Automated soldering process and apparatus

An automated program controlled discrete soldering system for plated holes of a printed wiring board, having a crucible for holding a discrete amount of solder and operable to travel in a substantial vertical path to a soldering position beneath the board. The solder is metered and deposited in the crucible prior to each application of laser energy to the crucible for melting the solder.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to soldering, and more particularly, to an 
improved method and apparatus for making soldered joints for assemblages 
of electronic components. 
2. Description of the Prior Art 
Methods of making soldered joints in devices where many electronic 
components are electrically joined to an interconnecting pattern on a 
common base of insulated material, such as printed wiring boards (PWB), 
may be classified as either discrete joint soldering, where each joint is 
soldered individually by a soldering iron, or batch soldering such as wave 
soldering, where the PWB with its full complement of components in 
position, is subjected to a molten wave of solder. Heretofore, because of 
its relative speed, wave soldering was widely utilized, particularly where 
a large number of soldered joints were required on a single PWB. However, 
there are certain disadvantages in wave soldering, for example, the 
individual components are usually mechanically fastened to the PWB in some 
manner, prior to being subjected to the molten wave of solder; the 
time-of-exposure to the molten solder must be long enough to heat the 
greatest thermal sink locality on the PWB, resulting in unnecessary 
thermal stressing of all other areas of the PWB and its associated 
components; and the exposure of the PWB to high temperatures requires 
special treatment to prevent moisture outgassing and associated 
delamination of the epoxy glass board material. Additionally, wave 
soldering may result in many defective or non-uniform joints. Therefore, 
in order to measure the actual efficiency of the wave-soldering method, 
consideration must be given to the time and effort required to 
mechanically fasten the components in position, the wave-soldering process 
itself, and the detection and repair of defective soldered joints. The 
main categories of defective joints made by wave soldering are: 
insufficient or excess solder, voids, no solder, or pinholes in the joint, 
and solder bridging between circuits, and to a lesser extent lifted solder 
pads. Discrete joint soldering, of course, is beneficial in that it 
permits optimization of the process parameters required for each joint on 
the PWB, resulting in high quality joints with minimum defects. However, 
to enable discrete joint soldering to be economically feasible when 
compared to wave soldering, certain problems must be overcome; 
particularly where the thermal energy required to heat the joint is 
conducted through the soldering tip or probe, such as a soldering iron. 
For example, the cross-section of the probe should be preferably 
approximately the same size as the diameter of the solder pad in order to 
restrict the heat applied exclusively to the target joint (typically 
0.050") diameter. This small size imposes practical limits on the rate at 
which the thermal energy can be transferred to the joint by conduction. 
This in turn, particularly for joints with large heat sinks (i.e. buss 
planes), creates a significant time lag when the thermal energy stored in 
an idle probe is initially drawn into the joint at a higher rate than can 
be resupplied from a remote power (heat) source. This time/temperature 
relationship results in uncontrolled and unnecessary heating of the 
surrounding board laminate and the associated component. Further, for 
discrete-joint soldering, the probe material should on the one hand be a 
good thermal conductor such as most metals, and on the other hand not 
corrode or degrade in any way that would have an adverse effect on 
conduction of heat into the joint. Maintenance of a constant, and 
therefore predictable, conductance factor, appears to be a requisite for 
an automated, discrete joint soldering process. 
In an attempt to overcome the disadvantages of discrete-joint soldering 
when performed with a soldering iron, it has been proposed to utilize a 
laser beam as the energy source for surface-mounted components, where the 
solder must be applied to the same side of the PWB as the components are 
mounted. However, as far as is known, a method and apparatus for making 
discretely soldered joints in through-hole PWB's automatically, and 
thereby overcoming the disadvantages of wave soldering, has not been known 
prior to the present invention. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, there is provided a 
method and apparatus for making discretely soldered joints in 
through-holes of PWB's where the thermal energy is applied to a precise 
amount of solder in a small area spaced from and adjacent the underside of 
the PWB opposite that on which the components are mounted. 
In another aspect of the invention, there is provided such a method and 
apparatus that includes at least one crucible for holding a pool of molten 
solder, with said crucible having a susceptor surface, and being 
dimensioned to apply solder to a single through-hole joint when the 
crucible and joint, under program control, are in a predetermined position 
relative to one another, and an energy beam is applied to the susceptor 
surface. After completing the soldering in one relative position, the 
crucible and PWB are repositioned relative to one another, under program 
control, to apply solder to another through-hole in the PWB. 
Also, the present invention includes provision for maximizing the 
reliability of delivery of metered soldered slugs automatically to the 
crucible; and a crucible structure that optimizes the precise control of 
the conversion of light energy to thermal energy, which further maximizes 
conformity of the soldered joints.

GENERAL DISCUSSION OF THE PREFERRED EMBODIMENT 
The method and system of the present invention, which is adapted to be used 
in a robotic printed wiring board (PWB) assembly and test station, 
involves a controlled, discrete-joint, soldering process where the solder 
process variables are precisely controlled to provide uniform, 
minimum-defect, soldered joints. In carrying out the process, a 
computer-controlled apparatus in utilized that feeds the solder, 
preferably with the flux in its hollow core, and severs a predetermined 
length (volume) of such solder. The metered solder is supported while 
being transported by slight air pressure introduced radially in a bore 
containing the solder slug; and then deposited in the crucible with aid of 
slight air pressure introduced above the slug carrying bore. 
The crucible is carried upwardly by a free piston, toward the underside of 
a PWB plated through-hole from which a short length of component lead wire 
may protrude. The lead wire, the plated through-hole, and the solder 
filler metal constitutes the soldered joint. The bottom of the crucible 
that carries the metered solder piece is a susceptor that converts light 
energy focused on it to thermal energy to heat the solder to a molten 
state for application to the joint. In turn, the molten solder comes into 
contact with the protruding wire and the plated hole to heat the wire and 
the through-hole plating, such that the molten solder is drawn from the 
crucible into the hole. Once the solder is in place, the crucible and the 
PWB are repositioned relative to one another for soldering another joint. 
The thermal energy which is created preferably by laser beam, particularly 
for applications where ambient heat must be avoided, is applied to a 
suscepting surface of a crucible which conducts the heat to the solder in 
the crucible, and then from the solder to the component lead wire and the 
plated hole to form the joint. Thermal energy, such as infrared radiation, 
also may be applied directly to the area surrounding the plated hole for 
localized preheating, to provide a more rapid soldering of each joint. 
The susceptor base of the crucible to which the thermal energy beam is 
applied has a surface area substantially larger than the area of the 
crucible to permit greater responsiveness to beam power changes, and 
faster temperature rise times by distributing the laser beam power over a 
larger area. The crucible is connected at its base to a cylindrical 
piston-like member slidably mounted in a vertical cylinder. The crucible 
and the cylindrical member form a free piston that is propelled under 
fluid pressure to the soldering position. The use of the free piston 
ensures the proper positioning of the molten solder for the discrete 
formation of each soldered joint. 
In the method and apparatus illustrated, solder is metered and heated for 
one joint at a time. The source of thermal energy is programmed to provide 
temperature control during the repetitive process of metering, melting, 
and applying the solder. In one application of the present invention, the 
laser beam parameters are controlled by computer software to provide the 
power, pulse length, or continuous beam required for the particular 
application. The axis of the energy beam is substantially coincident with 
the axis of travel of the piston. The crucible may be preferably of any 
suitable graphitic material which has a relatively rapid rate of heat 
transfer and is a good suscepting material for infrared energy. When a 
laser for infrared beam such as is created by a CO.sup.2 laser is 
utilized, it has been determined that graphite or silicon-carbide is 
particularly desirable in that it does not react with solder or flux, is 
non-wetting, and does not oxidize below 750.degree. F. It also has the 
high thermal conductivity necessary to provide uniformity in the soldered 
joints. The detailed portion of the system involving the structure of the 
crucible and structure and function of the free piston arrangement is the 
subject of another inventive entity. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 through 4, a soldering apparatus 10 is comprised of a 
generally C-shaped supporting frame 11 of rectangular cross-section having 
an upper or top portion 12, fastened to a back or vertical portion 13, by 
screws 14. The back portion 13 in turn is fastened to a base portion 15 by 
screws such as 16. The base 15 may be supported or fastened to a work 
surface (not shown) to stand upright as viewed in the drawings. 
Secured to the base portion 15 adjacent one end thereof by screws 17 is a 
solder applying assembly 18 that melts, carries and applies metered solder 
to the underside of a PWB 20 (see FIG. 3). Fastened to the back portion 13 
of the frame 11 is a solder feeding assembly 21, that feeds flux-core 
solder wire 22 to a shearing and carrying mechanism 23 fastened to the 
upper frame portion 12 by screws 24. The solder 22 is fed from a spool 
such as 25 rotatably fastened to the back frame portion 13. 
The solder feeding assembly 21 is comprised of a stepper motor 26 that 
drives a serrated edge pulley 27 through a shaft 28. The motor 26 is 
supported by a plate 30 through which its shaft 28 (FIG. 4) extends. A 
pair of spaced arms 31 are pivotally attached at one end of the plate 30 
by a pin 32; the other end of the arms 31 rotatably support therebetween 
an idler pulley 33 on a shaft 34. The solder 22 on the spool 25 is guided 
through a tube that extends through a member 36 attached to the plate 30 
and then between the pulleys 27 and 33. The arms 31 are urged pivotally 
about the pin 32 by a spring 37 such that the pulley 33 engages the solder 
at the periphery of the pulley 27. The solder 22 fits in the groove of the 
pulley 33 and is gripped by serrations on the periphery of the pulley 27 
to be fed as governed by the stepping motor 26. The solder 22 is guided 
through a second tube 38 and fed through a bore 40 in the upper frame 
member 12 to be engaged by a shear 42 of the assembly 23. 
The shearing assembly 23 is comprised of the shear 42 which is slidably 
mounted between spaced guides 41 in contact with the upper surface of the 
member 12. The solder slug is advanced from the bore 40 into bore 43. 
Above bore 43 in the shear is a cover member 44 that prevents the metered 
solder from leaving the bore 43 during a first portion of its travel. 
After the solder piece is sheared, a slight gas pressure is created in the 
bore 43 of the member 41 via tube 45 to cause the solder piece to levitate 
in the bore while being positioned in registry with crucible 50. Also, 
slight gas pressure is created above the solder piece in the bore 43 via 
tube 46 to help dislodge the solder from the bore 43 when it is in 
registry with the crucible 50. A hydraulic or pneumatic cylinder 51 is 
fastened to the upper frame portion 12 by screws 52; and has a piston rod 
53 connected to the slidable shear member 42 through a connecting member 
55 to move the shearing member 42 into and out of shearing position. 
The solder melting and applying assembly 18 is comprised of a metallic 
piece or supporting member 60 which may be substantially rectangular in 
configuration. The lower portion of the piece 60 which is fastened to the 
base member 15 of the frame 11 by the screws 17 (see FIG. 2) has a slot 61 
that mates with a slot 62 in the base 15 to form a rectangular opening, 
the axis of which is parallel to the base member (see FIG. 3). 
Communicating with the slot 61 at its top wall is a bore 63, an 
intermediate portion 64 of lesser diameter and an upper portion 65 of 
least diameter. The portion layer 63 of the bore houses an hydraulic or 
pneumatic cylinder 66 having a piston carried member 67 that engages a 
ball 68 to sealingly close and open the lower end of the upper portion 65 
of the bore. The ball 68 is forced by the member 67 to engage an O-ring 69 
to seal the upper portion 65. The bore comprised of the portion 63, 64 and 
65 is preferably circular in cross-section. The cylinder 66 extends into 
the slot 61 to accommodate a tube 71 for introducing fluid to apply 
pressure to operate the ball valve 68. 
Formed in the upper portion of the piece 60 is a cylindrical bore 72 having 
a portion of reduced diameter to form a shoulder 73. The upper portion of 
the bore 72 communicates with the exterior surface of the piece 60, and 
has a vertical axis that extends substantially normal to the path of the 
shear member 42. The lower end of the bore 72 communicates with a 
cylindrical opening 74 having an axis that extends normal to the axis of 
the opening 72. The opening 74 has a shoulder 75 to support a lens such as 
76 that seals the opening by an O-ring 79. The outer end of the opening 74 
is threaded to receive a threaded annular ring or plug 80 for securing the 
lens 76 in position. The piece 60 also has an opening the axis of which 
intersects the opening 72 and the opening 74 at an angle of 45.degree. to 
the axis of the opening 72 and the opening 74. A deflecting mirror 81 fits 
at the juncture of 72 and 74 and is sealed therein by a threaded plug 82 
that engages an O-ring 83. The face of the mirror 81 is at such an angle 
that a beam of energy entering the opening 74 through the lens 76 is 
deflected to be coincident with the axis of the opening 72 and then go 
through a tapered bore 85 of a sleeve 86 that fits in the opening 72. The 
lower end of the sleeve 86 rests against the shoulder 73. The upper end of 
the opening 72 has a linear 87 having a lower edge that fits in an annular 
groove 88 at the upper end of the sleeve 86. 
A cylindrical piston 90 preferably made of graphite slidably fits in the 
linear 87 in the upper end of the cylindrical opening 72. The sleeve 86 
has a port 91 that communicates at one end with the upper surface 92 of 
the sleeve 86 and is offset and radially spaced from the axis of the 
central opening 85 of the sleeve 86. The port 91 is open at its other end 
with the peripheral surface of the sleeve 86 and an opening 93 in the 
metallic piece 60 for introducing fluid pressure beneath the piston 90 by 
way of a tube 94 for lifting the crucible 50 to its respective operating 
positions as described hereinafter. 
Referring to FIGS. 5, 6 and 7, the piston 90 is substantially hollow and is 
comprised of a lower tubular skirt portion 101, preferably made of 
graphite, an intermediate portion 102 bonded to the skirt 101 and has a 
central opening 105 that is cylindrical throughout a portion of its length 
and is frustoconical at its upper portion 104. The upper smaller diameter 
portion of the opening 105 of the member 102 is threaded to receive the 
crucible 50 which forms the upper portion of the piston 90. The crucible 
50 includes a chamber 106 that is of a depth and diameter to contain a 
sufficient quantity of solder for a single joint. The chamber 106 is 
slightly tapered internally, which aids in causing the molten solder 
sphere to assume a position towards the top of the chamber. The depth of 
the crucible chamber 61 is also sufficient to receive a typical protruding 
lead wire of the mounted component that may vary in length. The crucible 
50 has a base portion 107 with a diameter substantially greater than the 
exterior diameter of the crucible chamber 106. The enlarged area of the 
base 107 includes the susceptor surface 108 to which the laser energy beam 
is applied. 
The intermediate portion 102 of the piston 90 is a thermal insulator. One 
type that proved to be operable is a glass ceramic, which is machinable 
and widely known by the trade name MACOR manufactured by the Corning Glass 
Company of Corning, N.Y. The crucible 50 and the skirt portion 101 are 
preferably made of graphite. The portion 102 acts as a transition piece 
that is mechanically attached to the skirt 101 and the crucible 50 for the 
thermal isolation of the crucible 50. The thermal expansion coefficients 
of the glass and graphite appear to be well matched, at least in 
isothermal expansion, such that the diametrical clearance between the 
sleeve 87, which is preferably pyrex, and the graphite skirt 101 is 
maintained over wide operating temperatures. The graphite skirt 101 has a 
passage 109 for venting nitrogen gas to release the pressure beneath the 
crucible 50 and prevent the piston 90 from being propelled beyond a 
predetermined limit, if unrestrained by a printed wiring board in 
soldering position. 
Referring to FIGS. 7A through 7D, the actual application of the solder is 
illustrated. In FIG. 7A, solder slug 22' is deposited in the crucible 
chamber 106, and if desired, the underside of the PWB board may be 
preheated by infrared energy as illustrated by the arrow 111, while the 
crucible 50 advances toward the PWB 20 as shown in FIG. 7B. The energy 
illustrated by a laser beam 112 is applied to the underside of susceptor 
surface 108 of the crucible 50, causing the solder to form a molten sphere 
115 in the chamber 106, as shown in FIG. 7C. As the piston 90 is raised 
further, the protruding component wire 114 of the PWB 20 is immersed in 
the molten solder sphere 115. This immersion causes a heat transfer from 
the molten solder 115 to the wire 114 and the metallic plating and pads of 
through-hole 116. Once the wire 114 and the plated through-hole 116 have 
reached the proper temperature, the ball of solder 115 is then drawn 
between the protruding wire 114 and the plated hole 116, as shown in FIG. 
7D, to form the soldered joint. In actual practice, the solder 115 travels 
through plated hole 116 of the PWB forming a frustoconical solder shape at 
the top of the PWB, and a similar frustoconical shape at its bottom or 
underside. Once the joint is complete, the crucible 50 is withdrawn and 
travels downwardly in the reverse direction from that previously described 
until it reaches the starting position as shown in FIG. 7A. The laser is 
then turned off and a programmed wait time is entered to permit the solder 
fillet to solidify (see FIG. 7D). 
As previously mentioned, the infrared beam 111 may be applied to the 
underside of the PWB board 20 at each joint just before the soldering 
cycle to minimize the soldering cycle time. In actual operation the entire 
soldering cycle from cutting and feeding the solder for one joint to the 
cutting and feeding of the solder for the next joint consumes 
approximately four seconds. The utilization of the auxiliary infrared beam 
111 shortens each cycle by approximately one-and-one-half seconds. 
A more detailed understanding of the invention may be had by describing the 
operation of the device in connection with FIGS. 8 and 9 in addition to 
the previously described figures. In FIG. 4 the flux core solder 22 on the 
spool 25 is advanced between gear 27 and pressure wheel 33 to cause a 
precise amount of solder to extend beyond the top of the member 12 into 
the hole 43 to be cut by the shearing member 42, which is operated by the 
double acting hydraulic or pneumatic actuator 51. In the retracted 
position as shown in FIG. 8A the piston rod 53 is in its extreme left-hand 
position as viewed in FIG. 3, and the shear hole 43 is aligned 
concentrically with the feed hole 40. Initially, according to one actual 
embodiment, the stepper motor 26 drives the solder into bore 43 0.003" for 
each of a programmed number of activating pulses and then stops. Then the 
shearing member 42 shears the solder 22' from 22 and moves it in the shear 
hole 43 (see FIG. 8B) across the upper member 12 in one mechanical motion 
to a point centered over the crucible chamber 106 of the crucible 50 as 
shown in FIG. 8C. To hold the solder 22' in the shear hole 43 until it has 
come to a full stop and is aligned with the crucible 50, a jet of low 
pressure air flows into the shear hole 43 through a radial inlet 43' 
located in the sidewall of the shear hole, and to which is attached a 
small flexible hose connected to an air supply by way of a tube 45 (FIG. 
8B). 
When the shear 42 cuts off the piece of solder, the piece of solder 22' is 
retained in the hole 43 by the slight positive pressure of the air. The 
air stream is directed upwardly and downwardly in the shearing hole, and 
the presence of the slug 22' in the hole 43 acts to modify or throttle the 
opposed streams. Applying Bernoulli's law relating to the energy 
relationship between internal fluid pressure and stream velocity, opposed 
forces develop on each end of the solder 22' in inverse proportion to the 
air stream velocity. When the slug 22' is centered on the jet of air 
entering through inlet 43', the two opposing air streams are equal, and 
the forces on the slug cancel. Due to the force of gravity acting on the 
slug 22' it, of course, tends to fall out of the hole 43. However, as the 
slug moves downwardly, the throttle action of the slug causes imbalance in 
the air streams with a greater proportion flowing upwardly. A point of 
equilibrium is reached where the pressure/force differential exerted 
upwards on the slug nulls the downward force of gravity and the slug 
levitates in the hole 43. Initially, or before hole 43 clears member 12, 
air entering hole 43 through inlet 43' vents totally at the top of hole 
43. To ensure that the solder contained in the shear hole 43 under 
pressure of the air does not leave the bore 43, the member 44 may be 
attached to the shear 42 to cover but not seal the shear hole 43. When the 
shear hole 43 is aligned with the chamber 106 of the crucible 50, the 
nitrogen venting valve 68 is closed causing nitrogen pressure to rise 
causing the piston 90 to rise to contact a small pin stop 140 projecting 
from the underside of the extended shear 140. This prevents the crucible 
50 from actually contacting the shear hole 43 assuring reliable transfer 
of the solder slug to the chamber 106 of the crucible 50. The air pressure 
in the tube 45 is "shut off" (FIG. 8C) and the piece of solder drops by 
the force of gravity in the chamber 106. A short pulse of air may be 
introduced into the top of the shear hole 43 by way of tube 46 to release 
the solder piece should it stick. 
Once the slug has been transferred to crucible 50, valve 68 is opened 
causing nitrogen to vent through port 65' and allowing piston 90 to return 
to start position. The shear member 42 is then retracted to its extreme 
left-hand position as viewed in FIG. 3, clearing the path such that the 
piston 90 can move upwardly without restriction. A sensing device (not 
shown) may be used to sense that the piston 90 is at the proper position 
and out of the path of travel of the shear 42 to receive the next slug of 
solder. 
The laser may now begin heating the crucible 50 and again closing valve 58 
to cause the piston 90 to raise the crucible to the plated hole 116 of the 
PWB. The laser power level and timing of the piston ascent is coordinated 
such that the solder slug is above liquidous temperature when the solder 
contacts the protruding wire 114. The laser power may be either 
programmably adjusted to compensate for the heat sinking effect of the 
joint, or a temperature sensing closed loop control of laser power may be 
used to achieve optimal conditions. Such conditions would be fast rise in 
temperature to the maximum tolerable in the crucible, with the laser power 
being varied in real time to maintain this temperature throughout the 
application of the solder. 
Above a temperature threshold of 750.degree. F., graphite chemically reacts 
with oxygen in the air. For this reason, nitrogen gas is continually 
supplied by way of the tube 94 to exclude air from the chamber, and thus 
inhibit the deleterious effects of oxygen on the hot suscepting surface 
when illuminated by the laser beam. The regulated nitrogen pressure 
provides the force for lifting the piston and crucible containing the 
solder into soldering position when the valve 68 is closed. At another 
time in the cycle, the piston member 67 is released to permit the pressure 
to dissipate through the passage 65 and its outlet bore 65', permitting 
the piston 90 to drop to its lowermost position in the cylinder in 
preparation for repeating the process. 
In commenting on the method of the present invention, the vertical laser 
beam alignment with the path of travel of the crucible permits the laser 
beam to be keyed "on" at any point in the soldering sequence, or left "on" 
at a lower power. If desired, it can begin heating prior to the depositing 
of the solder slug to reduce cycle time, for example. The susceptor 
surface area of the crucible being larger than the crucible chamber 
permits a reduction of beam power density by distributing the energy over 
a larger area. The fact that the crucible is carried by a free piston 
assures the proper soldering position for the molten solder regardless of 
any mechanical deviations in the vertical position of the PWB board. 
In connection with the configuration of the chamber 106 in the crucible 50, 
such area and configuration relative to the volume of solder that is 
utilized for each joint, results in a joint with the proper fillet both on 
top and below the PWB, without leaving any residue of solder in the 
chamber 106. It should be pointed out that the described embodiment is 
what may be termed a nonsolder-wetting crucible that has the advantage of 
a relatively simple design and fabrication, as well as being unaffected by 
the solder alloy, flux or process temperature. The use of a non-wetting 
crucible in contact with the solder renders it important that the solder 
volume required to form an acceptable joint be predetermined and metered; 
and dispensed to the joint such that all the supplied solder remains with 
the joint when the crucible 50 is withdrawn from the proximity of the 
joint. As previously described, this non-wetted crucible lifts toward the 
PWB 20 to a position where the solder does not lose contact with chamber 
106 as it travels into the plated hole of the PWB; thus preventing a 
premature termination of feed to the joint. During experimentation, a 
solder-wetted surface of a nickel plated metallic crucible 50 was utilized 
where from 50% to 100% excess solder was brought into contact with the 
joint. With this arrangement, the crucible did not approach the PWB closer 
than the end of the projecting lead wire. When the crucible was withdrawn 
from the joint, the solder column existing between the PWB pad and the 
crucible rapidly necked down and then parted. The joint inherently 
retained a volume sufficient to be acceptable; however, the excess solder 
remained with the crucible and required removal prior to the next cycle. 
The material from which the crucible is made is important in that it should 
present a stable and predictable coupling factor for the particular laser 
wave length utilized. It also should present a relatively high coupling 
factor for the sake of process efficiency, and to minimize the effects of 
any reflected laser energy. The desired low friction instant response of 
the free piston has been achieved by adapting a commercially available 
pneumatic dash pot to this application. The dash pot is manufactured by 
Airpot Corporation, Norwalk, CT. It is composed of a pyrex glass cylinder 
to which has been carefully matched a graphite piston, normally with a 
small piston rod attached. In our application, the piston rod was removed 
and the face of the piston bored out to provide a clear path through the 
piston zone for the laser beam. 
With respect to the limitations applied to the beam of laser energy, an 
initial energy input of one level is desired to rapidly raise the 
temperature of the crucible solder to a temperature in excess of 
500.degree. F. Then energy at another level is applied to maintain the 
temperature. For certain applications, an increase in the input power may 
be required to compensate for the thermal sinking effect when the liquid 
solder is brought into contact with the joint. Control is also required to 
compensate for the rise in temperature of the joint at a rate of energy 
transfer as the solder wets to the extremity of the joint on the component 
side of the PWB. The maximum process temperature is limited by the 
particular flux that is utilized; and the rate at which the crucible can 
be brought to working temperature is limited to the maximum beam power 
density which the suscepting material can survive. 
Referring to FIG. 9, a power supply 120 controls current through the laser 
generator 95. The laser beam power is controlled by a microcomputer 127 in 
real time through a binary coded decimal output port and D/A converter 
123. X-Y positioner 130 controls the positioning of the PWB through a 
computerized numerical control referred to generally at 131. The solder 
advance is achieved through a stepping motor and pneumatic valve interface 
module generally referred to at 132, which is controlled by the 
microcomputer 127. The individual components of the system are all well 
known in the art including the manner and method of controlling a laser. 
Appendix A is a print-out of a program for automatic operation of the 
present method of the invention according to one embodiment thereof. 
The present invention thus provides virgin solder at each joint with the 
volume of solder and flux accurately metered. The thermal energy can be 
adjusted for various joints, and to provide minimum process time, with 
minimum heat bloom to areas surrounding the joint. 
Although the invention has been described in connection with one specific 
embodiment, it is understood that variations and modifications may be made 
without departing from the scope and spirit of the appended claims. 
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