The present invention relates to a method for wave soldering printed circuit boards wherein a solder coating is applied only where needed.
A usual automatic wave soldering apparatus includes a pair of endless chain conveyors driven to advance a printed circuit board at a constant speed from the entrance to exit ends of the apparatus. With the printed circuit board held by gripping fingers, the board is first carried to a fluxer where a foam or spray of flux is applied to the underside of the board. The printed circuit board is then carried over preheaters where the temperature of each board is elevated to approximately 110° C. to 130° C. so as to evaporate excess flux solvent, activate the flux and minimize thermal shock to the printed circuit board. After the printed circuit board is brought to such a preheat temperature, the board is passed over a solder reservoir to receive solder. The board is finally transported to a cool down zone where the solder is cooled to solidify.
Typically, pin grid alley modules and dual in-line packages are loaded onto one side of a printed circuit board, with their terminals or leads projecting downwardly through apertures in the printed circuit board. To increase packaging density, a number of surface mounted devices and connectors are loaded onto the other, underside side of the printed circuit board. Problems have arisen from wave soldering such a printed circuit board. Too much heat occurs on the underside of the board, when contacted with a solder wave, and tends to damage the surface mounted devices.
Attempts have been made to locally apply solder to preselected conductor areas on a printed circuit board, but not to those areas where surface mounted devices and connectors are mounted. In one known method, flux is locally spayed onto preselected conductor areas on a printed circuit board. The fluxed board is then preheated by a stream of warm gas to evaporate flux solvent. Thereafter, the board is positioned over a plurality of solder wave nozzles arranged within a solder reservoir. At this time, the solder wave nozzles are brought into alignment with a plurality of sets of preselected areas on the printed circuit board. A pump is arranged within the solder reservoir to force heated molten solder to flow upwardly through the solder wave nozzles so as to form solder waves. The preselected areas on the printed circuit board are contacted with the respective solder waves to make soldered joints. The height of the solder waves is then lowered until it becomes equal to the surface level of the molten solder within the solder reservoir. The position of the board is maintained until the solder solidifies. Finally, the board is delivered to a cool down zone wherein the printed circuit board is cooled.
This known method has proven to be effective for eliminating heat damage to sensitive electronic components such as surface mounted devices, but the method suffers from certain disadvantages. One problem is the attachment of solder oxides, better known as dross, and carbonized flux to soldered joints on a printed circuit board. Part of the molten solder tends to remain on the inner wall of the solder wave nozzles when the molten solder is immediately returned to the solder reservoir. The solder, typically composed of tin and lead, has a tendency to oxidize in the atmosphere. The resulting oxides are detrimental to the quality of the soldered joints. Also, part of the flux which has previously been applied to the soldered joints could be attached to the inner wall of the solder wave nozzles. This flux is carbonized by the heat from the molten solder. The resulting carbide could be attached to the inner wall of the solder wave nozzles. The oxides and the carbide will be separated from the inner wall of the solder wave nozzles when the molten solder within the solder reservoir is pumped up through the solder wave nozzles to process a next printed circuit board. The oxides and the carbide could be attached to soldered joints formed on the next printed circuit board. Another problem is the formation of icicles, solder bridges and other imperfections in the solder. When the conductor areas on the board are not sufficiently preheated, the solder solidifies before the conductor areas are completely wetted. This results in faulty solder connections. Also, such imperfections occurs when the solder wave is detached from the printed circuit board too fast.
Accordingly, it is an object of the present invention to provide a method for locally applying solder to preselected conductor or solderable areas on a printed circuit board, which prevents the occurrence of icicles, bridges and other solder imperfections, and which can force solder into through holes and other hard-to-reach areas to be soldered.