Abstract:
A vacuum pressure furnace and/or a cooling plate for a vacuum pressure furnace is described, having a cooling channel or tube that selectively circulates a liquid coolant at a reduced temperature. The cooling channel “snakes” back and forth through a target plate assembly to conduct heat from the target plate assembly and back to the coolant. The target plate assembly includes a plurality of clamp members that are screwed over portions of the cooling channel and to a bottom of a plate member of the assembly, enclosing portions of the cooling channel. Thermal sheets or foil are wrapped around the cooling channel, thereby bridging any gaps between the components that may occur during temperature changes due to thermal expansion/contraction.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 62/351,817 filed Jun. 17, 2016 entitled Apparatus for Rapid Cooling of Substrates Utilizing a Flat Plate and Cooling Channels, which is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Vacuum pressure furnaces allow a workpiece to be heated and cooled in various pressure environments, and can be used for a variety of purposes, including the production of electronics. These vacuum pressure furnaces can be particularly useful in achieving void-free die soldering of electronic components, as well as hermetic package sealing of electrical circuit components. Hermetic package sealing typically uses glass, ceramic, or metal (e.g., solder) to create a barrier to gas, preventing moisture or other harmful agents from otherwise damaging sensitive electrical circuit components. 
         [0003]    Vacuum pressure furnaces often have a computer control system that is connected to a heating system, a cooling system, and a pressure-vacuum system of a furnace chamber. In some furnace designs, the furnace chamber has a bottom target plate on which one or more workpieces are placed. Such a plate is heated via heating elements (e.g., on the walls of the furnace chamber) and may optionally include a cooling system comprising a cooling pipe or channel containing circulating, temperature-controlled medium. In this regard, the computer system can selectively heat and cool an electronic workpiece within a pressure-controlled environment (e.g., from a vacuum to high pressure). 
         [0004]    Since many electronics with hermetic packaging are formed in a full vacuum, the heat transfer mechanism of the furnace occurs almost entirely through radiation between the heating elements or target plate, and the electronic workpiece. While the heating elements can be quickly and readily heated up to provide direct radiant energy to the electronic workpiece, the target plate relies on conduction heat transfer between the plate and the cooling channel. Hence, to achieve relatively quick, efficient heat transfer, good contact between the cooling channel and the target plate is necessary. 
         [0005]    Prior furnaces have relied on joining the cooling channel and the target plate using mechanical brazing or welding. Some applications of the furnace require relatively fast cooling of the joining solders and pastes used to join substrates layers of the electronic workpiece. However, the rapid heating and cooling creates a significant amount of stress on the mechanical brazing or welded joints between the target plate and the cooling channel. For that reason, many prior art furnaces rely on separate chambers for rapid heating and cooling. 
         [0006]    For example, U.S. Pat. No. 6,796,483 filed Sep. 28, 2004, which is incorporated herein in its entirety, describes a furnace (oven) design with a different station for rapid cooling of an electronic workpiece. As discussed below, the present invention uses a novel method of cooling which is different than this invention. 
       SUMMARY OF THE INVENTION 
       [0007]    One embodiment is directed to a vacuum pressure furnace and/or a cooling plate for a vacuum pressure furnace. A cooling channel or tube selectively circulates a liquid coolant at a reduced temperature. The cooling channel is positioned in an undulating or alternating wave-like pattern through a target plate assembly to conduct heat from the target plate assembly and back to the coolant. The target plate assembly includes a plurality of clamp members that are screwed over portions of the cooling channel and to a bottom of a plate member of the assembly, enclosing portions of the cooling channel. Thermal sheets or foil are wrapped around the cooling channel, between the clamp member and the plate member, thereby bridging any gaps between the components that may occur during temperature changes due to thermal expansion/contraction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other aspects, features and advantages of which embodiments of the invention are capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which: 
           [0009]      FIG. 1  a view of a vacuum pressure furnace according to the present invention. 
           [0010]      FIG. 2  illustrates a view of a vacuum pressure furnace according to the present invention. 
           [0011]      FIG. 3  illustrates a view of a furnace chamber for a vacuum pressure furnace according to the present invention. 
           [0012]      FIG. 4  illustrates a view of a furnace chamber for a vacuum pressure furnace according to the present invention. 
           [0013]      FIG. 5  illustrates a view of a target plate with a cooling channel according to the present invention. 
           [0014]      FIG. 6  illustrates a view of a target plate with a cooling channel according to the present invention. 
           [0015]      FIG. 7  illustrates a cross-sectional view of a tare plate according to the present invention. 
           [0016]      FIG. 8  illustrates a view of a clamp member for a plate assembly according to the present invention. 
           [0017]      FIG. 9  illustrates a view of a clamp member for a plate assembly according to the present invention. 
           [0018]      FIG. 10  illustrates a plate member of a plate assembly according to the present invention. 
           [0019]      FIG. 11  illustrates a cooling channel according to the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0020]    Specific embodiments of the invention will now be described with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The terminology used in the detailed description of the embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, like numbers refer to like elements. 
         [0021]    In one embodiment, the present invention is directed to a vacuum pressure furnace  100  that can more efficiently cool down its furnace chamber  104  while maintaining a higher reliability of the its cooling system. As seen in  FIGS. 1 and 2 , the furnace  100  includes a furnace chamber  104  and a lid  106  that closes over and seals the furnace chamber  104 . A user interface  108  (e.g., a display and keyboard) allows a user to program a sequence of conditions (or combination thereof) within the furnace chamber, such as a period of increased temperature, a period of cooling, a period of vacuum (low pressure), and a period of high pressure. 
         [0022]    As best seen in  FIGS. 3 and 4 , the furnace chamber  104  includes a target plate  102  on which one or more electronic workpieces (e.g., an electronic sensor component) can be placed. Heating elements  110  are located along each of the walls of the furnace chamber  104  to provide a desired amount of heat to the target plate  102  and the workpiece. 
         [0023]    The pressure within the chamber  104  can be adjusted by a pressure controller which is connected to and controls a vacuum pump, an attached gas supply, and valves capable of adjusting the chamber  104  between a vacuum, ambient atmospheric pressure, and high pressure. Different types of gas (e.g., nitrogen, argon, and helium) are also commonly used in the chamber  104  during operation. 
         [0024]    Cooling within the chamber  104  occurs when a liquid or air cooling media is pumped or otherwise circulated through the tubular cooling channel  112 . The cooling channel  112  extends through the plate assembly  102  at the base of the chamber  104  and further extends out of the chamber  104  to a pumping mechanism (e.g., a fluid pump) and a heat exchanger. Hence, cool media is passed through the plate assembly  102  and furnace chamber  104 , thereby decreasing the temperature of the plate assembly  102 ; then passes back to the heat exchanger which again decreases the media&#39;s temperature. Preferably, the cooling channel  112  is generally tubular and composed of a heat tolerant metal. While the cooling channel  112  is described as a single tubular channel loop, it should be understood that several, separate cooling loops are also possible. 
         [0025]    Since the plate assembly  102  and cooling channel  112  are subject to rapid heating and cooling, they tend to expand and shrink, depending on the temperature. For example, the plate assembly  102  may increase/decrease in length, width, and height, and the cooling channel  112  may increase/decrease radially and along its length. Additionally, if the cooling channel  112  and plate assembly  102  are composed of different material, they may increase/decrease in size at different rates due to differences in their coefficient of thermal expansion. This difference in expansion/contraction can create stress on components, particularly if welded joints are used. The plate assembly  102  addresses these thermal expansion/contraction issues with a design that both allows expansion/contraction of components without causing stress, and by maintaining contact between the plate assembly  102  and the cooling channel  112  for efficient heat transfer. 
         [0026]    The plate assembly  102  includes a plate member  116  having a generally flat top surface ( FIG. 6 ) that provides a work surface for placement of electronic workpieces. The bottom surface of the plate member  116  includes a plurality of parallel, grooves or recessed areas  116 A (e.g., 10 channel) that are aligned along the width of the plate member  116  ( FIG. 10 ), in which the cooling channel  112  is positioned. Specifically, the cooling channel  112  has a plurality of relatively straight regions  112 B that are connected by a plurality of curved regions  112 A ( FIG. 11 ). The curved regions  112 A are curved about 180 degrees and are connected in an alternating pattern, such that the straight regions  112 B form a generally uniform and parallel pattern. The straight regions  112 B are positioned in the recessed areas  116 A of the plate member  116 , while the alternating curved regions curve around a recessed end area  1166 . Since the recessed areas  116 A are located at the edge of the plate member  116 , the cooling channel  112  are provided room to longitudinally expand and contract during heating and cooling without imparting undue stress on the plate assembly  102  components. 
         [0027]    The cooling channel  112  is further held against the plate member  116  by a plurality of clamp plates  114 , as seen in  FIG. 5 . Each clamp plate  114  includes a relatively straight groove  114 A having a rounded bottom, shaped with a similar arc to that of the cooling channel  112  and thereby allowing the clamp plate  114  to mate with the straight region  112 B of the cooling channel  112 . In this respect, the groove  114 A and the recessed area  116 A form an enclosed passage for the cooling channel  112 . Further, the use of screws  118  to secure the clamp plates  114  further reduces any stress-related damage that might be otherwise cause if the components were instead welded together. 
         [0028]    The clamp plate  114  also includes flange portions  114 B along each side of the groove  114 A. The flange portions  114 B include apertures (e.g., two on each flange portion  1146 ) that can be aligned with apertures through the plate member  116 , allowing screws to be screwed through both to secure both components together, as seen in  FIG. 5 . In one embodiment, each straight, recessed area  116 A accommodates two linearly adjacent clamp plates  114 . However, different numbers of linearly adjacent clamp plates are possible (e.g., 1, 3, 4, 5, of 6). 
         [0029]    As best seen in the cross-sectional view of the plate assembly  102 , the straight region  112 B (and optionally the curved regions  112 A) of the cooling channel  112  are wrapped with one or more layers of a thermally conductive sheet  120  (e.g., a copper sheet or similar thermally conductive material). During cooling, the cooling channel  112  may radially contract to a greater extent than the plate assembly  102  (i.e., the plate member  116  and the clamp plates  114 ). However, the thermally conductive sheets  120  tend to bridge this displacement, maintaining an efficient heat conduction path between the cooling channel  116  and the plate assembly  102 . 
         [0030]    In one example embodiment, the plate member  116  and clamp plates  114  are composed of graphite, the cooling channel is composed of stainless steel, and the thermally conductive sheets  120  are composed of copper. Additionally, the coolant circulating through the coolant channel is water or air. In another example embodiment, the plate member  116  has a length of 30 cm and a width of 30 cm (though a wide range of sizes are possible according to the present invention). 
         [0031]    In operation, a user programs the vacuum pressure furnace with a “recipe” or program specifying one or more periods of a desired heating temperature, pressure (or lack thereof), and cooling temperature. A workpiece is added to the chamber  104  and placed on the top surface of the plate assembly  102 . The lid  106  is closed over the chamber  104  and the program is executed. The workpiece can, for example, be an electronic substrate, a cover disposed on the substrate, and solder material in between the two. In another example, the workpiece can be a silicon wafer being annealed. 
         [0032]    In one example program, the chamber  104  is reduced to a low or vacuum pressure, and the heating elements  110  are activated to increase the temperature of the workpiece accordingly. Next, the heating elements  110  are deactivated and the vacuum pressure is either maintained or a specific gas is delivered into the chamber  104 . Finally, cooling media is circulated through the cooling channel  112 , reducing the temperatures of the outer portion of the channel  112 . This reduced temperature is conducted through thermally conductive sheet  120  and on to the plate assembly  102 . Finally, the reduced temperature of the plate assembly  102  is conducted to the workpiece. 
         [0033]    Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.