Non-contact detection of liquefaction in meltable materials

The melting of solder in a reflow soldering operation is detected by examining the rate of change of the solder temperature or by detecting a change in a surface reflectance characteristic of the solder which occurs upon melting.

BACKGROUND OF THE INVENTION 
This invention is directed to a method of determining the exact moment when 
a material is undergoing a change of state, e.g., in industrial or 
scientific processes. The invention is more particularly directed to the 
detection of the exact moment of melting in solids which are being heated 
to their melting points. It applies also to the detection of "freezing" in 
liquids which are being cooled to their solidification points. 
The invention is especially applicable to the electronics manufacturing 
industry, particularly in the manufacture of printed circuit boards and 
the like where molten solder is applied at an electrical connection in 
order to form a permanent mechanical and electrical bond between two 
conductors. However, it should be appreciated that there are a wide 
variety of uses for the invention and it will be described in the context 
of printed circuit board manufacturing by way of example only. 
The process of applying solder to form a permanent mechanical and 
electrical bond between two conductors on a printed circuit board is 
carried out in various ways which will be familiar to practitioners of the 
soldering art and will not be discussed at length here. For small-scale 
production, solder joints are individually hand-soldered; for large-scale 
production, an entire circuit board containing hundreds or thousands of 
solder-joints-to-be can be soldered in one step by wave-soldering or by 
reflow soldering. In the former, after certain preparatory steps, the 
board is passed over the surface of a molten solder bath where the solder 
is caused to adhere to local areas at the intended joints. In reflow 
soldering, individual solder pads are formed at the desired locations by 
use of molten solder which is then allowed to solidify. The desired 
electrical conductors are then placed in mechanical contact with their 
proper pads and the entire board is raised to the desired temperature 
either by radiant heating or by various other methods. Careful control is 
required of heating rates and temperatures. 
The preferred embodiment of our invention is addressed to the process of 
reflow soldering. A particular problem in this process is that all solder 
joints on a given board do not always have the same amount of solder or of 
adjoining metal in contact with the solder. The result is that various 
solder joints will have different heat-input requirements, whereas 
standard radiant or convective heat-input methods will tend to overheat 
the smaller joints while underheating the larger ones. This problem is 
partly overcome by yet another method of reflow soldering, notably vapor 
phase soldering, but this method is not yet in wide use and it carries 
other problems which remain to be solved. 
In industrial processes such as reflow soldering in which materials are 
being heat-processed, it is important to know the exact moment when a 
given solid turns to liquid or vice-versa. It is also advantageous to be 
able to make this determinaion without making physical contact with the 
sample and without having to know the radiant emissivity of the surface. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method and 
apparatus for non-contact detection of liquefaction in metals, and more 
particularly a method and apparatus for detecting the reflow of solder in 
a solid solder joint undergoing melting. 
It is a further object of this invention to provide such a method and 
apparatus wherein the beginning and end points of reflow can be accurately 
detected. 
In a first embodiment of the present invention, the surface of a material 
being heated to its melting point is monitored by an infrared detection 
system whose output signal is related to surface temperature. A signal 
processor recognizes the slight decrease in heating rate which is known to 
occur as the solid turns to liquid due to the latent-heat-of-fusion 
phenomenon. The moment at which the transition occurs is automatically 
signalled by the processor, and the re-solidification of the material can 
also be recognized by the use of the inverse of the heat-of-fusion effect. 
In a specific application of the invention to reflow soldering of lap 
joints, an integral, self-dividing solder preform can be used. The preform 
comprises an appropriately-shaped strip of solder placed between a row or 
group of leads and their respective pads, with reflow soldering then being 
carried out by means of laser beam irradiation. As the preform becomes 
molten, it will separate itself into individual solder masses, one at each 
solder joint, through a combination of surface tensile forces and 
repulsion by the substrate surface. This characteristic can be enhanced by 
selective application of solder flux to the areas to be soldered or by 
application of a solder-repellent material to areas of the substrate 
between solder joints. 
In a further embodiment of the invention, solder reflow can be detected by 
monitoring surface reflectivity characteristics.

DETAILED DESCRIPTION OF THE INVENTION 
The invention makes use of a well known physical phenomenon dealing with 
solids which are being heated to their melting points. During heating, the 
temperature of the solid continues to rise until melting begins. At this 
point, the further addition of heat causes no further temperature increase 
until the material is fully melted, after which the liquid will be 
observed to increase in temperature. 
To facilitate a full understanding of the invention, the physical 
principles by which the solid/liquid change of state can be recognized 
will first be discussed. 
The words "melting" or "fusion" are used to describe the process in which 
matter is transformed from the solid to the liquid phase. For most 
materials, particularly crystalline ones such as ice, many metals, and so 
forth, the transition occurs abruptly at a well defined temperature. This 
temperature is a constant for each material, provided that the pressure is 
not changed. By contrast, most amorphous materials such as glass, various 
waxes and non-crystalline metals pass through a gradual change of state 
over a broader temperature range. Our invention deals primarily with those 
materials which melt abruptly. 
It is a matter of common experience that the addition of heat to a material 
will raise its temperature. It is not often realized that, for a material 
in a given state, the rate of temperature increase is almost exactly 
proportional to the rate of heat input, whether material is a solid, a 
liquid, or a gas. 
As a heated solid reaches its melting temperature, the further addition of 
heat energy serves to overcome the molecular binding forces which hold the 
material rigid. Heat therefore seems to disappear within the material, for 
a time, without causing a further temperature increase. A well-defined 
amount of thermal energy is required in order to melt each gram of a given 
material. This is called the "latent" heat of fusion and it is added to 
the molecular energy of the material as it becomes liquid. This energy is 
then restored to the environment during the reverse of the process. 
A similar phenomenon occurs when a liquid or a solid turns to gas or 
"vapor". Heat energy is again used in separating molecules so that no 
external temperature increase is seen during vaporization. For a given 
material, the amount of heat needed per gram is again fixed. It is called 
the "latent heat of vaporization" and is not necessarily related to the 
latent heat of fusion for the same material. For example, in the case of 
pure water, the latent heat of fusion is 80 calories per gram and the 
latent heat of vaporization is 539 calories per gram, both measured at 
atmospheric pressure. 
Once a material has undergone a full transition from one phase to another, 
the further addition or extraction of heat will result in the expected 
temperature change. 
FIG. 1 illustrates an idealized surface-temperature history of a solid 
which is melted with a constant rate of heat input and is then frozen with 
a constant heat withdrawal rate. The sloping lines show a constant rate of 
surface temperature change; the level lines show heat entering or leaving 
the substance without causing a temperature change. The upper two sloping 
lines represent the heating and cooling of the material in the liquid 
state. These may or may not have the same slope as the lower sloping 
lines, depending upon whether the specific heat of the material changes 
during the transition. 
FIG. 1 assumes that the thermal changes which occur with time do so 
instantly throughout the material; that is, there are no thermal gradients 
and all parts are at the same temperature at any one time. Melting and 
freezing thus occur uniformly throughout the sample. 
In the practical case, materials to be melted are heated non-uniformly, 
usually from a surface, and time is required for heat to penetrate the 
whole volume. Thermal gradients exist, and various parts of the material 
are at different temperatures at any one time. The part nearest the heat 
source is the first to melt. Beneath the molten part is a "phase front" or 
transition boundary, between the liquid and solid states, which progresses 
through the material as the heat penetrates more deeply. The abrupt 
heating-rate discontinuities of FIG. 1 are softened by the fact that the 
change of state occurs at different times in various parts of the 
material. This can occur at the surface as well, so that the detector is 
viewing both phases at once. The result is seen in FIG. 2, and this 
typifies an actual laboratory result where a solid material is heated at 
one surface and is then allowed to cool normally while the surface 
temperature is observed. 
Another typical principle upon which the invention relies is the infrared 
measurement of surface temperatures for various materials. This is a 
science which is more than a century old and which is amply described in 
the technical literature. It will be discussed only briefly here by saying 
that all surfaces of temperature greater than absolute zero emit 
electromagnetic radiation in accordance with their surface temperature. 
For surfaces below incandescent temperatures, the radiation is primarily 
infrared and falls into certain wavelength regions according to known 
physical laws. This makes it possible for one to measure the amount of 
emitted radiation and to relate it to the surface temperature. The basic 
method is applicable to incandescent objects as well, the difference being 
that the emitted radiation will include visible light as well as infrared. 
This method of temperature measurement and its various forms are referred 
to by various names such as radiation thermometry, optical pyrometry, 
infrared radiometry, and so forth. The advantages offered are non-contact 
sensing (which can be from a convenient distance), high response speed and 
minimal disturbance of the temperature and process being measured. 
An important consideration in radiation thermometry is that all surfaces at 
a given temperature do not emit the same intensity of radiation. Certain 
surfaces, such as black ones, and certain surface configurations, such as 
cavities or flat but roughened surfaces, are better emitters than most 
light-colored or shiny flat surfaces. This property is described in terms 
of a quantity called "emissivity", which is measured on a scale from zero 
to one (or 0 to 100%). The lower value would be for an ideal non-emitter 
and the upper one is for a perfect emitter, neither of which cases 
actually occurs in nature although they can be approached quite closely. 
Emissivity is important because is must be taken into account when one 
attempts to relate the measured surface radiation to the actual surface 
temperature. Quite often, the emissivity of a target surface is unknown or 
is not known to the accuracy desired, which leaves varying amounts of 
uncertainty in the resulting temperature values. This point is mentioned 
here as a prelude to our later discussion in which we indicate that the 
use of our invention does not require a knowledge of target emissivity, 
which is an important advantage. Indeed, if the actual melting point of 
the target material is already known from separate measurements, then the 
emissivity can be determined via our method. 
Another physical principle, which is used in a preferred embodiment of our 
method, concerns the use of radiant energy to heat a target surface while, 
at the same time, monitoring the surface temperature by measuring its 
infrared radiation. The heating-while-measuring process poses the problem 
that, unless preventive measures are taken, radiation from the heat source 
can be reflected by the target surface directly into the radiation 
detector where it can be mistaken for thermal radiation from the target, 
leading to measurement errors. 
As a general case, various workers solve this problem by ensuring that the 
heating radiation is in a different wavelength band from that to which the 
detector is sensitive. This is most conveniently done by optical 
filtering, which is a technique well known to physicists and others; the 
method is especially simplified if a single-wave-band laser is used as the 
heating source, for its wavelength content can be removed from the 
detector spectrum by a fairly simple filter. 
Other solutions to the heating-while-measuring problem involve the use of 
shields or baffles to obstruct heat-source radiation from the detector 
path, or to heat one part of the target while observing another part. 
Another possible method is to time-share the processes such as by 
obscuring the heat source momentarily while making a temperature 
measurement. When this is to be done on a rapid and repetitive basis, it 
can be done by use of various or rotating choppers, which are known in the 
optical art. 
A preferred embodiment of the invention comprises at least the following 
components: 
1. A mechanical positioning means for placing a joint which is to be reflow 
soldered on the axis of an optical system. This may be a manually 
controlled positioner or it may be a computer-controlled automatic 
positioning table for the rapid sequential soldering of a large number of 
joints on a given circuit board. In either case, the stepping resolution 
and positioning accuracy should be compatible with the physical dimensions 
of the joint to be soldered; typically, their values should be a fraction 
of these dimensions; 
2. Means for bringing radiant power, typically via a focused laser beam, to 
the surface of the joint to be soldered, and to do so for a controlled 
exposure duration or, alternatively, for a programmed series of pulses of 
equal or variable pulse durations and pulse separations. Use of a single 
exposure contemplates the possibility that the radiant power intensity 
itself may be automatically varied throughout the exposure; 
3. An optical detection system, typically sensitive to infrared radiation, 
configured so as to receive a portion of the thermal energy which is 
radiated by the heated target, and providing an electrical output signal 
which is related, in a known way, to the amount of emitted target 
radiation. A requirement on the optical system is that it must in some way 
be made insensitive to the heating radiation from the laser or other 
source. This is done preferably by use of optical filtering, although 
chopping or other obvious means could be employed; and 
4. A signal processing system which, by either analog or digital means, can 
recognize discontinuities in heating rates associated with melting or with 
freezing. 
These items will be discussed in turn as they relate to our preferred 
embodiment of the invention. With the exception of our signal processing 
means, the individual methods per se comprise "state-of-the-art" 
technology and are known to practitioners in the respective fields. 
The first above-named element of the present invention, i.e., the 
mechanical positioning means, may comprise an automatic "XY" positioning 
table. Such tables, with computer controllers, are used widely throughout 
industry with far-ranging applications. Their function is to impart a 
prescribed motion to an item which is being processed in some manner or 
other. An example would be a machining operation in which a block of metal 
is sequentially brought to various positions under one or several cutting 
tools where it is milled, drilled, tapped, etc., to form a complicated 
shape such as an automotive engine block. Positioning tables range in size 
from a few square inches. such as are used in the automatic assembly of 
microscopic electronic parts, to many square feet as used in the garment 
industry for cutting stacks of cloth into intricate shapes to be made into 
wearing apparel. For precision applications, tables can be made which move 
in steps of 0.001" or less and have a positioning repeatability of 
one-tenth that amount. 
Typical table parameters which are envisaged for use with our invention 
would be: 
Table travel: From a few inches in the X and Y directions, for a small 
circuit board, to several feet in each direction for a multiplicity of 
larger boards; 
Stepping resolution and repeatability: From 0.001" to 0.005" for the 
former, for most applications, with a repeatability from one-tenth to 
one-half that of the stepping resolution; 
Table speed: For adjacent targets which are spaced, typically, 0.050" to 
0.100" apart, the table acceleration, speed and deceleration should be 
sufficient to allow from five to ten targets per second to be momentarily 
positioned, at rest, under a designated optical axis, assuming that no 
time is taken for target processing. In the actual case of reflow 
soldering, the table will halt for a finite time during the heating step. 
With regard to the second above-named element of the invention, i.e., the 
radiant power source, another art which has become highly developed in the 
past decade is that of using focused laser beams to provide intense local 
heating of various targets. Such beams can be used for either for the 
removal of metal or ceramic, for example, from a part which is to be 
drilled, scribed, cut, and so forth, or merely to provide a temperature 
increase such as for melting, heat-treating, welding, and so forth. Many 
types of lasers are commercially available for these applications. They 
differ among themselves in beam-power level (from a few watts to several 
thousand), wavelength band (with single or multiple wavelengths covering 
the spectrum from ultraviolet through infrared), and in whether they are 
designed to provide a steady beam (CW) or a pulsed one. Other parameters 
of interest to laser engineers include spatial mode structure (single vs. 
multiple), beam divergence, power output stability, beam diameter, 
longitudinal mode spacing, amplitude noise and ripple, tube lifetime, and 
many others. 
Among the lasers widely used for heat-processing are the Nd:YAG 
(neodymium-doped yttrium-aluminum-garnet) laser operating in the 
near-infrared at 1.06 micrometers (.mu.m) and the carbon dioxide 
(CO.sub.2) laser operating at the greater infrared wavelength of 10.6 
.mu.m. Various considerations such as cost, lifetime, required power, 
etc., enter into the user's choice between these lasers. One consideration 
is that, for metal-working use, most metals are somewhat better optical 
absorbers at the shorter than the longer wavelength, which makes the "YAG" 
laser somewhat more efficient as a heating source. 
Our invention envisages the use of a Nd:YAG laser with a beam power in the 
range of thirty to fifty watts, with a multi-mode spatial structure, 
operated CW (continuous wave) and with an optical system that can deliver 
between one and ten watts of beam power onto a typical solder-joint 
location. 
As a further feature of the invention, we include a length of optical fiber 
in the path between the laser and the focusing optics in order to 
homogenize the intensity over the cross section of the laser beam and thus 
to remove the uneven power density distribution which is inherent in most 
laser beams. It is desirable for the fiber path to be curved, with a 
cumulative bend of 90 degrees or more, to ensure efficient scrambling of 
the laser-beam rays as they proceed throughout its length by means of 
multiple internal reflections. 
The fiber is made typically from glass or a related material having high 
transparency at the laser wavelength. A reasonable fiber diameter would be 
about 0.025". This is large enough so that the beam which emerges from the 
laser is conveniently focused onto one end face by conventional optical 
methods, and it is thin enough to provide mechanical flexibility. This 
allows the laser to be placed in an out-of-the-way location and also 
provides vibration isolation between the laser and XY table whose 
"start/stop" motion can be a vibration source. 
The output end of the fiber serves as a secondary source which is imaged, 
via conventional optics, onto the target to be heated. Also by 
conventional optical means, the secondary source may be apertured so as to 
provide a square, rectangular or other shape more suited to the shapes of 
non-disclike targets, such as the elongated lap joints which are used with 
flat-pack integrated circuits. Alternatively, beam-spot elongation in the 
target plans can be achieved by the addition of a cylindrical lens element 
in conjunction with the focusing lens. Methods of doing so will be 
familiar to lens-system designers. Electromechanical means of rotating the 
cylindrical element through 90 degrees or other angles in order to 
accommodate randomly oriented lap joints will be familiar to mechanical 
designers. 
Regarding the choice of the optical radiation detection system to be used 
as the third above-named element of the invention, a variety of methods 
are available in the art, varying in temperature range, temperature 
resolution, response speed, cost and so forth. A preferred form uses a 
lead sulfide detector with conventional glass optics, a radiation chopper, 
and suitable electronics, as known in the art. Another preferred form, at 
higher cost but having finer temperature resolution and being useful with 
targets at lower temperatures, uses infrared optics with a cryogenically 
cooled detector of indium antimonide. This may be used without a chopper, 
and the infrared optics comprises one or more lenses of infrared 
transmitting materials such as sapphire, arsenic trisulfide, zinc selenide 
or many others which are known. Alternatively, reflecting optics may be 
used, as is often done in these cases. When no chopper is used, a separate 
blackbody radiation source can be used to provide a reference temperature. 
In the cases of the lead sulfide and indium antimonide detectors, response 
speeds in the range from microseconds to a few milliseconds are easily 
achieved by suitable circuitry design. This is more than adequate to 
follow the warmup of a typical solder joint which is being reflowed 
because this process requires a great many milliseconds to occur. Such 
response speeds are equally attainable with other detectors as well. 
With regard finally to the fourth element of the invention, the signal 
processing system may be of the analog or digital type. In either case, 
its function is to identify irregularities in the heating curve of the 
solder joint, such irregularities accompanying the solid/liquid 
transition. Its manner of operation will be more fully explained through 
later reference to the drawings describing our preferred embodiment of the 
invention. For the present, we will indicate that the recommended analog 
method of processing the infrared signals involves the use of the second 
time-derivative of the thermal signal and the fact that a non-zero second 
derivative signifies the presence of a departure from a constant heating 
rate. In the case of digital signal processing, a moment-by-monent 
computer evaluation of the first derivative, or slope, of the heating 
curve is carried out. Each new value is compared with the previous one, 
and a difference exceeding a predetermined threshhold is taken to signify 
an irregularity in the heating curve. 
FIG. 3 is a general diagram of a digital version of the present invention. 
In FIG. 3, the heating source is the Nd:YAG laser 10 whose output beam is 
focused by a glass lens 1 into an optical fiber 2 where it emerges from 
the other end with a given divergence angle. It is then incident upon 
glass lens 3 where it is rendered somewhat more parallel and proceeds to a 
dichroic mirror (or "beam splitter") 4 which reflects it downward upon an 
infrared transmitting lens 5. The function of lens 5 is to focus the laser 
beam upon the target mounted on the XY table, and also to collect and to 
direct upward some of the thermal radiation which leaves the target as it 
becomes warmed. This radiation is at greater wavelengths than the YAG 
radiation, being typically in the range from about two to five .mu.m. The 
dichroic mirror 4 is specially designed as to be largely transparent to 
the longer wavelength region, so that most of the thermal radiation passes 
through it and into a second infrared transmitting lens 6 where it is 
focused upon the infrared detector. 
Dichroic beam splitters of the type used here are available from many 
optical interference filter suppliers who provide filter-design and 
fabrication services. 
Because the YAG laser beam is invisible, the red-light beam from a helium 
neon (HeNe) laser 12 is added to it to render the focused spot visible. 
This is helpful during manual programming operations when the various 
solder-joint locations are being entered into the computer. This is done 
by use of table-control keys which move the table so that each solder 
joint in turn is located on the optical axis, whereupon its location is 
automatically entered into the computer when the operator presses another 
key. 
The infrared detector signal is preamplified and is digitized by an 
analog-to-digital (A/D) converter 14 whereupon it enters the computer 16. 
The computer is responsible for several control and data processing 
functions: 
1. It "memorizes" the target locations in terms of XY-coordinate positions 
of the table and it sequentially drives the table to each position when 
ready; 
2. At each new target position, the computer operates the YAG-laser shutter 
18 for a specified exposure duration, after which it commands the table to 
move to the next position; 
3. It monitors the detector signal and carries out the necessary 
processing; which includes: 
a. A determination of the moments when a solder surface has begun to reflow 
and has completed its reflow; 
b. A determination of the time when the molten solder has reached some 
thermal level at some predetermined value above the reflow level; 
c. A determination of whether accidental laser-beam damage may be 
occurring. This may occur due to a programming error or a system 
malfunction which causes the laser beam to impinge on something other than 
a solder joint, or it may occur due to a glancing reflection of the laser 
beam from a shiny solder surface and onto the substrate. It can also occur 
if debris or other easily burned material rests on or near the solder. In 
any event, the excessive resultant heating is recognized by the computer 
as an unusually high detector signal. The computer then causes the laser 
shutter 18 to be closed prematurely and it signals the operator. 
The computer communicates with the external world via a keyboard, a video 
display, a printer and, if appropriate, another computer. 
The manner in which the computer digitally determines the times referred to 
in (a) and (b), above, will be described after some preparatory remarks 
about the analog determination of reflow phenomena. 
For illustrative purposes, there are shown in FIGS. 4 and 5 two types of 
solder joints which are amenable to being reflowed by laser beams, 
comprising a lap joint (shown in FIGS. 4A and 4B before and after reflow, 
respectively) and a leadless chip carrier (LCC) joint (shown in FIGS. 5A 
and 5B before and after reflow, respectively). In FIGS. 4A and 4B, the 
integrated circuit (IC) is initially secured to the substrate, often by 
use of adhesive beneath the case, such that the contacting surfaces of its 
electrical leads rest on their respective solder pads, which are rigid. 
The IC is held tightly against the board such that its electrical leads 
are under slight compression. During reflow, the contacting portions of 
the leads sink into the solder, as shown in FIG. 4B, becoming partly or 
fully immersed and relieving the pressure stresses. The IC is then secure 
when the resulting joints have cooled. 
A similar situation occurs with the LCC of FIGS. 5A and 5B which is also 
pressed against the solder pads prior to reflow. During reflow, the molten 
solder wets the metallization and surface tension forces some of it upward 
to form a fillet on the vertical face. 
FIG. 6 illustrates a preferred embodiment of a radiation thermometer 
incorporating a reflow-sensing capability using an analog method, as 
opposed to the digital method of FIG. 3. The detector 20 is of lead 
sulfide and is chopped at 400 Hz. The resulting AC signal is amplified and 
is converted to a D.C. signal through an "amplifier-converter board" (ACB) 
22, after which the signal is "linearized" or processed in circuit 24 so 
as to correspond to temperature values rather than to detector-output 
values. Thereupon it is further amplified in amplifier 26 and is directed 
to the reflow sensing circuit 28. 
An emissivity conrol 30 at the ACB 22 is an amplifier gain adjustment which 
is used in order to take account of the emissivity of the solder. 
A detailed diagram of the reflow sensing circuit is shown in FIG. 7. The 
two electrical stages at the upper left, denoted by components Z7 and Z8, 
comprise a pair of electronic differentiators in series. They provide, at 
terminals 32 and 34, the first and second time derivatives of the signal 
entering at terminal 36. At left center in FIG. 7 are shown the respective 
derivatives associated with a particular input waveform. The functions of 
the remainder of the circuit will be apparent to electrical engineers. The 
various parts include comparators, "flip-flops" and so forth which are 
used for timing, thresholding, latching and other functions associated 
with the routine use of the system. For convenience, the operation of the 
circuitry of FIG. 7 will be briefly described. 
Upon the application of heat to the solder, the input to differentiating 
amplifier Z7 will rise as shown in the upper waveform in FIG. 7. When the 
temperature rises to a level in the vicinity of the melting temperature of 
the solder, amplifier Z7 will provide a low-level output which will be 
provided to the inverting input of amplifier Z8 and to the non-inverting 
input of amplifier Z1. The low-level input to the amplifier Z8 will result 
in a positive spike at the Z8 output, in response to which amplifier Z2 
will provide a clock signal to the clock terminal of flip-flop Z6A. Due to 
the low-level signal present at the D input terminal of the flip-flop Z6A, 
the Q output of the flip-flop will remain low. 
When the solder reaches its melting temperature, the temperature increase 
will temporarily halt while the solder undergoes a phase transition from 
solid to liquid. At this time, amplifier Z7 will provide a high-level 
output which will result in a similar high-level output from amplifier Z1. 
The high-level Z1 output will be inverted by NAND gate Z3A, thereby 
providing a second low-level input to NAND gate Z4B which, in turn, 
provides a high-level signal to disable the reset terminal of circuit Z5A. 
In response to the high-level output of amplifier Z1, the circuit Z5A 
provides at its Q output terminal a 5-second pulse which activates 
transistor Q.sub.1 to energize the "solder melting" LED. The 5-second 
pulse is also provided as the D input to the flip-flop Z6A. 
At the end of solder melting, the temperature will begin to increase and 
the input signal at terminal 36 will correspondingly increase as shown in 
the upper waveform in FIG. 7. At this time, the output of amplifier Z7 
will again fall to a low level to cause the outputs of amplifier Z1 and 
NAND gate Z3A to become low and high respectively. The downward transition 
in the output of amplifier Z7 will also result in a positive spike in the 
output of amplifier Z8, thereby resulting in a further clock pulse 
provided by amplifier Z2 to the clock terminal of flip-flop Z6A. Assuming 
that the melting has been completed within the 5-second duration of the 
pulse from circuit Z5A, i.e., within 5 seconds of the commencement of 
melting, the D input to the flip-flop Z6A will still be high, so that the 
clock signal will result in a high-level Q output. With both inputs at a 
high level, the NAND gate Z4B will provide a low level output, thereby 
reverse-biasing the diode CR2 and permitting the circuit Z5A to be reset 
by a low-level reset signal at a timing determined by the delay circuit 
R13, C13. 
The low-level output of NAND gate Z4B will result in a high-level output 
from gate Z4C to the set terminal of flip-flop Z6B, thereby activating 
transistor Q.sub.4 and energizing the "solder liquid" LED and closing an 
appropriate relay. 
Depressing the "START" button will reset both of flip-flops Z6A and Z6B, 
thereby preparing the system for subsequent operation. 
The waveforms shown at the left in FIG. 7 are presented again in FIG. 8, 
with slight modifications. The various parts of Curve (a) in FIG. 8 are 
labeled so that the reader may associate them with the various stages of 
solder reflow. This curve is idealized here, consisting of straight lines 
and sharp junctures. Curve (b) in FIG. 8 shows the first derivative with 
positive polarity, the inverse of that in FIG. 7. Also, the 
second-derivative spikes of Curve (c) in FIG. 8 are shown idealized, being 
more pronounced than in the FIG. 7 case. 
The reader should note that the first positive-going spike of Curve (c) is 
of no significance with regard to reflow. It simply indicates the start of 
warming. This spike is electronically ignored by the circuitry of FIG. 7. 
Turning now to FIG. 9, which is an approximation of a typical reflow curve 
for a solder joint, this represents the amplified and filtered detector 
signal which is to be sampled and digitized by A/D converter and then 
examined in the computer in the preferred embodiment of the invention. 
Shown above the first portion of the curve is a sequence of hypothetical 
signal values which might result from such a sampling. Below the curve are 
shown the increments in the successive signal values. 
The manner in which the computer identifies the moments of the start and 
end of reflow is a differential one in which it examines the sequence of 
increments. As the curve starts upward, it is noted that an increasing 
progression occurs in the values of the increments. In this progression, 
the computer subtracts each increment value from the succeeding values and 
recognizes that the difference is a positive number. As reflow begins and 
the slope of the curve starts to decrease, a sequence of increments will 
be found where the difference between successive increments is a negative 
number. At the first occurrence of this, the computer signals the start of 
reflow. It continues to observe negative differences until reflow is 
complete and the curve starts upward again. Upon recognizing the first 
positive change in the incremental values, the computer signals the 
completion of reflow. 
The differential method is an approximate simulation of the analog method 
of taking derivatives, as described earlier. The extent to which it is 
approximate is determined by the sampling time which is selected; as the 
sampling interval is decreased, the simulation becomes more exact. 
As was the case in the analog embodiment, the computer is instructed to 
ignore the first "upturn" in the curve after the laser shutter opens and 
normal warming begins; otherwise, this could be mistaken for the "end of 
reflow". 
An important feature of the invention is that, once reflow is completed, 
the method allows one to continue to heat the molten solder to a precise 
temperature, safely above the reflow point, without one's having to enter 
into the computer an emissivity value for the solder. Instead, assuming 
that the composition and therefore the exact melting point of the solder 
are known, the computer is able to derive its own emissivity value once 
reflow has been completed. It does this by noting the radiometric signal 
value at the moment when reflow is complete. Thereupon it can either 
calculate (by algorithm) or look up (in a "table") the radiometric value 
which a standard blackbody radiation source would have at precisely the 
solder melting point. By dividing the observed value by the blackbody 
value, it arrives at the emissivity value. Thereafter, it can convert 
radiometric values to actual temperature values while the molten-solder 
temperature continues to rise, and it can cause the laser shutter to close 
exactly when some predetermined temperature value above the melting point 
has been reached. 
Alternatively, the user may wish simply to specify that, after reflow has 
occurred, the heating continue to be applied until the radiometric signal 
is some pre-established percentage above the melting-point value. 
By use of the reverse of the reflow-detection process, the computer can be 
made to recognize when solidification has occurred, in case it is desired 
not to disturb the molten solder by accelerating the XY table toward the 
next target position. 
The foregoing comprises a description of a preferred embodiment of the 
invention. It will be apparent that many modifications of this embodiment 
are possible without departing from the spirit of the invention. For 
example, the fixed optical parts and movable table of FIG. 3 can be 
replaced by a fixed table and movable optical parts. A flexible, infrared 
transmitting optical fiber, can be added between the infrared detector and 
the lens/dichroic mirror system so that the latter may be moved while the 
other system parts remain stationary. Also, the optical system can be 
inverted so that its axis is directed upward, or in any other direction, 
in case it is desired to test samples from some direction other than 
vertically downward. Furthermore, the system can be configured so that the 
heat-injection axis and the infrared-detection axis are directed toward 
each other, instead of being coincident. By this means, one could heat a 
sample at one surface and monitor the heating rate at the opposite 
surface. Extending this idea, the aforesaid axes may be of arbitrary 
inclination to each other so that heat-injection and infrared-detection 
may be from arbitrary directions. 
A second means of reflow detection will now be described, which is to be 
included within the scope of the invention. This means makes use of a 
reflectance method by which a change in the physical properties of a 
solder surface is detected at the moment of solid-to-liquid transition. 
This visible change is a matter of common experience to laser-beam 
solder-reflow technologists. Most often it occurs in the form of a change 
from a slightly granular solid surface to a perfectly glossy molten one. 
It also sometimes happens during melting that the surface contour will 
change. In either case, use can be made of a narrow beam of light directed 
at the surface and of a photodetector so positioned as to receive some 
portion of the light reflected from the surface. Prior to melting, the 
surface reflections will remain unchanged and the detector signal will be 
constant. At the moment that the solid-to-liquid transition occurs at the 
surface, a discontinuity will be noted in the detector signal. Most often, 
this will be a signal increase due to the enhanced reflectance of the 
molten surface. Conversely, it can be a decrease due to a change in 
surface contour, and this will depend upon the optical geometry of the 
system. 
It is anticipated that there will be occasions in which, by chance, the 
aforesaid increase and decrease will exactly cancel each other, leaving no 
net signal change for reflow detection. This occurrence will be 
forestalled by the use of two or more detectors disposed at slightly 
different locations, and monitored by separate electronic means, so that 
at least one of them will register the desired glossy reflection. In any 
event, by use of a multiplicity of detectors, at least one of them is 
certain to detect either an increase or decrease in the reflected light 
intensity, be it due to a glossy reflection or to a change in surface 
contour. The detection of a signal change in any one of a group of 
separate detectors can be carried out by standard electrical practices. 
FIGS. 10A and 10B illustrate a hypothetical solder joint before and after 
reflow, respectively. The jagged-line contour in FIG. 10A represents the 
crystalline surface of solid solder and shows that the reflected rays 
spread throughout a small angle, with some of them bypassing the detector. 
In the molten case, the reflection is more mirror-like so that the 
detector receives more reflected power. 
FIG. 11 illustrates an embodiment of the reflectance method, using a single 
detector and being incorporated into the existing concept which was shown 
in FIG. 3. The rightward-proceeding YAG laser radiation impinges upon the 
first dichroic mirror 40 and is largely deflected downward to the target 
42 where the absorbed portion causes heating and where the remainder is 
reflected in various directions. A portion of it proceeds upward through 
the lower lens and most of it is reflected leftward by the first dichroic 
mirror 40, where it serves no purpose. However, the mirror 40 is slightly 
transparent to the 1.06-micrometer laser radiation and so a portion of 
this radiation proceeds upward through the upper lens 46 to the second 
dichroic mirror 48. This mirror serves the purpose of separating the 
1-.mu.m radiation from the longer-wave thermal radiation which is emitted 
by the heated target. The thermal radiation proceeds to the infrared 
detector 50, as in FIG. 3, whereas the 1-.mu.m radiation is diverted to a 
near-infrared detector 52, preferably of silicon, which serves as the 
reflectance sensor for reflow. 
Finally, in an application addressed to the reflow soldering of lap joints 
such as on flat pack integrated circuits (IC's), leadless chip carriers 
(LCC's) and similar devices, we include as a part of our invention a 
concept which we shall call the integral, self-dividing preform. This is 
an appropriately shaped strip of solder which is placed between a row or a 
group of device leads and their respective pads, preparatory to reflowing. 
The device is secured to the substrate, as is discussed in connection with 
FIG. 5, and reflow is carried out by laser-beam means. As the preform 
becomes molten, it will separate itself into individual solder masses, one 
at each joint, through a combination of surface tensile forces and of 
repulsion by the substrate surface. This arises because molten solder is 
attracted to metallic surfaces, particularly if they are treated with 
solder flux, and it is repelled by unfluxed non-metallic surfaces, 
especially if a "solder mask" has been applied (this is an adhering sheet 
of solder-repellent material with cutouts for the desired solder 
positions). 
In the case of a flat pack IC, which has rows of electrical leads issuing 
from two opposite sides, one suitably shaped preform would be placed 
beneath the contacting parts of each row of leads. For the LCC, which is 
square and which has rows of contacts on four sides, a square, hollow 
preform of the proper size would be placed under its perimeter, protruding 
slightly so as to be accessible to the reflow laser beam. 
In order to carry out reflow by laser beam, one might hope to irradiate one 
electrical contact at a time, proceeding to the next one only after the 
first one has reflowed. A glance at FIG. 5, however, will remind the 
reader that, during conventional reflowing when all joints are molten 
together, the contacts settle into the solder, which is a desirable 
condition. In laser-beam reflow, the same result must be achieved, and 
this can occur only if all joints are molten at the same time. 
My invention teaches two methods whereby simultaneous reflow can be brought 
about, one using time-sharing of the laser beam and the other using 
spatial division of the beam. 
In time-sharing the laser beam is brought to the target locations 
successively and repeatedly by use of scanning mirrors or other known beam 
deviators. While the beam is in transit, the laser shutter is closed. It 
opens only when the target is reached and for a specified time period in 
which the beam spot "dwells" on the target. Each exposure therefore stores 
a finite amount of energy in the joint-to-be, with the amount dependent 
upon the product of the laser-beam power and the exposure duration. The 
beam then impinges upon the remaining targets in succession, during which 
interval the previous targets are drained of some of their thermal energy. 
However, with a laser beam of sufficient power, the application of 
repeated energy pulses will result in a net temperature increase at each 
target and, after a sufficient interval, reflow of all targets will be 
achieved. 
Should it happen, as is often the case, that the connections external to 
the solder joints differ in their thermal capacities, then the jonts which 
experience less heat-sinking risk being overheated while those with 
greater heat-sinking are still approaching reflow. In this event, the 
power pulses into each joint can be metered, under computer control, based 
upon the heating needs of each joint as determined by the thermal 
detection system which concurrently scans the joints with the laser beam. 
Metering may be carried out by control of either the pulse duration or the 
laser-beam power by means of various modulators which are in common use. 
In the event that all joints share a common heat-sinking value, then they 
may be heated simultaneously and steadily by beam division or beam 
shaping, as it may be called. Various prism and/or mirror devices, 
branched optical fiber bundles, beam splitters and so forth are known in 
the art by which a beam of optical radiation may be divided into a number 
of less powerful beams having cross-sections of arbitrary shape. As a 
simple example, an ordinary laser beam which has a round cross section can 
be converted into a fan-shaped beam, having a rectangular cross section, 
by causing it to impinge upon a cylindrical lens whose axis is at right 
angles to the beam. A simple example of a cylindrical lens would be a rod 
of solid glass. The lens diameter would be comparable to the beam diameter 
or greater. By use of such a lens, what would normally be a spot in a 
target plane can be converted to a line. The orientation of the line will 
be perpendicular to the direction of the lens axis and is thus readily 
controlled. Thus, by use of beam splitters, a laser beam might be divided 
into four beams of equal intensity and these could be directed to four 
cylindrical lenses or other "spot-to-line" converters, and four "lines" of 
radiation could be made to impinge upon the four rows of contacts at an 
LCC, for example. The continuous lines of radiation could either be masked 
into spots, one for each contact, so that undesired heating of the 
substrate between contacts would be avoided, or the lines could be broken 
into spots by the focusing action of a row of small, conventional lenses 
used in conjunction with each cylindrical lens.