WO1989000905A1 - Printed circuit board solder coating method and apparatus - Google Patents

Abstract

An improved continuous process and apparatus for applying a solder coating to copper-coated holes and tracks on a printed circuit board (54) is provided. In the system which employs a bath of molten solder and means to contact the surface of the printed circuit board with solder as the board is moved through the molten solder, air knives (62, 70) that are remotely adjustable are used to blow hot air across the board being coated to clear the holes in the board and to level the solder. The adjustable air knives are attached to an air level frame (170). The frame is secured to a solder pot (40) located inside the main frame of the solder coated apparatus. The interiorly mounted air knives are adjusted remotely by a handle (158, 160) which rotates a pinion and through racks secured to air knives pivotable supporting plates adjust the distance of the knives from the printed circuit board. To preclude introduction of contaminants into the solder, weirs (52a, 52b) over which fresh clean solder that is fed into an upper part of the solder pot circulate downward and toward the molten solder exit such that the printed board enters and leaves through clean solder. To prevent slippage of the printed circuit board exiting on an incline from the solder pot, positive grip spaced board gripping rolls (42a, 42b) are used. A substantially contained heated atmosphere for optimum solder deposition and to prevent blowing about of contaminants from knife air velocity is obtained by confining shields (130) positioned at the side and top of the solder pot and air leveling sections.

Description

PRINTED CIRCUIT BOARD SOLDER COATING METHOD AND APPARATUS

The present invention relates to an improved method an apparatus for the continuous application of a solder coating to printed circuit boards.

BACKGROUND OF THE INVENTION

The continuous solder coating of printed circuit board and apparatus for applying a solder coating to the copper- coated tracks and holes; i.e., the conductive portions of a circuit board, in a mu tiple station operation are known. A system of this kind, and with respect to which the present invention is an improvement, is disclosed in U.S. Fa ents 4,277,518 and 4,383,494.

As shown in the continuous solder coating operation of those patents, a printed circuit board is first suitably pretreated, before solder coating, includinα applying a conventional flux at a flux station in an apDlic^>.b^"1e operati such as by spraying, roller coatinα, dipping, etc. In the next stage the flux treated circuit board is preheated to activate the flux and to sufficiently preheat the copper coated holes and tracks on the circuit board. The heatinα rate is moni ored to prevent damage from excess heat. The thus preheated printed circuit board is then introduced at a controlled rate of movement to, and through, a solder coatin station. Λt the solder coating stage the cleansed, preheate printed circuit board is conveyed throuαh a solder pot containing molten solder into which the circuit board is ful immersed.

The printed circuit board exiting the solder coating station is then processed at a solder leveling station to remove excess solder, utilizing a combination of air knives, and under controlled temperatures to provide a uniform precision applied solder layer. The processed solder coated circuit boards, thus suitably coated, are washed and dried. SUMMARY OF THE INVENTION

In the system of the prior art, it has been found that the withdrawal of the printed circuit board from the molten solder bath, the regulation of removal of excess solder from the coated board by the air knives and the control of contaminants in the solder coating operation are less than satisfactory.

It is accordingly an object of the invention to provide an improved method and apparatus for continuously applying a solder coating to copper coated tracks and holes of a circuit board in a more efficient and precise manner that results in less contamination in the system, a more uniform deposited solder layer of the desired thickness; and in the production of a substantially greater percentage of product that meets the necessary high standards.

It is a more particular object of the invention to provide a continuous solder coating system with an improved leveling stage arrangement in which internally mounted air knives are remotely adjustable from the exterior of the system in a convenient manner to move the knives toward and away from the printed circuit board an accurately measurabl desired distance and thereby to more efficiently clear the holes in the circuit boar and to produce the desired optimum thickness of the leveled applied solder. It is another object of the invention to provide an improved system which results in the introduction of less contamination into the solder bath and on the solder coated circuit board by a modification of the feed system wherein the point of feed of fresh solder is modified in combinatio with the provision of weirs in the solder pot such that the circuit board encounters clean fresh solder both in enterin and leaving the solder pot.

It is a further object of the invention to prevent back slippage of the coated printed circuit board as the board is withdrawn from the solder pot by and from the solder leveling stage by the provision of a positive gripping drive means to transport the printed circuit board It is still another object of the invention to provide an improved system that minimizes contamination and formation of "solder balls" as the printed circuit board passes through the air leveling stage by locating the air knives such that the stream of air is located at the optimum point relative to the exiting circuit board.

It is yet another object of the invention to shield and thereby better regulate the heated atmospheric conditions at the solder deposition stage and at the air leveling stage to enhance the oven-like ambient temperature conducive to more even flow of solder and to confine contaminants that are blown about by the contiguous air knives as the board is undergoing the leveling operation.

Additional objects and advantages will become apparent from the ensuing description which is provided in greater detail.

BRIEF DESCRIPTION OF THE DRAWING

In the description which follows reference is made to the enclosed drawings in which: Fig. .1 depicts a schematic flow diagram of a continuous system for applying a solder coating to a copper-coated surface of a printed circuit board.

Fig. 2 is a schematic front elevational view showing mechanism for the adjustability of air knives in relation to the main frame of the solder coating system and so as to adjust the distance of the knives from the path plane of the circuit board.

Fig. 3 is a schematic plan view looking down on the adjustable air knives mechanism of Fig. 2. Fig. 4 is a schematic longitudinal- elevational. view of the solder coating station wherein the printed circuit board is coated with solder in a continuous operation and showinσ spaced grip rollers, contiguous to the molden solder pan that assure more positive withdrawal of the coated board from the molten solder and confining shields to control, the atmosphere and optimize flow of solder. Fig. 5 is a detailed elevational view partially broken away showing the location and direction of the adjustabl e air knives of the air leveling stage.

DETAILED DESCRIPTION OF THE INVENTION

The continuous system with reference to which the present invention is an improvement is shown generally in its various stages by reference to Fig. 1 of the drawing. These stages, in the main, perform various operations on the circuit board and include: flux coating, preheating, solder coating, leveling, cooling, washing and drying.

As shown in Fig. 1 of the drawing, the circuit board 54 first treated at a flux coating station 12 where the circuit boards are fed via a conveyor 14 and pass between coating rolls 16. The coating rolls 16 receive a convention liquid flux from transfer rolls 18 which convey the flux fro pickup rolls 20 that are partiall immersed in flux within containers 22. The flux, if desired, may be applied by another procedure, such as spraying or dipping.

On leaving the flux coating station 32, the circuit boards are preferably preheated at station 24 where the flux is activated by heating to a board surface temperature of the order of about 150 to 200 degrees F. (65-95 degrees C) . From the preheat station 24, the circuit boards are transported on a conveyor 26 through an oven 28 at a speed and temperature such that copper-coated holes and tracks on the circuit board reach the desired temperature at about the same time. The heating rate at the preheat station 24 shoul not be too rapid because of the risk that the tracks formed on the board and the board itself may be adversely affected b excess heating during the time required to bring the holes to the desired temperature. The speed of the conveyor 26 may be controlled by a motor 30.

After leaving the preheat station 24, the circuit boards may be transported on a speed regulated conveyor 32 that is driven bv motor 34. It may be desirable to run the conveyor 26 at a controlled rate; e.g., at a slowed speed thereby gradua ly heating the circuit boards at the preheat station 24. In subsequent stations, however, it may be desirable to move the circuit boards at a faster rate to reduce the time duration of the circuit board in the solder bath. The speed change conveyor 32 may be inclined downwardly at a slight angle toward the right such that the circuit boards pass beneath a guide roll. 36 at a desired angle to enter the solder station 38. The solder station 38 employs a solder pot or pan 40 which contains molden solder and is provided with a belt 42 continuously moving through the molten solder within the pan. The belt 42 may pass over a bed 44 whose lower surface has a configuration which generally defines an arcuate path for the belt as it moves through the solder. The belt 42 may be formed of a corrosion resistant inert material, such as stainless steel, and have an open mesh configuration whereby the belt contracts the circuit board without retaining solder on the belt. The belt, for example, may be of a kind described more fully in U.S. Patent No. 4,277,51.8. The arcuate path of movement of the belt 42 as it passes through the molten solder imparts a desirable serpentinelike movement to the circuit boards which are contacted by the belt and insure that the circuit boards are completely immersed and wetted in the molten solder at the soldering station 38.

A separator or flow guide 46 which functions as a baffle for the flow of the molten solder may be employed in the pan 40 to control the flow of fresh molten solder over the weirs 52a and 52b at the left and right extremities, respectively, of the member 46 such that substantially clean fresh molten solder is continuously presented to contact the circuit board at the point of entry and again at the point of egress of the circuit board from the solder pot 40.

As the circuit boards move through the solder station 38 solder, which is displaced by the volume of the circuit boards, exerts a buoyant force against the circuit boards and holds the boards in contact with the belt 42. The speed of the belt 42 and speed and direction of movement of the molten solder within the solder pan 40 above the baffle member 46 may be regulated to maintain contact between the circuit boards 54 and the belt 42. The molten solder is appropriately pumped to provide a desired flow direction and flow velocity and volume over the weirs 52a and 52b within the pan 40 in the area above the flow separator element 46. The movement of belt 42 in passing throuσh the molten solder within the pan 40 tends to generate a flow of molten solder with the solder in proximity to the belt so that the flow effected by the solder feed to the left is partially offset over weir 52a and is supplemented over weir 52b. Depending upon the size of the circuit boards passing through the solder station 38, the density of the boards and the speed of movement of the belt 42, the movement of the molten solder within the pan 40 may be controlled by location of the solder feed inlet 126 and velocity of the feed stream.

Weirs 52a and 52b are preferably formed at the entrance and exit ends respectively and at least part of the fresh moten solder is introduced at a suitable point 126 approximately at the center in the solder pot so that fresh solder flows over, and reverses direction after passing around the end of, each of the weirs 52a and 52b. The circuit board on which solder is applied enters and leaves the solder pot throuσh this surging relatively clean solder at "the weir points" 52a and 52b, respectively.

To preclude slippage back of the circuit board at upward inclined exit from the solder pot, a set of spaced rollers 42a and 42b are provided to positively grip and advance the circuit boards from the solder stage 38.

After passing from the soldering station 38, the circuit boards 54 enter a solder removal station 55. A conveyor generally indicated as 56 transports the board 54 past an optional suction chamber 58 where a negative pressure is applied against the underside of the circuit board such that the board tends to remain in place on the conveyor. A optional suction chamber 60 may also be positioned above the board 54 while an adjustable air knife arrangement, the details of which are provided hereafter, directs a thin band of high velocity air against the surface of the board. Air which is discharged through the air knife 62 is preferably heated, as by a heater 63, so that the discharged air has an elevated temperature of the order of about 350 degrees F (178 degreesC) . On leaving the solder station 38, the board 54 may have a temperature of the order of 450 degrees F. (234 degrees C), plus or minus 50 degrees F. (30 degrees C) . It is desirable that air discharged against the circuit board 54 by the air knives 62 and 70 be heated to prevent thermal shock of the solder coating on the board.

Only the copper-coated tracks and holes of the circuit board are soldered. Solder which adheres to other portions of the board must, therefore, be dislodged by compressed air directed against the board by the air knives. The solder which is removed from the circuit board may pass into the suction chambers 58 and 60 for subsequent recovery in any convenient manner, such as by use of a screen. Blowers 64 and 72 may be used to generate a reduced pressure for the suction chambers 60 and 58, respectively.

The adjustable air knife leveling apparatus at station 55, which is situated contiguous to but follows in sequence the solder station 38, clears the holes that are formed in the circuit board and levels and removes excess deposited solder from the board surface. The adjustable air knife arrangement comprises at least one, but preferably a pair of knives (as best shown in Fig. 2 and Fig. 5) , one knife 62 above and positioned so as to direct a relatively high velocity, preferably heated, air stream substantially perpendicularly at the plane of the upper surface of the moving circuit board 54, and one knife 70 below positioned slightly downstream from the first and aimed so as to direct the air stream at the undersurface of the circuit board 54 at a shallower ang e with respect to the plane of the passing circuit board. In performance the upper knife 62 directs a stream of air of sufficient velocity and direction to level the surface deposition as well as to clear solder from the holes in the circuit board. The lower knife 70 directs air that functions to level and clear excess solder from the under surface of the circuit board 54 and at an angle which precludes re-introduction of solder into holes which may be formed in the circuit board. In operation, the distance of the air knives 62 and 70 from the circuit board surfaces is adjusted remotely by means of a distance increment measuring dial with handles 158 and 160 on a gear boxes 165 and 166. Rotation of the handles 158 and 160 a measured amount effects a corresponding rotation of related pinions and racks that are connected to pivotally mounted knife supporting plates 141 and 143. As described further hereafter, when the racks are moved through pinion rotation, the knife support plates is pivoted and moves the knife, located at the opposite end of the support plate from the pivot point, closer to or more removed from the surface of the circuit board.

Referring more particularly to Figs. 2, 3 and 5 of the drawing, it is seen that the leveling apparatus comprises an air level frame 170 which carries the air knives 62 and 70 and the related plates 141 and 143, respectively, and associated functional elements. The air leveling frame 170 is dimensioned so as to fit within the outer dimension of the main frame 172 within which the solder pan 40 is mounted. The leveling frame 170 has mounted thereon in pivotal relation, at 145, the upper knife mounting plate 141 and the lower knife mounting plate 143. Each of these plates are conveniently formed so as to have a generally U-shaped cross section with the lower plate 143 fitting within the sides of the (slightly larger) U-shaped upper knife support plate 141 and 143 pivoting at the point 145. The upper knife 62 mounted transversely in the upper plate 141 is so positioned and is at an angle so that it directs air downwardly at a suitable distance from the top surface of the passing circui board 54; i.e., the air stream exits less than about 0.75 inch, and preferably less than about 0.25 inch from the surface of the circuit board and at an angle near perpendicular to the plane of the circuit board 54; i.e., at an angle of about 85 to 90 degrees to the plane of the circuit board. This angle serves to effectively level the deposited molten solder and clear the holes in the circuit board of molten solder, which may have flowed into the holes, by blowing any molten solder present in the holes downward through the holes.

The lower knife 70, having its heated air stream directed upwardly against the passing circuit board 54, is similarly mounted on the lower plate 143 which also pivots on the frame 170 at point 145. The knife 70 is positioned so that its heated air stream impinges on the undersurface of the passing circuit board 54 at a distance of less than about one inch and preferably at a distance of less than 0.50 inch from the undersurface of the board and at an angle of less than about 70 degrees with the place of the passing circuit board and preferably at an angle which may vary between 35 and 60 degrees. The bottom knife 70 is preferably set slightly downstream from the upper knife; i.e., behind the upper knife, such that the direction line of the stream of heated air coming from its "blades" intersects the direction line of the stream from the upper knife at the midpoint of the printed circuit board 54 which norma ly has a thickness of about 1/16 inch.

To adjust the distance of the air knives 62 and 70 from the surface of the circuit board 54, racks 147a and 147b mounted on the upper plate 141, and racks 149a and 149b, mounted on the lower plate 143, respectively, and which are actuated by corresponding pinions, are conveniently utilized. It will be apparent that other known equivalent actuating mechanisms that provide a similar movement may also be employed. As shown, each of the knife support plates 141 and 143 are provided with separate racks, preferably one rack, 147a and 147b, on each side of the top plate 141 and racks 149a and 149b for the lower plate 143. The dual racks provide more reliable and uniform movement and as they mesh, respectively, which pinions 148a and 148b on shaft 155 for the upper air knife support plate 14.1 and with pinions 150a and 150b on the shaft 153 for the lower knife support plate 143.

To adjust the distance of the knives 62 and 70 from the surfaces of the circuit board 54, upper and lower handles 158 and 160 mounted on gear boxes 165 and 166 connected, respectively, to shafts 155 and 153 and which may conveniently include a coupling 154, are rotated a measurabl distance, as measured by micrometer-like distance measurement setting dials 159 and 161. Rotation of the shafts 153 or 155 moves the corresponding pinions thereon which mesh with the racks mounted on the respective knife support plates and effect a movement of the respective plate at the pivot point 145 of the upper plate 141 and lower plat 143. This adjusts the distance of the knife secured at the opposite end of the plate (from the pivot) the desired measurable distance. It is seen that this air leveling arrangement conveniently permits the ready adiustment remotely of the distance from the surface of the circuit board of the air knives by closely measurable amounts, without exposing the operator to the excessively hot conditions prevailing near the molten solder coated circuit board.

To permit a more efficient and contaminant free product it has been found advantageous to control the flow of the incoming relatively uncontaminated molted solder so that a printed circuit board entering the solder pot 40 encounters substantially fresh solder and, similarly, that the printed circuit board exiting the solder pot 40 is likewise flushed with substantially clean, fresh molten solder. For this purpose, the solder pot 40 is provided with a flow guide or baffle-like element comprising a separator with weirs which controls the flow path of the introduced molten solder so that the molten solder flows outwardly above and from the center of the baffle 46 and over the left and right weirs 52a and 52b. This baffle or separator element 46 with a wei at the left and right ends thereof is shown in Fig. 1 and

Fig. 4. The entrance port 126 for the fresh molten solder i preferably located near the midpoint above the separator 46 so as to effect the desired flow pattern over each of the weirs 52a and 52b taking into account the influence to flow created by the conveyor belt 42 which maintains the circuit board submerged in the solder pan 40 at the solder station 38. After passing over the weirs 52a and 52b, the molten solder flow reverses direction below the flow guide or baffle 46 and is withdrawn throuσh an exit port 127.

It has also been found that the achievement of proper leveling of the solder deposited on the printed circuit board is affected by control of the ambient temperature and air movement at the solder station. Otherwise a frosty solde deposit on, and/or improperly cleared holes in, the circuit board results. To obviate the conditions leading to these disadvantages, suitably located shields are positioned at the top and sides of the solder pot and air leveling sections as best shown at 130, 131, and 132 in Fig. 4. The shields 130, 131 and 132 effectively confine the ambient atmosphere during the critical formation of the solder layer on the circuit board, especially the atmosphere contiguous to the air leveling station, to more closely control the ambient temperature and shield the molten solder deposit from disruptive air currents. The result is to provide an effective temperature controllable oven-like atmosphere from v/hich contaminants, that are blown about as the circuit board is being leveled, can be substantially excluded. To avoid the occurence of undesirable slippage or temporary stalling in the continuous movement of the circuit board, which is most often likely to occur as the circuit board is transported at an upward incline from the solder deposition station to the leveling station and from the leveling station to the air cool station, circuit board gripping rollers are provided. This slipping or stalling in the movement of the circuit board occurs because of low friction conditions and the upward inclines in the motion path; thus, the circuit board does not always move with a positive motion but slides and stalls or moves backward. It has been found that this drawback is remedied by gripping the boards so as to propel them at least during the incline interim. For this purpose, paired spaced rollers 42a and 42B (as best seen in Fig. 4) , for example, one pair of which may be driven by a drive chain and the second pair by a spur gear (not shown) connected to the first pair, may be used. In conveying the circuit board from the leveling section, an additional set of gripping rollers 42c are employed. This set of rollers is preferably arranged to be spaced a suitable distance; e.g., of the order of 0.05 to 0.02 inch apart so that the rollers lightly grip the circuit board but do not "iron" or otherwise distort the deposited solder or introduce blockage into the holes on the board surface as the circuit board passes between the spaced rollers.

After passing the section in which the circuit board is leveled by the air, knives, the board is conveyed to a cooling section 74 as by means of conveyor 65. The conveyors 56 and may be specially constructed to reduce frictional contact between the conveyor surfaces and the circuit boards to minimize removal of solder from the circuit boards by frictional contact with the conveyor surfaces.

As an option, a suction chamber 60 positioned adjacent the upper surface of the board may be used and aids in imposing a negative pressure against the upper surface to hold the board in contact with the conveyor 65.

The air knife 70 may be heated within a heater 71 whil a blower 72 may be used to draw air from the suction chambers 58 and 60. The blower 64 draws air from the suction chamber 60. Solder which is removed from the circuit boards by the air knives may, at least partially, be drawn into the suctio chambers 58 and 60.

After leaving the leveling section 55, the circuit boards enter an air cooling station 74. In passing through air cooling station 74, the circuit boards may be transporte on a conveyor 76 through a chamber 78. Air may be drawn across the circuit boards within the chamber 78 with the air being discharged by a plurality of fans 80. To coordinate the speed of the belt 42 and speed of the conveyors 56, 65 and 76, all may be driven by a common source such as a motor 82 through a common drive train (not shown) or by individual synchronized motors. After leaving the air cooling station 74, the circuit boards are water scrubbed at station 86 where the boards may pass between a plurality of drive rolls 88. Rotatable brushes 90 may be positioned to contact the circuit boards while water is discharged against the circuit boards through conduits 92 and then disposed of as through a suitable drain 94. The boards may be conducted to a drying station 96, which may include a conveyor 98 that passes between air conduits 100, where air is discharged against the boards to dry them.

It will be apparent that various changes may be made in the details of the inventions herein described without departing from the spirit and scope of the invention. The enumeration of details herein, except to the extent recited in the appended claims should, therefore, not detract from the purview of the invention.

Claims

What is claimed is:

1. In a continuous process for coating a circuit board with solder to protect copper tracks and the surface of holes thereon from oxidation and, in subsequent soldering operations, to mount components on the circuit board and wherein the surface of a printed circuit board is coated by immersion in a substantially horizontal path in a molten bath of solder so that the molten solder contacts each surface of the circuit board and the solder on the coated board is thereafter leveled in a subsequent stage with air knives, the improvement comprising: leveling and removing solder from the upper surface of the circuit board by a first adjustable air knife positioned contiguous to the circuit board emerging from the molten bath of solder and so that air impinges on the upper surface of the circuit board and in a direction substantially perpendicular to the plane of the circuit board and so that molten solder present in holes in the circuit board is expelled downward and out of the holes; and subjecting the undersurface of the printed circuit board to a second remotely adjustable air knife so that air impinges on the undersurface of the circuit board and at a shallower angle with the plane of the board and so that the leveling action of said second air knife precludes re-introduction of molten solder into holes in the circuit board while leveling the solder at the undersurface of the circuit board; and remotely adjusting the distance from the surface of said circuit board surface, of said first and said second air knives.

2. The process of claim 1 wherein the first air knife directs air on the upper surface of the board at an angle of at least about 85 degrees with the plane of the circuit board.

3. The process of claim 2 wherein the second air knife is offset so as to impinge air at the undersurface mostly concurrently but at a point subsequent to that at which air is expelled by the first knife on the upper surface and wherein the air from the second knife is directed at the undersurface of the circuit board at an ang e not greater than 60 degrees.

4. The process of claim 1 wherein the molten bath is provided with a continuous flow of fresh molten solder and a continuous withdrawal of contaminated solder and wherein weirs in the molten solder bath direct the f ow of the fresh molten solder so that a circuit board on the ingress and egress to the solder bath is exposed to substantially fresh solder.

5. The process of claim 1 wherein positive movement means is provided to propel the coated circuit board emerging from the molten solder bath and through the air knife leveling stages.

6. The process of claim 1 wherein the ambient temperature and air current atmosphere of the molten solder immersion bath and subsequent leveling stage is controlled by coating the printed circuit board in a shielded confinement.

7. A multiple station apparatus for applying a protective coating of solder on the conductive portions of a printed circuit board including stations for coating the circuit board by immersion in a molten bath of solder so that the molten bath of solder contacts each surface of the circuit board and the molten solder is thereafter leveled at a subsequent air leveling station with air knives, the improvement comprising: a first remotely adjustable air knife positioned so as to direct air on the upper surface of the printed circuit board at a leveling station contiguous to the station at which the circuit board is withdrawn from its immersion in the molten solder and so that air from said first air knife impinges downward on the upper surface of the circuit board in a direction that is substantially perpendicular to the plane of the circuit board and such that molten solder present in the holes of the circuit board is expelled downward out of the holes; a second remotely adjustable air knife positioned so that air impinges on the undersurface of the circuit board and at a shallower angle with the plane of said circuit board than the angle of said first air knife and so that the leveling action of said second air knife precludes re-introduction of molten solder into holes in the circuit board; while leveling the solder at the undersurface of the circuit board; and means remotely positioned from said first and second knives for adjusting the distance of said first and second air knives from the surface of the circuit board.

8. The apparatus of claim 7 wherein the means for remotely adjusting the distance of said first and second air knives comprise a first and second actuating handle respectively.

9. The apparatus of claim 8 wherein the means for adjusting the distance of said knives from the surface of the circuit board comprises an upper and a lower pivotal mounting plate on which said upper and lower knives, respectively, are carried and wherein the plates are connected to a rack secured to said plates and wherein a pinion actuated by a handle meshes with and moves said rack when the handle is actuated to provide the desired predetermined distance of the knife from the surface of the board.

10. The apparatus of claim 7 wherein the said first air knif is positioned so as to direct air on the upper surface of the circuit board at an angle of at least about 85 degrees with the plane of the circuit board. ii. The apparatus of claim 10 wherein the second air knife is offset in the path of travel of the circuit board so as to impinσe air at the undersurface at a point subsequent to that at which air is expelled by the first air knife on the upper surface and wherein the air from the second knife is directed at the undersurface of the circuit board at an angle not greater than 60 degrees. 12. The apparatus of claim 7 wherein the molten bath is provided with means to supply a continuous flow of fresh molten solder and with means to withdraw substantially contaminated molten solder and wherein weirs in the molten solder bath direct the flow of fres molten solder so that the printed circuit board is exposed to substantial y fresh solder at a point of introduction into the molten solder bath and to substantially fresh solder again at a point of emergenc from the molten solder bath. 13. The apparatus of claim 7 wherein positive gripping mean are provided to grasp and positively to move the printed circuit board emerging from the molten solder bath station and following the air knife leveling station. 1.4. The apparatus of claim 7 wherein enclosing means are provided at the molten solder coating station to control the ambient atmospheric temperature of the coating station, and to substantia y shield the station against air currents from the air leveling station.