WO2010088237A1

WO2010088237A1 - Method and apparatus for measuring part temperature through a conveyorized thermal processor

Info

Publication number: WO2010088237A1
Application number: PCT/US2010/022175
Authority: WO
Inventors: Casimir W. Kazmierowicz; Miles Moreau
Original assignee: Embedded Designs Inc.
Priority date: 2009-01-30
Filing date: 2010-01-27
Publication date: 2010-08-05

Abstract

A fixture for standardizing temperature profiling of parts conveyed through a conveyer oven is disclosed. The fixture retains the parts in a desired orientation relative to the fixture and the conveyer and mounts thermocouples that measure the temperature of the surface of the part in predetermined locations. A docking station assists with loading parts such as PCBs, photovoltaic wafers and the like into the fixtures disclosed in some embodiments; the docking station is not necessary in other embodiments. Various TC assembly and TC bead configurations are disclosed.

Description

Method and Apparatus for Measuring Part Temperature Through a

Conveyohzed Thermal Processor

Technical Field

The present invention relates to an apparatus and method of profiling the temperature in a thermal conveyer oven to help determine the optimal process configurations and settings for conveyerized thermal processors, and more particularly, to apparatus for maintaining optimal thermocouple positioning and contact with parts conveyed through the oven.

Background

Thermal conveyer ovens, sometimes also called furnaces, are used in several different industries, and especially in the electronics industry. Broadly speaking, thermal conveyer ovens have multiple heating zones, and often cooling zones — the zones may be isolated from one another by curtains of various descriptions. A conveyer travels through the multiple zones, carrying the part that is being thermally treated.

Thermal conveyer ovens are used for a wide variety of purposes within the electronics industry. A solder reflow oven is one example of a thermal conveyer oven in the electronics industry. Electronic components such as integrated circuits may be mounted to underlying circuit boards with solder paste — the IC is soldered to the PCB as the components are conveyed through the oven. In a typical solder reflow oven, the PCB enters one end of the oven and moves at a constant speed through a series of temperature controlled zones. The component is soldered to the PCB during the time that it is in the oven. Generally speaking, the process within a conveyor oven includes a preheat phase, a dwell phase, a reflow phase and a cooling phase. During the preheat phase the solder is heated from ambient temperature to an elevated preheat temperature. This causes solvents in the solder to volatilize. In the dwell phase the solder is allowed to "dwell" for a preset period of time and at a temperature at which the flux in the solder becomes active. Next, in the reflow phase, the solder is heated above the melting temperature of the solder for a predetermined period of time that is sufficient to permit wetting of the solder. Finally, during the cooling phase the solder joint solidifies to electrically bond the IC to the PCB. It is of obvious importance to understand the temperature of the oven at each of the various zones, and how that temperature is manifest on the surface of the PCB, or part. Thermal profiling is the process of plotting temperature vs. time of the PCB as it travels through the oven. The PCB thermal profile is determined by temperature, time, and heat transfer rate in a solder reflow oven.

PCB temperature is typically measured by attaching thermocouples to different areas on the PCB. The thermocouples (TCs) are selectively positioned on the PCB to find the highest and lowest peak temperatures. The highest peak is found near its bare edges and the lowest peak is found at larger components, near the center of the PCB. Typically, 30-36 gauge TC wire is used. There are a variety of ways to attach the TC to the PCB. As one example, the TC may be attached to the PCB with Kapton Tape. Another example is to use thermally conductive adhesive such as high temperature epoxies or high temperature solder. Still another method entails placing the TC onto the surface of a wafer (such as a photovoltaic wafer during the metallization process) with a weight. This method has been used in the solar cell industry. While each of these methods is used widely, they each can be difficult to use for a variety of reasons. For example, even though Kapton tape is the quickest and simplest method of attaching thermocouples to a PCB, this method is known to generate serious random temperature errors in the temperature profile. The most reliable method of attaching TCs to a PCB is use of 10-88-2 solder with a melting point of 286°C. However, this method requires a sacrificial PCB.

Regardless of the method used to attach the TCs to the part, the TC wires are attached to a data logging device that travels through the oven with the part that is being treated. The data logger contains hardware and software that records and stores the temperature and time information within the oven. That information may then be downloaded to a computer to determine and optimize the temperature profile of the oven.

The placement of three TCs on a PCB and a plot of the resulting thermal profiles may be plotted on a Cartesian coordinate graph having time as the x axis and temperature as the y axis. The second TC, "TC2", typically enters the oven 5" behind the first TC, "TC1 ," and the third TC, "TC3" enters the oven 3" behind TC2. This separation is typical and causes the resulting temperature vs. time plot to be somewhat confusing.

By plotting the same data with distance on the x axis and temperature on the y axis the graph becomes more instructive and useful. In this instance, TC2 can be identified as an area of high thermal inertia, and TC1 as an area of low thermal inertia.

The temperature along the linear length of the oven conveyor defines the oven profile. The oven profile is primarily affected by the oven zone setpoint temperatures. However, the oven profile is also affected by other factors, including the air flow inside the oven, by the total thermal mass of the load and its spatial distribution, and by the ambient room temperature.

The time that the PCB is subjected to a given oven temperature can be changed by varying the speed of the conveyor. The slower the conveyor speed, the more time the PCB will have to reach equilibrium with the actual oven profile.

In theory, if the conveyor were to move infinitely slowly, the PCB profile would match the oven profile. In practice, there are many conveyor oven processes where the conveyor speeds are so slow that the oven profile is used to approximate the product profile. In the electronics industry, such processes include thick-film resistor firing and silver-glass die attach

The heat transfer rate is a measure of how quickly heat is transferred from the oven to the PCB. The greater the heat transfer rate, the lower the oven temperature required to raise the temperature of a given PCB a given amount in a given time. As the size and complexity of PCBs increase and the component density becomes less evenly distributed, ovens with a relatively low heat transfer rate begin experiencing problems. These ovens tend to overheat the low thermal mass areas of the PCB in order to insure adequate reflow of the more thermally massive components. Many of the newer oven designs boast a more efficient heating system with a higher heat transfer rate.

The ideal PCB thermal profile is usually based on a triangle of three factors: peak temperature, maximum slope, and time above reflow temperature. A plot of this data with time on the x axis and temperature on the y axis typically includes values for "peak", which indicates the peak temperature in degrees C for each PCB location; "max slope", which indicates the maximum change in temperature in degrees C per second; and "seconds over 183" (or some other reflow temperature value), which indicates the number of seconds each PCB location spends above the reflow temperature of 183°C.

The original device used to profile conveyor ovens was a strip chart recorder. Using this device, one or more thermocouples were trailed behind the PCB through the oven. The temperature of these thermocouples was plotted in real-time, on calibrated paper as the PCB traveled through the oven.

The strip chart recorder printout of multiple TCs was difficult to analyze. Oven manufacturers then began offering profiling systems that could monitor up to three thermocouples and would display the output on a computer screen. These systems had the advantage of allowing data to be manipulated and analyzed on a computer.

The problems associated with trailing multiple wires through the oven gave rise to the invention of portable, battery powered, thermal profilers that could withstand solder reflow temperatures. These profilers typically monitor three to six TCs and travel with the PCB through the oven — the profiler is best separated from the PCB to eliminate any possibility that the thermal mass of the profiler will alter the temperature profile measured by the TCs on the surface of the part. When the portable unit exits the oven, it is plugged into a standard PC and the profile data is displayed on the computer screen.

Although a great improvement over the first two methods, the portable profilers have the disadvantage of delayed data. The thermal data is no longer displayed as it is collected, thus, there is no way of knowing if the PCB or portable profiler is overheating. If the oven is too hot or the conveyor speed too slow, or if the units become stuck within the oven for any reason, both the PCB and the portable profiling device can be damaged beyond repair. The cost to replace a damaged portable profiling device is fairly high, and the portable profilers cannot be used in higher temperature applications without significant added insulation.

With the foregoing discussion serving as a background, it will be quickly apparent that regardless of the particular task for which the oven is being used, accuracy, precision and reliability of thermal profiling is a very important aspect of optimizing thermal treatment of parts. As a corollary, by reducing process variability the operator can determine critical parameters to make sure that the oven is operating within specifications. As noted, the primary function of temperature profiling is to give the operator the surface temperature profile for the part as it moves through the oven on the conveyer. By knowing the temperature profile of the oven and how that is manifest on the surface of the part, the operator may control the over operating conditions — i.e., the temperature and time profiles — in order to optimize thermal processing.

One factor that introduces variability into the process of thermal profiling, and therefore creates uncertainty in the results of the process, relates to the method and manner in which TCs are placed onto the PCB. The three methods described above — i.e., Kapton tape, epoxy and weights — are standards in the industry, but each method is known to have limitations that affect the reliability, precision and accuracy of the temperature measurements. There is a need therefore for methods and apparatus that improve the accuracy, precision and reliability of temperature measurement on the surface of a PCB, wafer or other part as it travels through a conveyer oven.

Summary of the Invention

The present invention is a fixture for holding and supporting parts and for positioning the TCs in optimal desired positions relative to the part. The fixture allows for maintaining contact between the TCs and the part as the fixture and part are moved through a conveyer oven. The thermocouple fixture is defined by a metal frame that is instrumented with thermocouple wires and wire/metal supports for retaining and supporting a wafer in the fixture. It is used to maintain accurate, reliable, and repeatable thermocouple contact and measurement of temperature on a part as it passes through a converyohzed thermal processor.

The metal frame has opposed arms that extend parallel to one another into a two-pronged fork — the arms are nominally about 7 inches apart with channels down each side to extend the thermocouple wire from the connectors. The arms are attached at a proximal end to a base member and a spreader bar extends between the arms at their distal ends, maintaining the arms in a spaced apart relationship. Mounted to the base member at one end of the metal frame are standard thermocouple connectors which plug into a temperature data logger.

The metal frame has relatively low mass so that it does not affect the thermal properties within the oven. Spring tensioned metal wire supports extend between the two arms of the frame to support the part. To allow for thermocouple placement at key areas of the part, the thermocouple wire is run through a small alumina tube (or other ceramic material or other high temperature insulator) which is inserted through a hole in the arm of the frame and runs perpendicular to the metal frame arm. The 2 wires that make up the thermocouple material (Alumel and Chromel for Type K thermocouples) is, in one embodiment, fed through 4 holes in the alumina tube in such a way as to maintain the thermocouple junction position even if the tube is rotated. In another embodiment the tube has only two holes therethrough. Metal collars hold the alumina tube in place along with a metal spring that runs under the base end of the tube and which is attached to the frame to keep a downwardly directed force on the extended end of the thermocouple junction at the opposite or distal end of the tube.

The thermocouple bead is specially formed and flattened to maximize the surface area of the thermocouple junction and maximize the accuracy of the measurement of the part location it is placed on. Various TC bead embodiments are disclosed. TCs may be used with the fixture of the present invention to measure the temperature of the air in the oven separate from the temperature on specific point on the part.

The frame is used to carry a silicon wafer, printed circuit board assembly or any other flat surfaced part and maintain placement of thermocouple sensors as the fixture and part go through a conveyorized thermal process. The thermocouple fixture is connected to a temperature data logger to record the temperature profile. There is substantial open space between the boundaries of the part that is being tested and the frame itself in order to eliminate or minimize temperature disturbance in the oven resulting from the mass of the frame.

For photovoltaic wafer temperature profiling, for example, the wafer would be placed in-between the wire supports, with the wire supports on each side of the wafer, to securely hold the wafer in place relative to the frame. Typically plural thermocouples extend over the upper surface of the wafer with the alumina tubes positioned on or over the surface of the wafer. TCs for measuring air temperature may also be used. The metal collars at the base of the ceramic tubes are locked into place using a set screw in the collar. This holds the thermocouple in position. The fixture is then plugged into the temperature data logger. A software package for temperature profiling is used to input the settings of the thermal processor and set up the temperature data logger and fixture for profiling. Once the steps are completed in the software the data logger is placed into a protective shield and the data logger along with the fixture plugged into it, are placed onto the conveyor of the thermal processor. At the exit of the thermal processor the data logger and fixture are retrieved and after removing the data logger from the shield it is plugged into the PC via USB cable and the temperature data is downloaded into the software.

Brief Description of the Drawings

Fig. 1 is a perspective view of a first illustrated embodiment of a fixture according to the present invention.

Fig. 1 A is a cross sectional view taken along the line 1 — 1 of Fig. 1.

Fig. 1 B is a close up view of a portion of the fixture of Fig. 1 , showing the support wires.

Fig. 1 C is a close up view of a thermocouple assembly.

Fig. 1 D is a close up side view of one arm of the fixture illustrating a tensioning spring used to apply tension to a support wire.

Fig. 2 is a perspective view of the fixture shown in Fig. 1 with a data logger attached to the fixture.

Fig. 3 is a side perspective view of a portion of the fixture shown in Fig. 1.

Fig. 4 is a top view of a portion of the fixture shown in Fig. 1 , and more particularly, a thermocouple and ceramic tube over a wafer.

Fig. 5 is a close up perspective side view of one of the metal arms used in an alternative frame design, showing a thermocouple collar and the supporting wires and tensioning springs.

Fig. 6 is a close up perspective side view similar to the view of Fig. 5. Fig. 7 is a close up perspective side view similar to the view of Fig. 5 illustrating the support wire routing.

Fig. 8 is a top perspective view of a docking station used with the fixture shown in Fig. 1.

Fig. 9 is a top perspective view of the docking station of Fig. 8 with a fixture loaded in the docking station.

Fig. 10 is an end perspective view of the docking station shown in Fig. 9, illustrating how the docking station facilitates loading the wafer into the fixture by raising the thermocouple tubes and spreading the support wires.

Fig. 11 A is a schematic side view illustrating one embodiment of a thermocouple tube with collars.

Fig. 11 B is an end view of the thermocouple tube shown in Fig. 11 A.

Fig. 12A is a schematic side view illustrating yet another embodiment of a thermocouple tube according to the invention, the tube containing one embodiment of a thermocouple configured for use with the present invention.

Fig. 12B is a top view of several embodiments of thermocouples configured for use with the present invention.

Fig. 13 is a top view of yet another embodiment of a thermocouple and thermocouple tube according to the present invention.

Fig. 14 is a close up end view of the thermocouple and thermocouple tube constructed according to the present invention.

Fig. 15 is a top view of an alternative and preferred illustrated embodiment of a fixture according to the present invention, illustrating the fixture with three TCs in contact with the part and one TC set up to measure air temperature.

Fig. 16 is a top view of an alternative embodiment for a thermocouple assembly comprising a ceramic tube with two thermocouple beads.

Fig. 17 is a top close up view of an alternative embodiment for a thermocouple having an extended tip portion.

Figs. 18 through 24 illustrate alternative embodiments of fixtures according to the present invention that include multiple thermocouples and which eliminate the need for a docking station for loading a wafer into the fixture. Fig. 18 is a top plan view of an alternative embodiment of a fixture according to the present invention with a typical photovoltaic wafer loaded into the fixture and the wafer supported by four support wires.

Fig. 19 is a close up perspective view of the fixture shown in Fig. 18.

Fig. 20 is a close up perspective view of the fixture illustrated in Fig. 18 with the wafer removed and illustrating a tool used to position and tighten the thermocouple assemblies.

Fig. 21 is a close up side view of one side of the fixture shown in Fig. 18, showing in particular from the outside of the fixture the collar that retains the ceramic tubes in place relative to the fixture.

Fig. 22 is a close up side view similar to the view of Fig. 21 , illustrating from the inside of the fixture the collar that retains the ceramic tubes in place relative to the fixture, and showing the collar with a split body to facilitate tightening the collar to secure the ceramic tube thereto.

Fig. 23 is a close up side view similar to the view of Fig. 21 and showing a ceramic tube installed in the fixture and held in the collar, with the TC wires installed with ceramic insulators to separate individual TC wires.

Fig. 24 is a top plan view of yet another alternative embodiment of a fixture according to the present invention, in which the support wires have been eliminated so that the part rests directly on the conveyer as it moves through the oven with multiple TCs deployed on the surface of the part. Detailed Description of the Illustrated Embodiments of the Invention

With reference to the drawings, a first preferred embodiment of a fixture 10 according to the present invention is illustrated in Figs. 1 through 4. Fixture 10 is defined by a generally U-shaped member 12 that has opposite arms 14 and 16 that are connected at their proximate ends 17 to a base member 18. An extension member 20 extends away from base member 18 and at the distal end 21 of the extension member are a series of prongs or electrical connectors 22. Electrical connectors 22 are configured for making an electrical connection to a data logger 24, as shown in Fig. 2. A spreader bar 26 extends between and connects the distal ends 27 of arms 14 and 16 — spreader bar 26 maintains the distal ends of the arms in a spaced apart relationship with the arms parallel to one another. Four thermocouple assemblies are identified with reference numbers 50, 52, 54 and 56. These TC assemblies are attached to the arms 14 and 16 and extend into the space between the arms. More specifically, and as detailed more thoroughly below, thermocouple assemblies 50 and 52 are attached to arm 14; thermocouple assemblies 54 and 56 are attached to arm 16. Each thermocouple assembly comprises a thermocouple that makes contact with the part that is being held in fixture 10 (identified herein as part 30 — part 30 may be a PCB, solar wafer, or other part to be tested), thermocouple wires, and a ceramic tube through which the thermocouple wires extend and various collars that attach the ceramic tube to an arm 14 or 16. The thermocouple wires run to the electrical connectors 22 as detailed below.

With reference to Fig. 1A, which shows a sectional view through arm 16, it will be appreciated that each arm 14 and 16 is defined by a channel- shaped length of metal.

Fig. 1 B is a relative close up of a portion of the fixture shown in Figs. 1 through 4, placed on a white background to illustrate the support wires that extend between arms 14 and 16 and which serve to retain part 30 in the fixture. More specifically, in the embodiment illustrated in Fig. 1 B, there are five support wires identified with reference numbers 60, 62, 64, 66 and 68. As detailed below, each of these support wires is adjustably tensioned with tension adjustment mechanisms.

Reference is now made to Figs. 1 C and 4 to describe in detail the thermocouple assemblies 50, 52, 54 and 56. Although the length of thermocouple assemblies 50 and 56 is greater than thermocouple assemblies 52 and 54, the structural components of each assembly is identical. Each thermocouple assembly comprises a ceramic tube 70 that has either two or four longitudinal bores extending completely through the tube — the tube 70 shown in Fig. 1C has four bores identified with numbers 78, 80, 82, and 84. The thermocouple wires 72 and 74 extend through two bores through the ceramic tube and form a thermocouple bead 76 at the terminal ends of the wires. The two wires are looped through the four bores in the tube 70 with the ends of the wires extending through the exposed, distal end 86 of the tube. The ends of the wires are welded together to define a thermocouple bead 76. The thermocouple bead 76 makes contact with part 30 in the assembled fixture 10 as shown in Fig. 1. The wires 72 and 74 extend through bores 80 and 84 through the length of the tube 70 and out the opposite end of the tube. From that point the wires run through the channel shaped arm 14 and to the electrical connectors 22, which as noted earlier connects to data logger 24. There are a variety of suitable methods and structures for attaching the TC assemblies 50 to arm 14. Importantly, the distal end 86 of the assembly must be free to move (under spring tension, as detailed below) in the direction that is generally transverse to the plane defined by the upper surface of part 30, as illustrated with arrow A in Fig. 1C. Because the distal ends 86 of the TC assemblies are movable in this direction (under spring force), the TC beads 76 are pressed onto the surface of the part 30 with force. This feature also allows the part 30 to be loaded into fixture 10, as detailed below.

Ceramic tube 70 is preferably alumina, although other ceramics will suffice. Each tube 70 is attached to an arm 14 or 16 through bores formed along the length of the arms. With reference to Fig 1C, it will be apparent that there are numerous bores (e.g., bores 88) formed along the length of arm 14. As such, the positions of the TC assemblies relative to one another and relative to the fixture 10 may be varied. As shown in Figs. 1 through 4, each TC assembly has a metallic shaft collar 90 circumferentially surrounding the tube 70 near the proximate end of the tube. Each shaft collar 90 has a set screw 92 that extends through the collar and bears against the tube 70 when tightened. With reference to Fig. 11 A, a second shaft collar 94 is positioned adjacent shaft collar 90. In the assembled TC assembly, the shaft collar 90 is positioned on tube 70 and the proximate end of the tube is then extended through a bore 88 in arm 14. The second shaft collar 94 is then placed over the tube and positioned near shaft collar 90 so that the arm 14 is between the two. The two collars are spaced apart slightly from the arm material so that the tube 70 is free to move in the direction of arrow A. It will be readily appreciated that there are numerous equivalent structures that are capable of attaching the tubes 70 to arms 14 and 16.

Fig. 3 illustrates collars 94 as they are positioned in arm 16. It may be seen that the upper portion of the collars defines a flattened section 95 that rests against the upper portion of the channel that defines the arm. This prevents the collar and the tube 70 from rotating relative to the sleeve.

As noted above and as best shown in Figs. 1 B and 1 C, a series of tensioned support wires 60, 62, 64, 66 and 68 are strung between arms 14 and 16. These wires serve to support and retain part 30 relative to the fixture 10 in a way that eliminates relative movement between the part and the fixture as it is conveyed through the oven. While there are five support wires shown in the illustrations, the precise number is not critical and more or fewer support wires may be used. The support wires are individually tensioned, with, for example, coil springs. The wires may have a spring on only one end, or on both ends, and the springs may include tension adjusting mechanisms. For example, with reference to Fig. 1 D, coil spring 97 has one end fixed to the interior portion of the channel defined by arm 14. Support wire 60 is attached to the opposite end of the coil spring and from there leads through an opening 96 (defined by a grommet) in arm 14. As illustrated in Fig. 1 B, support wire 60 extends across the fixture 10, through a similar opening in arm 16 and is either attached to a tensioning spring identical to coil spring 97, or has its end fixed to the arm. Either way, the spring or springs apply tension to the support wire. In some cases it is desirable to allow for adjustment of the amount of tension on the support wires. In that event a tension adjustment mechanism such as a screw tensioner or turnbuckle 98 (shown schematically in Fig. 1 D).

Support wires 62, 64, 66 and 68 may be attached to arms 14 and 16 in the same manner just described, or in alternate arrangements. For example, support wires 62 and 64 may be defined by a single length of wire that has at least one of its ends, or both, attached to springs that may be fitted with tension adjustment mechanisms. The same is true for support wires 66 and 68: the wires may be either individually strung or strung as pairs of support wires defined by a single length of wire, or even three or more support wires defined by a single wire. Again, springs may be attached at one or both ends of the support wires, and the springs may be fitted with tension adjustment means such as screw tensioners.

As noted previously, TC assemblies 50, 52, 54 and 56 are attached to the arms 14 and 16, respectively, in such a manner that the distal ends 86 of the ceramic tubes 70 (i.e., the end of the tubes near the thermocouple beads 76) are urged under spring force in the direction toward part 30. Attention is now turned to Figs. 5, 6 and 7, which show a close-up of a slightly different embodiment of arm 14 from that shown in Figs. 1 through 4. In the embodiment shown in Figs. 5, 6 and 7 the collar 94 does not include a flattened portion 95, and the channel defined by arm 14 is slightly wider than the channel shown in Figs. 1 through 4. Support wire 62 has one end attached to the non-fixed end of coil spring 97, as described above. The wire then extends through an opening in arm 14 and across the fixture to arm 16, as also described above. This puts the support wire under tension. As shown in Fig. 5, the path defined by support wire 62 is diverted so that the wire is routed around the lower side of collar 94, which has a circumferential groove 98 through which support wire 62 extends — the groove holds the wire in place relative to the collar. As a result of the tension on the support wire and the routing of the wire over the lower side of the collar, spring force is applied to the collar in the direction illustrated with arrow B. Because collar 94 is at the proximal end of tube 70, the spring force applied to the collar results in spring force applied to the distal end of tube 70 in the opposite direction — i.e., the direction of arrow A in Fig. 1C, so that the TC bead 76 is pushed toward part 30. Also visible in Fig. 5 are TC wires (e.g., 72, 74) as they lead through arm 14 toward base 18 and ultimately to electrical connectors 22.

Fig. 6 is similar to Fig. 5 in that it shows one preferred structure and method for applying spring force to the TC assemblies with the support wires. Fig. 7 illustrates the same functionality, except that the TC assembly has been slid a short distance out of the arm 14 to expose tube 70. Support wire 62 has been routed around tube 70 rather than collar 94. It will be appreciated that this structure also applies spring force to the proximal end of tube 70 in the direction of arrow B, but since the support wire has been diverted relatively less than in Fig. 5, the amount of tension will be less. The amount of spring force applied to the distal ends 86 of the TC assemblies may be varied and adjusted by adjusting the tension on the support wires as detailed above, and also by varying the position at which the support wire contacts the TC assembly (e.g., Fig. 5 versus Fig. 7). Likewise, the length of the TC assembly and tube 70 will dictate the amount of spring force that needs to be applied to the TC bead 76 in order to hold the bead firmly against the part 30. Generally speaking, the spring force applied to the TC assemblies with relatively long tubes 70 (i.e., TC assemblies 50 and 56) is greater than that applied to the TC assemblies with shorter tubes.

It will be appreciated that there are numerous equivalent methods of applying spring tension to the TC assemblies so that the distal ends of the tubes 70 are forced downwardly onto the part. For example, other spring arrangements such as leaf springs and circular springs could be utilized. Those of ordinary skill in the art will recognize other equivalent methods for this function.

A docking station 100 for use in loading a part 30 into fixture 10 is shown in Figs. 8, 9 and 10. Docking station 100 comprises a central plate 102 with upright sections 104 and 106 on opposite sides of plate 102. Upright extensions 104 and 106 are transverse to the plane of plate 102 and include laterally aligned notches 108 — that is, for each notch 108 in extension 104 there is a laterally aligned notch 108 in extension 106. Notches 108 define a first depth in the extensions 104 and 106. The extensions also include laterally aligned notches 109, which define a second depth in the extensions. The notches 109 do not extend into the extensions as deeply as notches 108. A toggle clamp 110 is positioned laterally outwardly of plate 102 and outward of the extensions 104 to define a channel 114 between the toggle clamp 110 and extension 104. A second toggle clamp 112 is similarly positioned on the opposite side of the docking station, to define a channel 116 between clamp 112 and extension 106. A pair of aligned notches 118 is found at opposite sides of the docking station 100 at one end thereof.

The docking station 100 serves as a template for holding fixture 10 in an orientation that allows a part 30 to be easily and reliably loaded onto the fixture. Figs. 9 and 10 illustrate a fixture 10 positioned in docking station 100 such that a part 30 may be loaded into the fixture. Arm 14 of the fixture 10 is positioned in channel 114 and the opposite arm 16 rests in channel 116 and spreader bar 26 is fit into the aligned notches 118. In this position, support wires 60, 64 and 68 fit into notches 108 — the deeper of the notches, and support wires 62 and 66 fit into notches 109 — the shallower of the notches. In addition, TC assemblies 50, 52, 54 and 56 also rest in notches 109. That is, the tubes 70 associated with each of these TC assemblies rests in a notch 109.

With the fixture in this position in the docking station 100, the toggle clamps 110 and 112 are lowered such that clamp 110 presses arm 14 into channel 114 and clamp 112 presses arm 16 into channel 116. As best illustrated in Fig. 10, as the fixture is pressed into the docking station by the toggle clamps, the arrangement of the TC assemblies and support wires in the notches in the docking station, described above, causes the distal ends of the TC assemblies to be raised upwardly, against the spring force applied to them by the support wires. Support wires 60, 64 and 68 are in the deep notches 108 and are not in contact with the docking station, and those wires are thus not moved when the fixture is secured to the docking station. However, support wires 62 and 66 are in the shallower notches 109 and are pushed upwardly in the same direction as the distal ends of the TC assemblies. The net result of this action is that the support wires are spread apart and the TC assemblies are raised, thereby opening the fixture from the end thereof so that a part may be slipped easily into the support wires. The part 30 is slipped into the fixture as shown in Fig. 1. Support wires 60, 64 and 68 extend below the part, and support wires 62 and 66 run over the upper surface of the part.

Once the part has been inserted into the fixture as explained above, the toggle clamps 110 and 112 are released and the fixture may be removed from the docking station. This causes the support wires 62 and 66 to move back to their original positions, thereby fixing the part between the five support wires. At the same time, the TC assemblies 50, 52, 54 and 56 return to their original positions and the TC beads 76 are pressed against the surface of the part.

The docking station thus facilitates easy insertion of parts 30 into the fixture. Many silicon wafers are quite fragile, especially many photovoltaic wafers. The docking station allows the wafers to be inserted into the fixture with minimal risk that the part will be damaged. It also allows for always positioning the part in the same position in the fixture, and in this way ensuring that the TC beads 76 are always at the same position on each part tested. Turning to Fig. 12B, the invention contemplates various thermocouple designs and orientations relative to the fixture. Specifically, there are four TCs shown and labeled with reference numbers 200, 202, 204 and 206. The bead 76 of each TC has been flattened — this allows for good contact between the TC bead 76 and the part 30. Various bead thicknesses have been tested, and in Fig. 12 the thickness of the beads 76 of the four TCs are as follows: Thermocouple Number Bead Thickness (in inches) TC 200 0.0065

TC 202 0.0050

TC 204 0.0045

TC 206 0.0040

The TCs 200 through 206 shown in Fig. 12B are not mounted in a ceramic tube in order to illustrate the TCs more completely. Fig. 12A also illustrates a fifth TC 208, which does include a ceramic tube 70. Experimental results have shown that the orientation of the thermocouples relative to the direction that the conveyer belt moves through the oven has an impact on the data. Specifically, in Fig. 12B assume in the first instance that the conveyer moves in the direction of arrow C. TCs 200 through 206 are all oriented generally transverse to the direction of conveyer movement, and TC 208 (Fig. 12A) is oriented parallel to the direction of travel. Next, in the second instance, assume that the conveyer is moving at 90 degrees to arrow C, in the direction of arrow D. For all five TCs in Fig. 12, a higher temperature is achieved and measured with the orientation of the TCs relative to the direction of conveyer travel shown with arrow D. It has been found that the TC 208 tends to be more sensitive than the TCs 200 through 206. Without being bound by any particular theory, it is postulated that because the relatively rapid rate at which the conveyer moves through the oven, a TC that is perpendicular to the direction of travel spends less time at any given point in the oven than a TC that is oriented parallel to the travel path. As a result, the TC that is oriented in the parallel direction will be exposed to the heating effect both earlier and later than the perpendicular TC. Although the effects attributed to differing orientations of the TCs relative to the direction of conveyer travel tend to be minimized when the TCs are in physical contact with a part 30, the effects are significant nonetheless. Assume that the four TCs 200, 202, 204 and 206 are attached to a data logger. In that case, the fifth TC 208 illustrates how the four TCs attached to the data logger is implemented in a fixture 10. The longer the tip of the TC and adjoining wire are exposed, in the direction of travel, the higher temperature that is achieved. This is a result of the wire spending more time at any given point in the oven and the higher temperature is gained from conduction of the lead wire to the tip.

It will be appreciated therefore that as an additional embodiment, it is possible to replace spreader bar 26 with an arm that is the same structural configuration as arms 14 and 16. One or more TC assemblies identical to those illustrated in Fig. 1 (e.g., TCs 50 through 56) and sprung in an identical manner could be attached to the arm that extends across the fixture between arms 14 and 16, with the TCs oriented parallel to the direction of conveyer travel. This third arm with additional TCs provides the advantage of further data points for the data logger and additional data points on the part 30.

Turning to Figs. 13 and 14 an alternate embodiment of a ceramic tube 250 is illustrated. The tube 250 is the same material as the tubes 70 described above. However, as shown in Fig. 14 the tube has only two longitudinal bores 252 and 254 through which the thermocouple wires 256 and 258, respectively, extend. The TC wires 256 and 258 define a TC bead 260. A slot 262 is cut in the end of the tube 250 such that the slot opens to the bore 254. This allows the TC wires 256 and 258 to be bent easily at roughly right angles so that the TC bead 260 is configured to make contact with the part 30. Any tendency of the TC bead 260 to twist or rotated tends to be minimized when the wires extend through the slot 262. In addition, it can be difficult and time consuming to thread the TC wires through the bores in the tube. By halving the number of bores from 4 to 2, it is easier to manufacture the assembly.

Various alternative embodiments are contemplated by the inventions described above. For example, in some cases it is desirable to have the part 30 supported by the conveyer itself (rather than separated and spaced apart from the conveyer as accomplished with fixture 10 detailed above), or by stand-offs on the conveyer. In these cases the bottom support wires (i.e., support wires 60, 64, and 68) may be removed or not utilized so that the part rests on the conveyer surface or the stand-offs, as the case may be.

With reference now to Fig. 15, a preferred embodiment of a fixture 300 according to the present invention is shown. In the embodiment of Fig. 15 there are three TC assemblies, 302, 304 and 306, each of which utilizes ceramic tubes 250 of the two-bore type described above, with the TC beads 260 in contact with the part 30. A fourth TC assembly 308 is identical in construction to TC 302, but is placed in fixture 300 to measure and profile the air temperature. It will be noted that the TC assemblies 302, 304 and 306 do not have a collar immediately adjacent the inner wall of the fixture arms (analogous to collars 90 described above with respect to the embodiment of Fig. 1 ). However, each of these TCs has spring tension applied to the distal end of the arm with the tensioned support wires, as detailed previously. TC 308 is not being pressed against part 30 and does include a collar 90.

Fixture 300 is identical in its essential features to the fixtures 10 described above, and thus has opposed arms 14 and 16, a spreader bar 26, and five support wires 60, 62, 64, 66 and 68, which are under spring tension. As with the prior embodiments, the lengths of the TC tubes may be varied widely to vary the point on the part that is being temperature-monitored, and the position of the TC assemblies in the fixture may be varied. This allows the fixture to accept parts of many different sizes and it provides a means for locating all three of the TCs 302, 304 and 306 along the same line running across the part 30.

An alternative embodiment of a thermocouple assembly 400 is shown in Fig. 16. In this case TC tube 402 is a four-bore tube that contains wires for two separate thermocouples, i.e., wires 404, 406, 408 and 410, which define two separate thermocouple beads 412 and 414 that extend from tube 402 along its length rather than at its distal end. With a TC assembly 40 the length of tube 402 may be adjusted so that the tube extends completely across the fixture from arm 14 to arm 16. A collar such as collar 90 would be fitted to each end of the tube to attach the tube to the fixture. Once a part is placed in the fixture, the tube 402 may be rotated to press the TC beads 412 and 414 onto the part and the collars are tightened to fix the position of the tube. The TC assembly 400 has several beneficial features. For example, because both of the TC beads 412 and 414 are aligned in the same position relative to the fixture, any temperature shadowing effect that might be experienced with TCs that are "downstream" of other TCs as the fixture moves through the oven will tend to be eliminated. Further, it is possible to load a part onto the fixture without use of a docking station since rotation of the tubes 402 moves the TC beads 412 and 414 out of the way so that the part may be inserted between the support wires.

Turning to Fig. 17, yet another alternative embodiment of a thermocouple 500 having an extended tip portion 502 is illustrated. TC 500 thus has a generally U-shaped extension 504 defined by opposed flattened arms 506 and 508 attached at their base ends to TC bead 510. This configuration is relatively simply manufactured by twisting two additional TC wires when twisting the TC prior to welding to form the bead, then separating the wires as illustrated prior to squeezing the TC to flatten it.

The TC 500 illustrated in Fig. 17 provides several advantages. Recall that a goal in thermal profiling is to place as much of the surface of the TC as possible onto the surface of the part to be measured in order to obtain the most reliable results. This is especially important where the surface of the part is electrically conductive or can generate a voltage by itself (as in the case of a silicon solar cell wafer). With TC 500 the U-shaped extension 504 is composed of one high temperature wire (preferably either chromel, alumel or silver). Since both of the opposed arms 506 and 508 are defined by the same material there cannot be any EMF (electromagnetic field) developed between the arms. As such, the U-shaped extension 504 can cover a relatively large portion of the surface of the part and conduct the heat generated in the part at the point back to the TC bead 510 where the added wire is attached thereto.

Experimental data indicates that better thermal contact between to the surface of the part may be achieved with a TC as illustrated in Fig. 17. The TC 500 is further beneficial for use an a TC to measure the air temperature (as with TC assembly 308 in Fig. 15) since the tip will get hot sooner as the TC enters or leaves a particular point in the furnace and thereby achieves a better average temperature. Standardization of the methods used during thermal profiling is important to have reliable, reproducible results. From the foregoing it will be appreciated that elimination of variability in test procedures is one important factor in generating accuracy and precision. The fixture 10 described herein eliminates variability in the manner in which the part 30 is held relative to the fixture and the conveyer belt, and in the location and manner in which the TC beads are placed in contact with the part. This greatly improves the standardization of thermal profiling.

With reference now to the series of figures in Figs. 18 through 24, two different embodiments of a fixture are illustrated in which the part may be loaded into the fixture without the use of a docking station. For purposes of continuity, parts shown in Figs. 18 through 24 that are structurally and functionally identical or equivalent to parts described above are given like reference numbers.

In Fig. 18, fixture 550 includes four thermocouples 552, 554, 556 and 558. TCs 552 and 554 are supported by a first ceramic tube 560 and TCs 556 and 558 are supported by a second ceramic tube 562. Tubes 560 and 562 are located in arms 14 and 16, respectively, so that the TCs are all aligned along a line transverse to the direction of travel of the fixture through the furnace (illustrated by arrow C in Fig. 18). Fixture 550 has four support wires 564, 566, 568 and 570, which may be tensioned in the manners described above. In a preferred embodiment, support wires 564 and 566 are defined by a single length of wire, and support wires 566 and 568 are likewise defined by a single length of wire. The preferred structure for providing tension on a pair of wires (e.g., support wires 562 and 564) is with a single spring attached to one end of the single length of wire, and an adjustable turnbuckle (as described above) on the opposite end of the wire.

The part 30, which in the illustration is a typical photovoltaic wafer, is supported in the fixture by the four support wires. The distance X in Fig. 18, which is the distance measured from base member 18 to the nearest support wire (in this case, support wire 570), is longer than the length of the part 30. As such, the part 30 may be loaded into the fixture 550 by first placing the part into the space between the base member 18 and nearest support wire 570, then sliding the part between the wires so that the part is completely supported by the fixture. This may be done in several different combinations of wire position relative to the part, one of which is shown in Fig. 19, where part 30 is inserted into the fixture with support wire 570 below the part, wires 568 and 566 above the part, and wire 566 below the part. It will be appreciated that when the loaded fixture 550 is moving through an oven, the TCs are facing toward the direction of travel. In other words, the leading edge of the fixture when it is in the oven is the end with spreader bar 26, and the TCs thus face this end of the fixture.

As noted, the four TCs (i.e., 552, 554, 556 and 558) are held in two separate ceramic tubes, 560 and 562. This is best illustrated in Figs. 20, 21 and 22. Each of the ceramic tubes 560 and 562 is supported in one of the respective arms 14 and 16 with a collar 571 that has a central bore 574 for receiving the base or proximal end of the ceramic tube. The collar is split on one side with a groove 576. A hex set screw 578 is threaded into the collar so that the nut extends across the groove 576 and thus allows for tightening and loosening the ceramic tube in the collar when the tube is inserted into the bore 574. It will be understood that prior to loading a part 30 into fixture 550 as described above, the hex set screws 578 may be loosed and the ceramic tubes 560 and 562 rotated relative to the collars to move TCs 552, 554, 556 and 558 upwardly and away from the plane in which the part will lie in the fixture. After the part has been inserted between the support wires as detailed above so that the part is supported by the fixture, the ceramic tubes are rotated in the opposite direction so that the TCs are pressing down onto the surface of the part (as in Figs. 18 and 19) and the hex set screws are tightened.

The fixture 550 of Figs. 18 through 22 has four TCs, two in each of the two ceramic tubes. As such, and as shown in Fig. 23, each ceramic tube has four separate TC wires leading out of the tube and running through the channels in the arms to the data logger connector prongs 22. These wires may be separated from one another as they lead out of the base of the ceramic tubes with individual insulating tubes, labeled in Fig. 23 with reference numbers 580, 582, 584 and 586.

Another embodiment of a fixture 600 is shown in Fig. 24. In some instances it may be desirable to have the part 30 rest directly on the surface of the furnace belt and thus not be supported away from the belt by support wires as with the fixture 550 described above. The fixture 600 facilitates this method of measuring temperature through the furnace. Thus, opposite arms 14a and 16a are attached to a base member 18 but the arms are shortened and there are no support wires extending between them. The fixture 600 includes four TCs that are identical in structure to the four TCs described above with respect to fixture 550. However, as noted, the fixture 600 has no support wires. As such, part 30 is laid directly on the furnace belt - in the illustration of Fig. 24 the surface of the furnace belt is represented by surface 602. The fixture 600 is positioned over the part as shown, and the two ceramic tubes 560 and 562 are rotated so that the four TCs 552, 554, 556 and 558 are pressing down onto the surface of the part and the hex set screws are tightened.

While the present invention has been described in terms of preferred and alternative embodiments, it will be appreciated by one of ordinary skill that the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents as defined in the appended claims.

Claims

Claims:

1. A fixture for retaining a part conveyed through a conveyer oven, comprising: a frame having a first arm and a second arm; plural wires extending from the first arm to the second arm; and at least one thermocouple (TC) mounted to one of the arms, said TC having a TC bead configured for making contact with the part.

2. The test fixture according to claim 1 wherein each of the first and second arms is attached at a proximal end thereof to a base member and the first and second arms are held in a spaced apart relationship.

3. The test fixture according to claim 2 wherein each of said plural wires is tensioned and said plural wires are configured for holding said part.

4. The test fixture according to claim 3 wherein said part is held in said test fixture such that the part is spaced apart from the first and second arms.

5. The test fixture according to claim 4 wherein said plural wires are tensioned under spring force.

6. The test fixture according to claim 5 including adjustable tensioning means for adjusting the tension on said plural wires.

7. The test fixture according to claim 1 wherein the TC bead has a force applied thereto for urging said TC bead against said part.

8. The test fixture according to claim 7 wherein the TC bead is at a distal end of said TC.

9. The test fixture according to claim 1 wherein at least one TC is mounted to said first arm and at least one TC is mounted to said second arm, each TC having a TC bead configured for making contact with the part.

10. The test fixture according to claim 2 wherein the base member extends transverse to the first and second arms, and including means for electrically connecting the at least one TC to a data logger.

11. The test fixture according to claim 10 including a spreader bar extending between said first and second arms and interconnecting said arms at distal ends thereof.

12. The test fixture according to claim 1 wherein said at least one TC includes plural TC beads.

13. The test fixture according to claim 1 wherein said each TC of said at least one TC further includes a ceramic tube having a proximal end attached to an arm.

14. The test fixture according to claim 13 wherein said ceramic tube includes a pair of bores extending longitudinally therethrough and wherein said TC includes at least two wires extending through said bores, said wires connected together to define said TC bead.

15. The test fixture according to claim 14 wherein said wires are further connected to a data logger.