Coating apparatus – With heat exchange – drying – or non-coating gas or vapor... – With housing surrounding or engaging coating means
Patent
1985-06-28
1987-01-13
Lawrence, Evan K.
Coating apparatus
With heat exchange, drying, or non-coating gas or vapor...
With housing surrounding or engaging coating means
118429, 228 20, B05C 1100, B05C 310
Patent
active
046355847
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an apparatus for tinning printed-circuit boards by immersion in a heatable vessel filled with molten solder, with an additional heatable vessel for a thermofor oil capable of being heated to a high temperature in which a squirting means in the form of two jet-nozzle tubes spaced apart from one another and equipped with opposed nozzles are provided, from which thermofor oil aspirated by a pump from the vessel can be squirted against the opposite flat sides of printed-circuit boards previously dipped into the solder vessel.
The selective tinning of printed circuit boards is performed by dipping the circuit boards, masked in accordance with the tin coating to be applied, into a bath of molten lead-tin solder after previously dipping them in a flux and preheating them. The circuit board removed again from the molten solder must be freed of adhering excess solder especially in solder trapped in the holes in the circuit boards, and the finally tinned areas of the circuit board are to bear a thin, uniform tinning without any unsightly irregularities along edges, at eyelets and on larger surface areas. To accomplish this, a variety of methods have been developed for treating the tinned circuit boards after their removal from the solder bath, e.g., by blowing heated air against them and, recently, immersing them into a bath of water-soluble thermofor oil heated at about 235.degree. C., while moving them within the bath between slotted nozzle tubes from which thermofor oil aspirated by a pump from the tank is squirted against the circuit board surfaces, thus flushing away excess solder, even out of the holes in the circuit board. The flushed-away solder gathers, due to its greater specific weight, on the bottom of the tank, whence it is pumped back by means of a special pump into the container holding the molten solder. Although this method achieves technically faultless results, the surface of the tinning can show textural signs of the lead-tin flux and may have unsightly irregularities at the edges and eyelets and also on larger surface areas, which may be attributable to the fact, among others, that, when the circuit boards are removed from the molten solder, a thin oxide skin can form on the tinning as a result of the possible access of atmospheric oxygen before the board is passed between the slotted-nozzle tubes.
It is consequently the object of the invention to create an apparatus for tinning printed-circuit boards, in which the after-treatment of the boards dipped into the molten solder is performed also by squirting heated thermofor oil against them, but in which the tinning finally obtained has a perfect appearance. Moreover, the apparatus is to have a simpler construction than the known apparatus.
Setting out from an apparatus of the kind mentioned in the beginning, this object is achieved in accordance with the invention in that the tank containing the thermofor oil and the solder tank are portions of a common treatment tank having a continuous interior in which the part filled with the specifically lighter, heated thermofor oil is situated above the tank portion filled with the specifically heavier molten solder, so that the circuit boards to be tinned pass, before and after they are dipped into the solder, through the thermofor overlying the solder. The circuit boards, therefore, after their removal from the solder bath, enter directly into the thermofor oil bath and thus pass between the jet-nozzle tubes, so that no intermediate cooling takes place and no atmospheric oxygen can reach them, resulting in the obtaining of a perfectly bright and uniform appearance of the tinned surfaces. The excess solder squirted away by the jet-nozzle tubes is returned due to its greater specific weight immediately into the molten solder situated in the bottom portion of the tank, so that the use of a special pump for returning the solder to the solder tank is unnecessary.
In order to keep the amount of the solder as low as possible, yet sufficiently deep to permit the complete immersion of the
REFERENCES:
patent: 20673 (1938-03-01), Meyers
patent: 3865298 (1975-02-01), Allen et al.
patent: 4315042 (1982-02-01), Spigarelli
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