Heating unit and method of making the same

Abstract

A heating unit includes an AlN substrate having a main surface on which an elongated heat-generating resistor is provided. A protection layer is formed on the main surface of the substrate for the heat-generating resistor. The protection layer includes a first cover layer covering the heat-generating resistor and a second cover layer covering the first cover layer. The first cover layer is made of crystallized or semi-crystallized glass having a higher crystallization temperature by at least 50° C. than the softening point of the glass. The second cover layer is made of non-crystalline glass.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a heating unit according to a first embodiment of the present invention;

FIG. 2 is a cross-sectional view showing a formation process of a heat-generating resistor by a manufacturing method of the heating unit of FIG. 1;

FIG. 3 is a cross-sectional view showing a formation process of a first cover layer by the manufacturing method of the heating unit of FIG. 1;

FIG. 4 is a cross-sectional view showing a formation process of a third cover layer by the manufacturing method of the heating unit of FIG. 1;

FIG. 5 is a cross-sectional view showing a formation process of a second cover layer by the manufacturing method of the heating unit of FIG. 1;

FIG. 6 is a cross-sectional view of a heating unit according to a second embodiment of the present invention; and

FIG. 7 is a cross-sectional view of a conventional heating unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cross-sectional view of a heating unit according to a first embodiment of the present invention. The illustrated heating unit A includes an AlN substrate 1, oxide layers 2, a heat-generating resistor 3, and a protection layer 4. The AlN substrate 1 has an upper or main surface 1a, and a lower or back surface 1b. The heating unit A is used in e.g. a printer to provide heat for fixing toner T on printing paper P. The printing paper P with the toner T transferred thereto is conveyed along the surface of the heating unit A under appropriate pressure provided by the pressure roller R, and the heat of the heating unit A fixes the toner T on the printing paper P.

The AlN substrate 1, made of aluminum nitride, is elongated in a direction perpendicular to the print paper conveying direction. The AlN substrate 1 is 7 to 14 mm in width and 0.5 to 0.7 mm in thickness. The aluminum nitride has excellent thermal response, and therefore the heat tends to spread substantially uniformly through the AlN substrate 1, which is advantageous in preventing the substrate from cracking. Also, the excellent thermal response permits locating the heat-generating resistor 3 on the back surface 1b of the AlN substrate 1 and utilizing the main surface 1a as the heating surface, as shown in FIG. 1. Although not shown in FIG. 1, the AlN substrate 1 includes an electrode layer for supplying power to the heat-generating resistor 3.

The heat-generating resistor 3 is for example a silver/palladium resistor containing 15 wt % or more of palladium, and is disposed on the back surface 1b of the AlN substrate 1, to extend along the lengthwise side of the AlN substrate 1. When power is supplied by a driving unit (not shown) to the heat-generating resistor 3 via the electrode layer which is not shown, the heat-generating resistor 3 generates heat at a predetermined calorific value. The heat-generating resistor 3 is formed by sintering a resistor paste to have a thick-film shape with a predetermined width. The foregoing weight ratio of the heat-generating resistor 3 is selected for efficiently discharge the gas generated from the reaction between the glass component of the resistor paste and the component of the AlN substrate 1, which takes place during the sintering process of the heat-generating resistor 3. Also, the thickness of the heat-generating resistor 3 may be appropriately determined according to the required calorific value, normally in a range of 7 to 23 μm, for example.

The oxide layer 2 is an aluminum oxide layer formed as a result of oxidation of the main surface 1a and the back surface 1b of the AlN substrate 1 during the sintering process of the heat-generating resistor 3. Also, the AlN substrate 1 may be intentionally heated before forming the heat-generating resistor 3, to form the oxide layer 2 in advance. The oxide layer 2 serves to prevent the reaction of nitrogen in the AlN substrate 1 and the glass component in the glass paste.

The protection layer 4 is formed of glass, and serves to protect the electrode layer (not shown) provided on the back surface 1b of the AlN substrate 1, and the heat-generating resistor 3. The protection layer 4 includes a first cover layer 41 that covers the heat-generating resistor 3, a second cover layer 42 that covers the first cover layer 41, and a third cover layer 43 formed on a region where the first cover layer 41 is not provided on the back surface 1b of the AlN substrate 1.

The first cover layer 41 is formed in a thick film of for example 20 to 40 μm in thickness, from glass paste predominantly composed of a material of crystallized glass of semi-crystallized glass, and is located to cover the heat-generating resistor 3 on the foundation of the heat-generating resistor 3 and a part of the back surface 1b of the AlN substrate 1. The crystallized glass or semi-crystallized glass predominantly composing the first cover layer 41 has a glass softening point of 740° C., and a crystallization temperature of 790 to 810° C. The crystallized glass or semi-crystallized glass generally has excellent heat resistance, and hence the first cover layer 41 is not fused even by direct application of the heat generated by the heat-generating resistor 3. Also, the difficulty for the heat from the heat-generating resistor 3 to be transmitted to the first cover layer 41 causes a majority of the calories is transmitted to the AlN substrate 1, thereby urging the heat increase on the surface of the AlN substrate 1.

The third cover layer 43 is provided in a region where the first cover layer 41 is not provided, on the back surface 1b of the AlN substrate 1, to surround the first cover layer 41. The third cover layer 43 is formed into a closely packed layer of approx. 10 to 25 μm in thickness, from glass paste predominantly composed of non-crystalline glass. The non-crystalline glass predominantly constituting the third cover layer 43 has a glass softening point of 780 to 810° C.

The second cover layer 42 is formed from glass paste predominantly composed of non-crystalline glass into a thick film with a smooth surface and, for example, 30 to 50 μm in thickness, to cover the first cover layer 41 and the third cover layer 43. Because of the smooth surface, the second cover layer 42 is less likely to be damaged by a foreign material such as dust, and besides prevents a foreign material such as moisture from intruding, because of being a closely packed structure of the non-crystalline glass. Also, to an outer face of the second cover layer 42, metallic parts such as a thermistor that controls the heating unit A, a thermoswitch and a thermal fuse for disconnecting the power when the control is disabled for some reason, are attached.

A manufacturing method of the foregoing heating unit A will now be described below.

FIGS. 2 to 5 are cross-sectional views showing processes in an embodiment of the manufacturing method of the heating unit A. The following description will be made referring to these drawings. Here, FIGS. 2 to 5 illustrate the AlN substrate 1 in the reverse orientation to FIG. 1. Accordingly, the upper side surface of the AlN substrate 1 in FIGS. 2 to 5 will be referred to as the back surface 1b, and the lower side surface as the main surface 1a.

Firstly, as shown in FIG. 2, the heat-generating resistor 3 is formed on a predetermined position on the back surface 1b of the AlN substrate 1. More specifically, a resistor paste including a resistor component constituted of silver/palladium, with 15 wt % or more of palladium in the resistor component, is applied to the predetermined position on the back surface 1b of the AlN substrate 1, in a form of a thick film by a printing method. The resistor paste is the dried, and sintered under a temperature of 700 to 850° C. Because of the above specified weight ratio of the palladium, the film formation of the silver by sintering is suppressed during the sintering process. Accordingly, the gas generated from the reaction of the glass component of the resistor paste and the component of the AlN substrate 1 can be efficiently discharged, and hence formation of void defect in the heat-generating resistor 3 because of foaming during the sintering can be prevented. Also, during the sintering of the resistor paste, the oxide layer 2 is also formed at a time in a region on the AlN substrate 1 where the heat-generating resistor 3 is not formed, in a thickness of approx. 1.0 to 10 μm. The oxide layer 2 serves to suppress the subsequent reaction between the AlN substrate 1 and the glass component. Here, it is preferable to form an interconnect pattern on the back surface 1b of the AlN substrate 1, for supplying power to the heat-generating resistor 3, in advance of this process.

Then as shown in FIG. 3, the first cover layer 41 is formed to cover the heat-generating resistor 3. At first, glass paste predominantly composed of crystallized glass or semi-crystallized glass material having a glass softening point of 740° C. and a crystallization temperature of 790 to 810° C. is heated up to 740° C. for softening, and printed in a form of a thick film to cover the heat-generating resistor 3. At this stage, the glass paste is to be applied to expose the left and right end portions (according to the orientation of FIG. 3) of the back surface 1b of the AlN substrate 1 as shown in FIG. 3. The glass paste thus applied is dried and then sintered at a temperature of 800 to 850° C., preferably at 810° C., to thereby crystallize the crystallized glass or semi-crystallized glass. The first cover layer 41 can be thus formed.

The above is followed by formation of the third cover layer 43 as shown in FIG. 4, in a region on the back surface 1b of the AlN substrate 1 not occupied by the heat-generating resistor 3 or the first cover layer 41. Firstly, glass paste predominantly composed of non-crystalline glass having a glass softening point of 780 to 810° C. is printed in a form of a thick film of approx. 10 to 25 μm in thickness, in a region on the back surface 1b of the AlN substrate 1 unoccupied by the first cover layer 41, to surround the first cover layer 41. Then the glass paste is dried, followed by sintering at 810° C., and cooling for hardening. Here, the sintering temperature may be altered as long as the temperature is higher than the glass softening point, and the difference is 30° C. or less.

Proceeding to FIG. 5, the second cover layer 42 is formed to cover the first cover layer 41 and the third cover layer 43. Firstly, glass paste predominantly composed of non-crystalline glass having a glass softening point of 700° C. or higher is printed in a form of a thick film, on the foundation of the first cover layer 41 and the third cover layer 43. The printed glass paste is then dried, and sintered at 800 to 850° C., followed by cooling for hardening. Preferably, the glass softening point of the non-crystalline glass employed as the second cover layer 42 is lower than the sintering temperature in this process, with a difference of 100° C. or less. It is preferable to attach, after this process, the metallic parts which are not shown, such as a thermistor that controls the heating unit A, a thermoswitch and a thermal fuse for disconnecting the power when the control is disabled for some reason, to the outer face of the second cover layer 42.

Through the foregoing process, the heating unit A can be efficiently manufactured. In addition to the above process, the manufacturing method may also include a process of coating the main surface 1a of the AlN substrate 1 with a smooth and heat-conductive resin, and a process of forming the oxide layer 2 in advance on the main surface 1a and the back surface 1b of the AlN substrate 1.

The heating unit A thus configured provides the following advantageous effects.

The first cover layer 41 is formed by sintering the glass paste including the material of crystallized glass or semi-crystallized glass at a sintering temperature higher than the glass softening point of the glass paste, but with a difference in a range of 50 to 70° C. This sintering temperature range includes the crystallization temperature of the crystallized glass or semi-crystallized glass predominantly constituting the first cover layer 41, and hence the first cover layer 41 is crystallized and hardened, during this sintering process. Since the crystallization temperature of the first cover layer 41 is higher than the glass softening point of the glass paste by 50° C. or more, the glass component in the paste flows, while the first cover layer 41 turns from the paste to the crystallized state. Accordingly, the first cover layer 41 is formed into a closely packed layer rather than a porous layer, and thus exhibits excellent electrical insulation performance. Besides, the second cover layer 42 and the third cover layer 43 are originally closely packed layers formed of the non-crystalline glass, and are hence excellent in electrical insulation. The heating unit A includes, therefore, the protection layer 4 which is excellent in electrical insulation between the metallic parts and the heat-generating resistor 3, thereby achieving a higher withstand voltage, thus minimizing the likelihood of being damaged by a surge originating from lightning or other reasons.

Also, since the sintering temperature of the first cover layer 41 is not more than 70° C. higher than the glass softening point of the glass paste, the glass component is kept form being excessively liquefied, and hence the reaction between the glass component and the component of the AlN substrate 1 can be suppressed. Accordingly, the first cover layer 41 suppresses the emergence of the void defect originating from the foaming. Further, the crystallized glass or semi-crystallized glass is generally excellent in heat resistance, and is not fused again once crystallized, and therefore the first cover layer 41 is not fused again during the sintering process of the second cover layer 42 and the third cover layer 43. The third cover layer 43 is formed by sintering the non-crystalline glass, the predominant component thereof, at a sintering temperature higher than the glass softening point of the non-crystalline glass but with a difference of 30° C. or less. In the case where the glass softening point and the sintering temperature are thus close, it takes shorter before the glass component is hardened after the sintering, and hence the glass component can only remain liquefied for a shorter time. Such arrangement allows suppressing the reaction between the glass component and the component of the AlN substrate 1, thereby preventing emergence of the void defect originating from the foaming. Also, the third cover layer 43 is formed in a thickness of approx. 10 to 25 μm, which allows shortening the time required for sintering and cooling. Further, since the third cover layer 43 is disposed adjacent to the first cover layer 41 to surround the same, the entirety of the back surface 1b of the AlN substrate 1 is covered with either the first cover layer 41 or the third cover layer 43. In other words, the back surface 1b of the AlN substrate 1 is covered with the first cover layer 41 and the third cover layer 43, both of which can suppress emergence of the void defect originating from the foaming. The second cover layer 42 is sintered on the foundation constituted of the first cover layer 41 and the third cover layer 43, and therefore the sintering process of the second cover layer 42 can be executed free from the reaction between the glass component and the component of the AlN substrate 1.

The second cover layer 42 is formed by sintering the glass paste predominantly composed of the non-crystalline glass having a glass softening point of 700° C. or higher, at a sintering temperature of 800 to 850° C. Limiting the difference between the glass softening point and the sintering temperature in a range of 100° C. or less allows suppressing, to a certain extent, the reaction between the glass component and the AlN substrate 1, in case where the first cover layer 41 or the third cover layer 43 should be chipped. Also, the third cover layer 43 may be softened during the sintering of the second cover layer 42. However, since the glass softening point of the non-crystalline glass predominantly constituting the third cover layer 43 is 780° C. or higher and the sintering temperature of the second cover layer 42 is 800 to 850° C., the third cover layer 43 remains softened for a short time only, and the foaming is suppressed to a minimal extent. Thus, the protection layer 4 of the heating unit A is least likely to incur the void defect originating from the foaming, and also excellent in strength. Besides, the outermost surface of the protection layer 4 is formed of the smooth non-crystalline glass, and hence there is little likelihood that an external foreign material gets caught by the protection layer 4, thereby peeling off and damaging the protection layer 4.

FIG. 6 illustrates another embodiment of the heating unit. In the heating unit B shown in FIG. 6, a part of the third cover layer 43 of the heating unit A according to the foregoing embodiment intrudes in the first cover layer 41. In the heating unit B thus configured, the contact interface between the third cover layer 43 and the first cover layer 41 is inclined, which is advantageous in isolating the second cover layer 42 from the back surface 1b of the AlN substrate 1. In the manufacturing method of the heating unit B, it is preferable to form the third cover layer 43 before forming the first cover layer 41. In the remaining portions, the heating unit B has the same structure as the heating unit A.

As still another embodiment, the third cover layer 43 of the heating unit A may be omitted, so that the protection layer 4 only includes the first cover layer 41 and the second cover layer 42. In this case, from the viewpoint of the withstand voltage, the heating unit of the same performance can be obtained with a simpler structure. However, since a part of the second cover layer 42 is in direct contact with the back surface 1b of the AlN substrate 1, the glass component and the component of the AlN substrate 1 are reacted during the sintering process of the second cover layer 42 thereby incurring the foaming, which is a drawback in comparison with the above embodiments.

The heating unit and the manufacturing method thereof according to the present invention are not limited to the foregoing embodiments. For example, in the manufacturing process of the heating unit A, the step of forming the first cover layer 41 and the step of forming the third cover layer 43 may be exchanged. Also, the shape of the first cover layer 41, the second cover layer 42 and the third cover layer 43 may be designed as desired.

Claims

1. A heating unit comprising: an AlN substrate including a main surface;an elongated heat-generating resistor provided on the main surface of the AlN substrate; anda protection layer for the heat-generating resistor;wherein the protection layer includes a first cover layer covering the heat-generating resistor and a second cover layer covering the first cover layer,wherein the first cover layer is made of crystallized or semi-crystallized glass having a higher crystallization temperature by at least 50° C. than a glass softening point, the second cover layer being made of non-crystalline glass.

2. The heating unit according to claim 1, further comprising a third cover layer that covers at least part of an exposed region of the main surface where the first cover layer is not provided, wherein the third cover layer is made of non-crystalline glass higher in glass softening point than the non-crystalline glass constituting the second cover layer, and wherein the second cover layer is provided on the first cover layer and at least part of the third cover layer.

3. A method of making a heating unit, the method comprising the steps of: sintering an elongated heat-generating resistor on an AlN substrate;sintering a first cover layer to cover the heat-generating resistor; andsintering a second cover layer to cover the first cover layer;wherein the first cover layer is made of crystallized or semi-crystallized glass having a higher crystallization temperature by at least 50° C. than a glass softening point of the glass, the sintering of the first cover layer being performed at a higher crystallization temperature by 50 to 70° C. than the glass softening point of the glass,wherein the second cover layer is made of non-crystalline glass, the sintering of the second cover layer being performed at a temperature higher by at most 100° C. than a glass softening point of the non-crystalline glass.

4. The method according to claim 3, wherein the glass constituting the first cover layer has a glass softening point of no lower than 740° C., the sintering temperature of the first cover layer being in a range of 800 to 850° C.

5. The method according to claim 3, wherein the non-crystallized glass constituting the second cover layer has a glass softening point of no lower than 700° C., the sintering temperature of the second cover layer being in a range of 800 to 850° C.

6. The method according to claim 3, further comprising the step of sintering a third cover layer on the AlN substrate before the sintering of the second cover layer, wherein the second cover layer is formed on the first cover layer and at least part of the third cover layer.

7. The method according to claim 6, wherein the third cover layer is made of a non-crystalline glass higher in glass softening point than the non-crystalline glass constituting the second cover layer, and wherein the sintering of the third cover layer is performed at a temperature higher by at most 30° C. than the glass softening point of the non-crystalline glass constituting the third cover layer.