Apparatus for spray coating flat surfaces

Apparatus for coating a workpiece such as a printed circuit board. Boards are arranged in closely spaced fashion upon a conveyor having an adjustable drive. A spray assembly for providing a low pressure spray having a substantially oval-shaped spray pattern is arranged upon guide tracks and moves in a reciprocating fashion in a direction transverse to the direction of movement of the conveyor. The speed of movement of the spray head and the extreme ends of the range of movement are adjustable by controlling the operation of a drive motor to create a substantially Z-shaped spray pattern on the workpiece with the size of the Z-shaped pattern being a function of the conveyor speed spray head assembly, drive speed and range of movement of the spray head assembly. Control may be exerted by a microprocessor based computer system receiving input information regarding size of workpiece, thickness of coating and the like for automatically controlling the conveyor motor and the spray assembly motor thus further simplifying the speed and ease of setup changes as well as significantly reducing waste of coating material and the like.

FIELD OF THE INVENTION 
The present invention relates to spray coating of flat surfaces with a 
paint-like material and more particularly to coating a printed circuit 
board with a liquid photoresist which is subsequently dried and imaged to 
produce a primary image or solder mask. The method and apparatus is also 
applicable for coating circuit assemblies with conformal coating material 
to protect circuits from mechanical damage and/or corrosion. 
BACKGROUND OF THE INVENTION 
A number of methods have been used in the past and are still presently in 
use for coating printed circuits. One of the most widely used methods is 
screen printing wherein a surface area of a board to be coated is defined 
by an opening in the screen device. Coating material is applied to the 
screen and is urged through the opening by the wiping action of a squeegee 
drawn across the screen. The amount of material deposited on the surface 
is determined by screen dimensions, material rheology and the speed of the 
squeegee traversing the screen. Whereas the screen method has the 
advantages of being accurate, fast and repeatable, disadvantages reside in 
the need to provide a specific screen for a specific board size, the 
complicated setup needed to establish machine parameters and the limited 
life of a screen. 
A second method in popular use is curtain coating. In practice, a web of 
liquid is caused to fall from a metering device in a known and controlled 
geometry. The web is situated transverse to a conveyor or similar handling 
mechanism. The workpiece passes beneath the web and intersects the falling 
liquid and collects a controlled amount of falling liquid along an upper 
surface of the workpiece. Whereas curtain coating has the advantages of 
high productivity and minimal setup when changing from one board size to 
another, disadvantages reside in the fact that the equipment is not easily 
turned off and on again for short-run work, the equipment is relatively 
expensive in terms of capital required and the technique requires a large 
charge of material for proper operation. When thin coatings are desired, 
there is a tendency to skip and thereby not completely and properly cover 
the surface to be protected. 
A third method of application is electrostatic spray which requires an 
electrically conductive material. A voltage is applied between the spray 
device and the surface being coated. An electric field created by the 
applied voltage causes the coating material particles to be accelerated 
and attracted to the surface being coated. Whereas advantages reside in 
the ability to completely coat the surface with negligible setup, the 
disadvantages are: high initial costs; tendency to waste material due to 
overspray; potential for electrical discharge and operator hazard to high 
voltage in the work area. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention is characterized by the employment of low pressure 
air spraying techniques for the application of liquid to the surface of a 
printed circuit board. The techniques provided are designed with an 
objective to require only minimal setup and/or equipment changes to 
accommodate changes in production. The system apparatus comprises a 
conveyor for carrying the workpiece, a spray gun for applying coating 
material, and an exhaust system to control air flow and overspray in the 
work area. 
Workpieces are preferably moved horizontally along a conveyor means which 
is preferably an open mesh structure having a depression in the center or 
on edge conveyors to provide support to the side edges of the part. Work 
is preferably placed in the center of the conveyor means with the leading 
and trailing edges of parts in close proximity to minimize spray losses 
between the parts or workpieces. 
The spray gun is positioned above the moving plane of the workpiece and is 
pointed downwardly toward the surface of the workpiece. 
The spray pattern of the spray gun is preferably oval in shape with a ratio 
of the oval dimensions being about five to one such that the width of the 
pattern is about 1/5 that of the length or longitudinal dimension, which 
is aligned to be substantially parallel to the direction of workpiece 
travel. 
The spray gun is mounted on a cart movable transverse to the direction of 
workpiece travel. The cart is driven by a variable speed motor. During 
operation the gun is caused to travel across the path of the workpiece 
while spraying the coating material onto the workpiece surface. At the 
extremes of travel of the spray gun, the drive motor is reversed causing 
the travel of the spray gun to reverse. Velocity and distance of travel of 
the spray gun and the speed of the workpiece results in a "Z-shaped" path 
described by the spray gun relative to the workpiece. The width and pitch 
of the Z-shaped path are determined by the relative speeds of the spray 
gun and the workpiece. The oval pattern of the spray and the pitch of the 
Z-shaped path are adjusted to eliminate apparent non-uniformity by causing 
successive passes of the spray pattern to overlap and eliminate 
non-uniform spray coating. 
The traverse mechanism determines the width of the spray pattern by 
reversing direction at the extremes of travel, which is adjustable. 
The design of the system is such as to enable setup changes to be made 
quickly and easily to accommodate changes in workpieces being processed. 
One preferred technique for accomplishing this result is to provide a 
proximity switch which detects the position of the moving cart and 
initiates a timer to control movement of the spray gun from a center 
position to the outer extreme positions. The timer is adjustable to adjust 
the distance of the extreme position from the center point. Speed of 
movement is adjusted by adjusting the drive motor. The timing functions 
may vary to accommodate asymmetric shapes of work. Given the velocity of 
the spray gun, the excursion, the speed of the conveyor and fluid 
properties are all known, this data may be manipulated by an embedded 
computer which computes speeds as a function of fluid thickness, for 
example, thus minimizing operator control and further expediting setup 
changes. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide method and 
apparatus for effectively coating printed circuit boards and the like and 
especially providing method and apparatus for effectively applying thin 
coatings of liquid. 
Another object of the present invention is to provide a novel spraying 
method for uniformly spray coating printed wiring boards and the like and 
which is designed to facilitate fast and easy setup changes. 
Another object of the present invention is to provide a novel spraying 
method for uniformly spray coating printed wiring boards and the like and 
which is designed to reduce costs for coating apparatus. 
Another object of the present invention is to provide a novel spraying 
method for uniformly spray coating printed wiring boards and the like and 
which is designed to minimize wasting of spray material. 
Still another object of the present invention is to provide a novel method 
and apparatus for coating printed circuit boards and the like including 
means for automatically traversing workpieces carried upon a conveyor with 
a predetermined coating spray pattern so as to traverse a substantially 
Z-shaped spray pattern relative to the workpiece wherein the shape of the 
pattern is a function of conveyor speed and a traversal speed of the spray 
gun and wherein the length and width of the pattern are quickly and easily 
adjustable thus significantly simplifying setup changes.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF 
FIG. 1 shows a system 10 embodying the principles of the present invention 
and comprising a conveyor belt 12 entrained about upstream and downstream 
rollers 14 and 16, respectively. The upper run 12a supports workpieces W 
which may be of any shape and size that can be accommodated by the 
conveyor system. In addition to closely spacing workpieces W in the 
direction of movement as shown by arrow A, workpieces W may be closely 
spaced side by side in a direction transverse to the direction of movement 
as shown in FIG. 1b. The belt may be fabric or wire of open or closed 
mesh. 
The conveyor is driven by an adjustable speed motor 18 whose speed may be 
adjusted, for example, by a potentiometer 20 to obtain a Z-shaped pattern 
of an adjustable size, as will be more fully described. The speed of motor 
18 may be controlled by either adjusting an input voltage or an input 
current. Any other adjustable speed motor may be used, if desired. 
The spray head assembly 22 is positioned above upper run 12a and includes a 
spray head 24 coupled to a liquid source 26 pumped into spray head 24 by a 
pump 28 to provide a low pressure liquid spray. As an alternative, liquid 
source 26 may be a stationary source (not shown) positioned at a location 
adjacent to the conveyor and coupled through a flexible tube 30 to spray 
head 24 by pump 28, as shown in FIG. 1c. The tube is sufficiently flexible 
so as to provide a continuous flow of fluid to spray head 24 without 
kinking. For example, an intermediate portion 30a of tube 30 may be coiled 
and be free to respectively expand and contract as shown by arrow sets B 
and C to accommodate reciprocating movement of the spray assembly in the 
direction transverse to workpiece movement (see arrow A) to accommodate 
movement of the spray assembly between its opposite extremes. The pump 28 
and/or a valve control 29 may be adjustable by an adjustable control 28a 
and valve control 29a (see FIG. 1c) to provide additional controls for 
obtaining a desired coating. 
The spray assembly may be provided with a chassis 22a upon which members 24 
and 26 and 28 are mounted. The chassis is provided with rollers R which 
extend into the grooves G of a pair of guide rails 32, 34 to guide 
assembly 22 and prevent lateral movement. The spray assembly is movable in 
a direction transverse to the direction of movement of conveyor belt upper 
run 12a as shown by arrows D in FIG. 1. 
A closed loop drive belt 36 is entrained about a pair of drive belt rollers 
38, 40 arranged at opposite ends of tracks 32 and 34 and extending between 
the tracks as shown in FIG. 1d. The rollers 38 and 40 are freewheelingly 
mounted. 
A drive motor 42 mounted at one end of the tracks 32 and 34 which extend, 
for example, beyond the right-hand end of workpiece conveyor belt 12 
drives roller 40 which in turn moves drive belt 36. The ends 36a and 36b 
of drive belt 36 are secured to opposite sides of spray assembly chassis 
22a. 
Motor 42 is provided with a speed adjustment means which may, for example, 
be an adjustable potentiometer 44 and further includes adjustable timing 
means 46 having an adjustment knob 46a and receiving a timer initiating 
signal from proximity sensor 48 which senses a probe or projection 22b 
extending downwardly from spray assembly 22 to pass in close proximity to 
sensor 48. Extremity sensors 50 and 52 are further provided at the extreme 
ends of the tracks for a purpose to be more fully described. The sensors 
may be proximity sensors, optical sensors or microswitches having switch 
arms which are closed when slidably engaged by probe 22b when it passes a 
switch arm. 
Exhaust system 27 shown in dotted fashion in FIG. 1 controls airflow and 
overspray. FIG. 1j shows an end view of the exhaust system which has an 
inverted U-shaped configuration and is positioned beneath upper run 12a. A 
vacuum is drawn through conduits 27a-27b, which are coupled to a vacuum 
(or blower source, not shown). Openings along side surfaces 27c and 27d 
draw air and overspray into the interior 27f of the exhaust system 
enclosure and out through conduits 27a, 27b. Filters may be employed as 
shown. If desired, the sides 27c, 27d and 27e may be completely open. 
The exhaust system is preferably used to support the guide tracks 32-34. 
The operation of the spraying system of the present invention is as 
follows: 
Workpieces are arranged upon the upstream (left hand) end of conveyor upper 
run 12a (see FIG. 1) and a position in close proximity so as to avoid 
wasteful use of spray. Depending upon the size and shape of workpieces, 
workpieces may be arranged side by side in a direction transverse to the 
direction of movement A of conveyor upper run 12a (see FIG. 1b). 
Motor 18 is energized and motor speed of the motor is adjusted at 
potentiometer 20 to adjust the linear speed of conveyor upper run 12a. 
Spray head 24 is arranged so that its oval-shaped spray pattern 24a (see 
FIG. 1a) so that the longitudinal axis 24b of the spray pattern is aligned 
substantially parallel to the direction of movement of workpieces 
represented by arrow A and so that the width dimension of the spray 
pattern represented by transverse axis 24c is substantially perpendicular 
to the direction A. A ratio of 5:1 of length to width of the spray pattern 
is preferred. The ratio may be modified, so long as the length dimension 
is greater than the width dimension. 
The timer 46 of motor 44 is adjusted to a predetermined time interval and 
the operating speed of drive motor 42 is adjusted by potentiometer 44. 
Assuming that the spray assembly 22 moves toward the right relative to FIG. 
1b, the probe 22b on passing proximity sensor 48 causes a pulse to be 
applied to timer unit 46. This pulse initiates the timer which times out 
according to the interval selected by adjustment knob or control 46a. Upon 
reaching the end of the selected time interval, the timer times out, 
automatically reversing the rotational direction of drive motor 42 thereby 
reversing the direction of movement of the spray assembly 22. 
In moving toward the left, the probe 22b again passes proximity sensor 48 
causing initiation of the operation of timer 46. When the time period 
times out, the timing unit reverses direction of motor 42 causing the 
spray assembly to move from left to right. 
Additional extremity sensors 50 and 52 may be provided, if desired. The 
extremity sensors can be utilized either to automatically stop motor 42 or 
alternatively to stop and reverse motor 42. Sensors 50 and 52 may be 
located at the extremities of the range of permissible movement thereby, 
for example, protecting against continuous movement of the spray assembly 
due to the failure of the proximity sensor 48 to detect the passage of 
probe 22b. 
As another alternative, the extremity sensors 50 and 52 may be slidably 
mounted along the support and track assembly and moved and manually moved 
to a desired position. 
As another alternative, shown in FIG. 1f, each of the extremity sensors 50 
and 52 may alternatively comprise a pair of sensors, for example, sensor 
assembly 50' may be comprised of a pair of sensors 50a' and 50b' spaced a 
predetermined distance apart by a spacing member 50c'. Similarly sensor 
assembly 52' may comprise a pair of sensors 52a', 52b' spaced apart by a 
spacer 52c'. Sensor assemblies 50' and 52' are movable along the track 
supporting assembly and, as spray assembly 22 moves toward the right in 
FIG. 1f, probe 22b passes sensor 52a' which causes the motor to 
decelerate. As the motor is decelerating, it comes in the vicinity of 
proximity sensor 52b' which causes the motor to stop and reverse 
direction. In moving from the right to the left, as probe 22b passes 
sensor 52a' the motor is accelerated to the maximum selected speed. The 
arrangement of FIG. If may be used either with or without a centrally 
located sensor 48. Alternatively, sensor assemblies 50' and 52' may 
consist of only one sensor, eliminating the decelerating sensor. 
FIG. 1g shows one embodiment of a timer arrangement. Timer 46 is comprised 
of a register 46a for inputting a digital representation of a numeric 
analog input through adjustable means 46b. 
Oscillator 47 applies incrementing pulses to counter 46c through gate 49 
responsive to an output from a bistable flip-flop circuit 51 which opens 
gate 49 to provide accumulating pulses to counter 46c. When the count in 
counter 46c is the same as the count in register 46a, comparator 46d 
develops an output at 46d-1 which is simultaneously applied to a reset 
input of counter 46c to reset the counter and is applied to a reset input 
for bistable flip-flop 51. 
The comparator may comprise, in addition to comparator 46d, a second 
comparator 46e which compares the count in counter 46c against a value 
which is a predetermined numeric quantity less than the value set into a 
register portion 46a-1 by input 46b whereupon a signal is developed when 
the count in counter 46c reaches its lower count which signal appears at 
output 46e-1 to cause the motor to slow down a predetermined time before 
reaching the count value set in to register 46a representing the extreme 
end of the range of movement selected according to the workpieces being 
treated. 
As shown in FIG. 1g, the system control may comprise a CPU 54 receiving 
variable input information from input means 56 and containing a stored 
program in a ROM and having a RAM memory. Suitable algorithms are provided 
in the CPU for automatically selecting the drive speed of motors 18 and 42 
by inputting information including the thickness of the desired coating, 
the nature of the coating material and the width of workpieces arranged in 
the direction transverse to the direction of movement of conveyor 12. This 
information is utilized to compute control signals provided to motors 18 
and 42. For example, motor speed S and motor direction D may be provided 
as outputs to motors 18 and 42 as shown in FIG. 1g. 
The conveyor belt 12 may be any suitable conveyor material. However, 
conveyor belt 12 is preferably an open mesh wire structure depressed in 
the center to provide support to the side edges of the workpiece. For 
example, the belt may be comprised of longitudinal wires 13' and cross 
wires 12', wires 12' being bent in a gently curved arrangement arranged to 
pass about sprockets 14', 14' as shown in FIG. 1e. 
As another alternative, the depression may be more pronounced and 
"squared-off" as shown by the conveyor belt 12w' in FIG. 1g. The roller 
14" may have a similar shape, or may be replaced with sprockets (see FIG. 
13). FIG. 1g shows a workpiece supported thereon. 
As a further alternative, the center section 12a" (FIG. 1g) may be removed 
whereupon the boards are supported by "edge" conveyors. By the use of 
clamping members at spaced intervals along the edge conveyor, only one 
edge conveyor need be used. See one such clamp 15" in FIG. 1g. 
The spray liquid may be a photoresist, a conformal protective coating or 
any other liquid coating typically applied to printed circuit boards. The 
boards may be coated before or after having components assembled thereon. 
A latitude of modification, change and substitution is intended in the 
foregoing disclosure, and in some instances, some features of the 
invention will be employed without a corresponding use of other features. 
Accordingly, it is appropriate that the appended claims be construed 
broadly and in a manner consistent with the spirit and scope of the 
invention herein described.