METHOD AND DEVICE FOR DRYING MATERIALS AND METHOD OF PRODUCING CIRCUIT BOARDS USING THE SAME

Abstract: The present invention relates to a method and a system of cleaning to remove cutting dusts and the like generated and adhering to a board material (1a) during drilling for electrical connection, and drying the board material, in a process of manufacturing a circuit board for small electronic equipment and the like. A large amount of board materials can be treated without receiving thermal damage by performing the steps of: placing sheets of the board material that have absorbed moisture resulting from cleaning, like a stack in a vacuum chamber (11); and drying the board material by repeating evacuation and pressurization under predetermined conditions while heating the board material. Thus, high-quality drilling can be achieved without losing quickness of laser machining, and highly reliable circuit boards can be obtained at low cost.

Description: FIELD OF THE INVENTION The present invention relates to a method and system of drying materials and a method of manufacturing circuit boards using the same. BACKGROUND OF THE INVENTION With recent downsizing and high density of electronic equipment, a double-sided or multi-layered circuit board is employed more frequently than a conventional single-sided board, as a circuit board on which electronic components are mounted, and a high density circuit board allowing a larger number of circuits to be integrated in a circuit board is developed. For the high density circuit board, instead of drilling of holes (through holes) in the board, which is conventionally operated, employment of laser machining capable of faster and more fine machining is developed (Y. Yamanaka et al. "Excimer Laser Processing In The Microelectronics Fields", for example). Also proposed is a circuit board in which layers are connected using fine holes made by laser and such connection means as conductive paste (shown in Japanese Patent Application Non-examined Publication No. H06-268345, for example). In the technique of making fine holes and connecting layers using conductive paste, a few foreign substances may cause contact failure. In this technique, the substrate material is drilled together with a film bonded thereto and the film is used as a mask for filling the conductive paste into the fine holes. Therefore, all the substrate material including the film must be kept clean. However, both drilling and laser machining produce a large amount of cutting dusts, and the cutting dusts may adhere to the board material and close the holes for electrical connection. In addition, a little dust in air may close the fine holes. To solve this problem, the board material is cleaned before the conductive paste is filled. However, because complete cleaning of the holes by dry method is very difficult, such new wet cleaning methods as ultrasonic cleaning are employed. These methods require drying after cleaning, in order to remove the moisture absorbed from the periphery of the machined holes or the outer ends of the board material. As shown in Fig. 4 (a), mask films 2a and 2b are bonded to substrate material 1. Provided at an end of substrate material 1 are peel off leading parts 3a and 3b for stripping mask films 2a and 2b after the conductive paste is filled. The leading parts 3a and 3b are made by stripping a part of each of mask films 2a and 2b. Because of such a structure, when sheets of substrate material 1 are stacked one on top of another and dried by heating using a conventional hot air convection method as shown in Fig. 4 (b), B-stage epoxy resin in the board material melts and stripping leading parts 43a and 43b and substrate material 1 melt together again, as shown in Fig. 4 (c). Therefore, it is difficult to treat and sufficiently dry a large number of sheets at a time. A small number of sheets could be treated with a vacuum dryer. However, it takes long time to dry a large number of sheets because sufficient latent heat cannot be given to the material under vacuum and thus calories sufficient to remove the moisture cannot be added. In improving reliability of connection in circuit boards, wet cleaning is more effective than dry cleaning. However, insufficient drying after cleaning would significantly affect the reliability of connection and insulation. For this reason, moisture must be removed completely. However, the conventional method is not efficient and may possess problems e.g. causing more damage to the board material. The present invention is directed to realize a board material having high-quality fine holes and to provide a method and a system of manufacturing highly reliable circuit boards at low cost. DISCLOSURE OF THE INVENTION In order to address the above-mentioned problems, a method of manufacturing circuit boards in accordance with the present invention includes a bonding step of bonding a film-like material to at least one side of a board material to form a substrate material having a film(s). The method of manufacturing a circuit boards also includes: a hole-forming step of drilling a through or a blind hole in the board material made of a plurality of materials; and a step of forming a connection means in the through or blind hole for mutually connecting circuits formed on or in the circuit board. The hole-forming step includes: a step of irradiating the board material with laser to form a through or blind hole shape; and a step of cleaning to remove foreign substances that have attached to the surface of the board material and the inner wall of the through or blind hole in the previous step. The method can provide a clean and highly reliable board material by efficiently drying the moisture absorbed by the board material, without damaging the board material, after the removal of water drops that have adhered to the surface of the board material in a cleaning process. Therefore, in accordance with the present invention, high-quality drilling can be achieved without losing high speed of the laser machining, and a highly reliable circuit board can be provided at low cost. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross sectional view showing a process of a method of manufacturing a circuit board in accordance with an exemplary embodiment of the present invention. Fig. 2 is a schematic diagram of a circuit board manufacturing system in accordance with the embodiment. Fig. 3 is a graph showing a profile of pressures in a vacuum chamber for use with a material drying method in accordance with the present invention. Fig. 4 is a cross sectional view showing a process of a conventional method of manufacturing a circuit board. DESCRIPTION OF THE PREFERRED EMBODIMENTS An exemplary embodiment of the present invention is hereinafter described with reference to Figs. 1 to 3. Figs. 1 (a) to (h) shows cross sectional views of a process of a method of manufacturing a multi-layered circuit board in accordance with the present embodiment. In Fig. 1, reference numeral 1 shows a substrate material that is a square plate of 250 mm on each side and approx. 150 µm thick, as an insulating substrate. Used as the substrate material is a resin impregnated substrate made of a composite material in which an non-woven fabric made of an aromatic polyamide fiber (hereinafter referred to as "aramid fiber") is impregnated with a thermosetting epoxy resin (hereinafter referred to as "epoxy resin"), for example. The epoxy resin in accordance with the present embodiment is a so-called "B stage" epoxy resin that contains an uncured portion called. Reference numerals 2a and 2b show strippable resin films for masking films, each of which is approx. 20 µm thick. On their surfaces that are to adhere to substrate material 1, thermosetting epoxy resin layers (not shown) of approx. 1 µm thick each are applied. Polyethylene terephthalate (hereinafter referred to as "PET sheet") is used as the strippable resin film, for example. Board material 1a is made by bonding PET sheets 2a and 2b to substrate material 1. At one end of board material 1a, peel off leading parts 3a and 3b are previously provided between substrate material 1 and PET sheets 2a and 2b so as to peel off PET sheets 2a and 2b afterwards. The peel off leading parts 3a and 3b are made by stripping off a part of each PET sheet from substrate material 1. Next, as shown in Fig. 1 (b), substrate 1a is irradiated with laser light 4 to form through hole 5. At this time, part of the thermosetting epoxy resin and aramid fiber in board material 1a is sublimed by heat, and scatters to its surroundings. However, the thermosetting epoxy resin and aramid fiber that have not been sublimed remain on the wall surface of the hole, as hard and brittle altered portions 6. Because the aramid fiber is more heat-resistant than the thermosetting epoxy resin and thus has a lower laser-machining rate, the aramid fiber is likely to remain without subliming. Therefore, the inner wall of the hole has rough surfaces as shown in the drawing. On the other hand, part of the thermosetting epoxy resin or aramid fiber that has scattered to its surroundings adheres to the surface of board material 1a or the inside of through hole 5 as cutting dusts 7. Next, as shown in Fig. 1(C), board material 1a is placed near ultrasonic vibrator 9 and vibrated in the water in ultrasonic cleaning vessel 8. The acoustic energy radiated from ultrasonic vibrator 9 vibrates board material 1a and altered parts 6 and cutting dusts 7 fall out or are peeled off from board material 1a. As a result, board material 1b having a proper hole shape as shown in Fig. 1 (d) can be obtained. Additionally, water drops adhering to the surface of board material 1b are removed by air blow and the like. However, at the end, in through hole 5 and peel off leading parts 3a and 3b of board material 1b, some moisture absorbed therein still remains. Next, as shown in Fig. 1 (e), two sets 10 of 180 sheets of board material 1 stacked are prepared and placed in vacuum chamber 11, and board material 1b is dried. Fig. 2 shows a schematic diagram of a dryer in accordance with the present invention. Pre-heater 31 are provided around vacuum chamber 11, and a vacuum system comprising a vacuum pump 32, and foreign substance removing filter 35 for protecting vacuum pump 32 is connected to the chamber. In addition, the dryer has hot air supplier 33 comprising hot air generator 34, foreign substance removing filter 35, and air dryer 36 for pressurizing the chamber. Board material 1b placed in vacuum chamber 11 is evacuated and dried while being heated by radiation heat radiated from pre-heater 31. Instead of pre-heater, microwaves can be used for drying. Next, an operation system of vacuum drying is described using Fig. 3. (1) After placement of the board material, the vacuum pump is actuated to evacuate the chamber to a pressure of approx. 100 Pa. (2) Next, the evacuation operation is stopped. (3) Hot air supplier 33 supplies dry air at a temperature of approx. 60 °C that has been filtered through an air cleaner to vacuum chamber 11 and the chamber is pressurized to atmospheric pressure. (4) After pressurizing, dry air is continuously supplied, and air in vacuum chamber 11 is circulated. Simultaneously evacuation using vacuum pump 32 makes air circulation efficient. (5) Dry air is circulated for approx. one minute after the pressurization. Then supply of dry air is stopped and the chamber is evacuated to a pressure of approx. 70 Pa. (6) The above-described steps (2), (3), and (4) are repeated. (7) After dry air is circulated for one minute after the pressurization, dry air supply is stopped and the chamber is evacuated to a pressure of approx. 40 Pa. (8) Next, the evacuation operation is stopped. (9) After hot air supplier 33 supplies dry air at a temperature of approx. 60 °C that has been filtered through the air cleaner to vacuum chamber 11 and the chamber is pressurized to atmospheric pressure, board material 1b is taken out of vacuum chamber 11. It goes without saying that further repeating of these steps will enhance the degree of vacuum and dryness of the board material. To obtain a required dryness, the number of repetition, ultimate pressure, temperature of dry air can be optimized according to the material. An example of optimized conditions and its results are shown below: First evacuation: degree of vacuum: 500 Pa → After reaching the ultimate pressure, dry air is introduced and the chamber is pressurized (at atmospheric pressure kept for one minute). Second evacuation: degree of vacuum: 300 Pa → After reaching the ultimate pressure, dry air is introduced and the chamber is pressurized (at an atmospheric pressure kept for one minute). Third evacuation: degree of vacuum: 150 Pa → After reaching the ultimate pressure, dry air is introduced and the chamber is pressurized (at an atmospheric pressure kept for one minute). Fourth evacuation: degree of vacuum: 150 Pa → After reaching the ultimate pressure, dry air is introduced and the chamber is pressurized (at an atmospheric pressure kept for one minute). Above four evacuating and pressurizing processes are repeated to next process. Nineteenth evacuation: degree of vacuum: 150 Pa → After reaching the ultimate pressure, dry air is introduced and the chamber is pressurized (at an atmospheric pressure kept for one minute). Twentieth evacuation: degree of vacuum: 150 Pa → After reaching the ultimate pressure, dry air is introduced and the chamber is pressurized (at an atmospheric pressure kept for one minute). → Completion In the above processes: temperature of dry air: 60 °C Total process time: approx. 40 minutes Results of dryness: Measuring samples were placed in each of the upper, middle, and lower parts of the right and left pallets in the chamber and the weight of the materials before cleaning, after cleaning, and after drying was measured. The results show that the weight after drying under vacuum is less than that of before cleaning and moisture has sufficiently be removed. Next, as shown in Fig. 1 (f), conductive paste 14 is filled in through holes 5 using such means as printing. As through holes 5 have a proper shape, the inside of through holes 5 can completely be filled with conductive paste 14 without hindrance. After the filling, mask films made of PET sheets 2a and 2b are stripped off. Next, as shown in Fig. 1 (g), board material 1b is sandwiched by metal foils 15 and heated and pressed using a hot press (not shown). This operation completely cures board material 1b that has been in the pre-preg state and electrically connects metal foils 15 bonded to the both sides of board material 1b together via conductive paste 14. Next, metallic foils 15 are patterned to a desired pattern and a double-sided circuit board that has circuit patterns 16 as shown in Fig. 1 (h) is obtained. The drying method in accordance with this embodiment can also be used for drying a double-sided circuit board that has been washed after the patterning. In this embodiment, a double-sided circuit board is described. However, it goes without saying that the method can be applied to a multi-layered circuit board by repeating the steps in accordance with the present invention a plurality of times. In the present invention, the case where conductive paste 14 is filled in through holes 5 in board material 1b is described. However, in the case where plated conductor is formed in through holes 5, the same effect can be obtained. INDUSTRIAL APPLICABILITY As described above, a method and a system of manufacturing a circuit board in accordance with the present invention comprise the following steps. A step of irradiating a board material with laser to form a through or blind shaped holes. A step of obtaining a desired through or blind shaped holes by water cleaning and removing altered substances in the form of powder or blocks that have scattered from the board material and adhered during or after the laser irradiation step. The altered substances are formed on the surface of the board material and the inner wall of the through-holes or blind holes. Removing water drops that have attached to the surface of the board material in the cleaning step. Drying the board material by the gross to dry the moisture absorbed in the board material in the cleaning process, without causing thermal damage to the board material, by repeating evacuation and pressurization cycles while heating the board material. Thus, high-quality drilling can be achieved without losing quickness of laser machining, and highly reliable circuit boards can be obtained at low cost.

Claims: A method of drying material comprising the steps of: evacuating air in a vacuum chamber containing said material under an atmospheric pressure to an ultimate pressure; and supplying air to said vacuum chamber to make a pressure in said chamber to an atmospheric pressure again, wherein said evacuating and supplying steps are operated at least once. The method of drying material according to claim 1, wherein said ultimate pressure is gradually decreased as a number of said operation of evacuating and supplying steps increases. The method of drying material according to claim 1 further comprising the steps of: supplying hot air into said chamber when said chamber is made to the atmospheric pressure again; and evacuating again said chamber while said hot air is supplied to said vacuum chamber; stop supplying said hot air after a predetermined time duration; and evacuating to said ultimate pressure, in said supplying step. The method of drying material according to claim 3, wherein said predetermined time duration is at least one second. The method of drying material according to claim 1, wherein said steps are performed while material in said vacuum chamber is heated. The method of drying material according to claim 3, wherein air pressure of said hot air supplied is lower than air pressure in a previous evacuating step in one of a evacuating step performed thereafter. The method of drying material according to claim 3, wherein a temperature of said hot air is at least 35 °C. The method of drying material according to claim 1, wherein said supplying of air to said vacuum chamber is performed from a plurality of points. The method of drying material according to claim 1, wherein said evacuating of said vacuum chamber is performed from a plurality of points. The method of drying material according to claim 1, wherein said air to be supplied is dry air. The method of drying material according to claim 10, wherein said dry air is passed through at least one of an air cleaner and a dehumidifier. The method of drying material according to claim 1, wherein said ultimate pressure is at most 200 Pa. The method of drying material according to claim 5, wherein said material is heated using radiation heat from a heater. The method of drying material according to claim 5, wherein said material is heated using dielectric heating by microwaves. A dryer comprising a material container of a sealable structure, wherein said container has an air outlet and an air inlet, said air outlet and air inlet are piped to a evacuator and an air supplier, respectively, and said container has a heating means. The dryer according to claim 15, wherein said evacuator and said air supplier can operate repeatedly in one of a simultaneous manner and an alternate manner. The dryer according to claim 15, wherein said air supplier has a means for heating air and a means for removing at least one of foreign substance and moisture in air. The dryer according to claim 15, wherein said container has a plurality of air inlets. The dryer according to claim 15, wherein said container has a plurality of air outlets. The dryer according to claim 15, wherein a means for removing at least one of foreign substance and moisture in air is placed between said air outlet and said evacuator. The dryer according to claim 15, wherein said heating means uses radiation heat from a heater. The dryer according to claim 15, wherein said heating means uses dielectric heating by microwaves. The dryer according to claim 15 further comprising the following operation systems: (1) a system for actuating said heating means to heat a material in said container; (2) a system for actuating said evacuator, and stopping the evacuating operation after said container has reached to a predetermined vacuum pressure; (3) a system for actuating said air supplier to supply air to said container; (4) a system for actuating said evacuator to exhaust the air in said container; (5) a system for stopping said air supply operation after a predetermined period of time; and (6) a system for stopping the evacuating operation after reaching to a predetermined vacuum pressure. The dryer according to claim 15 comprising an operation system wherein said operations (1) to (6) are repeated a plurality of times. The material dryer according to claim 15, wherein said predetermined vacuum pressure in operation (6) of Claim 23 can be set lower than said predetermined vacuum pressure in operation (2) of Claim 23. The dryer according to claim 24, wherein said predetermined vacuum is set lower every time the number said repeating of said operation to be performed a plurality of times increases. A method of manufacturing a circuit board comprising the steps of: bonding a film shape material to at least one surface of a substrate material to form a board material with a film; forming one of a through hole and a blind hole by irradiating said board material with a film with laser; cleaning said board material to remove at least one of altered parts and altered substances formed on an inner wall of one of said through hole and said blind hole, and to remove foreign substance adhering to a surface of said board material with a film; blowing gas onto the surface of said board material with a film to remove water drops adhering to said surface; drying moisture adhering to and absorbed in said board material with a film, using the method of drying according to claim 1; and forming an electrical connection means in one of said through hole and said blind hole. A method of manufacturing a circuit board comprising the steps of: bonding a film-shape material to at least one surface of a substrate material to form a board material with a film; forming one of a through hole and a blind hole by irradiating said board material with a film with laser; filling one of said through hole and said blind hole with conductive paste; drying a double-sided circuit board having circuit patterns on both surfaces thereof, using the method of drying according to claim 1; and laminating said double-sided circuit board and said board material to form a multi-layer circuit board, said board material being filled with conductive paste and said film-shape materials on the both sides of said board material being peeled off. A method of manufacturing a circuit board comprising the steps of: removing moisture of a circuit board having a circuit pattern on at least one surface thereof; bonding a board material to at least one surface of said circuit board; laminating said circuit board and said substrate material by further bonding a metal foil on said board material, and heating and pressing said circuit board and said substrate material at the same time; and forming a means for electrically connecting said circuit board and said metal foil, wherein moisture contained in said circuit board is removed using the method of drying according to claim 1. The method of manufacturing a circuit board according to claim 27, wherein said substrate material comprises a prepreg, said prepreg being made by impregnating a reinforcing material with a thermosetting resin and making the resin into B stage. The method of manufacturing a circuit board according to claim 30, wherein said reinforcing material is one of a glass fiber fabric and a glass fiber non-woven fabric. The method of manufacturing a circuit board according to claim 30, wherein said reinforcing material is one of an aromatic polyamide fiber fabric and an aromatic polyamide fiber non-woven fabric. The method of manufacturing a circuit board according to claim 27, wherein said forming an electrical connection means is a method of filling said hole with paste containing conductive particles. The method of manufacturing a circuit board according to claim 27, wherein forming an electrical connection means is a method of forming a plating layer on an inner wall of said hole. The method of manufacturing a circuit board according to claim 27, wherein one of said board material and said circuit board is dried under a state of stacking sheets of one of said board material and said circuit board one on top of another. The method of manufacturing a circuit board according to claim 27, wherein said film-shape material is coated with a thermosetting epoxy resin on a surface thereof.

Inventors: KISHIMOTO KUNIO, MIURA AKIHIRO, NAKAMURA SHINJI, NISHII TOSHIHIRO, TAKENAKA TOSHIAKI, KISHIMOTO, KUNIO, MIURA, AKIHIRO, NAKAMURA, SHINJI, NISHII, TOSHIHIRO, TAKENAKA, TOSHIAKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD
Application year: 2001
Main classification: H05K3
IPC codes: F26B5, H05K3