1. Field of the Invention
The present invention relates to a reflow furnace in which a printed circuit board with solder paste or solder cream put thereon and having electronic components put in position on the solder cream is heated and cooled to join the electronic components to the circuit board.
2. Related Background Art
In the reflow furnace, a printed circuit board having electronic components in position thereon is heated while it is transported by a conveyor to melt solder cream on the circuit board and join the electronic components to the circuit board. In general, the reflow furnaces can be classified into two major types: air reflow furnaces using air as atmosphere and nitrogen reflow furnaces supplied with gaseous nitrogen as an inert gas to increase the nitrogen content in its atmosphere. However, both types of reflow furnaces basically have a common configuration.
Generally, the reflow furnace has the inner space thereof divided into a plurality of chambers by a plurality of partition walls located at intervals in the direction of transporting a printed circuit board. These chambers are controllable in temperature and flow rate of atmosphere in the furnace independently from each other. Typically, the chambers are used as preheating chambers for preheating the circuit board, heating chambers for melting solder cream present on the circuit board, etc.
Japanese Laid-open Patent Publication JP 2002-134905 discloses a reflow furnace including a plurality of chambers separated from each other and in which in-furnace atmosphere is circulated and blown vertically onto a printed circuit board. For example, hot air is blown onto a printed circuit board in the preheating and heating chambers and, in case of a reflow furnace having a cooling chamber next to a heating chamber, unheated atmosphere is blown onto the printed circuit board in the cooling chamber to cool it.
In the conventional reflow furnaces, increased flow rate of atmosphere blown vertically onto a printed circuit board in each chamber often moves electronic components from their proper positions on the circuit board.
FIG. 20 graphically illustrates measured velocity of atmosphere in a conventional reflow furnace. In FIG. 20, the line A indicates the velocity of atmosphere blown vertically onto a printed circuit board (vertical blow), and the line B indicates the velocity of atmosphere laterally blown on electronic components to the circuit board (horizontal blow). In FIG. 20, reference numeral 100 indicates an inlet wall having formed the inlet of the reflow furnace, 101 indicates partition walls between adjacent chambers, and 102 indicates an outlet wall having formed the outlet of the reflow furnace.
To obtain the data shown in FIG. 20, vertical walls 105 were placed to partition the portion of the circuit board 4a from its front and back in its transport (or conveying) direction as shown in FIG. 21A, and an anemometer S was placed on the circuit board 4a to detect the velocity of the vertical blow. To detect the horizontal blow, a cover 106 in the shape of a rectangular bracket was placed at a height of 3 mm from the circuit board 4a to orient its lateral projections across the transport direction of the circuit board 4a, and the anemometer S was placed on the circuit board 4a under the cover 106, as shown in FIG. 21B. The anemometer S used in the experiments had the following specification, and the reflow furnace was driven at 60 Hz.                (1) Manufacturer of the anemometer S Japan CANOMAX Co., Ltd.        (2) Type of the anemometer S Linear output type ANEMOMASTER Model 6141        (3) Sensitive portion of the Probe Platinum wire-wound resistor        (4) Response speed Slow        
As will be seen from the measured data shown in FIG. 20, the velocity of the horizontal blow varied largely near the inlet wall 100, partition walls 101 and outlet wall 102, and the variation in velocity of the horizontal blow was about 2.0 m/sec.