Method of mounting terminal to flexible printed circuit board

Abstract: For soldering a terminal (14) to a land (13) formed on an FPC (12) made of a material having a softening point below 230°C, such as PET, by using a flow soldering or reflow soldering technique, an opening (17) larger than the land (13) is formed in a thermal insulation holder (11) including a phenolic paper base copper clad laminate board or the like so that the land (13) and the terminal (14) are within sight, and the thermal insulation holder (11) is held in contact with a surface of the FPC (12) to which solder is applied so that the land (13) and the terminal (14) are located within the opening (17) to perform the soldering using the flow soldering or reflow soldering technique, whereby molten solder readily reaches the land (13) through the opening (17) which need not be worked into a conventional trapezoidal configuration in vertical section, thereby preventing poor soldering.

Description: The present invention relates to a method of mounting a terminal to a flexible printed circuit board and, more particularly, to a method of soldering a terminal to a land formed on a flexible printed circuit board made of a material having a softening point below 230°C. Flow soldering and reflow soldering techniques using conventional solder are not applicable to a flexible printed circuit board (referred to hereinafter as an FPC) having a substrate made of polyethylene terephthalate (referred to hereinafter as PET) since the softening point of PET ranges from 220°C to 230 °C which is substantially equal to the melt temperature of the conventional solder. Soldering by local heating using solder having a low melting point below 200°C may be considered but presents the problems of costs and safety depending on the types of materials to be added to lower the melting point of the solder. An approach for soldering a terminal to a land on the FPC without using low-melting-point solder has been proposed which is disclosed in JP-A-5-218632 (1993). Referring to Fig. 11, a thermal insulation holder 1 including a phenolic paper base copper clad laminate board, a paper base epoxy resin board, or a glass fabric base epoxy resin board is prepared. A land 3 is formed on a FPC 2 made of PET, and a terminal 4 is received through the FPC 2 generally centrally of the land 3. An opening 5 which is substantially equal in size to the land 3 is then formed in the thermal insulation holder 1. The thermal insulation holder 1 is brought into contact with a surface of the FPC 2 to which solder is applied so that the land 3 and the terminal 4 are located within the opening 5. With the contact maintained, soldering is performed. To facilitate the introduction of solder into the opening 5, the opening 5 is designed to be equal in size to the land 3 on its contact surface side and to be greater than the land 3 on the opposite side thereof. That, is, the opening 5 is of a trapezoidal configuration in vertical section. In the disclosed method, however, the opening 5 formed in the thermal insulation holder 1 has the size similar to that of the land 3 on the FPC 2 and is very small. Thus, when the FPC 2 is brought into contact with molten solder 6 in a solder tank, with the thermal insulation holder 1 positioned downside, to perform soldering, the solder 6 does not reach the land 3 due to the surface tension of the molten solder 6 in some cases, resulting in poor soldering. Soldering a multiplicity of terminals such as connector terminals requires an equal number of lands and, accordingly, a large number of openings corresponding to the respective lands. Thus, fabrication of a multiplicity of openings of a trapezoidal cross-sectional configuration adds to the number of fabrication steps and fabrication costs. DE-A-2833480 discloses a method of mounting a terminal made of a conductive material to an FPC by soldering the terminal to a land formed on the FPC. The holder for said FPC (which can be connected to said FPC before or after soldering by means of an insulating adhesive film) is made of a material having high heat conductivity. In a first aspect the present invention provides a method of mounting a terminal made of a conductive material to a flexible printed circuit board made of a material having a softening point below 230°C by soldering the terminal to a land formed on the flexible printed circuit board. According to the present invention, the method comprises the steps of: forming an opening rectangular in vertical section and larger than the land in a flat heat insulating holder, the heat insulating holder being for contacting a surface of the flexible printed circuit board to which solder is applied; and bringing the heat insulating holder into contact with the surface of the flexible printed circuit board so that the land and the terminal are located within the opening, and soldering the terminal by a flow soldering or reflow soldering process, with the contact maintained and the terminal extending through the land into the opening. In a second aspect the present invention provides a method of mounting a plurality of terminals made of a conductive material to a flexible printed circuit board made of a material having a softening point below 230°C by soldering the terminals to a plurality of lands formed on the flexible printed circuit board, respectively. According to the present invention, the method comprises the steps of: forming a rectangular opening being rectangular in vertical section and slightly larger than a minimum rectangle defined to surround a row or a rectangular array of the lands in a heat insulating holder, the heat insulating holder being for contacting a surface of the flexible printed circuit board to which solder is applied; and bringing the heat insulating holder into contact with the surface of the flexible printed circuit board so that the lands and the terminals are located within the opening, and soldering the terminals by a flow soldering or reflow soldering process, with the contact maintained and the terminals extending through the lands into said opening. Preferably, thickness of the heat insulating holder is about one-tenth to seven times the thickness of the flexible printed circuit board. In a third aspect the present invention provides a method of mounting a terminal made of a conductive material to a flexible printed circuit board made of a material having a softening point below 230°C by soldering the terminal to a land formed on the flexible printed circuit board. According to the present invention, the method comprises the steps of: forming a through aperture having a size substantially equal to the size of the land in a heat insulating film provided in contact with a surface of the flexible printed circuit board to which solder is applied; forming an opening larger than the land in a heat insulating holder, the heat insulating holder being for contacting a surface of the heat insulating film which is opposite from the flexible printed circuit board; and soldering the terminal by a flow soldering or reflow soldering process, with the heat insulating film in contact with the surface of the flexible printed circuit board so that the land and the terminal are located within the through aperture and with the heat insulating holder in contact with the surface of the heat insulating film so that the land and the terminal are located within the opening. Preferably, the heat insulating holder is provided with a plurality of guide pins in corners thereof, and the flexible printed circuit board and the heat insulating film have through apertures formed therein for the guide pins; and the guide pins are respectively inserted into the through apertures to bring the heat insulating holder into contact with the heat insulating film and to bring the heat insulating film into contact with the flexible printed circuit board. Preferably, the heat insulating holder is formed with a guide frame for receiving the flexible printed circuit board and the heat insulating film therein; and the heat insulating film comes into contact with the flexible printed circuit board, and the heat insulating holder comes into contact with the heat insulating film. According to the first aspect of the present invention, the opening of the heat insulating holder is rectangular in vertical section and larger than the land on the flexible printed circuit board to permit molten solder to readily reach the land through the opening, preventing the occurrence of poor soldering. The opening need not be worked into a conventional trapezoidal configuration in vertical section, thereby simplifying the step of forming the opening in the thermal insulation holder. The formation of one opening slightly larger than the minimum rectangle defined to surround the row or the rectangular array of lands in the heat insulating holder enables the soldering of the plurality of connector terminals by using the flow soldering or reflow soldering process. The thickness of the heat insulating holder is one-tenth to seven times the thickness of the flexible printed circuit board. The short distance between the surface of the molten solder and the land allows the molten solder to readily reach the land through the opening, preventing the occurrence of poor soldering. The heat insulating film is provided in contact with the solder-applied surface of the flexible printed circuit board, and the heat insulating holder is provided in contact with the surface of the heat insulating film opposite from the flexible printed circuit board. The provision of the heat insulating film between the flexible printed circuit board and the heat insulating holder prevents the deterioration of the thermal insulation effects in the case of the thin heat insulating holder to perform the soldering by using the flow soldering or reflow soldering process. Further, the molten solder may readily reach the land through the opening and through aperture, preventing the occurrence of poor soldering. The contact between the heat insulating holder and the heat insulating film and the contact between the heat insulating film and the flexible printed circuit board may be readily made by inserting the plurality of guide pins formed in the corners of the heat insulating holder into the through apertures for the respective guide pins formed in the flexible printed circuit board and heat insulating film in accordance with the fifth aspect of the present invention, or by forming the guide frame on the heat insulating holder for receiving the flexible printed circuit board and the heat insulating film therein. It is therefore an object of the present invention to prevent the occurrence of poor soldering without difficulty. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a first preferred embodiment according to the present invention; Fig. 2 is a bottom view of the first preferred embodiment; Fig. 3 is a cross-sectional view of a second preferred embodiment according to the present invention; Fig. 4 is a perspective view of the second preferred embodiment; Fig. 5 is a bottom view of the second preferred embodiment; Fig. 6 is a cross-sectional view of a third preferred embodiment according to the present invention; Fig. 7 is a cross-sectional view of a fourth preferred embodiment according to the present invention; Fig. 8 is an exploded perspective view of the fourth preferred embodiment; Fig. 9 is an exploded perspective view of a fifth preferred embodiment according to the present invention; Fig. 10 is an exploded perspective view of an sixth preferred embodiment according to the present invention; Fig. 11 is a cross-sectional view of the background art; and Fig. 12 illustrates the operation of the background art. DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Preferred Embodiment) Referring to Figs. 1 and 2, an opening 17 is formed in a 2.6-mm thick, flat heat insulating holder 11 including a phenolic paper base copper clad laminate board, a paper base epoxy resin board, or a glass fabric base epoxy resin board and to be brought into contact with a surface of an FPC 12 made of PET to which solder is applied. The opening 17 is greater in size than a land 13 formed on the FPC 12 and has a rectangular configuration in vertical section. Preferably, the size of the opening 17 is about 2 to 3.5 times the area of the land 13. The heat insulating holder 11 is brought into contact with the solder-applied surface of the FPC 12 so that the land 13 and a terminal 14 made of a conductive material are located within the opening 17. With the contact maintained, soldering is performed by using the flow soldering or reflow soldering technique to mount the terminal 14 to the FPC 12. The opening 17 in the heat insulating holder 11 is greater than the land 13 on the FPC 12 to permit molten solder to readily reach the land 13 through the opening 17 without the need to form the opening 17 into the conventional trapezoidal configuration in vertical section, insuring prevention of the occurrence of poor soldering. Additionally, there is no need to work the opening 17 into the conventional trapezoidal configuration in vertical section. This simplifies the step of forming the opening 17 in the thermal insulation holder 11. (Second Preferred Embodiment) Referring to Figs. 3 to 5, the present invention may be applied to the soldering of a plurality of terminals 14 mounted to a connector housing 18. Formed in the thermal insulation holder 11 is a rectangular opening 19 slightly greater than a minimum rectangle defined to surround rows of lands 13. In this manner, the formation of the opening 19 slightly greater than the minimum rectangle defined to surround the rows of lands 13 insures the soldering of the plurality of connector terminals 14 to the FPC 12 by using the flow soldering or reflow soldering technique. The need to form an opening for each connector terminal in the conventional manner is eliminated, but only one opening 19 is required for the plurality of connector terminals 14. This simplifies the step of forming the opening 19. (Third Preferred Embodiment) As shown in Fig. 6, the plurality of terminals 14 mounted to a connector housing 26 are soldered in a similar manner to the second preferred embodiment. A heat insulating holder 25 for use in the soldering has a thickness ranging from 30 µm to 2 mm which is about one-tenth to seven times the thickness of the FPC 12. Formed in the heat insulating holder 25 is a rectangular opening 27 slightly greater than a minimum rectangle defined to surround rows of lands 13. This allows molten solder 28 to readily reach the lands 13. Thus, like the second preferred embodiment, the third preferred embodiment insures soldering of the plurality of connector terminals 14 to the FPC 12 by using the flow soldering or reflow soldering technique. The present invention is similarly applicable to the soldering of leads of a device in place of the terminals. (Fourth Preferred Embodiment) With reference to Figs. 7 and 8, through apertures 31 having a size substantially equal to that of the lands 13 on the FPC 12 are formed in a heat insulating film 30 made of a high-heat-resistant material such as polyimide, polyethylene terephthalate, polyparabanic acid, aramid, polyamide, polysulfone, polyether sulfone, polyphenylene sulfide, polyether etherketone, polyether imide, polyarylate, polyethylene naphthalate, fluorocarbon polymer or the like. The heat insulating film 30 is provided in contact with the solder-applied surface of the FPC 12 so that the lands 13 and the terminals 14 are located within the through apertures 31. An opening 34 greater than the lands 13 are formed in a heat insulating holder 33 including a phenolic paper base copper clad laminate board, a paper base epoxy resin board, or a glass fabric base epoxy resin board. The heat insulating holder 33 is provided in contact with the opposite surface of the heat insulating film 30 from the FPC 12 so that the lands 13, the terminals 14 and the through apertures 31 are located within the opening 34. For contact between the heat insulating holder 33 and heat insulating film 30 and contact between the heat insulating film 30 and FPC 12, the heat insulating film 30 and the heat insulating holder 33 are bonded respectively to the FPC 12 and the heat insulating film 30 with a pressure-sensitive adhesive or an adhesive. The molten solder 28 readily reaches the lands 13 through the opening 34 and the through apertures 31 to prevent poor soldering because of the provision of the heat insulating film 30 in contact with the solder-applied surface of the FPC 12 so that the lands 13 and the terminals 14 are located within the through apertures 31 of the heat insulating film 30 and the provision of the heat insulating holder 33 in contact with the opposite surface of the heat insulating film 30 from the FPC 12 so that the lands 13 and the terminals 14 are located within the opening 34. The provision of the heat insulating film 30 between the FPC 12 and the heat insulating holder 33 allows soldering by the flow soldering or reflow soldering technique without deterioration of thermal insulation effects in the case of the thin heat insulating holder 33. Bonding the heat insulating film 30 and the heat insulating holder 33 respectively to the FPC 12 and the heat insulating film 30 with a pressure-sensitive adhesive or an adhesive facilitates the contact between the heat insulating holder 33 and the heat insulating film 30 and the contact between the heat insulating film 30 and the FPC 12. The high-heat-resistant material for the heat insulating film 30, such as polyimide, polyethylene terephthalate, polyparabanic acid, aramid, polyamide or the like, provides for the repeated uses thereof. (Fifth Preferred Embodiment) In the fifth preferred embodiment according to the present invention, as shown in Fig. 9, the heat insulating holder 33 is implanted with guide pins 36 in four corners thereof, respectively, and through apertures 37 for the guide pins 36 are formed in the FPC 12 and the heat insulating film 30, respectively. The contact between the heat insulating holder 33 and the heat insulating film 30 and the contact between the heat insulating film 30 and the FPC 12 may be made with the guide pins 36 received through the through apertures 37. The fifth preferred embodiment provides effects similar to those of the fourth preferred embodiment. (Sixth Preferred Embodiment) In the sixth preferred embodiment according to the present invention, as shown in Fig. 10, a guide frame 39 is formed on the heat insulating holder 33. The guide frame 39 receives the FPC 12 and the heat insulating film 30 therein for contact between the heat insulating holder 33 and the heat insulating film 30 and contact between the heat insulating film 30 and the FPC 12. While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.

Claims: A method of mounting a terminal (14) made of a conductive material to a flexible printed circuit board (12) made of a material having a softening point below 230°C by soldering the terminal (14) to a land (13) formed on the flexible printed circuit board (12), said method comprising the steps of: forming an opening (17) rectangular in vertical section and larger than said land (13) in a flat heat insulating holder (11), said heat insulating holder (11) being for contacting a surface of said flexible printed circuit board (12) to which solder (28) is applied; and bringing said heat insulating holder (11) into contact with said surface of said flexible printed circuit board (12) so that said land (13) and said terminal (14) are located within said opening (17), and soldering said terminal (14) by a flow soldering or reflow soldering process, with the contact maintained and the terminal (14) extending through said land (13) into said opening (17). A method of mounting a plurality of terminals (14) made of a conductive material to a flexible printed circuit board (12) made of a material having a softening point below 230°C by soldering the terminals (14) to a plurality of lands (13) formed on the flexible printed circuit board (12), respectively, said method comprising the steps of: forming a rectangular opening (19) being rectangular in vertical section and slightly larger than a minimum rectangle defined to surround a row or a rectangular array of said lands (13) in a heat insulating holder (11), said heat insulating holder (11) being for contacting a surface of said flexible printed circuit board (12) to which solder (28) is applied; and bringing said heat insulating holder (11) into contact with said surface of said flexible printed circuit board (12) so that said lands (13) and said terminals (14) are located within said opening (19), and soldering said terminals (14) by a flow soldering or reflow soldering process, with the contact maintained and the terminals (14) extending through said lands (13) into said opening (19). The method in accordance with claim 1 or 2, wherein the thickness of said heat insulating holder (11) is about one-tenth to seven times the thickness of said flexible printed circuit board (12). The method in accordance with claim 1, 2 or 3, wherein said flexible printed circuit board (12) is made of polyethylene terephthalate. The method in accordance with anyone of claims 1 to 4, wherein said heat insulating holder (11) includes a phenolic paper base copper clad laminate board, a paper base epoxy resin board, or a glass fabric base epoxy resin board. A method of mounting a terminal (14) made of a conductive material to a flexible printed circuit board (12) made of a material having a softening point below 230°C by soldering the terminal (14) to a land (13) formed on the flexible printed circuit board (12), said method comprising the steps of: forming a through aperture (31) having a size substantially equal to the size of said land (13) in a heat insulating film (30) provided in contact with a surface of said flexible printed circuit board (12) to which solder (28) is applied; forming an opening (34) which is larger than said land (13) in a heat insulating holder (33), said heat insulating holder (33) being for contacting a surface of said heat insulating film (30) which is opposite from said flexible printed circuit board (12); and soldering said terminal (14) by a flow soldering or reflow soldering process, with said heat insulating film (30) in contact with said surface of said flexible printed circuit board (12), so that said land (13) and said terminal (14) are located within said through aperture (31) and with said heat insulating holder (33) in contact with said surface of said heat insulating film (30) so that said land (13) and said terminal (14) are located within said opening (34). The method in accordance with claim 6, wherein said heat insulating holder (33) is provided with a plurality of guide pins (36) in corners thereof, and said flexible printed circuit board (12) and said heat insulating film (30) have through apertures (37) formed therein for said guide pins (36), and wherein said guide pins (36) are respectively inserted into said through apertures (37) to bring said heat insulating holder (33) into contact with said heat insulating film (30) and to bring said heat insulating film (30) into contact with said flexible printed circuit board (12). The method in accordance with claim 6, wherein said heat insulating holder (33) is formed with a guide frame (39) for receiving said flexible printed circuit board (12) and said heat insulating film (30) therein, and wherein said heat insulating film (30) comes into contact with said flexible printed circuit board (12), and said heat insulating holder (33) comes into contact with said heat insulating film (30). The method in accordance with any one of claims 6 to 8, wherein said heat insulating film (30) is made of polyimide, polyethylene terephthalate, polyparabanic acid, aramid, polyamide, polysulfone, polyether sulfone, polyphenylene sulfide, polyether etherketone, polyether imide, polyarylate, polyethylene naphthalate, or fluorocarbon polymer. The method in accordance with any one of claims 6 to 9, wherein said heat insulating holder (33) includes a phenolic paper base copper clad laminate board, a paper base epoxy resin board, or a glass fabric base epoxy resin board. The method in accordance with any one of claims 6 to 10, wherein said flexible printed circuit board (12) is made of polyethylene terephthalate. The method in accordance with any one of claims 6 to 11, wherein said heat insulating film (30) and said heat insulating holder (33) are bonded respectively to said flexible printed circuit board (12) and said heat insulating film (30) with a pressure-sensitive adhesive or an adhesive.

Inventors: NODA HARUO, NODA, HARUO, Noda, Haruo, c/o Sumimoto Wiring Sys, Ltd
Applicants: SUMITOMO WIRING SYSTEMS, SUMITOMO WIRING SYSTEMS, LTD.
Application year: 1995
Main classification: H05K3
IPC codes: H05K1, H05K3, B23K3