Method of inspecting crack in ceramic substrate

Abstract: A method is provided for inspecting for cracks in a ceramic substrate for example a crack which is not exposed on the surface. The method does not leave a crack undetected, and can make objective judgement possible by expressing the result of inspection as numerical data is provided. Conductors (1,4) are disposed on both faces of a ceramic substrate (3), one or both of the conductors being a conductive liquid (1), and the insulation resistance value or an electrically characteristic value which depends on the insulation resistance value is measured with the conductive liquid or other conductors electrically connected to the conductive liquid acting as electrodes.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of inspecting for cracks in a ceramic substrate, and in particular, a ceramic substrate used in manufacturing a sensor, a resistor element, an actuator for an ink-jet print head, etc. Description of Related Art Conventionally, a ceramic substrate coated with a conductive film has been used as an electrode, a resistor, a lead, an output terminal, etc. in a sensor, a resistor element, an actuator for an ink-jet print head, etc. As a method of manufacturing a ceramic substrate coated with a conductive film, a method of applying by printing conductive paste on a ceramic substrate is appropriate. It is because a uniform film having a complicated pattern can be efficiently formed and the conductive film can be adjusted to have a wide range of thickness by adjusting the viscosity of the conductive paste. In order to meet the recent demands for a miniaturized and lightened actuator for an ink-jet print head, etc., thinning of a ceramic substrate has been carried forward, and recently, a ceramic substrate which is 10 µm or less in thickness has become available. This is accompanied by a fear that handling of a ceramic substrate in manufacturing may cause a crack in the ceramic substrate. If a ceramic substrate is cracked, not only the mechanical strength of the ceramic substrate is so lowered that it can not be put to practical use, but also leakage of ink may be caused. As a method of inspecting any crack in a ceramic substrate, conventionally, there are known a red check test, a method of inspection by visual observation or with microscope using liquid penetrant (liquid penetrant test) such as a fluorescent penetrant test, a method of inspection by detecting leakage of gas such as gaseous helium and air, a method using an image processor, etc. However, the liquid penetrant test suffers from a problem that it depends to a great extent on the skillfulness of the inspector, cracks are likely to be left undetected, and the result of the inspection is difficult to express as numerical data. Further, in case a ceramic substrate applied to, for example, an actuator for an ink-jet print head has a multilayer functional film formed thereon, there is a problem that, according to the method using liquid penetrant, it takes the liquid penetrant much time to ooze. The method of inspection by detecting leakage of gas has a problem that, since closed space is necessary, there is technical difficulty and the scale of the apparatus has to be large. Further, the method using an image processor has a problem that, depending on the resolution of the apparatus, a microscopic crack may be difficult to detect. Still further, as a defect common to the convention methods on the whole, there is a problem that a crack which is, for example, shut by a functional film such as a conductive film and thus, which is not exposed on the surface of a ceramic substrate can not be detected. JP-A-7-161570 describes a method of detecting internal cracks in a multilayer ceramic capacitor, positively and quickly without breaking the capacitor, in which at least one of capacitance and dielectric loss is measured and compared with the respective value for a capacitor having no internal crack. Machine Design Vol 53 (1981) Cleveland, Ohio, USA, page 40, describes a pin hole detector for sensing coating flaws in paint, plastics, varnish and epoxy on conductive substrates, which employs as a detector probe a sponge moistened with tap water. One lead is connected to the bare conductive substrate and another to the sponge. The sponge is passed over the dry coating, and water penetrating any defect completes an electrical circuit to operate an audible atarm and a light-emitting diode. JP-A-63177051 describes a method of confirming the position and size of pinholes in insulating layers on a substrate. By applying a voltage through the insulating layer between an electrode located between substrate and insulating layer and an anode in a plating liquid, the position and size of each pinhole can be detected visually by the resulting electrolytic precipitation. SUMMARY OF THE INVENTION In view of the above, an object of the present invention is to provide a method of inspecting for cracks in a ceramic substrate which reduces or eliminates the inconveniences mentioned above, and for example detects even a crack that is not exposed on the surface of a ceramic substrate, does not leave a crack undetected, and can make objective judgment possible by expressing the result of inspection, as numerical data. According to the present invention, there is provided a method of inspecting a ceramic substrate for a crack, as set out in claim 1. One electrode may be the whole or a part of a container containing the conductive liquid. Still further, the electrically characteristic value may be dielectric loss. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view schematically showing an example of an Inspection apparatus used in a method of inspection for a crack according to the present invention; Fig. 2 is an enlarged view of the circled portion of Fig. 1; Fig. 3 is a graph showing results of measurement according to the method of inspection for a crack of the present invention; and Fig. 4 is a schematic view showing another example of an Inspection apparatus used in the method of Inspection for a crack according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of inspection for a crack of the present invention, by disposing conductors on both faces of a ceramic substrate and by measuring an insulation resistance value or an electrically characteristic value which depends on the insulation resistance value with the conductors being as the electrodes, a crack in the ceramic substrate is detected. Here, conductive liquid is used as one of the conductors. Since liquid is used as one of the conductors, in case there is a crack, the conductive liquid soaks into the crack due to capillary phenomenon to come in contact with the conductor on the opposite face, thereby decreasing the insulation resistance value. On the contrary, in case there is no crack, the insulation resistance value does not change. Therefore, by using the conductive liquid, whether there is a crack or not can be grasped objectively and quantitatively. The principle of the method of the present invention as mentioned in the above requires that a conductor on one face of the ceramic substrate is the liquid, but the conductor on the other face of the ceramic substrate is a film. However, in order to inspect efficiently whether there is a crack or not, it is preferable that the conductor disposed on the other face has a plane electrically connected portion which can be in close contact with the face of the ceramic substrate. Further, more preferably, a plane conductor having an area and a shape substantially similar to those of the ceramic substrate is used. Still further, if a plastic material is used as the conductor, it can also be applied to inspection of a bent ceramic substrate. Thus a conductive film is provided on one face of the ceramic substrate and is used as an electrode. Specifically, one of the electrodes is a conductive film provided on one face of the ceramic substrate while the other of the electrodes is a conductor formed of the conductive liquid disposed on the other face of the ceramic substrate. By measuring the insulation resistance value between the two faces in this way, a crack coated with the conductive film can be detected. In the method of inspection for a crack of the present invention, another conductor electrically connected to the conductive liquid may be used as an electrode. For example, the whole or a part of a container containing the conductive liquid may be formed of a conductive material such that the container or the conductive portion of the container is used as an electrode. It is preferable that the specific resistivity of the conductive liquid used in the present invention is small. Specifically, the specific resistivity is preferably 0 - 20 Ω · m, and is, more preferably, 0 - 2 Ω · m. It is also preferable that the surface tension of the conductive liquid is small. Specifically, the surface tension is preferably 0 - 80 dyn/cm, and is, more preferably, 0 - 50 dyn/cm. If the surface tension is more than 80 dyn/cm, the wettability is so small that the capillary phenomenon is difficult to occur, thereby lowering the precision of detecting a crack. The viscosity of the conductive liquid is preferable 0 - 1,000 mPa · s, and is, more preferably, 1 - 300 mPa · s. If the viscosity is more than 1,000 mPa · s, the fluidity of the liquid is so small that bubbles are likely to be contained therein. Still further, it is preferable that the conductive liquid has a high washability and a low foamability. Further, a small amount of a defoaming agent such as alcohol may be added to the conductive liquid optionally. If the conductive liquid has a low washability, a component of the liquid may remain on the ceramic substrate, causing a stain or defective insulation. If the conductive liquid has a high foamability, bubbles between the conductive liquid and the ceramic substrate makes the electric resistance large, thereby increasing noise in the measurement. For the reasons mentioned in the above, as the conductive liquid used in the present invention, synthetic detergent, conductive coating, etc. are preferable, and in particular, synthetic detergent having a low foamability is preferable. As the other materials electrically connected to the conductive liquid and, in case the conductive liquid is used only on one side of a ceramic substrate, as a material for the conductor disposed on the opposite side to the conductive liquid, Au, Ag, Pt, Cu, Al, SUS, etc. are used. Preferably, a material having a small specific resistivity is used. Further, although an insulation resistance, a dielectric loss, Q (quality factor), etc. can be the electrically characteristic value which depends on the insulation resistance value, from the standpoint of simplicity and stability, it is preferable that dielectric loss is measured. Since the relationship between the insulation resistance value R and the value of dielectric loss D can be expressed as D(tan δ)=1/ωCR, if there is a crack, the value of dielectric loss increases. Preferably, which of the electrically characteristic values is selected to judge whether there is a crack or not is appropriately decided taking into consideration the electric characteristics of the conductive liquid, the structure of the ceramic substrate to be inspected, etc. Although it is preferable that the measured voltage when the dielectric loss is measured is in the range to which an ordinary LCR meter can be set, that is, from 5 mV to 2 V, around 1.0 V which is popularly used is sufficient. Further, the measured frequency is preferably 100 Hz - 100 kHz, and more preferably, 300 Hz - 1 kHz. The present invention is now described in more detail with reference to embodiments shown in the figures. However, the present invention should not be construed to be limited to these embodiments. (Example 1) According to the method of inspection for a crack of the present invention, measurement was conducted to inspect a ceramic substrate for a crack. Fig. 1 shows an inspection apparatus used in the measurement. Fig. 2 is an enlarged view of a part shown in Fig. 1. In Fig. 1, a ceramic substrate 3 is mounted on a container 2 formed of a conductive metal filled with conductive liquid 1 such that a face of the ceramic substrate 3 which does not have a conductive film 4 is in contact with the liquid 1. The conductive film 4 and the container 2 are connected to an LCR meter via leads 6. As shown in Fig. 2, if there is a crack, since the liquid 1 goes up through the crack 5 due to capillary phenomenon to come in contact with the conductive film 4, the liquid 1 is electrically connected to the conductive film 4 to decrease the insulation resistance value, thereby increasing the value of dielectric loss. The inspection was made by measuring dielectric loss under the following conditions: the measured voltage was 1 V; the measured frequency was 1 kHz, the bias voltage was 0 V; and the specific resistivity, the wettability expressed as the surface tension, and the viscosity of the conductive liquid 1 were 1.0 Ω · m, 28.0 dyn/cm, and 50 mPa · s, respectively. (Examples 2 - 10) In the same way as in Example 1, measurement was conducted to inspect other ceramic substrates for a crack. Fig. 3 shows the results of the measurement in the inspection of the ceramic substrates for a crack. In addition, results of inspection according to the liquid penetrant test are put in parentheses in Fig. 3. "OK" means that the judgement was that there is no crack, while "NG" means that the judgement was that there is a crack. (Example 11) Fig. 4 shows another example of an inspection apparatus used in the method of inspection for a crack according to the present invention. In Fig. 4, the ceramic substrate 3 is connected in series to a reference resistance R ref . Voltage V C of the ceramic substrate 3 when constant voltage V S is applied is measured. By calculating insulation resistance R of the ceramic substrate 3 according to the following equation: R = (V S - V C ) / V C × R ref , a crack can be detected with simplicity. As a result of the measurement in the inspection for a crack with the apparatus shown in Fig. 1 or Fig. 4 according to the method mentioned in the above, the same results as in the inspection according to the liquid penetrant test were obtained. Further, since it takes only several dozen msec to measure one sample, the efficiency of inspection was greatly improved compared with the liquid penetrant test. By using the method of inspecting a ceramic substrate for a crack according to the present invention, even a crack which is coated with a conductive film and is not exposed on the surface of a ceramic substrate can be detected, a crack is not left undetected, and objective judgement is made possible since the result of inspection is expressed as numerical data. Further, since it takes only several dozen msec to measure one sample, mechanized and high-speed processing can be conducted.

Claims: A method of inspecting a ceramic substrate (3) for a crack, comprising the steps of disposing a first conductor in the form of an electrically conductive liquid (1) in contact with a lower face of the substrate (3), while there is a second conductor in the form of an electrically conductive film (4) in contact with an upper face of the substrate (3), measuring an insulation resistance value between said upper and lower faces of the substrate or an electrical characteristic value which is dependent on said insulation resistance value using as electrodes said conductive film (4) and either said conductive liquid or a conductor (2) connected to the conductive liquid (1). A method according to claim 1 including the step of applying said conductive film (4) to said substrate (3). A method according to claim 1 or 2 wherein said conductor (2) connected to the conductive liquid (1) is employed as one said electrode and is a container for the conductive liquid (1). A method according to any one of claims 1 to 3 wherein dielectric loss is detected as said electrical characteristic value dependent on the insulation resistance value. A method according to any one of claims 1 to 4 wherein the specific resistivity of said conductive liquid (1) is in the range 0 to 20 Ω.m. A method according to any one of claims 1 to 5 wherein the surface tension of said conductive liquid (1) is in the range 0 to 0.08 N/m (0 to 80 dyn/cm). A method according to any one of claims 1 to 6 wherein the viscosity of said conductive liquid (1) is in the range 0-1000 mPa.s.

Inventors: KATAKURA TAKAHIRO, MIYATA KEIZO, OKUMURA MOTONORI, SHIMOGAWA NATSUMI, TAKAHASHI NOBUO, USUI MINORU, WADA TAKAICHI, KATAKURA, TAKAHIRO, MIYATA, KEIZO, OKUMURA, MOTONORI, SHIMOGAWA, NATSUMI, TAKAHASHI, NOBUO, USUI, MINORU, WADA, TAKAICHI
Applicants: NGK INSULATORS LTD, SEIKO EPSON CORP, NGK INSULATORS, LTD., SEIKO EPSON CORPORATION
Application year: 1997
Main classification: G01N27
IPC codes: G01N27