The present invention relates to a vapor reflow type soldering apparatus wherein an article to be processed on which paste of soldering material is printed is subjected to vapor of a thermal medium and heated to melt the soldering material while being delivered by a conveyor and, more particularly, to an improved vapor reflow type soldering apparatus for soldering works of printed circuit boards carrying electronic parts thereon.
A technique of the aforementioned type as disclosed in, for example, Japanese Patent Unexamined Publication Nos. 60-106502 and 63-90362.
These conventional vapor reflow type soldering apparatus involve the following problems:
a) If a printed board carrying electronic parts thereon is to be processed, the temperature of the board exposed to saturated vapor of a thermal medium will be drastically increased because the condensation heat transfer rate of the thermal medium is extremely high. However, the temperatures of members of small heat capacities such as, for example, electrodes of small-sized chips and IC leads, will be raised even more drastically in comparison with the temperature of the printed wiring board. As a result, problems arise with respect to soldering operation such as tombstone effect of chips and wicking effect by molten soldering material, thereby degrading the reliability of the soldering works.
b) In order to prevent the above-described troubles in relation to soldering works, it is known that the printed wiring board of a relatively large heat capacity is heated from the lower surface thereof so that the temperatures of the chip electrodes and IC leads can later be raised through heat transfer. In this case, however, a problem arises in that the configuration and size of a lower vapor supply hole becomes very large. Besides, if the above-mentioned board is packaged on both sides, parts carried on the lower surface thereof will be exposed to the thermal medium vapor for a long period of time, thus resulting in a problem concerning heat resistance of the parts.
Other improvements of a soldering apparatus of this type have been proposed in the U.S. patent application Ser. Nos. 425,025 (on Oct. 23, 1989) and 426,434 (on Oct. 25, 1989).