Abstract:
A high power resistor includes a resistance element with first and second leads extending out from the opposite ends thereof. A heat sink of dielectric material is in heat conducting relation to the resistance element. The heat conducting relationship of the resistance element and the heat sink render the resistance element capable of operating as a resistor between the temperatures of −65° C. to +275° C. The heat sink is adhered to the resistance element and a molding compound is molded around the resistance element.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a high power resistor having improved operating temperature range and method for making same. 
     The trend in the electronic industry has been to make high power resistors in smaller package sizes so that they can be incorporated into smaller circuit boards. The ability of a resistor to perform is demonstrated by a derating curve, and a derating curve of typical prior art devices as shown in FIG.  9 .  FIG. 9  shows a derating curve  68  having a horizontal portion  70  which commences at −55° C. and which extends horizontally to +70° C. The resistor then begins to reduce in efficiency as shown by the numeral  72 , and at +150° C. it becomes inoperative. 
     Therefore, a primary object of the present invention is the provision of a high power resistor having an improved operating temperature range, and a method for making same. 
     A further object of the present invention is the provision of a high power resistor which is operable between −65° C. and +275° C. 
     A further object of the present invention is the provision of a high power resistor which utilizes an adhesive for attaching a heat sink to the resistor element. 
     A further object of the present invention is the provision of a high power resistor and method for making same which utilizes an anodized aluminum heat sink. 
     A further object of the present invention is the provision of a high power resistor and method for making same which utilizes an improved dielectric molding material surrounding the resistor for improving heat dissipation. 
     A further object of the present invention is the provision of a high power resistor and method for making same which provides an improved operating temperature and which occupies a minimum of space. 
     A further object of the present invention is the provision of an improved high power resistor and method for making same which is efficient in operation, durable in use, and economical to manufacture. 
     BRIEF SUMMARY OF THE INVENTION 
     The foregoing objects may be achieved by a high power resistor comprising a resistance element having first and second opposite ends. A first lead and a second lead extend from the opposite ends of the resistance element. A heat sink of dielectric material is capable of conducting heat away from the resistance element and is connected to the resistance element in heat conducting relation thereto so as to conduct heat away from the resistance element. The heat conducting relationship of the resistance element and the heat sink render the resistance element capable of operating as a resistor between temperatures of from −65° C. to +275° C. 
     According to one feature of the present invention the heat sink is comprised of anodized aluminum. This is the preferred material, but other materials such as beryllium oxide or aluminum oxide may be used. Also, copper that has been passivated to create a non-conductive outer surface may also be used. 
     According to another feature of the present invention, an adhesive attaches the heat sink to the resistance element. The adhesive has the capability of permitting the resistor to produce resistively throughout heat temperatures in the range of from −65° C. to +275° C. The adhesive maintains its adhesion of the resistance element to the heat sink in the range from −65° C., to +275° C. The specific adhesive which is Applicant&#39;s preferred adhesive is Model No. BA-813J01, manufactured by Tra-Con, Inc. under the name Tra-Bond, but other adhesives may be used. 
     According to another feature of the present invention a dielectric molding material surrounds the resistance element, the adhesive and the heat sink. Examples of molding compounds are liquid crystal polymers manufactured by DuPont (having an address of Barley Mill Plaza, Building No. 22, Wilmington, Del. 19880) under the trademark ZENITE, and under the Model No. 6130L; and a liquid crystal polymer manufactured under the trademark VECTRA, Model No. E130I, by Tucona, a member of the Hoechst Group, 90 Morris Avenue, Summit, N.J. 07901. 
     The method of the present invention comprises forming a resistance element having first and second opposite ends and first and second leads extending from the first and second opposite ends respectively. A heat sink is attached to the resistance element in heat conducting relation thereto so as to render the resistance element capable of producing resistance in the temperature range of −65° C. to +275° C. 
     The method further comprises forming the resistance element so that the resistance element includes a flat resistance element face. The method includes attaching a flat heat sink surface to the flat resistance element face. 
     The method further comprises using an adhesive to attach the heat sink to the resistance element. 
     The method further comprises molding a dielectric material completely around the resistance element, the adhesive, and the heat sink. 
     The method further comprises forming a pre-molded body on opposite sides of the heat sink before attaching the heat sink to the resistance element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the high power resistor of the present invention. 
         FIG. 2  is a perspective view of a strip of material having the various resistor elements formed thereon. 
         FIG. 3  is a perspective view of a similar resistance element such as shown in  FIG. 2 , but showing the pre-molded material and the adhesive material applied thereto. 
         FIG. 4  is a sectional view taken along line  4 — 4  of FIG.  3 . 
         FIG. 5  is a perspective view similar to  FIG. 3  showing the adhesive applied to the resistance element. 
         FIG. 6  is a view similar to  FIGS. 3 and 5  showing the heat sink in place. 
         FIG. 7  is a perspective view of the resistor after the molding process is complete. 
         FIG. 8  is a derating curve of the present invention. 
         FIG. 9  is a derating curve of prior art resistors. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings the numeral  10  generally designates a resistor body made according to the present invention. Resistor body  10  includes leads  24 ,  26  which extend outwardly from the ends of a dielectric body  16 . The leads  24 ,  26  are bent downwardly and under the bottom surface of dielectric body  16 . An exposed heat sink  18  is shown on the top surface of the body  10 . 
       FIG. 2  illustrates the first step of development and manufacture of the present invention. An elongated strip  20  includes a plurality of resistor blanks  36  extending there from. Strip  20  includes a plurality of circular indexing holes  22  which are adapted to receive pins from a conveyor. The pins move the various blanks  36  to each of various stations for performing different operations on the blanks  36 . 
     Each blank  36  includes a pair of square holes  23  which facilitate the bending of the leads  24 ,  26 . Between the leads  24 ,  26  is a resistance element  28 , and a pair of weld seams  34  separate the resistance element  28  from the first and second leads  24 ,  26 . Preferably, the first and second leads  24 ,  26  are made of a nickel/copper alloy, and the resistance element  28  is formed of a conventional resistance material. 
     Extending inwardly from one of the sides of the resistance element  28  are a plurality of slots  30  and extending inwardly from the opposite side of resistance element  28  is a slot  32 . The number of slots  30 ,  32  may be increased or decreased to achieve the desired resistance. The resistance is illustrated in the drawings by arrow  38  which represents the serpentine current path followed as current passes through the resistance element  28 . Slots  30 ,  32  may be formed by cutting, abrading, or preferably by laser cutting. Laser beams can be used to trim the resistor to the precise resistance desired. 
       FIG. 3  shows the next step in the manufacturing process. The blank  36  is pre-molded to form a pre-mold body  40 . Pre-molded body  40  includes a bottom portion  42  (FIG.  4 ), upstanding ridges  44  which extend along the opposite edges of the resistance element  28 , and four lands or posts  46  at the four comers of the resistance element  28 . Extending inwardly from the upstanding ridges  44  are two spaced apart inner flanges  48  which form slots  50  around the opposite edges of resistance element  28 . A pair of V-shaped bottom grooves  52  extend along the under surface of the bottom portion  42  of the pre-mold  40 . 
       FIG. 5  is the same as  FIG. 3 , but shows an amount of adhesive  54  which has been applied to the central portion of the resistance element  28 . The adhesive should have the properties of maintaining its structural integrity and maintaining its adhesive capabilities in the range of temperatures from −65° C. to +275° C. An example of such an adhesive is an epoxy adhesive manufactured by Tra-Con, Inc., 45 Wiggins Avenue, Bedford, Massachusetts 01730 under the trademark TRA-BOND, Model No. BA-813J01. 
     Referring to  FIG. 6 , a body  56  of anodized aluminum is placed over the adhesive  54  so that it is in heat conducting connection to the resistance element  28 . Thus heat is conducted from the resistance element  28  through the adhesive  54 , and through the anodized aluminum heat sink  56  to dissipate heat that is generated by the resistance element  28 . 
     After the heat sink  56  is attached to the resistance element  28  as shown in  FIG. 6 , the entire resistance element  28 , pre-mold  40 , adhesive  54 , and heat sink  56  are molded in a molding compound to produce the molded body  58 . The molded body  58  includes an exposed portion  18  so that heat may be dissipated directly from the heat sink  56  to the atmosphere. 
     The molding compound for molding the body  58  may be selected from a number of molding compounds that are dielectric and capable of conducting heat. Examples of such molding compounds are liquid crystal polymers manufactured by DuPont at Barley Mill Plaza, Building 22, Wilmington, Del. 19880 under the trademark ZENITE, Model No. 6130L; or manufactured by Tucona, a member of Hoechst Group, 90 Morris Avenue, Summit, N.J. 07901 under the trademark VECTRA, Model No. E130I. 
     The leads  24 ,  26  are bent downwardly and curled under the body  16  as shown in FIG.  1 . 
       FIG. 8  illustrates the derating curve produced by the resistor of the present invention. The derating curve is designated by the numeral  62  and includes a horizontal portion commencing at −65° and remaining horizontal up to +70° C. Then the derating curve declines downwardly as designated by the numeral  66  until it reaches  0  performance at +275° C. Thus the device of the present invention operates as a resistor between the temperature ranges of −65° C. to +275° C. 
     As can be seen by comparing  FIG. 8  to  FIG. 9 , the performance of the resistor of the present invention commences at 10° below the lowest temperature of the average prior art device and functions as a resistor up to 125° higher than the capabilities of prior art resistors. The resistor of the present invention will function in this temperature range to produce ohmage in the range of from 0.0075 ohms to 0.3 ohms, and to dissipate heat up to approximately 5 or 6 watts. 
     The invention has been shown and described above with the preferred embodiments, and it is understood that many modifications, substitutions, and additions may be made which are within the intended spirit and scope of the invention. From the foregoing, it can be seen that the present invention accomplishes at least all of its stated objectives.