Patent Publication Number: US-2023154656-A1

Title: Making an aluminum nitride heater

Description:
This application claims priority as a Divisional Application of U.S. Application Serial No. 16/782,287, filed Feb. 5, 2020. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to a heater for a variety of uses. The heater defines an essentially pure aluminum nitride base having thick film printing, including resistive and conductive layers and overlayers of glass. 
     BACKGROUND 
     Heaters have many and diverse applications in the industries of automotive vehicles, appliances, and consumer items, to name a few. They serve as cabin heaters in electric and hybrid vehicles, water heaters for dishwashers and washing machines, and sources of heat for hair dryers/straighteners, cooking apparatuses, and space heaters, etc. In imaging devices, such as printers and copiers, heaters exist in fusers to melt and fix toner on media. However, when narrow media (e.g., envelopes) are imaged, portions of the fusers extending beyond the media quickly overheat due to low thermal mass components which can become damaged. Some printers having a belt fuser heated by an aluminum oxide, thick-film printed, ceramic heater slow from 70 pages-per-minute print speed for full-sized media to 7 ppm for narrow media to prevent fuser damage by better controlling temperature, but at the expense of printing speed. 
     Positive Temperature Coefficient (PTC) heaters have been tested as a means of preventing damage to fusers during printing of narrow media. Some have suggested that PTC heaters are desirable for 2 main reasons: 1) PTC materials have a Curie point in a temperature region that can eliminate damage to other components, theoretically allowing narrow media to be printed at higher speeds than occurs with aluminum oxide heaters; and 2) the lower Curie point of the PTC materials serves as a safety feature. However, the inventors note that PTC materials alone are not advantageous because they have relatively low thermal conductivity. For PTC materials to be more effective, heat must be dissipated quickly from the PTC materials. Furthermore, PTC materials, when cold, have a severely low resistance - creating an extremely high in-rush current. 
     An aluminum nitride substrate, thick film heater, has been also mentioned for preventing damage to fusers during printing of narrow media. However, aluminum nitride heaters to date have been limited to hybrids consisting of about 80% aluminum nitride and about 20% aluminum oxide. Such materials have a thermal conductivity of about 80 W/mK compared to aluminum oxide which has a thermal conductivity of about 23 W/mK. Thus, the inventors propose an essentially pure aluminum nitride substrate, thick film heater, having a significantly higher thermal conductivity with a thermal conductivity of about 200 W/mK. No one as of yet, however, has been able to successfully thick-film print a large area resistor heater on such a substrate without Kirkendall voids (e.g., voids or gaps between the thick-film printed conductor and resistor layers on the substrate). In turn, voids such as these lead to extremely high electrical resistance causing heater failure. Furthermore, existing glass pastes cannot be used for thick-film printing di-electric glasses or cover glasses serving as electrical insulation layers over the conductor and resistor layers due to large voids in the glass. The inventors identify one reason for this as current manufacturing processes have difficulty in out-gassing nitrogen gas during high temperature firing/heating (typically 850° C.) of the substrate having the conductor, resistor, and glass paste materials. 
     A need exists to overcome the foregoing. The inventors further note that any solutions in the technology of heaters should further contemplate the competing design constraints found in power consumption, safety features, warm-up characteristics, operating temperatures, heating speeds, thermal conductivity, materials, costs, electrical requirements, construction, materials to-be-heated, temperature control, installation/integration with other components, size, shape, and dimensions, and the like. 
     SUMMARY 
     A heater includes an aluminum nitride base having equal to or less than 1% impurities, especially none of polybrominated biphenyl (PBB), polybrominated diphenyl ether (PBDE), hexabromocyclododecane (HBCDD), polyvinyl chloride (PVC), chlorinated paraffin, phthalate, cadmium, hexavalent chromium, lead, and mercury. At least one longitudinally extending resistive trace of silver and palladium overlies the base as does a conductor of silver and platinum or silver and palladium that electrically connects to the resistive trace to apply an external voltage to the resistive trace for heating thereof. At least four, but optionally five layers of glass overlie the resistive trace and conductor, but not an entirety of the conductor. A first two consecutive layers of the glass layers define a first glass having a solid content of more than 65% and a viscosity of 100 Pa · s or less. The following two or three consecutive layers of the five layers define a second glass dissimilar to the first. 
     A method of making a heater includes thick film printing an aluminum nitride base having equal to or less than 1% impurities. The base is fired in a heating unit before any layering of the base. Thereafter, on a topside and backside of the base a conductor layer from a conductive paste is layered and allowed to settle and dry before firing the base having the conductor layers. Next, a resistive layer is layered on the base from a resistor paste such that the resistive layer connects to the conductor layer on the topside of the base. The resistor paste is allowed to settle and dry and then the base with the conductor and resistor layers is fired in the heating unit. At least four layers of glass are layered next over the resistive layer. Each instance thereof includes layering a glass, drying the glass and firing the base including the glass so layered. Settling, drying, and firing profiles are also noted under various embodiments. 
     The embodiments noted herein eliminate life-limiting Kirkendall voids in thick film printing of silver platinum (AgPt) conductors and silver palladium (AgPd) resistors at the interface of the conductor and resistor layers. The embodiments also eliminate large, prohibitive voids in layers of glass. 
     Further embodiments teach blends of resistor paste of about 80% silver and about 20% palladium for thick film printing on an aluminum nitride base, but still supporting a 115 volt resistor. The embodiments pioneer the use of relatively low palladium content unlike typical prior pastes of about 45% silver and 55% palladium. Pastes for conductor layers include content of about 93% silver and about 7% palladium or platinum and being essentially free of Kirkendall voids at the juncture of the resistor to conductor. The embodiments overcome problems noted with typical prior conductive pastes. 
     Still further embodiments note the use of a relative low firing temperature for a thick film di-electric (“cross glass”) and a cover glass overlying the resistor and conductive layers. As has been practiced in the past, typical large area ceramic heaters utilized a glass firing temperature of approximately 850° C. Whereas, embodiments of the present disclosure separate the “cross glass” (higher di-electric properties) from a cover glass (lower di-electric properties but improved surface quality) and fire the cross glass at a peak temperature as low as about 835° C. while the cover glass could be fired as low as about 810° C. Without being bound by theory, the lower firing temperatures are believed to be fundamental to preventing large voids in any of the glass layers (leading to poor di-electric strength) and guaranteeing no Kirkendall voids between the resistive and conductive layers. The inventors also found the lower temperatures of firing the glass helped in two ways. First, silver migration was diminished between the silver content of each of the resistor and conductor material. Second, the oxidation of the palladium in the resistor and conductor materials was lowered in those embodiments containing palladium, where palladium oxidation and reduction rates are known to relate to Kirkendall voids. 
     In even further embodiments, oxidizing or plasma treating the surface of the aluminum nitride base further contributes to eliminating the deleterious effects of nitrogen out-gassing during later instances of firing the base which occurs during print, dry, and firing sequences of thick film printing. 
     The embodiments of the invention support many forms of heaters for use in many and diverse applications. In one design, the heater is contemplated for use as a cabin heater for Electric Vehicles (EV) and hybrid vehicles. Traditional cabin heaters today utilize a series of PTC heaters embedded in a radiator type arrangement, i.e. an array of aluminum finned heat spreaders attached to PTC heaters. Air flows through the aluminum fin heat spreaders - removing heat from the PTC. However, a major disadvantage of a PTC heater (only) arrangement is that it heats relatively slowly. Auto users complain about the relatively slow warm-up time of the PTC only cabin heaters which is attributed to a relatively high in-rush of current. Developers then must be careful not to drain the battery of the vehicle or use excessive fuel to handle the excessive in-rush current. With the embodiments of the present heater, however, the essentially pure aluminum nitride heater supports thermal conductivity of about 200 W/mK and can serve as both a pre-heater and heat spreader to the PTC heater array - eliminating any slow warm-up of cabin heaters. It also supports power designs of about 1200 Watts or more. 
     In other applications, an essentially pure aluminum nitride heater facilitates the printing of narrow media in an imaging device without the problems of the prior art and can bear the duties of warming up a belt and back-up roller in a belt fuser in imaging devices. Such a heater can also pre-heat a series of PTC elements and then be turned off. Heating of the PTC can commence until the in-rush current is within 10% - 20%, for example, of the steady state current. Such a dual combination heating scheme allows for much quicker warm-up times. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES: 
         FIG.  1 A  is an exploded view of an aluminum nitride heater according to a representative embodiment of the present invention; 
         FIG.  1 B  is a non-exploded view of the aluminum nitride heater of  FIG.  1 A ; 
         FIG.  1 C  is a backside view of the aluminum nitride heater of  FIG.  1 B ; 
         FIGS.  2 A- 2 F  are diagrammatic views of a representative sequence of printing, drying and firing layers on a substrate when forming an aluminum nitride heater; 
         FIGS.  3 A- 3 E  are diagrammatic views of a representative sequence of printing and drying layers on a substrate when forming an aluminum nitride heater; 
         FIGS.  4 A and  4 B  are diagrammatic views of a representative sequence of only firing a substrate with an optional layer(s) when forming an aluminum nitride heater; 
         FIGS.  5 A- 5 E  are diagrammatic views of a representative sequence of printing, drying and firing patterned layers on a substrate, such as resistive traces and conductors, when forming an aluminum nitride heater; 
         FIGS.  6 A- 6 I  are diagrammatic planar views of a representative sequence of patterning a plurality of top layers on a base forming an aluminum nitride heater; 
         FIG.  7    is a diagrammatic view of a large substrate for dicing into plural, individual aluminum nitride heaters; and 
         FIG.  8    is a graph of a representative heating profile according to embodiments of the invention for firing in a heating unit a base or substrate with or without overlying layers. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS.  1 A and  1 B  teach a heater  10  for a variety of uses. The heater includes an essentially pure aluminum nitride base or substrate  12 . Essentially pure is at least 5% impurities or less, but equal to or less than 1% is preferred. In one embodiment, the impurities of the base do not include any of polybrominated biphenyl (PBB), polybrominated diphenyl ether (PBDE), hexabromocyclododecane (HBCDD), polyvinyl chloride (PVC), chlorinated paraffin, phthalate, cadmium, hexavalent chromium, lead, and mercury. The shape of the base includes a longitudinally extending solid of a generally rectangular shape having a length (1) and width (w) dimension and a thickness (t). After separating by dicing in  FIG.  7    from a saw  15  along dashed lines  17  and  19  from a larger wafer  20 , for example, representative dimensions of each heater  10  include a thickness in a range of about 0.5 - 0.7 mm, a length in a range of about 150 - 160 mm, and a width in a range of about 6-8 mm. 
     With continued reference to  FIGS.  1 A and  1 B , each heater  10  includes at least one resistive trace  22  on a topside  24  of the base. Connected to each resistive trace at interface  25  is a conductor  26 . During use, the conductor  26  receives power from an external voltage source to power the resistive trace(s)  22 . In turn, the resistive trace heats and provides heating to the device in which it is used, such as for a cabin heater in an electric or hybrid vehicle or a fuser in an imaging device. In one embodiment, the external source is 115 VAC. In others, it is 12 VDC, 350 VDC, 650 VDC or 800 VDC. In any, the resistive trace and conductor support the voltage and lack Kirkendall voids at the interface  25 , by way of the methods of manufacturing the heater as described below. In dimensions, the thickness of the resistive trace is about 10 - 13 µm on the aluminum nitride base and has a length of about 135 - 145 mm and a width of about 4.5 - 5.5 mm. The conductor has a thickness of about 9 - 15 µm on the aluminum nitride base, a length of about 11 - 13 mm, and a width of about 4.8 - 5.8 mm. Also, the resistive trace has a resistance of about 10 - 12 ohms at 195° C. The resistive trace is formed from a resistor paste of about 80% silver and 20% palladium while the conductor is formed from a conductive paste of silver and palladium or platinum. In one embodiment, pastes for conductor layers include content of about 93% silver and about 7% palladium or platinum. 
     Overlying each resistive trace and at least a portion of the conductor, but not an entirety of the conductor as it needs to connect to the external power source, is at least four layers of glass  30  ( 30 - 1 ,  30 - 2 ,  30 - 3 ,  30 - 4 ,  FIG.  1   a   ). The glass is any of a variety but the first two consecutive glass layers  30 - 1 ,  30 - 2  are of a first type, while the next two  30 - 3 ,  30 - 4  are of a different type. The first type defines a cross glass layer, while the different type defines a cover glass layer. Any of the four glass layers define a glass having a viscosity of 100 Pa · s or less. More particularly, the viscosity exists at 90 Pa · s or less, especially 65 Pa · s or less. Its solid content, on the other hand, exists at 65% or more. In various specific embodiments, the glass is purchased commercially from AGC, Inc. (formerly the Asahi Glass Company) as seen in Table 1. Its properties are also noted.  
     
       
         
          TABLE 1
           
               
               
               
               
             
               
                 AGC, Inc. Glass Paste ID 
                 Thixotropic Index 
                 Viscosity (Pa · s) 
                 Solid Content (%) 
               
             
            
               
                 AP5717B10 
                 2.0 - 2.4 
                 
                   100 
                 
                 66 
               
               
                 AP5717B13 
                 1.6 
                 89 
                 69 
               
               
                 AP5717B14 
                 1.4 
                 61 
                 72 
               
            
           
         
       
     
     In any layer of glass, the dimensions include a thickness in a range of about 10 - 13 µm on the aluminum nitride base, a length in a range of about 135 - 145 mm, and a width in a range of about 4.5 - 5.5 mm. In one embodiment, the first two consecutive layers  30 - 1 ,  30 - 2  of the at least four glass layers together have a thickness of about 24 µm. The next two consecutive layers  30 - 3 ,  30 - 4  and a fifth layer of glass (not shown until  FIG.  6 I ) together have a combined thickness of about 65 µm. The fifth layer of glass also overlies the base and resistive and conductive layers and is similar in composition to any of the cover glass layers. 
     With reference to  FIG.  1 C , a bottom or backside  40  of the base  12  optionally includes one or more thermistors  50 . They interconnect with a same or different conductor  26  of the topside. They are positioned to measure the temperature of the heater  10  and the conductor  26  connects the thermistors to external sources to measure, store and control the temperature. 
     With reference to the Figure sets of 2A et seq., 3A et seq., and 4A et seq., the general process steps for fabricating the heater  10  of  FIGS.  1   a  and  1   b    will be described. They include one or more of thick-film printing, settling, drying, and firing or heating. As shorthand from the industry, they are generally known as print, dry, and fire, or PDF. 
     In more detail, the  FIGS.  2 A- 2 F  show printing, drying, and firing. In  FIG.  2 A , a base or substrate, such as the essentially pure aluminum nitride base  12 , is provided. In  FIG.  2 B , thick-film printing of the substrate includes providing a mesh stencil  60  upon and through which a paste  62  is applied. In the instance of layering a resistor, conductor or glass, a resistive paste, a conductive paste or a glass paste is applied. In  FIG.  2 C , a leveling device  64 , such as a squeegee or other scraper, levels the paste on a surface  66  ( FIG.  2 B ) of the base. In  FIG.  2 D , the paste so applied is allowed to settle on the base forming a layer  70  upon removal of the stencil. This settling occurs typically for about five to ten minutes at room temperature, e.g., 20° - 25° C. In  FIG.  2 E , the base and layer is provided to a curing or drying unit  80 . The drying unit typifies a box oven or blast furnace and the base is provided to the unit along a conveyor, typically. The drying unit begins drying the layer  70  at around room temperature followed by a curing or drying cycle of about 30 minutes reaching temperatures of 140°- 160° C. In one embodiment, the drying cycle includes applying infrared heat or hot air (both given generically as heat  82 ) for a period of time of about  30  total minutes at a temperature profile of the drying unit beginning at about 25° C. and ramping up to about 80° C. for about 10 minutes, ramping up again to about 160° C. for about 10 minutes and cooling down to below 50° C. After that, the base  12  with layer  70  is fired in a heater or firing unit  80 ′. In some instances, the firing unit  80 ′ is the same unit as the drying unit  80 , but having different heating profiles. In others, the firing unit  80 ′ is different from the drying unit  80  and the base advances from one unit to the next along a conveyor, typically. In any, the heating profile for heating the base depends upon which type of layer is most recently printed and dried thereon, e.g., resistive layer, conductive layer or glass layer. 
     In  FIG.  8   , a representative heating profile for any layer is shown in graph  100 . Namely. the heating profile for a un-layered base or resistive or conductive layer is shown by the solid line  102 , whereas a dashed line  104  depicts the heating profile for glass. In general, the heating profile of the heating unit includes a total heating time of about  40  total minutes starting at about 25° C. and ramping up to a peak temperature (part of zones 5-8) by 20 minutes and maintaining the peak temperature for at least 10 minutes and decreasing the temperature of the heating unit (post zone 8) for at least 10 minutes thereafter. Cooling continues even further thereafter (post zone  12 ) until completely cooled. For an un-layered base or the resistive or conductive layers, the peak temperature reaches about 850° C. The glass layers, on the other hand, have a peak temperature of 830° C. or 810° C., depending on which layers. Table 2, infra, provides a representative embodiment of which glass layers heat at which temperatures. 
     With reference to  FIGS.  3 A- 3 E , instances are shown of thick-film printing a base  12  to form and dry a layer  12  thereon. The views are similar to  FIGS.  2 A- 2 E , except there is no instance of firing the base/layer(s) in a firing unit. Rather, the processing steps only include printing and drying. Similarly, too,  FIGS.  4 A and  4 B  show the mere firing of a substrate  12  in a firing unit  80 ′, but without any instance of printing or drying a layer on the base or substrate. 
     With reference to  FIGS.  5 A- 5 E , a sequence of events depicts the printing, drying and firing steps of processing, but for a patterned layer overlying a base. That is,  FIG.  5 A  shows a base  12 . In  FIG.  5 B , the mesh stencil  60 ′ includes a patterned layout  61  for receiving (1) a paste  62  and leveling (2) therein by the leveling device  64 , but whereas a remainder of the stencil includes a masked portion  65  preventing application of the paste  82  to the base  12 . In  FIG.  5 C , the result is given with a base  12  having patterned layers thereon. In this instance, two longitudinally extending resistive traces  91  reside on the surface  66  of the base  12  in the pattern matching the patterned layering  61  of the stencil  60 ′. Of course, any patterned shapes are possible. Settling of the patterned layer then occurs for about five to ten minutes at room temperature and are similar to that of  FIG.  2 D . Heating of the base and patterned layers next occurs in  FIG.  5 D , including either curing and or firing in a drying and or heating unit  80 / 80 ′. In  FIG.  5 E , the patterned layer of the base  12  is further shown with another patterned layer  93  representing a conductive layer connected to a resistive layer at an interface  25 . Again, any patterning of layers is contemplated herein. 
     With the principles of any instances of printing, drying and firing on a base, reference to  FIGS.  6 A- 6 I  show one embodiment of forming an aluminum nitride heater according to the invention. At  FIG.  6 A , an essentially pure aluminum nitride base  12  is provided. The base has 5% or fewer impurities, especially 1% or less. A surface  66  of the base is optionally pretreated by oxidizing the surface or providing a plasma treatment according to known techniques. The base is then fired according to the heating profile  102  of  FIG.  8   , up to a peak temperature of 850° C. 
     In  FIG.  6 B , a conductor layer  26  is patterned on a topside  24  of the base by thick-film printing and drying. The conductor layer is formed from a conductive paste. The past is a blend of silver and platinum or silver and palladium. The silver comprises more than 90% of the paste. In one design, the paste is about 93% silver and about 7% palladium. In  FIG.  6 C , on a backside  40  of the base  20 , another conductor layer  26  is patterned by thick-film printing and drying. The paste is the same as the topside paste and the backside is used to secure thermistors, e.g.,  FIG.  1 C , such as by resistance-welding thermistors to the conductor layer. Thereafter, the base  12  with top and backside conductor layers are fired. The firing takes the form of the heating profile  102  of  FIG.  8    and reaches a peak temperature of about 850° C. In alternate embodiments, the processes of  FIGS.  6 B and  6 C  could be reversed with the latter occurring first. 
     In  FIG.  6 D , a resistive trace  22  is patterned and connects to the conductor layer  26  at an interface  25 . The trace is formed by thick-film printing with a patterned stencil and allowed to settle into place at the interface whereupon it is dried. The trace, formed also of a blend of silver and palladium, representatively comprises 80% silver and 20% palladium. Thereafter, the trace together with the base and the top and backside conductor layers is fired in a firing unit. The firing takes the form of the heating profile  102  of  FIG.  8    and reaches a peak temperature of about 850° C. 
     In  FIG.  6 E , a first glass layer  30 - 1  is patterned over the resistive trace and portions of the conductor. The first glass layer is patterned by thick-film printing, then dried and fired. The heating profile takes the form of the dashed line  104  in  FIG.  8    and reaches a peak temperature of about 830° C. The glass layer is typified as a cross glass layer formed from a paste sold by AGC, Inc. as AP5717B14. It has thixotropic index of 1.4, a viscosity of about 61 Pa · s and a solid content of more than 70%, especially 72%. Similarly, in  FIG.  6 F , a second glass layer  30 - 2  is patterned over the first glass layer  30 - 1  and also covers the resistive trace and portions of the conductor. The second glass layer is patterned by thick-film printing, then dried and fired. The heating profile takes the form of the dashed line  104  in  FIG.  8    and reaches a peak temperature of about 830° C. The second glass layer is also a cross glass layer formed from a paste sold by AGC, Inc. as AP5717B14. 
     In  FIG.  6 G , a third first glass layer  30 - 3  is patterned over the second glass layer and resistive trace and portions of the conductor. The third glass layer is patterned by thick-film printing, then dried and fired. The heating profile takes the form of the dashed line  104  in  FIG.  8    and reaches a peak temperature of about 830° C. The glass layer in this embodiment, however, is cover glass layer formed from a paste sold by AGC, Inc. as AP5717B13. It has thixotropic index of 1.6, a viscosity of 90 or less, about 89 Pa · s and a solid content of about  69 . Similarly, in  FIG.  6 H , a fourth glass layer  30 - 4  is patterned over the third glass layer and resistive trace and portions of the conductor. The fourth glass layer is patterned by thick-film printing, then dried and fired. The heating profile takes the form of the dashed line  104  in  FIG.  8    and reaches a peak temperature of about 830° C. The glass layer is cover glass layer formed from a paste sold by AGC, Inc. as AP5717B13. It too has thixotropic index of 1.6, a viscosity of 90 or less, about 89 Pa · s and a solid content of about  69 . 
     In  FIG.  6 I , a fifth glass layer  30 - 5  (optional in some embodiments, hence the dashed lines) is patterned over the fourth glass layer and resistive trace and portions of the conductor. The fifth glass layer is patterned by thick-film printing, then dried and fired. The heating profile takes a form similar to the dashed line  104  in  FIG.  8   , but reaches a peak temperature lower than any other temperature, at around 810° C. The glass layer is cover glass layer formed from a paste sold by AGC, Inc. as AP5717B13. It has thixotropic index of 1.6, a viscosity of 90 or less, about 89 Pa · s and a solid content of about  69 . 
     In table form, as a series of processes # 1-11, Table 2 shows the making of an essentially pure aluminum nitride heater as a technical specification. Namely: 
     
       
         
          TABLE 2
           
               
               
               
               
               
             
               
                 # 
                 Process Step 
                 Sequence 
                 Process Temp (°C) 
                 Spec 
               
             
            
               
                 1 
                 Fire Base 
                 F 
                 850 
                   
               
               
                 2 
                 Conductor Topside 
                 PD 
                   
                   
               
               
                 3 
                 Conductor Backside 
                 PD 
                   
                   
               
               
                 4 
                 Fire 
                 F 
                 850 
                   
               
               
                 5 
                 Resistive trace 
                 PDF 
                 850 
                   
               
               
                 6 
                 Uniformity Checked 
                   
                   
                   
               
               
                 7 
                 COG1 
                 PDF 
                 830 
                 Total thickness 
               
               
                 8 
                 COG2 
                 PDF 
                 830 
                 24 microns 
               
               
                 9 
                 OG1 
                 PDF 
                 830 
                 Total thickness 65 microns 
               
               
                 10 
                 OG2 
                 PDF 
                 830 
               
               
                 11 
                 OG3 
                 PDF 
                 810 
               
               
                 Notes: #1 is optional, #2 and #3 can be reversed, PD = Print, Dry, F = Fire, PDF = Print, Dry, Fire, COG1 = 1 st  “Cross Glass” layer, COG2 = 2 nd  “Cross Glass” layer, OG1 = 1 st  Cover Glass layer, OG2 = 2 nd  Cover Glass layer, OG3 = 3rd Cover Glass layer. 
               
            
           
         
       
     
     Thereafter, upon cooling, the resistive trace of the heater becomes tested under voltage conditions of 1.75 KVAC applied to the conductor layer. Resistance of the trace is tested cold at room temperature and upon heating the heater to about 200° C. Its resistance should be about 10 ohms at room temperature and about 11 ohms upon heating. A range of +/- 2 ohms is acceptable. 
     The foregoing description of several structures and methods of making the same has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the claims. Modifications and variations to the description are possible in accordance with the foregoing. It is intended that the scope of the invention be defined by the claims appended hereto.