Sensing of solder melting and solidification

A method is provided for non-contact and contact sensing of phase changes of a solder material. By adding solder to a preexisting solder joint or substrate, an infrared sensor with limited resolution capability is able to discriminate between various solder characteristics at the solder enhanced site and other thermally distracting components when the solder transitions from a solid to a liquid phase or from a liquid to a solid phase. One type of contact sensing of solder reflow is accomplished by holding a thermocouple against a solder joint. When the pre-existing solder volume is insufficient to produce the desired results, additional solder is added to the solder joint or lead. The additional solder may be solid solder, a solder pre-form, or solder paste. Contact sensing may also be achieved by placing a spring loaded probe against the solder and detecting the probe's movement as the solder softens. In another contact reflow detection technique, a thermocouple sensor is housed in a protective sleeve fillable with solder paste or molten solder. The sensor is placed against a substrate adjacent a solder joint, and is heated simultaneously with the solder joint. Detection of solder reflow in the sleeve by the thermocouple signals reflow of the adjacent solder joint.

FIELD OF THE INVENTION 
The present invention relates to a sensing technique for detecting solder 
melting and solidification, and more particularly to contact and 
non-contact methods of determining the occurrence of a phase change in 
solder. 
BACKGROUND OF THE INVENTION 
The technique of reflow soldering electrical components or devices on a 
circuit board is well known. Generally, one or more electrical leads from 
a device are placed on top of one or more solder pads located on a circuit 
board. The entire circuit board, or the area immediately surrounding a 
particular device, is then heated until the solder comprising the 
interconnect between the leads and the circuit pads changes phase from 
solid to liquid or melts. When the solder melts the device leads and the 
underlying circuit pad are wetted, thereby forming a solder joint. Heating 
is then stopped, and the circuit board and solder joint are cooled or 
allowed to cool until the solder solidifies, at which point the device is 
fully attached. Device removal is merely a reversal of the above steps. To 
wit, the solder joint is heated until the solder touching the lead is 
heated to a temperature above its melting point, at which time the lead 
may be easily withdrawn from the solder, and the device removed from the 
circuit board. Delay or failure to detect solder reflow and subsequent 
temperature increases, results in excessive or unduly prolonged heating of 
the solder, and causes timing inefficiency, equipment damage, and poor 
process results. 
For optimum wetting between the materials involved, the lead/pad interface 
and solder should be heated to a temperature above the melting point of 
the solder. Therefore, after reflow is sensed, the temperature must be 
monitored to establish that a target temperature above the melting point 
of the solder has been reached. In the case of eutectic or near-eutectic 
tin/lead solder, a temperature approximately 30.degree. C. above the 
melting point is desirable. During device removal a temperature 30.degree. 
C. above the solder's melting point allows the solder to reach a lower 
viscosity so that excess solder is not removed from the circuit pads by 
adhering to the leads of the device as the device is removed from the 
circuit during a replacement operation. In prior art methods, only solder 
temperature, not the phase change phenomena itself, has been monitored to 
determine when the correct temperature has been reached. 
Generally unsuccessful attempts have been made to use remote or non-contact 
infrared sensors for the measurement of the temperature of an object 
within the sensor's field of view by detecting infrared emissions from the 
object. In the case of soldered electronic devices, the sensor is directed 
at a solder joint to detect temperature. Temperature monitoring with an 
infrared sensor and other methods of detecting and monitoring solder, such 
as detecting a change in surface reflectance as part of a soldering 
technique, are presented in U.S. Pat. Nos. 4,657,169 to Dostoomian et al.; 
4,696,104 to Vanzetti et al.; and 4,696,101 to Vanzetti et al. These prior 
art methods were developed to monitor solder phase changes for relatively 
large solder joints found on typical J-lead chip carriers and leadless 
chip carriers (LCC). However, the inaccuracy and unreliability of these 
techniques has engendered little interest in the electronics industry. 
FIG. 1 is a side view of a device 10 having a 50 mil pitch J-lead 12. For a 
J-lead 12 of this width, an electrical connection to a substrate 14 is 
made with a solder joint 16 that is 0.030 to 0.035 inches wide. FIG. 2 is 
an end view of FIG. 1 which further illustrates the general nature of the 
solder joint 16. With a J-lead 12 equipped device 10, the solder wicks up 
the J-lead 12 and around the perimeter of the lead 22, thereby presenting 
a substantial amount of solder for a sensor to view. This connection 
technique produces a viewing surface or solder joint 16 of approximately 
4.times.10.sup.-4 in.sup.2. 
FIG. 3 is a side view of a device 10 having 50 mil pitch leadless chip 
carrier leads 18. As with the J-lead 12, the LCC is electrically connected 
to the substrate 14 with a series of solder joints 16 that are typically 
0.030 to 0.035 inches wide. The solder joint 16 extends vertically from 
the surface of the substrate 14 up the metalized castellation 18 of the 
device 10 and forms a relatively large bulbous type solder joint 16 when 
heated. FIG. 4 is an end view of FIG. 3 which further illustrates the 
general nature of the solder joint 16. The solder joint 16 of the LCC 
device 10 presents an even larger solder volume for sensor viewing than 
the J-lead 12, the LCC solder joint 16 presenting a surface of 
approximately 9.times.10.sup.-4 in.sup.2. 
While an infrared sensor may function properly in wide or coarse pitch 
applications, fine pitch devices having a lead width in the range of 0.015 
to 0.002 inches wide are more becoming important and form a necessary part 
of many new designs. Fine pitch devices typically present a reduced volume 
and reduced surface, a scant 1.times.10.sup.-4 in.sup.2 for viewing a 25 
mil pitch device, and correspondingly less for a smaller lead width. As 
lead pitch gets finer, there is less and less distance between center 
lines of adjacent leads and the viewing area is correspondingly smaller. 
As the solder volume gets smaller, the change in rate of temperature rise, 
which is interpreted to detect phase change, is affected by the solder 
being heat sunk by adjacent masses. Eventually, the diminutive volume of 
solder is so masked by heat sinking that phase changes are not detectable. 
Accordingly, prior art non-contact measurement systems are of little or no 
use with fine pitch devices due to limitations of field of view, volume 
(therefore mass) of solder, and state of the art sensors. Specifically, in 
non-contact detection of infrared energy the amount of solder in the field 
of view of the sensor appears quite small in relation to other items in 
the field of view, such as metal leads, the device package, and the 
circuit substrate. Therefore, the infrared emission from the solder joint 
is small in comparison to the other infrared emissions received by the 
sensor. This multiplicity of strong emissions masks the relatively weak 
signal from the solder joint at which the sensor is directed. Thus, the 
insufficiently restricted field of view of the sensor, or its inability to 
discriminate between objects in its field of view, results in its 
inability to sense anomalies in the rate of heating of solder materials. 
In FIG. 5 for example, a fine pitch device 20 having "gull wing" leads 22, 
only has a thin coating (if any) of solder on top of each lead 22. This 
creates problems in sensing the anomaly of solder melting because the thin 
layer of solder more specifically represents the temperature of the 
underlying metal of the lead 22. Furthermore, the solder develops 
intermetallics with the base metal of the lead 22 so that the solder's 
melting point is no longer that of the solder originally deposited on the 
substrate. FIG. 6 is an end view of the solder joint 16 of FIG. 5, and 
FIG. 7 is a top view of the same solder joint 16, both views illustrating 
the nature of the solder joint 16. 
The reduction in viewing surface presented to an infrared sensor in the 
fine pitch device 20 virtually eliminates the observability of the change 
in emissivity, reflectance, and shape of the solder while it melts using 
prior art techniques and devices. Even if the field of view is reduced to 
a 0.001 inch spot on the solder joint, the decreased solder volume used 
with fine pitch devices causes the solder temperature to be effectively 
masked by the surrounding thermal masses that are in intimate contact with 
the solder. 
Ideally, a monitoring sensor used in the process would instantly detect and 
continue to accurately monitor the phase change event itself, and use that 
information to trigger subsequent events and calibrate subsequent 
measurements. The present invention fulfills the above need, and overcomes 
present sensing difficulties by enhancing the emissions from the area of 
the solder joint and by enhancing a sensor's ability to discriminate 
between thermal sources. 
SUMMARY OF THE INVENTION 
In surmounting the foregoing disadvantages, the present invention provides 
a method of sensing solder melting and solidification. Embodiments of the 
method are provided for both contact and non-contact sensing and 
monitoring of various solder characteristics which signal phase changes of 
a solder material. This sensing method is valuable in either component 
installation or removal applications. 
In one embodiment of the method, adding solder to a preexisting solder 
joint enables an infrared sensor with limited resolution capability to 
discriminate between the solder signature of the augmented solder joint 
and other thermally distracting components when the solder transitions 
from a solid to a liquid phase or from a liquid to a solid phase. The 
supplemental solder may be a paste, solid solder, or a solder preform, and 
have either similar or different material properties than the preexisting 
solder to which it is added. The sensor may monitor a single solder 
characteristic or a combination of factors, such as temperature, 
emissivity, reflectance, texture, volume, or shape. 
Another embodiment of the method involves inserting a thermocouple into a 
mass of supplemental solder paste placed on a lead. The embedded 
thermocouple instantly and accurately measures solder temperature for the 
detection of phase change. 
Yet another embodiment of a contact method for detecting reflow uses a 
spring loaded probe that sinks into the solder as at softens or melts. 
Movement of the probe is monitored by a secondary sensor, such as a 
photosensor, a linear variable differential transformer, or a pressure 
sensor. 
Still another embodiment of the method incorporates a small thermocouple 
junction in a protective sleeve having a cavity filled with solder paste. 
The sensor is placed adjacent a solder joint to be monitored and the area 
is heated. The solder paste is then monitored for a phase change.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 8, a method of enhancing an infrared sensor's ability to 
detect phase changes of solder in a solder joint 16 for a fine pitch 
device 20 is shown. An infrared sensor 24 with limited thermal 
discrimination capacity, such as is known to those familiar with the art, 
is directed toward a solder joint 16 to be monitored. While the following 
discussion focuses on infrared sensors 24, the teachings of the present 
invention are equally applicable to sensing techniques with other optical 
sensors. 
A small amount of supplemental solder, hereinafter referred to as sensing 
solder 26, is selected for use based upon the solder characteristics or 
properties a given sensor will monitor, such as: emissivity, reflectance, 
temperature, texture, shape, and volume. When an infrared sensor 24 is 
used, the sensing solder 26 is chosen for its emissivity and reflectance 
characteristics. The sensing solder 26 is then placed on an easily 
accessed portion of a lead 22 or leads of the fine pitch device 20 within 
the field of view of the infrared sensor 24. The sensing solder 26 may 
either be in direct contact with a solder joint 16 to be monitored or 
simply in contact with the lead 22 which is in contact with the solder 
joint 16, as is shown in FIG. 8, wherein the sensing solder 26 has been 
applied to "toe" of a gull wing lead 22. Addition of the sensing solder 26 
either creates a viewing surface where hitherto one had not existed, or 
enlarges an extant viewing surface by increasing the total volume of 
solder at the solder joint 16. In either circumstance, an increased 
infrared signal is emitted from the enhanced solder joint 16, thus 
facilitating detection of an anomaly when the point of reflow or phase 
change is reached. 
The sensing solder 26 may be solid solder applied with a soldering iron or 
a similar device, or a solder preform of an exact or customized size and 
shape which is placed over, around, or adjacent the lead 22. 
Alternatively, the sensing solder 26 may be a solder paste or cream that 
may be precisely dispensed through a syringe and applied with a needle or 
other similar means such as a "doctor blade" to a single lead 22, an 
entire line of leads, or around the four sides of a lead or leads. 
Typically, the solder paste comprises solder metal in extremely small 
particulate form in a suspension of flux to permit extrusion though a 
small diameter orifice such as a syringe or needle. The ability of solder 
paste to increase sensor sensitivity at the point of reflow or phase 
change is significant because the solder paste has notably different 
emissivity characteristics than the original solder joint 16. 
The enhancement method of adding sensing solder to or near the lead 22 is 
particularly suited for rework operations, as some significant amount of 
the added sensing solder 26 is withdrawn from the substrate 14 when the 
device is pulled away or removed from it. Conversely, when a replacement 
fine pitch device 20 is placed onto the vacated position on the substrate 
14, any remaining sensing solder 26 which was not removed during the 
removal process augments the quantity of solder forming the solder joint 
16 for the replacement fine pitch device 20. 
Adding sensing solder 26 to a lead 22 or a solder joint 16 is effective for 
enhancing or facilitating contact sensing as well as non-contacting 
sensing. FIG. 9 illustrates a method of contact sensing using an exemplary 
contact sensor 28. The contact sensor 28 comprises a thermocouple 30 with 
an exposed thermocouple junction 32, and is mounted in a protective tube 
34. The protective tube 34 provides electrical and thermal insulation for 
thermocouple wires 31. Sensing solder 26 is applied to the lead 22 as 
described with respect to FIG. 8, and the exposed thermocouple junction 32 
is immersed in the sensing solder 26 which is heated by a thermal source 
36. 
Both the thermocouple 30 and the thermocouple junction 32 must be quite 
small to be completely surrounded by the sensing solder 26. Also, a very 
small thermocouple 30 is able to respond rapidly to changes in temperature 
of the sensing solder 26 and does not draw more than a nominal amount of 
heat away from the site being monitored, precluding an adverse impact on 
temperature measurement accuracy. Furthermore, the thermocouple 30 must be 
shielded so that it measures only the temperature of the sensing solder 26 
and is not responding directly to the heat being applied from the thermal 
source 36, or heat withdrawn by a heat sink such as cooling air. Mineral 
oxide or other suitable insulators may be used for the body of the 
protective tube 34, its lining, or its covering. 
When the heat is applied to the area surrounding the lead 22, by hot gas or 
infrared for example, the solder in the solder joint 16 melts at the same 
time the sensing solder 26 does when the thermal mass of the sensor 
matches the thermal mass of the solder joint 26 being measured. Free from 
other thermal distractions, the thermocouple 30 provides accurate and 
continuous monitoring of a phase change as it occurs. It is also 
contemplated that other temperature sensors can be used in the above 
described contact sensing method, such as: a resistance thermometer; a 
sensor which measures the resistance between two probes implanted in the 
solder joint 26; or a sensor which measures the change in the velocity of 
sound waves through the solder joint 26 as the solder liquifies. In some 
situations, the solder joint 26 is sufficiently large to obtain accurate 
contact measurements of the anomaly by pressing a thermocouple against the 
surface of the solder joint 26. 
An alternative method of contact sensing which has utility with or without 
the addition of sensing solder 26 is shown in FIG. 10, wherein a 
spring-loaded probe 38 is shown in contact with the solder joint 16. The 
solder joint 16 shown in FIG. 10 is sufficiently large and accessible to 
place a pointed tip 40 of the spring-loaded probe 38 thereon, yet too 
small to provide a distinguishable thermal source for a remote sensor. 
When the solder joint 16 is heated to its melting temperature, the probe 
tip 40 penetrates the melted solder an amount sufficient to cause a moment 
arm 42 attached to the probe tip 40 to move. The movement of the moment 
arm 42 is then detected by a secondary sensor 44 such as: a photoelectric 
sensor, a linear variable differential transformer, or a pressure sensor. 
Penetration of the probe tip 40 indicates the onset of solder melting, 
which may be useful in itself, but it may also be used to initialize a 
time measurement for determining solder temperature which is required for 
a subsequent process step. 
Due to the minuscule size of a solder joint on fine pitch devices 20, 
movement of the probe tip 40 will obviously be slight and difficult to 
detect by any means. Accordingly, it is desirable to amplify the movement 
of the probe tip 40 to ensure that the secondary sensor 44 detects its 
motion. FIG. 11 depicts a method for so doing. The probe tip 40 is shown 
connected to the moment arm 42 which is positioned on a pivot 46 to 
mechanically amplify or exaggerate the movement of the probe tip 40. The 
moment arm 42 is thus divided into a short moment arm 48 on the probe tip 
side of the pivot 46, and a long moment arm 50 on the opposite side. A 
force F is applied to the long moment arm 50 in the direction shown which 
causes the probe tip 40 to be gently pressed against the solder joint 16. 
When the probe tip 40 moves, its motion is magnified by the ratio of 
length of the long moment arm 50 to the short moment arm 48 and a motion M 
is created. 
FIG. 12 depicts yet another embodiment of a contact sensor, a shielded 
thermocouple 52 that is especially useful for applications requiring 
determination of the temperature at which a fine pitch device 20 should be 
removed from a substrate 14 during rework. Unlike the contact sensors 28 
and 38 of FIG. 9 and 10 respectively, which are placed directly against or 
into the sensing solder 26 or a solder joint 16, the shielded thermocouple 
52 is placed on the substrate 14 adjacent the solder joint 16 to be 
monitored. 
The shielded thermocouple 52 comprises a small gauge thermocouple 54 
mounted in a protective tube 56 like the protective tube 34 of FIG. 9. The 
thermocouple junction 58 extends slightly beyond the end of the protective 
tube 56 and a sleeve 60 made from a material which is wet by solder, such 
as copper, surrounds this the thermocouple junction 58 to form an end 
cavity 62. In the exemplary embodiment, the protective tube 56 which 
houses thermocouple wires 64 may be in the range of 0.125 to 0.625 inches 
in diameter. The sleeve 60 is as small as possible, typically 0.500 to 
0.750 inches in length and 0.050 to 0.100 inches in diameter. The open end 
of the sleeve 60 is cut or ground at an angle to permit complete contact 
of the entire open end against the substrate 14 when the shielded 
thermocouple 52 is held pencil like by an operator. In the exemplary 
embodiment, the angle is 45 to 60 degrees, however, the cut angle may vary 
as desired or the shielded thermocouple 52 may be bent or curved. 
Additionally, the shielded thermocouple 52 may be supported by a stand 
instead of a hand. 
The diameter and wall thickness of the sleeve 60, and the volume of sensing 
solder 26 are selected to be thermally similar to the solder joint 16 to 
be measured. Next, a small amount of sensing solder 26 is inserted into 
the open end of the sleeve 60 by gently squirting the sensing solder 26 
into the end cavity 62, or by dipping the sleeve 60 into a bulk reservoir 
of molten sensing solder, so that it makes contact with the thermocouple 
junction 58. The sleeve 60 of the shielded thermocouple 52 is then placed 
in contact with the substrate 14 adjacent a solder joint 16 whose 
temperature is to be measured. 
When heat from a thermal source 36 is directed toward the sleeve 60 and the 
solder joint 16, the sensing solder 26 and the solder joint 16 are heated 
simultaneously. When the sensing solder 26 melts, the melting is detected 
by the thermocouple 54 and indicates that the solder of the solder joint 
16 has also melted, thus signaling that the fine pitch device 20 can be 
removed from the substrate 14. 
The effectiveness of the various embodiments of the invention is more fully 
appreciated when considered with respect to the following time and 
temperature graphs depicting solder anomaly sensing test results. FIG. 13 
is a graphic depiction of anomaly sensing of solder using the shielded 
thermocouple 52 of FIG. 12 and the contact sensor 28 of FIG. 9. The upper 
curve 66 represents the readings from the shielded thermocouple 52, and 
the lower curve 68 represents readings from the contact sensor 28. The 
flat portion of each curve from A to B indicates a phase change in the 
solder. Both of the sensors 28 and 52 sensed the solder anomaly, although 
at apparently different temperatures. Computer software can be used to 
interpret the information and recalibrates the temperature at which the 
anomaly occurs to the melting point of the solder joint 16 (182.degree. C. 
for example). 
FIG. 14 is a graphic depiction of the contrast in solder anomaly detection 
capability between the contact sensor 28 of the invention and a prior art 
infrared sensor 24 without the addition of sensing solder 26. The infrared 
sensor 24 used for the test had a field of view of 0.016 inches and was 
directed at a solder joint having a lead 22 that was 0.016 inches wide and 
0.100 inches long. The lower curve 70, representing the contact sensor 28, 
clearly shows the solder anomaly designated by the flat portion of the 
lower curve 70 from A to B. However, while the infrared sensor 24 detected 
the increasing temperature as indicated in the upper curve 72, it was 
unable to isolate the heat signature of the solder joint 16 from other 
thermal distractions, and was thus not sufficiently sensitive to sense the 
solder anomaly. 
FIG. 15 graphically depicts the enhancement in detection capability of an 
infrared sensor 24, when used in conjunction with additional sensing 
solder 26 having the same characteristics as the solder of a solder joint 
16. The upper or relatively smooth curve 74 represents temperature 
measurement with the contact sensor 28 according to the method of the 
invention, and the lower or irregular curve 76 depicts the measurements of 
the infrared sensor 24 which was aimed at the additional sensing solder 
26. Although the infrared sensor curve 76 exhibited significant noise 
characteristics, the anomaly from A to B was still recognizable. 
Appropriate filtering and/or software can compensate for the noise. 
FIG. 16 illustrates the remarkable improvement in detection capability of 
an infrared sensor 24 when used with solder paste as the sensing solder 
26, as taught with respect to the embodiment of the invention illustrated 
in FIG. 9. Again, the smooth curve 78 depicts the measurements of the 
contact sensor 28 of the invention, and the lower curve 80 depicts the 
measurements of the infrared sensor 24. The infrared signal as shown of 
the lower curve 80, however, was different than a straight temperature 
measurement. The temperature first appeared to drop, then rise rapidly and 
significantly, then fell off again after solder reflow. This "spike" 82 
from A to B was the result of the infrared sensor 24 having detected the 
change in emissivity and shape of the solder paste which occurred during 
the melting period of about five seconds. Even though the spike 82 was not 
a true temperature measurement, it still signaled that reflow had 
occurred. 
These and other examples of the concept of the invention illustrated above 
are intended by way of example and the actual scope of the invention is to 
be determined solely from the following claims.