PRINTING METHOD AND APPARATUS

Abstract: Regarding filling of a paste, a temperature gradient at a paste-pressing face is managed by a heater incorporated in a filling head. Alternatively, a nonvolatile component is applied to a printing face of a base material before the paste is filled, thereby preventing a volatile component of the paste from evaporating with time. A viscosity of the paste is thus secured. Meanwhile, a change of the quantity of the paste is coped with by changing a moving speed of the filling head or a contact angle of the filling head to the printing face, or by supporting the filling head at the side in touch with the printing face.

Description: TECHNICAL FIELD The present invention relates to a printing method and a printing apparatus for pressing and filling a printing material into a through hole of a circuit board to be printed by a filling head. BACKGROUND ART Recently, circuit boards have been changed from single-face boards having circuits formed on respective one faces, to double-face boards to multilayer circuit boards so as to achieve compact and high-performance apparatuses. Many of the boards adopted at present are advanced multilayered ones. In the meantime, a printing technique using a filling head, i.e., squeegee is employed in order to press and fill a paste to opening parts such as via holes or through holes in these circuit boards, or to press and fill the paste to opening parts of a metal mask to form circuits on the board. A lot of conditions are involved in the printing technique, for instance, a hardness of the squeegee, an angle of the squeegee to a printing face, a pressure of the squeegee to the printing face, a filling speed of the squeegee to opening parts of the printing face, etc. Even a slight error in setting of the conditions adversely influences the quality of printing films, and therefore the set conditions are adjusted based on experiences at the production line thereby to improve a yield in the paste filling process and quality of products. Some adaptations are also made to the aforementioned squeegee in the filling process. For example, the squeegee is improved in shape to be a flat one, a sharp one or a squeegee having a curved contact face to the printing face. In one arrangement, a heater is built in the squeegee to heat the squeegee itself to a fixed temperature thereby lowering a viscosity of the paste and increasing filling efficiency for the paste to the openings of the printing face. The viscosity of the paste increases in proportion to an operating time of the squeegee. For stabilizing the viscosity, therefore, the paste is exchanged every fixed time interval, or an organic solvent is added to the paste to adjust the viscosity. Meanwhile, the filling amount of the paste to the openings, e.g., via holes and through holes, etc. or the openings of the metal mask is controlled and stabilized by filling the paste up to a surface of the openings. An example of the conventional circuit board-manufacturing method will be described with reference to Figs. 13A, 13B. Figs. 13A, 13B respectively represent examples of a manufacturing method and a manufacturing apparatus for circuit boards in the filling process according to the conventional art. In Figs. 13A, 13B, 300 denotes a metal mask placed on a base material 301. 303 and 305 denote squeegees which move on the metal mask 300 while keeping touch with the metal mask 300. 304 denotes a paste and 302 a stage. The base material 301 is disposed in touch with on the stage 302. The metal mask 300 is arranged via a slight space above the base material 301, onto which the paste 304 is supplied. The squeegees 303, 305 located above the metal mask 300 can move in an up/down direction and a printing direction of the squeegees 303, 305. The paste is filled in the thus-constructed apparatus in a process described herein. The base material 301 is firstly set at a predetermined position on the stage 302. A through hole 301a formed in the base material 301 is registered with a hole 300a in the metal mask 300, and the metal mask 300 is set above the base material 301. The paste 304 is supplied onto the metal mask 300. The squeegees 303, 305 are lowered to hold the base material 301 with impressing a predetermined pressure to the material 301. Thereafter, the squeegees 303, 305 are moved in the printing direction to move the paste 304 on the metal mask 300, thereby performing printing to the base material 301. The paste 304 is pressed and filled in the through holes 301a formed in the base material 301 in this manner. In the above paste filling process in the conventional art, a filling shape or the filling amount of the paste 304 is greatly changed even by mechanical errors, resulting in blanking, insufficient printing or the like filling failures. When the flat squeegee 305 shown in Fig. 13B is used, a contact angle of the squeegee to the printing face of the metal mask 300 or base material 301 is hard to fix and maintain, and the amount of the paste filled in the through hole 301a varies large, which is considerably difficult to control properly, which is an important issue. In the case of the square squeegee 303 as shown in Fig. 13A, the contact angle of the squeegee to the printing face of the metal mask 300 or base material 301 is easy to set, whereas a front end part of the square squeegee 303 in touch with the printing face of the base material 301 is not sufficiently sharp and consequently, the paste 304 is left on the metal mask 300 or base material 301. The front end part of the square squeegee can hold only a small amount of the paste 304, making it necessary to frequently replenish the paste 304, thus deteriorating productivity as an issue. When a viscosity of the paste 304 is increased because of the evaporation of volatile components or when the paste of a high viscosity is used, the paste is filled considerably short if kept under the same condition as originally properly set beforehand. In other words, the filling condition should be re-set during the operation although this takes a long time, which is another issue. In the case where a heater is incorporated in the squeegee 303, 305, this arrangement merely lowers the viscosity of the paste 304, not making an effective influence on rolling or filling of the paste 304 and raising a necessity for setting and adjusting the condition again. The poor productivity still remains to be improved as an issue. JP 11-33394 A discloses a similar forming method for providing a through hole wherein a metal or plastic jig having holes at through hole portions is mounted between a substrate and a printing machine. A squeegee is moved, and the substrate is coated between pastelike conductors in accordance with the pattern of a mesh screen. Further, JP 11-84996 A discloses a through-hole printer, wherein a printing mask is made to overlap a substrate from the upside, and a squeegee is made to move as being pressed against the mask. Finally, US-A-4 817 524 representing the closest prior art from which the invention proceeds discloses a similar screen printing method and apparatus in which the ink pressure is held constant by varying the squeegee speed and/or the squeegee angle during a print stroke. The ink is pumped through a mesh by the squeegee in order to deposit the ink onto the surface of a substrate. An important function of moving the ink over the screen with the squeegee is the generation of hydraulic pressure of sufficient magnitude to pump the ink through the openings of the mash, wherein the adjustment of the pressure is initially carried out only when starting the printing. SUMMARY OF THE INVENTION An object of the present invention is therefore to provide a printing method and a printing apparatus whereby a sufficient filling amount of a printing material into through holes of a circuit board to be printed is secured in conformity with various filling conditions, thereby to improve a yield, quality and productivity of printing by the printing material. In order to achieve the above and other objects, in accordance with a first aspect of the present invention, there is provided a printing method for pressing and filling a printing material into a through hole of a circuit board to be printed by a filling head, wherein a flow pressure of the printing material is kept roughly constant during printing of the printing material, characterized in that the filling head is in direct contact with the circuit board. In accordance with a second aspect of the present invention, there is provided a printing apparatus for pressing and filling a printing material by a filling head into a through hole of a circuit board to be printed, which is provided with a holding device for keeping a flow pressure of the printing material roughly constant during printing of the printing material, characterized in that the filling head is in direct contact with the circuit board. Further advantageous embodiments of the invention are defined in the dependent claims. According to the present invention, during filling of the printing material, the flow pressure of the printing material is kept roughly constant and the filling head is in direct contact with the circuit board, so that a sufficient filling amount of the printing material is secured into through holes of the circuit board to be printed in conformity with various filling conditions, thereby improving a yield, quality and productivity of printing and avoiding the forming of voids. According to a preferred embodiment of the invention, when the filling head moves thereby to press and fill the printing material into through holes of the circuit board, at least either one of the contact angle of the filling head to the printing face of the circuit board or a moving speed of the filling head is changed in accordance with a change of quantity of the printing material, thereby to keep the flow pressure of the printing material roughly constant. So, the flow pressure of the printing material is kept roughly constant to cope with an increase of a viscosity of the printing material with time or a decrease of the filling/pressing force due to the consumption of the printing material. A predetermined amount of the printing material is stably secured at all times, thereby improving continuous printing efficiency. According to a further preferred embodiment of the present invention, the printing material is printed while the filling head is supported at a side of a printing material-pressing face in touch with the printing face of the circuit board, thereby to keep the flow pressure of the printing material roughly constant. The side apart from the printing face may be rendered a free end and deformable, so that a required flexibility is secured. The printing material is controlled not to leak when pressed by the filling head at the side in touch with the printing face, although a necessary minimum elasticity is ensured for the filling head to trace the uneven printing face of the circuit board. The flow pressure of the printing material is thus maintained roughly constant. A predetermined amount of the printing material is stably filled without any special control means against an increase of the viscosity of the printing material with time and a reduction in volume of the printing material because of the consumption. Continuous printing efficiency can be improved. In accordance with a still further preferred embodiment of the present invention, a temperature gradient is set to a printing material-pressing face of the filling head so that a part of the pressing face in touch with the printing face of the circuit board is higher in temperature than a part of the pressing face apart from the printing face of the circuit board and the filling head, thereby to keep the flow pressure of the printing material roughly constant. The temperature at the side of the pressing face in touch with the printing face is made high and the temperature is decreased as the pressing face is far from the printing face. As a result, the viscosity of the rolling printing material of a part in touch with the printing face is lowered, thereby to improve fluidity of the printing material and to maintain the flow pressure of the printing material nearly constant. At the same time, the temperature at the side of the pressing face away from the printing face of the circuit board and the filling head is made low, whereby the fluidity is limited thereby to increase a hydraulic pressure of the pressing face of the part in touch with the printing face. Accordingly, not only a predetermined amount of the printing material is stably filled, but the evaporation of volatile components is restricted at the low temperature part. The hole of the printing material can be maintained at a predetermined viscosity at ease and continuous printing efficiency can be improved. With the use of a temperature-adjusting element for applying the above-described temperature gradient, and when the temperature-adjusting element is constituted to set an optional temperature gradient to a distance of the pressing of the filling head from the printing face of the circuit board, circuit boards of various conditions can be manufactured with high quality in accordance with a variety of conditions of the printing material and filling conditions of the printing material. In accordance with a still further preferred embodiment of the present invention, a pretreatment material is applied before the printing material is filled by the filling head to the printing face of the circuit board, thereby to keep the flow pressure of the printing material roughly constant. The effect of applying the pretreatment material, e.g. a nonvolatile organic substance, is applied onto the circuit board to be printed before the printing material is filled, an increase of the viscosity of the printing material can be supressed thereby to maintain the flow pressure of the printing material nearly constant and improve continuous printing performance. Although a volatile component, if there is any, evaporates after the printing, the nonvolatile component adsorbs to the printing face which results in a change of a composition of the printing material. As such, the nonvolatile component is applied beforehand to prevent the change of the composition. BRIEF DESCRIPTION OF DRAWINGS These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings in which: Fig. 1 is a diagram of a part with a paste-filling function of a circuit board-manufacturing apparatus in which a printing apparatus for executing a printing method in a first embodiment of the present invention is installed; Fig. 2 is a block diagram of a control circuit related to the paste-filling part of the apparatus of Fig. 1; Fig. 3 is a diagram of a part with a paste-filling function of a circuit board-manufacturing apparatus in which a printing apparatus for executing a printing method in a second embodiment of the present invention is installed; Fig. 4 is a block diagram of a control circuit related to the paste-filling part of the apparatus of Fig. 2; Fig. 5 is a diagram of a part with a paste-filling function of a circuit board-manufacturing apparatus in which a printing apparatus for executing a printing method in a third embodiment of the present invention is installed; Fig. 6 is a diagram of a part with a paste-filling function of a circuit board-manufacturing apparatus in which a printing apparatus for executing a printing method in a fourth embodiment of the present invention is installed; Fig. 7 is a diagram explanatory of an arrangement of a low temperature heater of a filling head in Fig. 6; Fig. 8 is a diagram of a temperature distribution of a face of the filling head of Fig. 6 in touch with a paste; Fig. 9 is a diagram of a part with a paste-filling function of a circuit board-manufacturing apparatus in which a printing apparatus for executing a printing method in a fifth embodiment of the present invention is installed; Fig. 10 is a diagram of a part with an applying function for an nonvolatile organic component in a paste of a circuit board-manufacturing apparatus in which a printing apparatus for executing a printing method in a sixth embodiment of the present invention is installed; Fig. 11 is a graph of results of a viscosity increase when the nonvolatile organic component in the paste is applied onto a base material and when the nonvolatile organic component in the paste is not applied onto the base material in the sixth embodiment; Fig. 12 is a diagram showing a state of the paste after filled according to a circuit board-manufacturing method representing a seventh embodiment of the present invention; and Figs. 13A, 13B are diagrams of conventional printing apparatuses. BEST MODE FOR CARRYING OUT THE INVENTION Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. In order to describe a printing method and a printing apparatus in embodiments of the present invention, a method and an apparatus for the manufacture of circuit boards with the subject printing method and printing apparatus provided will be depicted below with reference to the drawings. (First Embodiment) Figs. 1, 2 are diagrams of an apparatus used in a filling process in order to manufacture circuit boards with a printing method and a printing apparatus according to a first embodiment of the present invention provided. As is schematically illustrated in Fig. 1, in the apparatus used in the filling process, a filling head 21 for filling a paste 22 which is an example of the printing material to a base material 4 which is an example of the body to be printed on a stage 5 is moved in a filling direction, e.g., by a pair of right and left transverse guides 25 parallel in the drawing to a printing face of the stage 5 and a transverse slider 26 guided by the guides. The filling head 21 is also moved up and down by a pair of vertical guides 27 arranged above the slider 26 in a vertical direction and a slider 28 guided by the guides 27. The filling head 21 is pivotally supported by a shaft 29 on the vertical slider 28 in a rotatable fashion. At this time, a part of the filling head 21 to be in touch with the base material 4 is a free end. A contact angle  of the filling head 21 to a printing face of the base material 4 is variable. The filling head 21 is rotated about the shaft 29 by driving of an actuator A1 coupled to the shaft 29 and set on the vertical slider 28. Moreover, the filling head 21 is moved up and down via the vertical slider 28 by driving of an actuator A2 coupled to the vertical slider 28 and mounted to the transverse slider 26. The filling head 21 is also moved right and left via the transverse slider 26 by driving of an actuator A3 coupled to the transverse slider 26. Each actuator is constructed by, for example, a motor, a ball screw shaft coupled to a rotary shaft of the motor and a nut screwed to the ball screw shaft and fixed to a driven member. Fig. 2 shows an example of a control circuit for controlling driving of the actuators A1, A2, A3 in the first embodiment. Each of the actuators A1-A3 is controlled by a control part 31 which is a microcomputer, and various control conditions including a changing condition for a moving speed (printing speed) of the filling head 21 are input through an operation panel 32. Information from sensors detecting an angle, a speed, a height of the filling head 21 to the printing face of the base material is input into the control part 31 and based on the information detected thereat, the control part 31 of the control circuit feedback-controls the actuators A1, A2, A3 so that information from sensors detecting an angle, a speed, a height of the filling head 21 to the printing face of the base material conform to set values. In the conventional printing method, a set angle of a squeegee to the printing face of the base material is kept constant during filling of the paste irrespective of the feeding amount of the paste. However, a hydraulic pressure generated in the paste 22 decreases when the amount of the paste 22 is reduced, with lowering a filling force. D.E. Riemar reports this holds a correlation as follows in "INK HYDRODYNAMICS OF SCREEN PRINTING", Proceedings of the 1985 ISHM Symposium, pages 52-58. p = k µ v V /sin 2  In the equation (1), p is a pressure generated in the paste 22, µ is a flow velocity of the paste 22, v is a moving speed of the filling head 21, V is a volume of the paste 22,  is a contact angle of the filling head 21 to the printing face of the base material 5, and k is a proportional constant. A pressure difference expressed by the equation (1) is a pressure difference necessary for filling the paste 22 to a through hole 4d as an example of the opening part formed in the base material 4. A paste filling state when filled in the through hole 4d of the base material 4 is detected by substituting the pressure difference to the technologically well known Hagen-Poiseulle's law. Therefore, in order to make the filled state of the paste 22 constant, it is important to always maintain the pressure p generated in the paste 22 to be constant during printing. Conventionally, the paste is generally supplied which is enough as much as to ignore the decreasing amount of the paste volume in an even continuous paste-filling operation. However, this cannot solve a viscosity change or a composition change of the paste itself. On the other hand, according to the method of the first embodiment, the reducing amount of the paste 22 in one printing operation, namely, in one stroke of the filling head 21 is formed beforehand, and the moving speed of the filling head 21 is increased from v to a speed obtained by an expression (2) below, thereby to cope with the volumetric change of the paste. The feeding amount of the paste 22 for each time can thus be reduced. v ( V / ( V-ΔV ) -1) The expression (2) is obtained by solving the foregoing equation (1) and the following equation (3). p = k µ v V - ΔV /sin 2 (-Δ) wherein ΔV>0. Since the moving speed of the filling head 21 is changed in accordance with the changing amount of the paste 22, the pressure generated in the paste 22 is maintained to be constant, and consequently the amount of the paste 21 filled in the through hole 4d of the base material 4 becomes constant. Changes of the filling force for the paste after 100 sheets are printed in the conventional method based on an initial feed amount of the paste and the method of the first embodiment (new technique) are confirmed in Table 1 in terms of a changing ratio of a filling depth of the paste. Regarding the above description of "based on the initial feed amount of the paste", this implies that the paste is not added after initially supplied once. The initial amount is compared with the amount of the paste after 100 sheets are printed, and the result is represented by the filling rate in Table 1. For instance, when the amount of the paste is 500g, a difference from a filling rate 100-99.5% is the changing ratio of the conventional method. In the method of the first embodiment, the amount of the paste is not changed and therefore the filling rate is 100% and the changing ratio before and after printing is 0%. Amount of paste Filling rate up to filling depth in conventional method Filling rate up to filling depth in new technique 500g 99.5% 100% 300g 99.2% 100% 100g 97.5% 100% 50g 94.9% 100% 30g 91.3% 100% As is clearly confirmed from Table 1, the speed control to the filling head 21 in the first embodiment is effective. As described hereinabove, since the moving speed of the filling head 21 is controlled by the control part 31 via the actuator A1 in accordance with the change of the amount of the paste 22 during printing, the pressure generated in the paste 22 is maintained to be constant, whereby the filling amount of the paste 22 to the through hole 4d of the base material 4 is uniformed. The paste can be continuously printed efficiently, with productivity improved. In the above-described first embodiment, the moving speed of the filling head 21 is controlled to maintain the pressure in the paste 22 to be constant. On the other hand, the angle of the filling head 21 to the printing face of the base material can be controlled by driving the actuator A1 in accordance with the changing amount of the paste 22, thereby to hold the pressure in the paste 22 to be constant. The same effect can be achieved in this manner as well. More specifically, Δ satisfying the following equation (4) is obtained and the actuator A1 is driven by the control part 31 so that the angle of the filling head 21 is reduced every Δ, whereby the same effect as achieved by the above speed control can be realized. sin(-Δ) = ( V -Δ V ) / V ) sin The contact angle  to the printing face of the base material 4 is automatically changed in the first embodiment by rotating the filling head 21 about the shaft 29 by the actuator A1. The filling rate is improved also by reducing the contact angle , thereby to cope with the change of the amount of the paste. Needless to say, when the contact angle  of the filling head 21, specifically, free end of the filling head in touch with the printing face is changed, a height of the free end is changed, which should be corrected by suitably driving the actuators A2 and A by the control part 31 to suitably move the filling head 21 up/down and right/left and maintain the filling head 21 at a predetermined height. This way of angle correction to a predetermined angle is unnecessary if the filling head shown in Fig. 5 is used. In the first embodiment, both the moving speed and the contact angle  of the filling head 21 may be adapted to meet the change of the amount of the paste. (Second Embodiment) Figs. 3, 4 are diagrams of an apparatus used in a paste-filling process in manufacturing of circuit boards in which there is provided a printing method and a printing apparatus according to a second embodiment of the present invention. The apparatus fundamentally operates the same effect as in the first embodiment. What is different from Figs. 1, 2 is that the contact angle  is changed to a predetermined value by the actuators A2, A3 while the actuator A1 for driving the filling head 21 is eliminated. An angle sensor 100 is installed instead to detect an angle of the filling head 21 about the shaft 29. It is so constructed that angle information detected by the angle sensor 100 is input to the control part 31 which in turn controls the angle of the filling head 21 detected by the angle sensor 100 to be the predetermined value, whereby the contact angle  is kept at the predetermined value. (Third Embodiment) As indicated in Fig. 5, in an apparatus used in a paste-filling process in a manufacturing apparatus for circuit boards in which there are provided a printing method and a printing apparatus according to a third embodiment of the present invention, a filling head 121 has a squeegee of a triangular prism laid laterally. Two sides of the filling head 121 defining a right angle of a right triangle in lateral cross section are set schematically in parallel to the lateral direction and vertical direction. A holder 41 for supporting the filling head 121 is fixed to the filling head 121 along the vertical side. As shown in Fig. 5, the holder 41 supports the vicinity of a part C at a paste pressing face 21a of the filling head 121 of the third embodiment which comes in touch with the printing face of the base material 4 on the stage 5, while a part D of the pressing face separated from the printing face is rendered a free end. More specifically, although a flat squeegee is employable in the third embodiment, the squeegee actually used in the embodiment is a slightly modified blade type of a triangular lateral cross section. A thickness of the squeegee is gradually reduced from the part C of the pressing face 21a in touch with the printing face to the part D apart from the printing face. In a supporting structure in the conventional flat squeegee type filling head, when the filling head 121 is lowered to be in touch with the printing face of the base material 4 thereby to impress a printing pressure to the base material 4 to fill the paste, the pressure by the holder is applied to the vicinity of the part D because the part D of the pressing face 21a apart from the printing face is supported, whereas the part C in touch with the printing face is free and easily deflected or broken. In other words, the pressure to the paste 22 leaks and is weakened at the part C although the pressure is important to fill the paste 22 to the through hole 4d of the base material 4. Stable filling of the paste results in a failure. Moreover, in the conventional filling head, the contact angle  defined between the filling head and the printing face decreases from an initially set value. Since the contact angle and the broken amount of a contact part of the filling head to the printing face vary depending on a material of the filling head and a structure of a printing material pressing part, adjustments are required. Such phenomenon occurs that the printing material filled in the through hole of the base material is undesirably scraped out unless the broken amount of the front end of the conventional filling head is restricted to a necessary minimum to let the filling head trace the uneven printing face of the base material. Meanwhile, the conventional square squeegee has its four corners rounded from the beginning and therefore scrapes the paste extraordinarily poorly. Although a required or larger amount of the printing pressure is needed due to the aforementioned poor scraping efficiency of the squeegee so as to compensate for the broken front end of the filling head, the scraping amount of the filled paste from the hole is instable. In contrast, according to the third embodiment of the present invention, the filling head 121 pressing and filling the paste 22 to the through hole 4d of the base material 4 is supported by the holder 41 at the part C of the pressing face 121a which is in touch with the printing face of the base material 4, and thus, the part D of the pressing face 121a away from the printing face becomes free and deformable. In consequence, a required flexibility can be secured for the filling head 121 and the part C in touch with the printing face can suppress the loss of the pressing force to the paste 22. A required minimum elasticity fit to the uneven printing face of the base material 4 can be secured by selecting an elastic material for the filling head 121. The filling head 121 of the embodiment can thus stably fill a predetermined amount of the paste 22 without a special control even if the viscosity of the paste 22 is increased with time or the amount of the paste 22 is reduced due to the consumption. Continuous printing performance can be enhanced. According to the third embodiment, the printing pressure acting to the printing face from the holder 44 via the filling head 121 is directly applied to the part C in touch with the base material 4, so that the contact angle can be less reduced even when the filling head 121 is deflected. Practically no deflection is generated at the contact part between the filling head 121 and the base material 4, as is observed in experiments. By employing the apparatus of the invention, conditions set for the filling head 121 which is the part with a function for filling the paste 22 to the through holes of the base material can be kept constant during printing of the paste. If the printing apparatus is adopted to a circuit board-manufacturing apparatus to print the paste to the boards, it becomes possible to maintain the set conditions for the filling head to be constant during manufacturing of the circuit boards, and produce circuit boards of constant quality. A manufacturing yield can be improved. (Fourth Embodiment) Fig. 6 is a schematic diagram of a process of filling a paste in a circuit board-manufacturing apparatus adopting a printing method and a printing apparatus according to a fourth embodiment of the present invention. As indicated in Fig. 6, a cylindrical filling head 221 is moved to press and fill the paste 22 into the through hole 4d of the base material 4 held at the stage 5. At this time, the filling head 221 is not rotated. The filling head 221 is made movable in an up/down direction and a filling direction shown by an arrow A in Fig. 6. In general, when the filling head 221 moves on the base material 4, the paste 22 present in the vicinity of a contact point between the filling head 221 and base material 4 is considerably hard to flow even if the viscosity of the paste 22 is lowered, due to a small movable area where the paste 22 is allowed to move and a small pressure gradient in the neighborhood of the filling head 221. Although the fluidity of the paste 22 can be improved by raising a temperature of the paste 22 itself, a life of the paste 22 is decreased if the paste 22 is of a thermoset type. What is needed to secure the fluidity required for the paste 22 to be filled in the through hole 4d of the base material 4 is only a part of the paste 22 which comes close to the through hole 4d of the base material 4. And, a force necessary for the paste 22 to be filled in the through hole 4d is obtained by a hydraulic pressure generated by the filling head 21 and through rolling of the paste 22 itself denoted by an arrow B (e.g., D.E. Riemer: "INK HYDRODYNAMICS OF SCREEN PRINTING", Proceedings of the 1985 ISHM Symposium, pages 52-58). The aforementioned rolling of the paste 22 is achieved when the paste 22 moves up from the printing face of the base material 4 along a paste pressing face 221a of the filling head 221. Therefore, if a predetermined temperature gradient is set to the pressing face 221a of the filling head 221, the fluidity of the paste 22 can be improved. More specifically, the temperature gradient of the pressing face 221a is set so that a temperature of the pressing face 221a is high at the side in touch with the printing face of the base material 4, and the temperature is lowered as the pressing face 221a is far away from the printing face of the base material 4. Thus, since the temperature of the pressing face 221a is high at the side in touch with the printing face of the base material 4, the viscosity of the rolling paste 22 decreased at the side, and therefore the fluidity of the paste 22 can be improved. On the other hand, since the temperature of the pressing face 221a at the side away from the printing face of the base material 4 and the filling head 221 is lowered, the fluidity of the paste 22 is restricted, whereby the hydraulic pressure of the contact part between the printing face of the base material 4 and the pressing face 221a is increased. Accordingly, a predetermined amount of the paste 22 is filled stably and at the same time, volatile components in the paste 22 are refrained by the low-temperature part from dispersing, making it possible to maintain the whole paste 22 at a predetermined viscosity easily. The paste 22 can show improved continuous printability. Table 2 indicates a filling depth of the paste and a temperature difference in the filling head 221 under general filling conditions (for instance, pressure of 2kgf/cm 2 , speed of 40mm/sec, amount of the paste (feeding amount) of 500g). Temperature difference (°C) Filling depth (mm) 0 5 5 10 10 12 15 13 As is understood from Table 2, the filling force is improved when the temperature difference is not smaller than 5°C (i.e., in the fourth embodiment wherein the temperature gradient is set to the pressing face) rather than when the temperature difference is 0° (i.e., in the conventional example using the filling head without the temperature gradient set to the pressing face). By way of example, the temperature difference is preferably 10°C when a solder paste is used. According to the fourth embodiment, an upper and a lower heaters 24, 23 are built in the filling head 221 so as to provide the pressing face 221a with the temperature gradient as above. In this arrangement, the lower heater 23 heats a part of the paste 22 in touch with the printing face, and the upper heater 24 heats so that a part of the paste 22 separated from the printing face and the filling head 221 becomes a lower temperature than a heating temperature by the lower heater 23. As a result, the heaters 23, 24 realize such temperature gradient at the pressing face 221a of the filling head 221 that the temperature of the pressing face 221a at the side in touch with the printing face of the base material 4 is high, and is decreased as the pressing face 221a is away from the printing face. It is generally known that volatile components if contained in the paste are lost subsequent to a temperature rise thereby to increase the viscosity of the paste. Since the part of the paste 22 away from the printing face and the filling head 221 to be exposed to the outside air is made lower in temperature than the part at the pressing face 221a in touch with the printing face of the base material 4 and even lower than an evaporation temperature of the volatile components, the paste 22 can be prevented from being hardened as a result of the evaporation of the volatile components. The continuous printability can be enhanced. Fig. 7 is a diagram explanatory of an arrangement of the upper heater 24 at the low-temperature side of the filling head 221. The upper heater 24 is generally disposed, as is clearly shown in Fig. 7, to pass the center of the filling head 221 in a low-temperature variable area approximately 30° above a plane in parallel to the printing face of the base material 4. A position of the upper heater 24 should be determined within the above range from a relationship with the amount of the paste, because the paste 22 generally does not move up to the pressing face 221a of the filling head 221 over the range. In case there is supplied a large amount of the paste, the upper heater 24 is preferably arranged slightly upward in the range. On the other hand, if the paste of a small amount is supplied, the upper heater 24 is preferably installed slightly downward within the range. When the paste 22 is a solder paste, it is preferred, e.g., that the lower heater 23 heats the paste up to 45°C, while the upper heater 24 heats approximately to a room temperature. If the paste 22 does not include volatile components, the paste may be 50 or 60°C. When the paste 22 is maintained at the room temperature or so by the upper heater 24, the upper heater 24 may be set at a lower temperature than the room temperature to slightly cool the paste 22 heated by the lower temperature 23. An apparatus shown in Fig. 9 which will be described later is preferably used if it is necessary to positively cool the paste 22. An example of the above-described temperature gradient is preferably a curve satisfying y=b-ax 2 as in Fig. 8 wherein b and a are constants optionally determined in accordance with characteristics of the paste 22. In this graph, a position of the lower heater 23 in Fig. 6 is indicated as an origin of an axis of abscissa, and an intersection point between the curve and the axis of abscissa is an arranging position of the upper heater 24. The above temperature gradient may be similarly achieved by setting only the lower heater 23 of a part heating the paste 22 or interposing a suitable count of heaters between the upper and lower heaters 24 and 23. The temperature gradient can be defined more forcibly and finely if a count of heaters is large. Alternatively, a continuous one heater may be incorporated from the lower heater 23 to the upper heater 24 to gradually reduce a thermal capacity (generating calory). As is indicated in Fig. 6, in the fourth embodiment, variable resistors (volumes) 23a, 24a are set in a energizing circuit of the heaters 23, 24, thereby regulating the generating amount of heat of the heaters 23, 24 to obtain the temperature gradient in various forms. Circuit boards of various conditions can accordingly be manufactured with high quality to meet a wide variety of conditions of the paste 22 and filling conditions of the paste 22. Although the heaters 23, 24 are used as an example of the temperature-adjusting element in the fourth embodiment, the element is not limited to this and any optional element is usable so long as it can adjust temperatures. (Fifth Embodiment) Fig. 9 is a schematic diagram of a process of filling a paste in a circuit-board manufacturing apparatus adopting a printing method and a printing apparatus according to a fifth embodiment of the present invention. A difference from Fig. 8 is that the upper heater 24 is eliminated, and a cooling device 200 is set at a position corresponding to the arranging position of the upper heater 24 to supply cool wind to the pressing face 221a of the filling head 221. In the fifth embodiment, while the lower heater 23 heats the part of the paste 22 in touch with the printing face, the cooling device 200 prevents the part of the paste 22 separated from the printing face and the filling head 221 from being raised to not lower than a necessary temperature because of heating by the lower heater 23. Owing to this arrangement, a required temperature difference is secured between the part of the paste 22 away from the printing face and the filling head 221 and the part of the paste 22 in touch with the printing face. (Sixth Embodiment) Fig. 10 is a schematic diagram of a filling apparatus which shows a pretreatment state before filling the paste to manufacture a circuit board with a printing method and a printing apparatus according to a sixth embodiment of the present invention provided. In the sixth embodiment, as shown in Fig. 10, before the paste 22 is pressed and filled by the filling head 21 to the printing face of the base material 4 on the stage 5, a pretreatment material 51 which is an nonvolatile component included in the paste 22 is applied to the printing face of the base material 4. The apparatus is equipped with the filling head 21 and also a pretreatment head 52 for applying the pretreatment material 51. In a process of filling the paste 22 by the filling head 21 to the through hole 4d of the base material 4, the same nonvolatile organic component as contained in the paste 22 is applied as the pretreatment material 51 by the pretreatment head 52 to the printing face of the base material 4 before the paste 22 is filled in the through hole 4d. When the pretreatment head 52 moves on the base material 4 while keeping touch with the base material 4 to apply the pretreatment material 51 to the printing face of the base material 4, the pretreatment material 51 is sufficiently scraped by the pretreatment head 52 from the printing face of the base material 4, but a part of the pretreatment material 51 adsorbing the printing face of the base material 4 is left on the printing face of the base material 4. The adsorption on this occasion is brought about on the printing face of the base material 4 also during the filling of the paste 22 to the through hole 4d, and causes an increase of the viscosity of the paste 22. For solving the viscosity increase of the paste 22, therefore, the same component as adsorbs to the printing face of the base material 4 is preliminarily applied to the printing face of the base material 4 before the paste 22 is printed. Since the component adsorbing to the base material 4 is supplied to the paste 22 during printing of the paste 22, the increase of the viscosity of the paste 22 can be prevented. Results of the viscosity increase of the paste 22 when the nonvolatile organic component in the paste 22 is applied as the pretreatment material 51 onto the base material 4 beforehand (represented by "treated" ) and when the pretreatment material 51 is not applied on the base material 4 beforehand (represented as "not treated") are shown in Fig. 11. As is apparent from Fig. 11, the viscosity of the paste is constant when the nonvolatile organic component of the paste is applied, while the viscosity of the paste is increased when the organic component is not applied. Therefore, the application of the nonvolatile organic component can be effective to restrict the viscosity increase of the paste. From the graph of Fig. 11, the pretreatment material 51 is considered to be scraped an equal amount to the paste 22 by the base material 4 and the pretreatment head 52. If the scraping is not carried out enough, the organic component remains on the surface of the base material 4 thereby to decrease the viscosity of the paste 22. Therefore, much care should be taken. As the above result, the viscosity increase of the paste 22 is restricted owing to the application of the nonvolatile organic component in the paste 22 onto the base material before the paste 22 is filled. The continuous printing efficiency can be improved. In the sixth embodiment, the filling head 21 and the pretreatment head 52 are preferably formed in the same shape, structure and material in order to make the scraping condition equal. An application head of the pretreatment head 52 is a polyurethane squeegee in the above sixth embodiment, similar to the filling head 21. However, the same effect is obtained even when the organic component is adsorbed onto the base material by vapor deposition or the like manner. (Seventh Embodiment) Fig. 12 is an explanatory diagram of a state where the paste is filled in a paste-filling apparatus for circuit boards with a printing method and a printing apparatus according to a seventh embodiment of the present invention provided. According to the seventh embodiment, as shown in Fig. 12, in a process to press and fill the paste 22 to the through hole 4d of the compressive base material 4, when the paste 22 is filled into the through hole 4d of the base material 4, 5-20% the through hole 4d is left at maximum without being filled with the paste 22, as designated by 61. Accordingly, when the base material 4 is compressed, the paste 22 is let to escape to the non-filled part 61, thereby preventing connection parts of layers of the circuit board from being separated each other by a repulsive force of the paste 22 if the paste 22 would not be allowed leeway. Concretely, after the paste 22 is filled in the through hole 4d of the base material 4, the base material is heated and pressed while held between copper foils, whereby a double-face board is formed. Then, the copper foils are etched in a predetermined pattern thereby to produce a circuit board. When the base material 4 is heated and pressed while held between the copper foils in this series of operations, if the base material 4 is compressed by a degree not smaller than a tolerance of the paste 22 filled in the through hole 4d of the base material, the paste 22 when hardened generates a force separating the copper foils from the base material 4, with bringing about a large deterioration of connection reliability of the layers. This is caused because a part of the paste 22 exceeding the tolerance cannot be escaped if the paste filled in the through hole 4d is compressed so much. The part leaks into the base material 4 if the base material 4 is porous, resulting in a decrease of an insulation resistance, etc. and worsening reliability as the circuit board. Such a matter should be avoided. When the non-filled part 61 is provided in the through hole 4d of the base material 4 to function as a space allowing the paste 22 to move thereinto, the paste 22 compressed not smaller than the tolerance is accommodated in the through hole 4d of the base material 4, so that the aforementioned issue can be eliminated. Table 3 shows good and defect connection resistances when an average diameter of the through hole 4d and a depth of the non-filled part 61 to the average diameter are changed. From Table 3 above, when the non-filled part is 5-20% to the average hole diameter is formed, the reliability of the connection resistance can be ensured. From the above result, when the paste 22 is pressed and filled so that the depth amount of the through hole 4d of the base material 4 becomes 5-20% to the average diameter of the through hole 4d of the base material 4, it can be presented that the connection parts of layers are separated by the repulsion force of the compressed base material 4 and the reliability is deteriorated. According to the embodiments of the present invention, the flow pressure of the paste is kept to be nearly constant while the paste as an example of the printing material is filled, as described in any of the first through seventh embodiments. The filling amount of the paste is thus secured enough to the through hole(s) of the base material as an example of the body to be printed in conformity with various filling conditions. The yield, quality and productivity can be improved in the present invention. According to the printing method and the printing apparatus of the embodiment(s) of the present invention, at least either the contact angle of the filling head to the printing face of the base material, or the moving speed of the filling head is changed in accordance with a predetermined condition, namely, a change of the amount of the paste as the filling head moves forward. Even the increase of the viscosity of the paste or the decrease of the filling and pressing force due to the consumption of the paste with time can be handled in this manner. A predetermined amount of the paste can be stably and surely filled thereby to improve continuous printing performance. In the printing method and printing apparatus of the embodiment(s) of the present invention, the filling head for pressing and filling the paste to the through hole(s) of the base material is supported at the side where the pressing face is in touch with the printing face of the base material. On the other hand, the filling head at the side separated from the printing face is rendered a free end and deformable, thereby securing necessary flexibility. While the pressure to the paste at the side in touch with the printing face is restricted not to leak, a necessary minimum degree of elasticity is secured to let the filling head trace the uneven printing face of the base material. A predetermined amount of the paste can be stably filled without any special control means provided against the increase of the viscosity or reduction of the volume through consumption of the paste with time. Continuous printing efficiency can be improved. In the printing method and printing apparatus of the embodiment(s) of the present invention, the temperature-adjusting element incorporated in the filling head pressing and filling the paste to the through hole(s) of the base material works to apply the predetermined temperature gradient to the pressing face of the filling head. That is, because of such temperature gradient that the temperature of the pressing face at the side in touch with the printing face is high and the temperature is decreased as the pressing face is separated away from the printing face, the viscosity of the rolling paste of a part in touch with the printing face is lowered thereby to improve the fluidity of the paste. At the same time, the fluidity at the pressing face at the side away from the printing face of the base material and the filling head is restricted because the temperature is lowered. Since the hydraulic pressure at the pressing face of the part in touch with the printing face is increased, not only a predetermined amount of the paste is stably filled, but the volatile component is refrained from evaporating at the low temperature part. The whole paste can be maintained at a predetermined viscosity easily and therefore continuous printing efficiency can be improved. When the heater as an example of the temperature-adjusting element is constituted to set an optional temperature gradient to a distance of the pressing face of the filling head from the printing face of the base material, high quality printing can be achieved in various conditions in conformity with a variety of conditions of the paste and filling conditions of the paste. In the printing method and printing apparatus of the embodiment(s) of the present invention, the pretreatment material is applied by the filling head and the pretreatment head, provided side by side, before the paste is filled to the printing face of the base material. That is, an effect of applying the nonvolatile organic substance can be gained in one filling process. If the nonvolatile organic component in the paste is applied onto the base material before the paste is filled, the viscosity of the paste is restricted from rising, therefore enhancing continuous printability.

Claims: A printing method for pressing and filling a printing material (22) into a through hole (4d) of a circuit board (4) to be printed by a filling head (21, 121; 221), wherein a flow pressure of the printing material is kept roughly constant during printing of the printing material, wherein the filling head (21; 121; 221) is in direct contact with the circuit board (4). A printing method according to claim 1, wherein at least either a contact angle of the filling head (21, 121, 221) to a printing face of the circuit board (4), or a moving speed of the filling head is changed in accordance with a change of quantity of the printing material, thereby to keep the flow pressure of the printing material roughly constant. A printing method according to claim 1 or 2, wherein the printing material is printed while the filling head (121) is supported at a side of a printing material-pressing face (121a) in touch with the printing face of the circuit board (4), thereby to keep the flow pressure of the printing material roughly constant. A printing method according to any one of claims 1 to 3, wherein a temperature gradient is set to a printing material-pressing face (221a) of the filling head (221) so that a part of the pressing face in touch with the printing face of the circuit board (4) is higher in temperature than a part of the pressing face apart from the printing face of the circuit board and the filling head, thereby to keep the flow pressure of the printing material roughly constant. A printing method according to any one of claims 1 to 4, wherein a pretreatment material is applied before the printing material is filled by the filling head to the printing face of the circuit board (4), thereby to keep the flow pressure of the printing material roughly constant. A printing apparatus for pressing and filling a printing material (22) by a filling head (21; 121; 221) into a through hole (4d) of a circuit board (4) to be printed, which is provided with a holding device for keeping a flow pressure of the printing material roughly constant during printing of the printing material, wherein the filling head (21; 121; 221) is in direct contact with the circuit board (4). A printing apparatus according to claim 6, wherein the holding device is a control part (31) which changes at least either a contact angle of the filling head (21, 121, 221) to a printing face of the circuit board (4), or a moving speed of the filling head in accordance with a change of quantity of the printing material. A printing apparatus according to claim 6 or 7, wherein the holding device is a supporting member (41) which supports the filling head (121) at a side of a printing material-pressing face (121a) in touch with the printing face of the circuit board (4). A printing apparatus according to any one of claims 6 to 8, wherein the holding device is a temperature-adjusting element (23, 24, 200) which sets a temperature gradient to a printing material-pressing face (221a) of the filling head (221) so that a part of the pressing face in touch with the printing face of the circuit board (4) is higher in temperature than a part of the pressing face apart from the printing face of the circuit board (4) and the filling head. A printing apparatus according to claim 9, wherein the temperature-adjusting element sets an optional temperature gradient to a distance of the pressing face of the filling head from a side in touch with the printing face of the circuit board (4). A printing apparatus according to any one of claims 6 to 10, wherein the holding device is the filling head and a pretreatment head applying a pretreatment material to the printing face of the circuit board (4) before the printing material is filled.

Inventors: HIGASHIDA TAKAAKI, IWAKI TAKAHIKO, OTANI HIROYUKI, HIGASHIDA, TAKAAKI, IWAKI, TAKAHIKO, OTANI, HIROYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: B41F15
IPC codes: H05K3, H05K1, B23K26, B41F15