METHOD AND APPARATUS FOR DRAWING THICK-FILM CIRCUITS

Abstract: A method of an apparatus for drawing a thick-film cir­ cuit on a substrate (2) by bringing a drawing nozzle (1), which has a paste-discharging port at the free end thereof, near the substrate (2) on which a thick-film circuit is to be drawn, and discharging paste from the discharge port onto the substrate (2) while moving the nozzle (1) relatively thereto. If the discharge rate of the paste is controlled in accordance with the relative speed of linear motion of the drawing nozzle (1) to the substrate (2), fluctuations of the thickness and width of the film which occur due to variations in this speed can be prevented, so that a highly accurate thick-film circuit can be formed rapidly.

Description: Technical Field The present invention relates to a method of and apparatus for drawing a thick film circuit wherein a thick film circuit including resistors, capacitors and electric conductors is drawn on a substrate by discharging a paste from a drawing nozzle with a discharge hole. Background of the Invention A thick film circuit is being adapted for many electronic equipment such as broadcasting radio receiver, television receiver, video tape recorder, communication equipment, and measuring equipment. Such a thick film circuit is generally formed using the screen printing method. More specifically, a desired pattern is formed on a substrate made of alumina ceramic using a conductor paste, resistor paste and so on, followed by drying and heating so as to manufacture a thick film circuit. However, the screen printing method uses a mask pattern, with the result that longer time is required for the exchange of mask pattern, and cleaning of mask pattern and squeegee, and thus it is unsuitable for small lot production and frequent changes of kind. On the other hand, a method, which involves drawing on a moving substrate using a paste directly extruded from a nozzle without employing the mask pattern, is reported by "1978 International Microelectronics Symposium of ISHM, Minneapolis, MN, September 17, 1978" and is introduced by a journal " Electronic Packaging and Production (January, 1981), Pages 98-109". In operation, a drawing nozzle having a discharge hole for discharging a paste is kept at a certain distance from a substrate and is relatively moved to apply the paste in a predetermined pattern of a thick film circuit. This drawing method does not require the mask pattern because the thick film circuit is drawn by means of NC control, thereby resulting in making possible to quickly change the kind and instantly modify the circuit. However, one of problems with such a drawing method is that, when continuously drawing a thick film circuit pattern having linear lines and curves, difficulty is encountered to draw curve portions of the pattern at a high speed due to inertia of the drive system of the nozzle. Therefore, to efficiently form the thick film circuit by the above-described drawing method, it will be required to draw linear portions at a high speed and draw curve portions at a relatively low speed. However, this technique does not produce satisfactory results because of the occurrence of variations of film thickness and line width caused by the difference of moving speed of the nozzle between linear portions and curve portions. This problem will be similarly experienced in a method in which the substrate is moved without moving the drawing nozzle. Therefore, according to the conventional techniques, it is difficult to form the thick film circuit at a high speed and with high accuracy. From EP-A-0 126 742, a document under Article 54 (3) EPC, a method and apparatus for drawing a thick film circuit on a substrate is known, wherein a paste is discharged depending on the control of the driving means for relatively moving the discharge nozzle with respect to the substrate. Contrary to the present invention, however, the discharge of paste is controlled in dependency of the topography of the substrate by controlling the vertical displacement of the nozzle. A further difference between this known method and the subject matter of the present application resides in that in the known case the speeds of the motors and therefore the relative moving speed is merely determined, but not detected. From US-A-3 961 599 a method of drawing a layer on an insulating chip is known. In this known method, a paste is discharged from a discharge hole provided on one end of a drawing nozzle which is moved close to said substrate and relatively thereto. The paste discharge amount is controlled in accordance with the relative moving speed. However, this known method is not used for drawing a thick film circuit with different orientations of the drawn conductive lines but only for drawing resistors of preset resistance values between two conductors so that the problem of curvature in the conductive lines cannot arise in this known case. An apparatus for carrying out the method is also disclosed in US-A-3 961 599. Disclosure of the Invention An object of the present invention is to provide a drawing method and apparatus which are capable of drawing a thick film circuit at a high speed and with high accuracy, irrespective of complexity of pattern. This object of the present invention is achieved by controlling the discharge amount of paste in accordance with relative moving speed between a drawing nozzle and a substrate in accordance with Claims 1 and 3. The present invention relates to a drawing method for a thick film circuit as defined in Claim 1. A drawing apparatus according to the present invention is defined in Claim 3. Brief Description of the Drawings Fig. 1 is a perspective view showing the whole arrangement of a drawing apparatus for a thick film circuit according to a first embodiment of the present invention; Fig. 2 is a perspective view illustrating a drawing nozzle of the drawing apparatus of Fig. 1; Fig. 3 is a cross-sectional view of the drawing nozzle of Fig. 2; Fig. 4 is a block diagram showing control system for the drawing apparatus of Fig. 1; Fig. 5 is an illustration useful for describing a drawing method by means of the drawing apparatus; Fig. 6 is a flow chart for describing the operation of the drawing apparatus of Fig. 1; and Fig. 7 is a cross-sectional view of a drawing nozzle according to another embodiment of the present invention. Most Preferred Embodiment of the Present Invention Fig. 1 illustrates the whole arrangement of a drawing apparatus according to an embodiment of the present invention. In the Figure, the reference numeral 1 represents a drawing nozzle having a paste therein and a discharge hole at the end thereof, numeral 2 designates a heat-resistant substrate for forming a thick film circuit made of aluminous ceramic or the like, numeral 3 depicts a stage for fixing the substrate 2, numeral 4 represents a X-Y drive table for horizontally moving the drawing nozzle 1. The X-Y drive table comprises a X-axis drive section 4a and Y-axis drive section 4b, a lower support plate 6a for supporting the drawing nozzle 1 and an upper support plate 6b having a nozzle lifter are attached to one end of the Y-axis drive section 4b. The reference numeral 7 represents a control unit having a microprocessor and a memory for controlling the X-axis drive section 4a and Y-axis drive section 4b of the X-Y drive table and for controlling the amount of paste discharged from the discharge hole of the drawing nozzle 1. The X-axis drive section 4a and Y-axis drive section 4b have respectively a X-axis drive motor 5a and a Y-axis drive motor 5b. The position and speed of the X-axis drive section 4a and Y-axis drive section 4b are determined by controlling the rotations of the X-axis motor 5a and Y-axis motor 5b in accordance with the instructions from the microprocessor of the control unit 7 on the basis of information from respective encoders encased in the X-axis motor 5a and Y-axis motor 5b. Numeral 8 designates a base for supporting the above-noted parts. The arrangement of the drawing nozzle 1 will be described with reference to Figs. 2 and 3. A paste discharge hole 10 is provided at the end of the drawing nozzle 1, and is connected to one end of a paste supplying pipe 11 which further acts as a lever. A discharge pump 12 is attached at the upper portion of the drawing nozzle 1. The other end of the pipe 11 is coupled to a paste tank 9 for storing a paste. The reference numeral 14 represents an arm having a supporting bar 15. The supporting bar 15 is disposed to cross the pipe 11 so as to serves as a fulcrum for the pipe 11 so that the pipe 11 is freely rotatable. A nozzle lifter 27 comprising a magnet is attached to the upper supporting plate 6b. The discharge pump 12 comprises a circular movable film 16 made of a metal, a movable coil 17 connected to the center of the movable film 16, and a circular cup-shaped magnet 19 having a cylindrical projection. The magnet 19 is attached to a pump cover 18 so as not to come into contact with the movable coil 17. The circumference of the movable film 16 is put between the pump cover 18 and drawing nozzle 1 with kept airtightness. A valve 20 made of plastic film to prevent the flowing back of the paste is attached to the connection between the pipe 11 and drawing nozzle 1. When drawing, the drawing nozzle 1 is biased by a spring (not shown) and maintained to the state shown in Fig. 3. In response to the completion of drawing, current is passed through the nozzle lifter 15 comprising a magnet, and thus the pump cover 18 made of a magnetic material is sucked. As a result, the drawing nozzle 1 is apart from the substrate and is moved upwardly. The control of the drawing apparatus will be described with reference to Fig. 4. The microprocessor, using the memory 22, controls the nozzle lifter 27, discharge pump 12, and the X-axis motor 5a and Y-axis motor 5b of the X-Y drive table 4. An interface 23 further serves as a digital-to-analog converter. A position indicating signal and a speed indicating signal for the X-Y drive table outputted from the microprocessor 21 is supplied through the interface 23 to a X-Y drive circuit 24 and then reach the X-axis motor 5a and Y-axis motor 5b, whereby the operation of the X-Y drive table 4 is controlled by the microprocessor 21. A signal indicative of the discharge amount of paste from the microprocessor 21 is supplied through the interface 23 to a frequency generator 25, followed by generating an alternating current signal having a frequency corresponding to the amount to be discharged before the alternating voltage is amplified by an amplifier 26 and then supplied to the movable coil 17 of the discharge pump 12. Fig. 5 is an illustration for describing the drawing by the above-mentioned drawing apparatus. In drawing, the discharge pump 12 is driven in response to a signal from the control unit 7. The paste 9 is pushed in the direction of arrow A, thereby discharging it from the discharge hole 10 on to the substrate 2. The drawing nozzle 1 is moved by the X-Y drive table 4 in the direction of arrow B to draw a thick film circuit on the substrate 2. In the case of the completion of the drawing, the discharge pump 12 is stopped and thereby the pushing in the direction of arrow A is released and the discharging of the paste is terminated. The energization off the nozzle lifter 27 results in the operation that the pump cover 18 is sucked by the nozzle lifter 27 and the drawing nozzle 1 is parted from the substrate 2 and is moved upwardly. The thick film circuit drawn on the substrate 2 is dried to evaporate a solvent contained in the paste. For example, in the case of resistor paste, a thick film resistor is formed on the substrate 2 by heating at the temperature of about 850 o C. An operation for drawing a thick film conductor by the above-noted drawing apparatus will be described with reference to the flow chart of Fig. 6. First, the paste tank 13 is filled with a silver-palladium conductive powder and a conductor paste whose principal component is glass frit and organic solvent. Secondly, as illustrated in Fig. 6, various data required for the drawing are stored in the memory 22 of the control unit 7 by means of a reader with a keyboard and a paper tape, or a reader with a magnetic disc (step 1). The moving speeds V xo and V yo of the X-axis drive section 4a and Y-axis drive section 4b, and the rotating speed (rpm) of each of X-axis motor 5a and Y-axis motor 5b are respectively calculated on the basis of the data, and the initial operating speed (= frequency) is calculated using a proportional constant and the calculated values of V xo and V yo (step 2). The drawing nozzle 1 is moved by the X-Y drive table 4 to the initial point of drawing on the substrate 2 and the electro-magnetic action of the nozzle lifter 27 is stopped to move downwardly the drawing nozzle 1 to the substrate 2 by the bias force of the spring (step 3). The X-Y drive table 4 is driven, while at the same time the discharge pump 12 is driven to discharge the paste from the discharge hole 10, whereby the paste is attached so as to draw a desired shape on the substrate 2 (step 4). In subsequent step 5, respective rotating speeds of the X-axis motor 5a and Y-axis motor 5b are detected by encoders respectively provided therewith, and the actual moving speed of the drawing nozzle 1 is calculated on the basis of X-axis moving speed V x (mm/sec) and Y-axis moving speed V y (mm/sec). If √ V xo ² + V yo ² < √ V x ² + V y ² , the control is performed so as to increase the operating speed of the pump 12 (step 6), If √ V xo ² + V yo ² > √ V x ² + V y ² , the operating speed thereof is decreased (step 7), and therefore the discharge amount Q (µℓ/sec) is kept so that Q = k √ V x ² + V y ² . The reference character k represents a proportionallity constant, and is determined on the basis of the line width and film thickness of the thick film circuit and the volumetric shrinkage rate due to the drying and heating of the thick film paste. In the next step 8, as well as in the previous step 7, the microprocessor 21 checks the terminal point of the drawing on the basis of the data obtained from the encoders of the respective X-axis motor 5a and Y-axis motor 5b. If so, the discharge pump 12 is stopped to terminate the discharge of the paste 9 and, at the same time, to terminate the operation of the X-Y drive table (step 9). Thereafter, the pump cover 18 made of a magnetic material is immediately sucked by the electro-magnetic action of the nozzle lifter 27 (step 10). In a step 11, it is checked whether there is the following drawing data or not. If there is the following data, the operational flow returns to the step 2 for drawing the thick film circuit. If not, the drawing of the thick film circuit is terminated. The control of the discharge amount Q of the paste 9 is performed by the alternating current source supplied to the movable coil 17 of the discharge pump 12 in accordance with a signal from the microprocessor 21. The frequency of the alternating current source is in the range of 100 Hz to 1 kHz. Even if the linear portions are drawn at the speed of 150 mm/sec and the curve portions are drawn at the speed of 30 mm/sec, the line width and film thickness of the continuously drawn thick film circuit pattern are respectively uniform and excellent in accuracy. In addition, the pattern has been drawn in a short time. A second embodiment of the apparatus of the present invention will be described with reference to Fig. 7. Fig. 7 illustrates a drawing head which comprises a three-movable film type paste discharge pump 37, a paste discharge hole 28 communicating with the discharge pump 37, and a paste supplying pipe 29. The three-movable film type paste discharge pump 37 comprises movable films 30a, 30b, 30c which are equal in size, movable coils 31a, 31b, 31c, magnets 32a, 32b, 32c, pump chambers 33a, 33b, 33c, and connection paths for connecting between the pump chambers 34a, 34b, 34c. The pump chambers 33a, 33b, 33c are respectively connected to the connection paths 34a, 34b, 34c in series, and the paste is extruded by the operation of the paste discharge pump 37 from the pipe 29 through the pump chamber 33a, connection path 34a, pump chamber 33b, connection path 34b, pump chamber 33c, and paste discharge hole 28. An alternating current source applied to the coil 31b is lagged with respect to that for the coil 31a by a phase difference of 2/3π , and an alternating current source applied to the coil 31c is lagged with respect to that for the coil 31a by a phase difference of 4/3π , thereby making possible to discharge the paste with high accuracy. In the above-described embodiments the movable film type pump has a movable coil as a driving source. However, it is possible to use a movable magnet as the driving source. Furthermore, it is also acceptable to use a ceramic vibrator as the driving source and control the discharge amount in accordance with the amplitude of an alternating current source. Furthermore, in the above-described embodiments, the drawing head is moved by the X-Y drive table. However, it is also acceptable that the substrate is moved by the X-Y drive table and, on the other hand, the drawing head is fixed. Industrial Application As obvious from the above description, according to the present invention, since the paste discharge amount from the drawing nozzle is controlled in accordance with the relatively moving speed of the drawing nozzle, a thick film circuit pattern having stable film thickness and line width can be drawn, irrespective of linear portions or curve portions. As a result, a complex pattern can be drawn at a high speed and with high accuracy.

Claims: A method of drawing a thick film circuit on a substrate (2) by discharging a paste from a discharge hole (10; 28) provided at one end of a drawing nozzle (1), said drawing nozzle being moved close to said substrate (2) and relatively moved with respect to said substrate, a relative moving speed of said drawing nozzle with respect to said substrate being detected, and a paste discharge amount Q from said discharge hole (10; 28) being controlled in accordance with the detected relative moving speed, wherein said discharge amount Q is determined to satisfy the following equation: Q = k √ V x ² + V y ² (Q : µℓ/sec, V x , V y : mm/sec, k : µℓ/mm) where k represents a proportionallity constant; V x and V y are unequal to zero and represent speeds of a X-axis drive section and a Y-axis drive section crossing at right angle each other which are provided in X-Y drive motor means (5a, 5b) for relatively moving said drawing nozzle (1) with respect to said substrate (2), wherein the respective rotating speeds of the X-Y drive motor means (5a, 5b) are detected by encoders respectively provided therewith. A method as claimed in claim 1, wherein said drawing nozzle (1) has a pump (12; 371) for controlling said discharge amount Q of the paste. An apparatus for drawing a thick film circuit on a substrate, comprising: a drawing nozzle (1) having a paste discharge hole (10; 28); X-Y drive means (5a, 5b) for relatively moving said drawing nozzle (1) with respect to said substrate (2) on which thick film circuit is drawn in a direction parallel to said substrate; up-and-down drive means (27) for relatively moving said drawing nozzle (1) and said substrate (2) in a direction perpendicular to said substrate (2); means for detecting a relative moving speed between said drawing nozzle (1) and said substrate (2); and a control unit (21; 22, 23, 24) for controlling a paste discharge amount Q from said discharge hole (10; 28), said X-Y drive means (5a, 5b) and said up-and-down drive means (27), said control unit controlling said paste discharge amount from said discharge hole in accordance with the detected relative moving speed, wherein the discharge amount Q of the paste is determined to satisfy the following equation: Q = k √ V x ² + V y ² (Q : µℓ/sec, V x , V y : mm/sec, k : µℓ/mm) where k represents a proportionallity constant; V x and V y are unequal to zero and represent speeds of a X-axis drive section and a Y-axis drive section crossing at right angle each other which are provided in said X-Y drive motor means (5a, 5b), and in that the respective rotating speeds of X-axis and Y-axis motors (5a, 5b) are detected by encoders respectively provided therewith. An apparatus as claimed in claim 3, wherein a control of the discharge amount Q is performed in accordance with instructions from a microprocessor (21) which is encased in said control unit (21 to 24). An apparatus as claimed in one of the claims 3 or 4, wherein said drawing nozzle (1) has a pump (12; 37) for controlling the discharge amount Q of the paste. An apparatus as claimed in claim 5, wherein said pump (12, 37) comprises at least one movable film (16, 30a, 30b, 30c) coming into contact with a paste in said drawing nozzle (1), a movable coil (17; 31a, 31b, 31c) connected to said movable film, a magnet (19; 32a, 32b, 32c) for vibrating said movable coil using an interaction relative to said movable coil, and a valve (20) disposed at a paste supplying opening of said drawing nozzle (1) for preventing a flowing-back of said paste. An apparatus as claimed in claim 6, wherein a paste discharge amount is controlled in accordance with a variation of the vibration frequency of said movable film (16; 30a, 30b, 30c) caused by varying the frequency of an alternating current for driving said pump (12; 37). An apparatus as claimed in one of the claims 6 or 7, wherein the inside of said drawing nozzle (1) is divided into three portions (33a, 33b, 33c) and connection paths (34a, 34b) are provided between said portions, each of said portions having a movable film (30a, 30b, 30c) coming into contact with the paste, a movable coil (31a, 31b, 31c) connected to said movable film, and a magnet (32a, 32b, 32c) for vibrating said movable coil by an interaction, each said movable coil (31a, 31b, 31c) being driven by alternating current sources which are respectively 2/3 π degrees out-of-phase so that said paste is supplied from said paste supplying opening (28) of said drawing nozzle.

Inventors: KUDO, SHINICHI, MAEDA, YUKIO, SAEKI, KEIJI
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1985
Main classification: H05K3
IPC codes: H05K3