1. Field of the Invention
This invention relates to a wave soldering method for soldering on a reflow side of a work-piece by jetting molten solder onto the reflow side, and a system used for the wave soldering method.
2. Description of the Related Art
Conventionally, a printed circuit board is soldered by a wave soldering method to improve processing efficiency. A wave soldering system 9 coventionally used for the wave soldering method is shown in FIG. 11. The wave soldering system 9 includes two solder baths 93, 94. A flux coating device 91 for coating flux onto a reflow side of a printed circuit board 8 and a pre-heater 92 for heating the printed circuit boad 8 are disposed on an upstream side of the solder bath 93. A transfer unit 93 is disposed above these parts to sequentially transfer the printed circuit board 8.
When a soldering process is carried out by the wave soldering system 9, the printed circuit board 8 is sequentially transferred by the transfer unit 95 to pass through above the flux coating device 91, the pre-heater 92, and the solder baths 93, 94. Accordingly, the back face (reflow side) of the printed circuit board 8 is soldered. Specifically, in the solder baths 93, 94, as shown in FIG. 12, molten solder 7 is jetted upward from a wave nozzle 930 which is disposed within the solder bath 93, using a solder circulating unit 97. The printed circuit board 8 is transferred with the reflow side 81 contacting the molten solder 7.
In the conventional method and system described above, however, the printed circuit board 8 can be warped by heat transmitted from the molten solder 7. When the printed circuit board 8 is thermally warped, the following deficiencies arise. First, as shown in FIG. 13, when the printed circuit board 8 is largely warped by the heat, the position of the printed circuit board 8 where the molten solder 7 is jetted thereon may be go down. In this case, an immersion depth of the printed circuit board 8 into the molten solder 7 is unnecessarily increased. Accordingly, bridges, solder short-circuited parts, and the like are formed on the printed circuit board 8, and as shown in FIG. 14, solder climbing deficiency such that the molten solder 7 reaches the surface of the printed circuit board 8 on an opposite side of the reflow side from its side portion arises. Further, when the printed circuit board 8 is warped so that a circumference portion thereof escapes upward as shown in FIG. 15, the circumference portion may not contact the molten solder 7, thereby causing a non-soldered portion of the printed circuit board 8.
Various types of wave soldering methods and systems have been proposed to stably perform wave soldering. For example, JP-A-7-131143 discloses a method for stabilizing a soldered sate by accurately detecting a height of jetted molten solder. This kind of method for controlling a molten solder level is also disclosed in JP-A-2-37964.
However, the deficiencies caused by thermal deformation of the printed circuit board itself cannot be solved by these conventional methods. Therefore, even when the height of the jetted molten solder is precisely controlled, it is difficult to prevent the solder climbing deficiency and the occurrence of non-soldered portion.