to the side surface  112  using an adhesive  113  (shown in FIG.  1 ). However, it should be appreciated that other methods may be used to attach the fluid ejector element  120  to the side surface  112 , such as, for example, welding, mechanical clamping, and the like. Additionally, it should be appreciated that the fluid ejector element  120  may be attached to other surfaces of the base  111 . 
       FIG. 3  shows a second exemplary embodiment of a fluid ejector assembly  200  that dissipates heat from the thermal fluid ejector head. As shown in  FIG. 3 , the fluid ejector assembly  200  includes a fluid ejector element  220  and a heat sink  210 . The heat sink  210  includes a number of extension pins  215  that extend outwardly from a base  211 . 
     In various exemplary embodiments, the fluid ejector element  220  is attached to the side surface  212  using an adhesive, welding, mechanical connectors, or the like. 
       FIG. 4  shows a third exemplary embodiment of a fluid ejector assembly  300  according to this invention. As shown in  FIG. 4 , the fluid ejector assembly  300  includes a heat sink  310  and fluid ejector element  320 . The heat sink  310  includes a base  311 . In various exemplary embodiments, the fluid ejector element  320  is attached to side surface  312  of the base  311 . In this exemplary embodiment, the heat sink  310  does not include pins or fins. The material in the base  311  provides adequate heat dissipation in certain working environments. 
       FIG. 5  shows a fourth exemplary embodiment of a fluid ejector assembly  400  according to this invention. As shown in  FIG. 5 , the fluid ejector assembly  400  includes a heat sink  410  and a print element  420 . The heat sink  410  includes a base  411  having two extending sidewalls  416  and  417  connected together by a connection wall  418 . 
     In various exemplary embodiments, the fluid ejector element  420  is attached to the side surface  412  using an adhesive, or similar bonding mechanism. It should be appreciated that, in the various exemplary embodiments of the heat sinks  110 ,  210 ,  310  and/or  410 , the orientation of filler materials used to tune the performance of the material used to form these heat sinks is oriented as outlined below with respect to  FIGS. 5 and 6 . 
       FIG. 6  is a schematic diagram illustrating a first exemplary embodiment of a technique for molding a heat sink usable according to this invention. As shown in  FIG. 6 , the one or more thermally conductive filler materials are oriented parallel to the die module. As a result, as shown in Table 1, coefficient of thermal expansion is obtained for the heat sink that is similar to that of the material used to make the die module. Thus, the bond between the substrate and the die module is not subjected to significant stress due to temperature changes. In addition, the oriented thermally conductive filler materials provide an effective heat sink for dissipating heat in the fluid ejector head. 
     As shown in  FIG. 6 , one exemplary embodiment of a heat sink molding apparatus  500  usable to form the heat sinks  110 ,  210 ,  310  and/or  410  includes sidewall channels  511 ,  512 ,  513  and  514 . The highly conductive polymer used to form the heat sinks  110 ,  210 ,  310  and/or  410  is injected into the molding apparatus  500  through a gate  520  and flows in the flow directions  521  and  522  through the channels formed by the side walls  511 ,  512 ,  513  and  514 . The flow directions  521  and  522  orient the filler material of the highly conductive polymer in an oriented flow area  523  so that the filler materials extend between a surface of the heat sink that receives heat from the fluid ejector head and one or more heat dissipations surfaces of the heat sink that dissipate the received heat into the environment around the fluid ejector head. 
       FIG. 7  is a schematic diagram illustrating a second exemplary embodiment of a technique for molding a heat sink usable according to this invention. As shown in  FIG. 7 , the one or more thermally conductive filler materials are oriented parallel to the die module. As a result, as shown in Table 1, coefficient of thermal expansion is obtained for the heat sink that is similar to that of the material used to make the die module. Thus, the bond between the substrate and the die module is not subjected to significant stress due to temperature changes. In addition, the oriented thermally conductive filler materials provide an effective heat sink for dissipating heat in the fluid ejector head. 
     As shown in  FIG. 7 , one exemplary embodiment of a heat sink molding apparatus  600  usable to form the heat sinks  110 ,  210 ,  310  and/or  410  includes sidewall channel  611 . The highly conductive polymer used to form the heat sinks  110 ,  210 ,  310  and/or  410  is injected into the molding apparatus  600  through a gate  620  and flows in the flow direction  621  through the channel formed by the sidewall channel  611  and out an exit gate  630 . The flow direction  621  orients the filler material of the highly conductive polymer so that the filler materials extend between a surface of the heat sink that receives heat from the fluid ejector head and one or more heat dissipations surfaces of the heat sink that dissipate the received heat into the environment around the fluid ejector head. 
     It should be appreciated that, in various exemplary embodiments of the orientation of the filler materials, various methods may be used to obtain a flow direction and are not limited to the exemplary embodiment as outlined with respect to  FIGS. 5-7 . For example, other exemplary embodiments of a heat sink molding apparatus can include a single elongate channel having a gate at one end. 
       FIG. 8  is a flowchart outlining one exemplary embodiment of a method for manufacturing a fluid ejector assembly according to this invention. As shown in  FIG. 8 , operation of the method begins in step S 100 , and continues to step S 200 , where thermally conductive filler materials are mixed with a polymer. Then, in step S 300 , the polymer containing filler materials are molded into a heat sink such that filler materials are oriented along a designed thermal flow direction. Next, in step S 400 , the heat sink is attached to a fluid ejector element. Finally, operation continues to step S 500 , where operation of the method ends. 
     While this invention has been described in conjunction with various exemplary embodiments, it is to be understood that many alternatives, modifications and variations would be apparent to those skilled in the art. Accordingly, the preferred embodiments of this invention, as set forth above are intended to be illustrative, and not limiting. Various changes can be made without departing from the spirit and scope of this invention.