Mass soldering system

Abstract

A soldering system is described in which a fluid stream is directed onto a soldered board substantially immediately following deposition of molten solder onto the board. Preferably, but not necessarily, the fluid, which may comprise a gas or mixture of gases, is heated prior to contacting the board. The impinging fluid stream relocates solder on, and/or blasts excess solder from the bottom of the board, and any interconnections, component leads and/or component bodies carried thereon before the solder solidifies as shorts, icicles or bridges. If desired, liquid droplets such as soldering oil may be included in the fluid stream.

What is claimed is: 
     
       1. A method of mass joining with solder printed circuit boards having mounted thereon components with leads protruding downward through holes in said board, said method comprising the steps of: depositing a quantity of molten solder onto the underside of said board and said downward protruding leads by passing the bottom surface of the board over a solder wave and substantially immediately following said depositing impinging a stream of heated air directly onto the molten solder deposited on the underside of said board and said protruding leads by means of an air knife blast directed towards the solder wave and impinging on the bottom surface of the board sufficiently soon after it leaves the solder wave so that the solder is still molten, .Iadd.the air stream being heated to a temperature above that of the molten solder, .Iaddend.the air knife blast having sufficient velocity to remove essentially all bridges and shorts but not sufficient velocity to adversely affect properly wetted joints.   
     
     
       2. A method according to claim 1, wherein said air stream is heated to a temperature in the range of .[.93°.]. .Iadd.290° .Iaddend.C. to 350° C. 
     
     
       3. A method according to claim 2, wherein said air stream is heated to a temperature in the range of about 290° C. to 300° C. 
     
     
       4. A method according to claim 1, further including the step of dispersing liquid droplets in said air stream, and directing the resultant dispersion onto said board and leads. 
     
     
       5. A method according to claim 4, wherein said liquid droplets comprise soldering oil. 
     
     
       6. A method according to claim 4, wherein said liquid droplets comprise a wetting agent. 
     
     
       7. A method according to claim 4, wherein said liquid droplets comprise a flux agent. 
     
     
       8. A method of reducing the incidence of solder shorts, icicling and/or bridging in a mass soldering process in which a quantity of molten solder is deposited onto the underside of a component-carrying printed circuit board and onto those parts of said components which extend below said board underside and at least partially fill with molten solder any holes in said board, said method comprising the steps of: depositing a quantity of solder onto the underside of said board and component parts by passing the bottom surface of said board over a solder wave and substantially immediately following said depositing impinging a stream of heated air directly onto the molten solder deposited on the underside of said board and said component parts by means of an air knife blast directed towards the solder wave and impinging on the bottom surface of said board sufficiently soon after it leaves the solder wave so that the solder thereon is still molten, .Iadd.the air stream being heated to a temperature above that of the molten solder, .Iaddend.the air blast having sufficiently low velocity to leave a solder coating in said holes and sufficiently high velocity to remove solder shorts, bridges and icicles remaining on said board as it leaves the solder wave, the air blast increasing forces tending to remove the solder shorts, bridges and icicles.   
     
     
       9. A method according to claim 8 wherein said air stream is heated to a temperature in the range of .[.93°.]. .Iadd.290° .Iaddend.C. to 350° C. 
     
     
       10. A method according to claim 9, wherein said air stream is heated to a temperature in the range of about 290° C. to 300° C. 
     
     
       11. A method according to claim 8, further including the step of dispersing liquid droplets in said air stream, and directing the resultant dispersion onto said board and leads. 
     
     
       12. A method according to claim 11, wherein said liquid droplets comprise soldering oil. 
     
     
       13. A method according to claim 11, wherein said liquid droplets comprise a wetting agent. 
     
     
       14. A method according to claim 11, wherein said liquid droplets comprise a flux agent. 
     
     
       15. Apparatus for mass joining with solder electrical and electronic components assembled in a circuit board wherein said components have leads which protrude downward through apertures in the board, and comprising in combination: a wave soldering station adapted to provide a wave of molten solder;   means for transporting said circuit board across said solder whereby a quantity of molten solder may be deposited onto said circuit board underside and said protruding lead; and   an excess solder removal station adjacent said wave soldering station, said excess solder removal station comprising at least one hot air knife disposed below the travel path of said board and adapted to direct a blast of heated air directly onto the molten solder deposited on said board underside, a source of pressurized air, means connecting said source and said hot air knife, and means for heating said air knife blast .Iadd.to a temperature above that of the molten solder .Iaddend.prior to directing the latter onto said board underside, the air knife blast being directed towards the solder wave and impinging on the bottom surface of the board sufficiently soon after it leaves the solder wave so that the solder is still molten, the air knife blast having sufficient velocity to remove essentially all bridges and shorts but not sufficient velocity to adversely affect properly wetted joints.   
     
     
       16. Apparatus according to claim 15, and further including means for introducing liquid droplets into said air stream. 
     
     
       17. Apparatus according to claim 16, including a source of said liquid, and wherein said means for introducing comprises at least one aspirator connected to said source of said liquid. 
     
     
       18. Apparatus according to claim 16, including a source of said liquid, and wherein said means for introducing comprises at least one atomizing nozzle connected to said source of said liquid. 
     
     
       19. The apparatus of claim 15, wherein said air knife is provided with valve means to give a continuous small flow of air through the knife; and means for detecting the presence of a board over the solder wave to open the valve and provided a full air blast to the board as it leaves the wave. .Iadd. 
     
     
       20.  A method according to claim 3, wherein said solder comprises 67/37 solder alloy, and said air stream is heated to a temperature of about 290° C. to 300° C..Iaddend. .Iadd.21. A method according to claim 10, wherein said solder comprises 67/37 solder alloy, and said air stream is heated to a temperature of about 290° C. to 300° C..Iaddend. .Iadd.22. Apparatus according to claim 15 wherein said hot air knife comprises an elongate hollow chamber having an inlet adapted for connection to a source of pressurized air, and an elongate outlet, and including resistance heaters mounted in the interior of said hollow chamber for heating said air to a temperature above that of said molten solder..Iaddend. .Iadd.23. Apparatus according to claim 15 wherein said hot air knife comprises a hollow elongate block having an inlet for connection to a source of pressurized air and an elongate outlet, and including a plurality of cartridge heaters mounted in said block and adapted to heat said air to a temperature to above that of said molten solder..Iaddend.