Reflow method and reflow device

Abstract: For reflow soldering, radiant heating is applied to one surface of a printed circuit board on which electronic components are placed and onto which cream solder is supplied and at the same time hot air is blown locally and roughly perpendicular to to-be-connected points on said one surface of the printed circuit board. This reflow method permits secure soldering even if the hot air is set at a temperature not exceeding the heat resistance of the electronic components, which is possilbe because of its combination with the radiant heat. Moreover, this reflow method can permit soldering in such a manner that only the to-be-connected points are heated selectively, because the hot air is blown locally and roughly perpendicular to the points to be connected. Thus, this reflow method prevents heat damage to other sections than the to-be-connected points and ensures that the solder at the to-be-connected points is melted.

Description: TECHNICAL FIELD The present invention relates to a reflow device wherein, by heating a printed circuit board on which electronic components are loaded and to which solder cream has been applied at locations to be connected, solder is melted and electronic components soldered. A device according to the preamble of claim 1 is known from US-A-5 128 506. BACKGROUND ART A variety of methods for reflow soldering technology to mount electronic components on printed circuit boards are already known. These methods include, for example, batch reflow soldering of surface mount components, local soldering of insert mount components after batch reflow soldering of surface mount components, and local reflow soldering of surface mount components and insert mount components. A conventional reflow device for batch reflow soldering of surface mount components is constituted so as to melt solder and to solder electronic components by uniformly heating a printed circuit board with the radiant heat of a panel heater and hot air heated to a prescribed temperature. Also, the reflow device according to the report of Japanese Patent Publication No. 7-73790 is an example of a reflow device for local heating and reflow of surface mount components and insert mount components. As shown in Figure 11, this reflow device (50) is equipped with the following: hot air chamber (53) installed on the lower side of the transport path (52) for the printed circuit board (51) which collects gas heated to a temperature at which solder cream melts, an upper plate (57) installed to cover the opening section (54) of the hot air chamber (53) and in which is formed a plurality of hot air blow holes (56) to blow the hot air (55) selectively on only the necessary sections of the printed circuit board (51), and means for transport (58) to supply and remove the printed circuit board (51). Because heating is local, this reflow device (50) makes possible the following processes: sectional soldering, soldering of components with low heat resistance, and concurrent soldering of lead components (59) and chip components (60) wherein the lead terminals protrude from the lower surface of the board. However, batch reflow soldering is not possible when using the aforementioned, conventional type of reflow device if electronic components with low heat resistance are present together on a printed circuit board. Those components with low heat resistance among the electronic components will be damaged if that printed circuit board undergoes batch reflow soldering with the reflow device; therefore, the components with low heat resistance among the electronic components must be soldered separately, resulting in the problem of decreased productivity of the mounting process. Meanwhile, the reflow device (50) shown in Figure 11 makes possible batch soldering for a printed circuit board (51) on which electronic components with low heat resistance are present together by locally blowing hot air without blowing the hot air to the locations of the electronic components with low heat resistance. Actually, batch soldering of the electronic components (lead component (59) and chip component (60), etc.) placed on the entire surface of the printed circuit board (51) is not possible for the following reason. In effect, reflow soldering electronic components such as lead component (59) and chip component (60), etc., with the reflow device (50) requires heating to temperatures in the vicinity of 230°C for 20 - 30 seconds. Hot air of a temperature of roughly 350°C or greater is necessary to ensure the quantity of heat necessary for that purpose. In hot air of such high temperatures, areas of the printed circuit board (51) in which the temperature rises easily reach temperatures approaching that of the hot air. Even heat resistant components (for example, the temperature of heat resistance for QFP is 250°C (QFP is heat resistant to temperatures of 250°C) cannot withstand that temperature. When the hot air temperature is reduced to 250°C, the temperature of heat resistance, in order to avoid [damage], the risk is that areas of the printed circuit board (51) in which the temperature rises with difficulty are not heated to the temperatures necessary for soldering within a prescribed period of time and high reliability soldering may not result. DISCLOSURE OF THE INVENTION The object of the present invention is to provide a reflow device which resolve the problems and make possible batch soldering with high reliability and without resulting in heat damage to electronic components on the printed circuit board. This object is solved by the device of claim 1. Preferred embodiments are disclosed in the dependent claims. In order to solve the problems, the reflow method of the present invention is a reflow soldering method in which hot air is blown locally and roughly perpendicular to locations to be connected on one surface of a printed circuit board, concurrent with radiant heating of the surface of a printed circuit board, on which electronic components are loaded and to which solder cream has been applied at locations to be connected. This reflow method permits secure soldering in such a manner that the temperature of the hot air does not exceed the temperature of heat resistance of the electronic components because of its combination with the radiant heat. Moreover, this reflow method can permit soldering in such a manner that only those points to be connected are heated selectively because the hot air is blown locally and roughly perpendicular to the points to be connected. This reflow method prevents heat damage to sections other than the points to be connected and ensures that the solder at the points to be connected is melted. With the reflow device of the invention, radiant heat from means for radiant heating is definitely radiated through the hot air header to the printed circuit board, while hot air is definitely blown from the mask plate locally and roughly perpendicular to the points to be connected. Also, the present invention can be constituted in such a manner that, in the aforementioned reflow device, a dispersion plate in which a plurality of through holes are dispersed is placed with space between [itself and] the mask plate inside the hot air header and hot air is supplied from means for supplying hot air into the space between means for radiant heating and the dispersion plate. With this reflow device, a flow of hot air inside the hot air header is homogenized with the dispersion plate, ensuring that the hot air is blown uniformly and appropriately against and roughly perpendicular to the points to be connected. Also, in the aforementioned reflow device, a black body paint coating may be present on the mask plate; therefore, the present invention can improve the quantity of radiant heat with this reflow device. Also, means for cooling to blow cool air to prescribed locations of the other surface of the printed circuit board, one surface of which is being heated with means for radiant heating and means for hot air heating, can be installed in the aforementioned reflow device; therefore, the present invention can contribute to the operation by blowing the cool air as mentioned above. Also, the aforementioned reflow device can be equipped with means for transporting, to transport the printed circuit board in the heating section, and lifting means for lifting and lowering boards, to lift the printed circuit board in the heating section to a prescribed heating position; therefore, the present invention can provide heating quickly and with good precision by lifting the printed circuit board to a heating position with this reflow device. Also, the aforementioned reflow device may have means for cooling installed in lifting means for lifting and lowering boards; therefore, the present invention can appropriately cool prescribed locations on the other surface of a printed circuit board with this reflow device. Also, the aforementioned reflow device may have an isolation plate to isolate the cooling atmosphere and the heating atmosphere surrounding the periphery of the printed circuit board, which is lifted to a prescribed heating position by lifting means for lifting and lowering boards; therefore, the present invention can heat and cool efficiently and with good precision and without intermixing of the cool air and hot air. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross section to show the basic constitution of the entire reflow device based on the embodiment of the present invention; Figure 2 is a cross section to show a printed circuit board in a state with surface mount components and insert mount components present together; Figure 3 is a cross section to show a preheating section of the reflow device shown in Figure 1; Figure 4 is a cross section to show the principal elements of a main heating section of the reflow device shown in Figure 1; Figure 5 is a cross section to show a basic constitution of a main heating section of the reflow device shown in Figure 1; Figure 6 (a) is a cross section to show the principal elements of a main heating section of the reflow device shown in Figure 1; Figure 6 (b) is a plane diagram to show the principal elements of a main heating element of the reflow device as seen from Line A-A in Figure 6 (a); Figure 7 (a), (c), and (b), (d) are respectively oblique views and cross sections of the arrangement of holes for local cooling of the reflow device shown in Figure 1; Figure 8 is a figure to show the structure of the vicinity of a cooling device in the reflow device shown in Figure 1; Figure 9 (a) and (b) are respectively cross sections of the board pushing sections of the reflow device shown in Figure 1; Figure 10 (a) is a side view of a printed circuit board on which electronic components are place to show the point at which temperature is measures; Figure 10 (b) is a figure to show the temperature characteristics of each point at which temperature is measured for the electronic components with respect to heating time; and Figure 11 is a cross section of the principal elements of a conventional reflow device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Below the embodiment of the present invention is explained using Figures 1 through 10. In Figure 1, (1) shows a printed circuit board which is preheated in a preheating section (4) and then supplied to a main heating section (5), where it undergoes reflow soldering; afterwards the printed circuit board (1) is transported out with the exit conveyor (6). As shown in Figure 2, surface mount components (2), such as QFP (2A) and chip component (2B), and insert mount components (3), such as aluminum electrolytic capacitor (3A) and switch component (3B), are placed together on the printed circuit board (1). This printed circuit board (1) is transported in the reflow device in a position such that the body sections of the insert mount components (3) are located on the lower side of the printed circuit board (1). As means for transporting, conveyors (7) and (8) pass through the preheating section (4) and the main heating section (5) respectively. The conveyor (7) transports the printed circuit board (1) at a prescribed speed through the preheating section (4). The conveyor (8) transports the printed circuit board (1) at a quick speed to and from a prescribed position in the main heating section (5). Means for pushing boards (9), to push the printed circuit board (1) which has reached the terminus of the conveyor (7) onto the conveyor (8), is placed between the conveyors (7) and (8). A board stopper (10), as a board stopping mechanism to stop the printed circuit board (1) at a prescribed position, is placed near the board transport surface of the main heating section (5). Placed below the board transport surface of the main heating section (5) are a cooling device (12), which supports the printed circuit board (1) while blowing cool air onto and cooling prescribed locations thereon; a ventilation fan (13) to ventilate this cooling device (12); and a raising and lowering cylinder (11), as means to lift and lower boards, for lifting and lowering the cooling device (12) and carrying the printed circuit board (1) to a prescribed heating position. Cooling fans (14) and (15), to move air above and below and cool the printed circuit board (1), are located at the terminus of the conveyor (8) on the downstream side of the main heating section (5). Also, exhaust ducts (16) are located between and at either end of the preheating section (4) and the main heating section (5). As shown in Figure 3, the preheating section (4) is constituted of a hot air heater (17), to produce hot air; a hot air recirculation fan (18), to send the hot air from one side and along the upper surface of the printed circuit board (1); a catalyst (19) located on the hot air recirculation path and which burns off fumes of the flux and solvents produced from the solder cream, for example, during the heating of the printed circuit board (1); a panel heater (20), located in the upper section of the board transport path of the conveyor (7), for radiant heating of the upper surface of the printed circuit board (1); and an atmosphere exhaust duct (21), located in the lower section of the transport path. As shown in Figure 4, the main heating section (5) has a panel heater (22), as means of radiant heating for the surface of the printed circuit board (1), and means for hot air heating (23), for blowing hot air locally and roughly perpendicular to the points to be connected on the surface of this printed circuit board (1). Means for hot air heating (23) is equipped with hot air header (25), forming a flat space (24) and touching the lower surface of the panel heater (22); a mask plate (26), placed on a surface of this hot air header (25) opposite the printed circuit board (1), wherein blow holes (27), corresponding to the points to be connected are formed, and on which a black body paint coating is made; and a hot air supply duct (28), to supply hot air within the hot air header from one side thereof. Within the hot air header (25), a dispersion plate (30), on which a plurality of through air holes (29) is dispersed, is placed with space between itself and the mask plate (26). Hot air is supplied from the hot air supply duct (28) to between the dispersion plate (30) and the panel heater (22). Hot air sources such as the hot air heater (35) and the hot air ventilation fan (36) pass through the hot air supply duct (28) as shown in Figure 5. Meanwhile, as shown in Figure 4, a hot air nozzle tube (31), to blow hot air uniformly from its entire length, is placed inside the hot air supply duct (28). Moreover, in the example shown in Figure 4, the hot air supply duct (28) is placed on one side of the hot air header (25), but the present invention is not limited to that. For example, as shown in Figure 6, the constitution may also be such that a hot air supply duct (33) is placed so as to surround the periphery of the hot air header (25) and a rectangular, annular hot air nozzle tube (34) is placed therein to send hot air from the hot air source to the right place. Such a structure can form a more uniform hot air and permit hot air heating with good precision. A section with small holes (37) for local cooling, in which small holes to blow cool air are installed, and a board support element (38), to support the printed circuit board (1), are placed in the cooling device (12). As shown in Figure 7 (a) and (b), the section with small holes (37) may be constituted with a flat plate (40) in which holes (39) are made, or may have a structure with nozzles (41), conforming to the height of the printed circuit board (1), mounted on a plate (42) for blowing cool air from holes (39) in the nozzles (41), as shown in Figure 7 (c) and (d). Here, the distance from the nozzles (41) to the cooled sections of the printed circuit board (1) may be adjusted. Also, blow holes may be made in a metal or resin block and the cool air blowing distance, corresponding to the height of the insert mount components (3), may also be adjusted by the carving of the block. As shown in Figure 8, a fan (45), to ventilate the lower surface of the board with some of the air stream by the wind direction plate (44), may also be installed in the cooling section where the cooling device (12) is installed. As shown by the solid line in Figure 4, an isolation plate (32) is installed at a position where it surrounds the periphery of that printed circuit board (1) when the raising and lowering cylinder (11) has raised the printed circuit board (1) to a prescribed heating position. This isolation plate (32) prevents the intermixing of the heating atmosphere for the upper section of the printed circuit board (1) and the cooling atmosphere from the cool air, as shown by the broken arrow (C) from means for cooling (12) in the lower section. As shown in Figure 9, means for pushing boards (9) is constituted of a chain (46), a driving motor (47), and a lever (48) affixed to the chain (46). The operation of the reflow device, constituted as noted above, is explained. The conveyor (7) transports the printed circuit board (1), on which surface mount components (2) and insert mount components (3) are present together, at a certain speed into the preheating section (4). Inside the preheating section (4), the entire upper surface of the printed circuit board (1) is heated to a temperature in the vicinity of 150°C with the radiant heat from the panel heater (20) and the side flow hot air stream. The load on the hot air heater (17) is reduced because the hot air of a certain temperature is recirculated, while regularizing the temperature of the atmosphere within the preheating section (4). The temperature of the body sections of the insert mount components (3), which are transported in a position on the lower surface of the printed circuit board (1), is increased by the conduction of heat from the lead sections on the upper surface of the printed circuit board (1). With the constitution in which the atmosphere exhaust duct (21) continually takes in the atmosphere of the lower surface of the printed circuit board (1), room temperature atmosphere is guided to the lower surface of the printed circuit board (1), draws heat from the body sections of the insert mounted components (3), and suppresses temperature increases of the body sections. Having completed preheating, the printed circuit board (1) reaches the terminus of the conveyor (7) where means for pushing boards (9) pushes the rear end of the printed circuit board (1) and smoothly transfers the printed circuit board (1) to the conveyor (8). Means for pushing boards (9) is established in this way because it is necessary to transport the printed circuit board rapidly; the conveyor (7) is moved at a speed of 0.8 m/min, for example, because the printed circuit board (1) is heated while it is transported continuously through the preheating section (4), through which the conveyor (7) runs, but the printed circuit board (1) is stopped at a prescribed position with the board stopper (10), raised to a prescribed heating position by the raising and lowering cylinder (11), and is heated in the main heating section (5), in which a separate conveyor (8) is operating. The printed circuit board (1), transported into the main heating section (5), is stopped at a prescribed position with the board stopper (10) and the conveyor (8) is also stopped. Then means for cooling (12) is raised by the raising and lowering cylinder (11) and the printed circuit board (1), which is supported on the upper end thereof, is raised to a prescribed heating position surrounded by the isolation plate (32). In this state, the upper surface of the printed circuit board (1) is heated through the heat header (25) with the radiant heat from the panel heater (22). At the same time, hot air is blown from the blow holes (27) of the mask plate (26), in means for hot air heating (23), locally and roughly perpendicular to the points to be connected of the surface mount components (2) and the lead sections, etc., of the insert mount components (3). Consequently, the points to be connected only can be heated quickly and with good precision to the prescribed temperature with hot air of a temperature which does not exceed the temperature of heat resistance of the surface mount components. The solder cream of the points to be connected melts and reflow soldering is performed. Also, the insert mounted components (3), positioned on the lower surface of the printed circuit board (1), are cooled with the section with small holes (37) for local cooling, in which holes (39) to blow cool air are installed corresponding to the position of the insert mounted components (3). With this structure, as shown in Figure 10 (a) and (b), the temperature T X of the point to be connected X in the lead of the QFP (2A), as one example of a surface mount component (2), and the temperature T Y of the point to be connected Y in the lead of the aluminum electrolytic capacitor (3A), as one example of an insert mount component (3), are 200°C or greater. The solder cream of the points to be connected melts satisfactorily and reflow soldering is performed with certainty. Meanwhile, the temperature T Z of the body section Z of the aluminum electrolytic capacitor (3A) becomes 160°C or less and heat damage to the aluminum electrolytic capacitor (3A) can be prevented with certainty. In this way, since the radiant heating and hot air heating act together concurrently, soldering can be performed with certainty even though the temperature of the hot air does not exceed the temperature of heat resistance of the electronic components. Meanwhile, hot air is blown locally and roughly perpendicular to the points to be connected and only those points to be connected are selectively heated. While heat damage to sections other than points to be connected is prevented, the solder of the points to be connected can be definitely melted again and result in soldering; soldering with high reliability is ensured. Also, damage to components with low heat resistance, such as the body sections of insert mounted components (3), among the electronic components can be prevented and batch reflow soldering of electronic components on the printed circuit board (1) can be performed. Also, since the mask plate (26) is installed between the panel heater (22) and the printed circuit board (1), the supply of radiant thermal energy to surface mount components (2), which are not subject to cooling, can be isolated and the temperature increase can be controlled. The isolation plate (32) can also prevent the intermixing of the cooling atmosphere and the heating atmosphere of the main heating section (5). When the prescribed heating operation is complete, the printed circuit board (1) is lowered to the position of the conveyor (8) once more and the level of the cooling device (12) is dropped below that. In effect, the cooling device (12) is lowered and the heating of the cooling device (12) itself by the radiant heat is suppressed by its separation of the panel heater (22). When the cooling device (12) begins to drop, the cooling of the insert mount components (3) is insufficient. To compensate for this, the lower surface of the printed circuit board (1) is ventilated with some air from the fan (45) by the wind direction plate (44). Moreover, when the printed circuit board (1) is transported from the main heating section (5), the wind direction plate (44) is set horizontally and the cooling device (12) is cooled with wind from the fan (45). Also, the printed circuit board (1) is transported from the main heating section (5) with the conveyor (8) and is cooled with the cooling fans (14) and (15). Then the printed circuit board (1) transfers to the exit conveyor section (6) from the conveyor (8) and is transported to the exit of the reflow device.

Claims: A reflow device for reflow soldering of electronic components (2,3) onto a printed circuit board (1), comprising: radiant heating means (22) for radiant heating one surface of a printed circuit board (1) on which electronic components (2,3) are placed and onto which solder cream is applied at to-be-connected points; and hot air heating means (23) for blowing hot air locally and roughly perpendicular to the to-be-connected points on said one surface of the printed circuit board (1); said radiant heating means (22) and hot air heating means (23) being juxtaposed in a heating section (5) of the reflow device, characterized in that said hot air heating means (23) comprises: a hot air heater (25) forming a flat space (24) contacting a surface of a panel heater of the radiant heating means (22) on the printed circuit board side, and including a mask plate (26) having blow holes (27) formed on the surface opposed to the printed circuit board (1) in correspondence to the to-be-connected points; and hot air supplying means (28) for supplying hot air into the hot air header (25) from the sides thereof. The reflow device according to claim 1, wherein the hot air header (25) includes therein a dispersion plate (30) which has a number of dispersedly formed through holes (29) and is spaced apart from the mask plate (26), whereby hot air is supplied from the hot air supplying means (28) into the space between the radiant heating means (22) and said dispersion plate (30). The reflow device according to claim 1, wherein the mask plate (26) is coated with a black body paint. The reflow device according to claim 1, further comprising cooling means (12) adapted for blowing cool air to prescribed locations on the other surface of a printed circuit board (1) than said surface which is heated by the radiant heating means (22) and the to air heating means (23). The reflow device according to claim 1, further comprising transport means (7,8) for transporting a printed circuit board (1) to the heating section (5), and lifting means (11) for lifting the printed circuit board (1) to a prescribed heating position in the heating section (5). The reflow device according to claim 5, wherein the lifting means (11) is provided with a cooling device (12). The reflow device according to claim 5, further comprising an isolation plate (32) for isolating cooling atmosphere and heating atmosphere by surrounding the periphery of a printed circuit board (1) when the printed circuit board (1) is lifted up to a prescribed heating position by the lifting means (11).

Inventors: ISHIMOTO KAZUMI, KITAGAWA TATSUAKI, MATSUSHIMA OSAMU, NAGAI KOICHI, NEMOTO SEIZO, TANIGUCHI MASAHIRO, UJI KAZUHIRO, YAMAZAKI OSAMU, ISHIMOTO, KAZUMI, KITAGAWA, TATSUAKI, MATSUSHIMA, OSAMU, NAGAI, KOICHI, NEMOTO, SEIZO, TANIGUCHI, MASAHIRO, UJI, KAZUHIRO, YAMAZAKI, OSAMU, Ishimoto, Kazumi, c/o Matsushita Electr.Ind.Co., Kitagawa, Tatsuaki, c/o Matsushita Electr.Ind.Co., Matsushima, Osamu, c/o Matsushita Electr.Ind.Co., Nemoto, Seizo, c/o Matsushita Electr.Ind.Co., Taniguchi, Masahiro, c/o Matsushita Electr.Ind.Co., Uji, Kazuhiro, c/o Matsushita Electr.Ind.Co., Yamazaki, Osamu, c/o Matsushita Electr.Ind.Co.
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1997
Main classification: B23K1
IPC codes: H05K3