Apparatus for solder nozzle height sensing

The method and apparatus of the present invention permit the accurate sensing of the height of a solder nozzle above a circuit board during operation of the solder nozzle. A non-contact limit switch, which provides an indication of the presence of a planar surface at a preselected proximity, is mounted in a fixed relationship with a solder nozzle which is manipulatable by means of a robotic arm. A reference surface is provided which includes a proximity sensor, such as a through beam fiber optic switch. The proximity sensor is mounted a predetermined distance above the reference surface and is utilized to detect the solder nozzle as it is moved toward the reference surface. By noting the position coordinates of the robotic arm at which the proximity sensor indicates the presence of the solder nozzle at the predetermined distance above the reference surface, and the position of the robotic arm at which the non-contact limit switch closes in response to the proximity of the reference surface, it is possible to accurately calculate a calibration offset value which may be utilized in conjunction with the output of the non-contact limit switch to precisely position the solder nozzle at a desired distance above a circuit board.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates in general to the field of automated solder 
deposition systems and in particular to an improved apparatus and method 
for depositing solder onto solder wettable contact pads on a circuit 
board. Still more particularly, the present invention relates to a method 
and apparatus for accurately maintaining a tool at a desired height above 
a circuit board or other planar member. 
2. Description of the Related Art 
Solder distribution onto mounting pads for surface mount boards has 
generally been accomplished in the semiconductor industry utilizing a 
screening process. In a screen process artwork and screens must be 
fabricated having the solder deposition pattern. Then, a precision 
alignment process is carried out wherein the solder is screened onto the 
surface mount pads. The solder paste used for this process requires a 
substantially long cure and bake time. Thus, in addition to the necessary 
complexity of the alignment process, this prior art technique is 
relatively time consuming. 
The prior art solder distribution technique utilizing screening is further 
complicated by a requirement that the pattern mix very fine lead pitch and 
width surface mound pads along with standard surface mount pads. For 
example, in the case of tape automated bonding, the pitches vary from four 
to twenty mils, while standard surface mount parts have pitches in the 
range of twenty to fifty mils. Thus, this process requires that different 
amounts of solder be distributed on various parts of the board. Given the 
precision required, it is common to utilize separate screening steps, one 
for very fine lead pitch and width surface mount parts and a second for 
the standard surface mount parts. There is also a possibility of damaging 
the solder deposited in a previous step when multiple screening operations 
are carried out. Additionally, screening fine line solder presents a 
problem because the solder paste tends to stick in the openings of the 
screen, as the openings get progressively narrower. 
The prior art also discloses other techniques for depositing solder across 
the surface of a printed circuit. Dip soldering and wave soldering are 
both techniques which are known in the prior art. Wave soldering involves 
pumping a molten solder through a nozzle to form a standing wave. In this 
process, the entire side of an assembly containing printed conductors with 
the leads from the circuit components projecting through various points 
generally travels at a predetermined rate of speed over the standing 
surface of the wave of molten solder. The lower surface of the assemblies 
is placed into contact with the upper fluid surface of the wave. 
By this technique, the solder wave in the first instance wets the joining 
surfaces and promotes through hole penetration. This in turn helps to 
assure the formation of reliable solder joints and fillets. Wave soldering 
is illustrated in U.S. Pat. Nos. 3,705,457 and 4,360,144. An example of an 
immersion technique is illustrated in U.S. Pat. No. 4,608,941 wherein 
panels are immersed in a liquid solder bath and then conveyed to an air 
knife which levels the molten solder on the panels. The air knife is 
therefore used to effectively clear the panels of excess solder and only 
the printed patterns retain the solder. 
Another example of a solder leveler is contained in U.S. Pat. No. 
4,619,841. The technique disclosed therein is used in conjunction with dip 
soldering techniques. Other techniques of selective deposition of solder 
onto printed circuit patterns are described in U.S. Pat. Nos. 4,206,254, 
4,389,771 and 4,493,856. 
U.S. Pat. No. 3,661,638 is also directed to a system for leveling and 
controlling the thickness of a conductive material on the walls of 
through-holes of a printed circuit board. That technique for removing the 
excess amount of conductive material employs heating to melt a conductive 
material after it has been deposited. Then, while the conductive material 
is in a plastic state, gyrating the board to cause the plastic material to 
move circumferentially about the through-hole and flow axially through the 
through-hole. 
More recently, several techniques have been proposed which utilize a solder 
nozzle which deposits solder onto solder wettable contact pads in 
substantially uniform amounts on each pad. The tool utilized with such a 
nozzle generally comprises a solder reservoir or plenum, a heating element 
to melt the solder, and at the bottom of the reservoir, a foot which 
contains the nozzle and which passes over the contact pads to be wetted 
with solder. Examples of these types of systems are disclosed in U.S. Pat. 
No. 4,898,117, filed Apr. 15, 1988, entitled "Solder Deposition System" 
and U.S. Pat. No. 5,042,708, filed of even date herewith entitled "Solder 
Placement Nozzle Assembly." 
Methods utilized to adjust the height of a solder nozzle, such as those 
described in the abovereferenced patent applications, are completely 
manual and typically utilize mechanical shims or feeler gauges of known 
thicknesses to measure the height between the tip of the solder nozzle and 
the circuit board or planar member upon which solder is to be deposited. 
Simple trial and error techniques were then utilized to adjust the nozzle 
height until it was within a desired range. This technique is quite time 
consuming and expensive. 
A problem which exists with such systems is that the utilization of tools 
which contact the soldering nozzle and the surface to be soldered 
generally cause flux to be forced out from between the solder nozzle and 
the surface to be soldered, resulting in flux covered set up tools and the 
spreading of flux onto the soldering nozzle and the surface being 
soldered. Another problem which exists with such systems is that the 
surface being soldered varies in thickness and flatness and therefore tool 
height needs to be measured at each solder application point. 
The environment near a solder nozzle is generally filled with flux, flux 
vapor, molten solder, solder oxides and solder vapor and as a result 
attempts at utilizing contact sensors to ensure proper solder nozzle 
height have been generally unsuccessful. 
It should therefore be apparent that a need exists for a method and 
apparatus which permits the accurate and efficient adjustment of a solder 
nozzle height in a solder deposition system. 
SUMMARY OF THE INVENTION 
It is therefore one object of the present invention to provide an improved 
automated solder deposition system. 
It is another object of the present invention to provide an improved method 
and apparatus for depositing solder onto solder wettable contact pads on a 
circuit board. 
It is yet another object of the present invention to provide an improved 
method and apparatus for accurately maintaining a solder nozzle at a 
desired height above a circuit board. 
The foregoing objects are achieved as is now described. The method and 
apparatus of the present invention permit the accurate sensing of the 
height of a solder nozzle above a circuit board during operation of the 
solder nozzle. A non-contact limit switch, which provides an indication of 
the presence of a planar surface at a preselected proximity, is mounted in 
a fixed relationship with a solder nozzle which is manipulatable by means 
of a robotic arm. A reference surface is provided which includes a 
proximity sensor, such as a through beam fiber optic switch. The proximity 
sensor is mounted a predetermined distance above the reference surface and 
is utilized to detect the solder nozzle as it is moved toward the 
reference surface. By noting the position coordinates of the robotic arm 
at which the proximity sensor indicates the presence of the solder nozzle 
at the predetermined distance above the reference surface, and the 
position of the robotic arm at which the non-contact limit switch closes 
in response to the proximity of the reference surface, it is possible to 
accurately calculate a calibration offset value which may be utilized in 
conjunction with the output of the non-contact limit switch to precisely 
position the solder nozzle at a desired distance above a circuit board. In 
one preferred embodiment of the present invention, the solder nozzle is 
repeatedly moved toward the reference surface during calibration to ensure 
that the calibration offset value obtained is statistically significant.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
With reference now to the figures and in particular with reference to FIG. 
1, there is depicted a partially schematic side view of the solder nozzle 
height sensor system of the present invention. As is illustrated, a solder 
deposition system 10 is depicted which includes a solder nozzle 12 and a 
plurality of supply tubes 14 and 16. As those skilled in the art will 
appreciate, supply tubes 14 and 16 may be utilized to supply inert gas or 
flux, as well as solder into a heated plenum to form a molten mass of 
solder. Solder nozzle 12 is then preferably moved over a plurality of 
wettable contacts on the surface of a circuit board or other planar 
member, such that a uniform amount of solder is deposited on each wettable 
contact. 
Solder nozzle 12 is preferably mounted to robotic arm 18, which is manually 
or electronically manipulatable under the control of an operator or a 
properly programmed robot control unit, such as robot control unit 32. As 
is illustrated, robot control unit 32 preferably provides a plurality of 
electronic signals which are coupled to various servos and motors which 
are utilized to manipulate robotic arm 18. In this manner, solder nozzle 
12 may be accurately and efficiently moved throughout a preprogrammed path 
above the surface of a circuit board or other planar member. Of course, 
variations in the thickness of a circuit board make it imperative that a 
means be provided to accurately adjust the height of solder nozzle 12 
above the surface of a circuit board, so that the amount of solder 
deposited on the wettable contacts thereon is uniform and of desired 
thickness. 
In accordance with an important feature of the present invention, a rigid 
bracket 20 is also mounted to robotic arm 18 in conjunction with solder 
nozzle 12. Rigid bracket 20 is preferably formed of a metallic material or 
other highly rigid substance and serves as a mounting point for mounting 
member 22. As is illustrated, mounting member 22 is preferably mounted in 
a downward direction from rigid bracket 20 and is generally oriented 
parallel to the axis of solder nozzle 12. 
At the lowermost end of mounting member 22 is a pneumatic switch nozzle 24. 
Pneumatic switch nozzle 24 is utilized, in the depicted embodiment of the 
present invention, as a non-contact limit switch and operates on 
principles well known to those skilled in the art of such limit switches. 
A flow of oxygen, nitrogen, or other suitable gas is preferably coupled to 
pneumatic switch nozzle 24 by means of pneumatic supply lines 26 and 28 
and is focused, utilizing nozzles or other techniques, onto the surface of 
a planar member toward which solder nozzle 12 is manipulated. 
As pneumatic switch nozzle 24 reaches a predetermined proximity to a planar 
member, such as reference surface 38, variations in the pressure of the 
gaseous material flowing through pneumatic switch nozzle 24 are detected 
by limit switch 30 and may be utilized to accurately indicate the presence 
or absence of a planar member within a preselected proximity. 
Pneumatic switch nozzle 24 and its associated limit switch 30 may be 
obtained commercially. One such product is marketed by the Nippon 
Pneumatic/Fluidics Systems Company, Ltd., as Model No. DAS-05-05. In the 
depicted embodiment of the present invention, the preselected proximity 
setting of pneumatic switch nozzle 24 is typically set to 0.040 inches. 
This distance is detectable to within an accuracy of plus or minus 0.0015 
inches, utilizing the device discussed above. 
As is illustrated, the output of limit switch 30 is coupled to processor 
34. Processor 34 may be any suitably programmed microprocessor based 
device which may be utilized in conjunction with robot control unit 32 and 
keyboard 36 to perform the rudimentary mathematical calculations necessary 
to achieve a desired solder nozzle height. 
Still referring to FIG. 1, a reference surface 38 is provided. Mounted 
thereon is a calibration jig 40. Calibration jig 40 is preferably a 
semi-circular or horseshoe shaped apparatus which encloses a gap, across 
which is provided an optic beam 42. In a manner which will be explained in 
greater detail herein, optic beam 42 is directed from one side of 
calibration jig 40 to the other side and crosses the aperture therebetween 
at a predetermined distance above the upper surface of reference surface 
38. 
Referring now to FIG. 2, there is depicted a partially schematic 
perspective view of reference surface 38 and a calibration jig 40, which 
may be utilized in the solder nozzle height sensor system of the present 
invention. As is illustrated, optic beam 42 is preferably provided by 
utilizing a plurality of fiber optic cables, such as fiber optic cable 44 
and fiber optic cable 46. Apertures are drilled through calibration jig 
end 50 and calibration jig end 52 of a sufficiently small diameter so that 
the diameter of optic beam 42 is small enough to register the presence of 
solder nozzle 12 within a desired degree of accuracy. In the depicted 
embodiment of the present invention, the diameter of optic beam 42 is 
preferably 0.3 millimeters or smaller, leading to a capability of 
detecting the presence of solder nozzle 12, within an accuracy of plus or 
minus 0.0005". 
As is illustrated, fiber optic cables 44 and 46 are both coupled to beam 
switch 48. Beam switch 48 preferably includes a light source for 
generating optic beam 42, which is coupled to one of the fiber optic 
cables depicted. The presence or absence of the beam being returned by 
means of the other fiber optic cable is then utilized to close or open a 
solid-state switching device. The output of beam switch 48 is then 
preferably coupled to processor 34 (see FIG. 1). 
Referring now to both FIGS. 1 and 2, the calibration procedure which is 
utilized with the solder nozzle height sensor system of the present 
invention will be illustrated. First, solder deposition system 10 is 
energized and solder nozzle 12 is brought up to operational temperatures. 
Those skilled in the art will appreciate that it is important to raise 
solder nozzle 12 to its actual operating temperature utilized during 
solder deposition, so that errors induced by thermal expansion or 
contraction of the material which forms solder nozzle 12 or the mounting 
brackets therefore may be substantially reduced. Thereafter, robotic arm 8 
is utilized to move solder nozzle 12 at a speed Which is substantially 
equal to the speed which will be utilized during actual solder deposition 
technique. 
Next, solder nozzle 12 and its associated pneumatic switch nozzle 24 are 
moved downward toward the upper surface of reference surface 38. Robot 
control unit 32, in conjunction with processor 34 and beam switch 48 are 
then utilized to stop the movement of solder nozzle 12 when optic beam 42 
is interrupted, indicating the presence of solder nozzle 12 at a 
predetermined distance above the upper surface of reference surface 38. 
The position of solder nozzle 12, as determined by robot control unit 32 
is then stored. 
Next, solder nozzle 12 is once again urged toward the upper surface of 
reference surface 38, until such time as a variation in the pressure of 
the gas flow being emitted from pneumatic switch nozzle 24 indicates that 
the upper surface of reference surface 38 is within a preselected 
proximity to pneumatic switch nozzle 24. At this point, limit switch 30 
associated therewith closes, and sends a signal to processor 34, which is 
utilized to stop the movement of robotic arm 18. The position Of solder 
nozzle 12, as determined by robot control unit 32 is then once again 
recorded. 
Next, in accordance with an important feature of the present invention, 
these two preceding steps are repeated a number of times and each new 
value for the position of solder nozzle 12 at the locations where optic 
beam 42 has been interrupted and when pneumatic switch nozzle 24 indicates 
the presence of the upper surface reference surface 38 at a preselected 
proximity are stored. Thereafter, these values are statistically averaged 
and a standard deviation is calculated to permit the system to determine 
whether or not the average values obtained are statistically significant. 
Finally, a calibration offset value, which may be utilized to automatically 
measure the offset from solder nozzle 12 to pneumatic electric nozzle 24, 
is calculated. This calibration offset value is, quite simply, the average 
position of solder nozzle 12 when optic beam 42 is interrupted, less the 
position of solder nozzle 12 at the point when pneumatic switch nozzle 24 
indicates the presence of the upper surface of reference surface 38 at a 
predetermined proximity, less the predetermined height of optic beam 42 
above the upper surface of reference surface 38. This calibration offset 
value is then stored and may be utilized to accurately determine the 
height of solder nozzle 12, in a manner which will be described below. 
The automatic positioning of solder nozzle 12 at a selected distance above 
a surface to be soldered is now accomplished following a procedure which 
will be discussed herein. Once again, after raising the temperature of 
solder nozzle 12 to an operating level and setting the speed of solder 
nozzle 12 to that which Will be utilized during normal soldering 
operations, solder nozzle 12 and pneumatic switch nozzle 24 are urged 
toward the surface to be soldered. Robot control unit 32 is then utilized 
to stop the downward motion of solder nozzle 12 when the variation in 
pressure within pneumatic switch nozzle 24, as detected in limit switch 
30, indicates that pneumatic switch nozzle 24 is within a predetermined 
proximity to the upper surface of the circuit board to be soldered. 
The location of solder nozzle 12 at this point, as sensed by robot control 
unit 32, is recorded. Next, an operator enters a desired solder offset 
height, via keyboard 36, and robot control unit 32 then adjusts the height 
of solder nozzle 12 by calculating a location corresponding to the desired 
height. This is accomplished by adding the robot location indicated when 
pneumatic switch nozzle 24 indicates the presence of a circuit board at a 
preselected proximity to the desired solder height offset and the offset 
calibration value previously calculated. 
In this manner, a desired solder nozzle offset from the upper surface of a 
circuit board may be accurately and efficiently maintained without the 
utilization of a contact switch. By providing a non-contact limit switch, 
such as pneumatic switch nozzle 24 and, in accordance with the method and 
apparatus of the present invention, accurately determining the calibration 
offset value which exists between the height of solder nozzle 12 and the 
height of pneumatic switch nozzle 24 it is possible to accurately position 
solder nozzle 12. This calibration offset value may be rapidly and 
efficiently calculated by processor 34 and thereafter utilized by robot 
control unit 32 to automatically adjust the height of solder nozzle 12 to 
a desired level. 
In view of the fact that this technique utilizes non-contact switching, it 
performs well in the highly volatile environment which generally exists 
near a soldering nozzle. Attempts at utilizing contact sensors to 
determine the height of a solder nozzle above a circuit board have 
generally met with failure due to the fact that such sensors quickly 
become contaminated in this hostile environment. By utilizing a 
non-contact pneumatic-electric switch, such as that comprising pneumatic 
switch nozzle 24 and limit switch 30, the system becomes self-cleaning in 
that a constant, low flow rate of oxygen, air or nitrogen passes through 
pneumatic switch nozzle 24, constantly cleaning the aperture. Testing with 
this system has proven that defect rates may be lowered dramatically with 
this technique. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment, it will be understood by those 
skilled in the art that various changes in form and detail may be made 
therein without departing from the spirit and scope of the invention. For 
example, this system will work well in any robotic system in which a drill 
or liquid dispenser must be brought to within a precisely selected height 
above a planar member which is contaminated by dust and/or corrosive 
gasses which would render a contact sensor inaccurate.