Abstract:
A solar cell module comprises a solar cell die that is soldered to a substrate. The substrate comprises one or more power contacts. A power conductor is soldered to a power contact, thereby electrically coupling the power conductor to the solar cell die. A pre-heat module heats a first side of the substrate at a first area to a first temperature for a first duration. Then, a solder heat source solders a power conductor to a power contact at a second area of the substrate at a second temperature for a second duration. The resulting solder connection at the power conductor is less prone to cold-solder defects. The temperature of the pre-heat module is controlled to promote curing of an RTV sealant used in the manufacture of the solar cell module. The temperature of the solder heat source is controlled to avoid burning and degrading of the RTV sealant.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of electrical coupling of an electronic component to a power conductor at a power contact on a substrate. More specifically, the present invention relates to pre-heating a substrate to protect the substrate from damage when a high temperature solder joint is made to a contact on the substrate and to avoid overheating of the solar cell and RTV adhesive during a solder reflow process. 
     BACKGROUND OF THE INVENTION 
     A solar cell module comprises a solar cell die soldered to a substrate having a plurality of electrical traces terminating in at least two power contacts output onto a surface of the substrate. A solar cell die housing, termed a cowling, is coupled to the substrate with a curable sealant such as a room temperature vulcanization (RTV) sealant. Curing of the sealant can be accelerated by application of heat, but the application of too much heat can degrade the bonding strength and change the properties of the RTV sealant. Also, excessive heat may expand the RTV rapidly, squeezing the RTV, leaking the RTV out to the cowling that may result in an electrical contact problem in addition to the leakage of the RTV. 
     After the solar cell die housing is coupled to the substrate, a power conductor is coupled to a power contact on the substrate, thereby electrically coupling the solar cell die to the power conductor via the power contact on the substrate. The temperature at the power contact during soldering can be as high as 220° C. for a tin/lead solder process, or higher for a lead-free process. If the substrate is cold, relative to the 220° C. solder temperature, then the resulting solder joint at the power contact can comprise a solder “cold-joint” that is subject to cracking and breaking. If the temperature of the solder at the power contact is held high long enough to overcome a cold-joint, then too much heat may be transferred to the substrate, thereby burning and degrading the RTV sealant that bonds the solar cell die housing to the substrate. Further, excessive heat applied to the substrate can result in increasing the size of solder voids in the solder joint between the solar cell die and the substrate. 
     To address these problems, it is desirable to control the temperature of the substrate in relation to the effect of temperature upon the RTV sealant in contact with the substrate and in relation to the temperature needed to make a high quality solder joint of the power conductor to the power contact on the substrate. 
     SUMMARY OF THE INVENTION 
     To avoid defects in a solder joint of a power conductor to a power contact on a substrate and avoid melting and reflowing of an RTV sealant that couples a solar cell die housing to the substrate, the substrate is pre-heated to a predetermined temperature in a first area of the substrate for a predetermined period of time. Pre-heating the substrate also pre-heats a power contact on the substrate. By pre-heating the substrate to a controlled temperature, the pre-heated substrate will avoid burning of an RTV sealant that couples a solar cell die housing to the substrate. Heating of the substrate is controlled by a first heat source. Heating of the solder joint that couples the power conductor to the power contact is controlled by a second heat source termed a “hot bar”. Pre-heating of the substrate before soldering to a power contact on the substrate can also be used with other types of electronic components that are soldered to a substrate, particularly components that comprise an RTV sealant that may burn and degrade at the temperature of soldering. 
     The systems and methods described herein are able to be incorporated with systems and methods of reducing a void in a solder joint between a solar cell die and a substrate as described in U.S. patent application Ser. No. 13/564,568 entitled “VACUUM REFLOW VOIDING REWORK SYSTEM”, filed on the same day as this application, by inventors Dason Cheung and Richard Loi, which is hereby incorporated by reference in its entirety for all purposes. 
     In a first aspect, a method of soldering a power conductor to a power contact, the method practiced upon an electronic component module having an electronic component soldered to a substrate at a first area of the substrate and having a power contact at a second area of the substrate, the method comprises pre-heating the first area of the substrate to a first temperature for a first duration and soldering the power conductor to the power contact at the second area of the substrate at a second temperature for a second duration. Preferably, the electronic component is a solar cell die and the electronic component module is a solar cell module. In some embodiments, the first temperature is in a range of 90° C. to 120° C. on the substrate surface during the first duration. In some embodiments, the second temperature is in a range of 215° C. to 245° C. In one embodiment, the second temperature heat source is set to approximately 330° C. to achieve the desired temperature range during the second duration. In a preferred embodiment, the first duration overlaps with the second duration. The method preferably further comprises increasing the first temperature to between 150° C. and 170° C. In one embodiment, the method further comprises reducing the pre-heating temperature at the first area, thereby controllably cooling the substrate, the power contact, the power conductor, and the solder joint of the power contact and the power conductor. The first area is a first surface of the substrate. Pre-heating the first area of the substrate to a first temperature results in heating of the power contact. In some embodiments, soldering the power conductor to the power contact at the second area of the substrate at the second temperature comprises heating a second surface of the substrate. In a preferred embodiment, the selection of at least one of the first temperature and the first duration promotes curing of a sealant in the electronic component module. In another preferred embodiment, the selection of at least one of the first temperature and the first duration avoids melting and reflow of a sealant in the electronic component module. 
     In a second aspect, a system for soldering a power conductor to a power contact of an electronic component module, the electronic component module comprising an electronic component soldered to a substrate at a first area of the substrate and having a power contact at a second area of the substrate, the system comprising a pre-heating source thermally coupled to the substrate and configured to pre-heat the first area of the substrate to a first temperature for a first duration and a soldering heat source thermally coupled to the power conductor and the power contact and configured to solder the power conductor to the power contact at a second temperature at a second area of the substrate for a second duration. In some embodiments, the the pre-heating source generates temperatures in a range of ambient temperature to 150° C. In one embodiment, the pre-heating source comprises a plurality of independently controllable heating modules. In some embodiments, the pre-heating source comprises a Peltier thermo-electric cooler. The pre-heating source is configured to heat the first area of the substrate on a first side of the substrate. In some embodiments, the soldering heat source is generates temperatures in a range of 200° C. to 400° C. In some embodiments, the soldering heat source is configured to heat the second area of the substrate on a second side of the substrate. 
     Throughout the disclosure, reference is made to a solar module comprising a solar cell die soldered to a substrate. One skilled in the art will recognize that the disclosure applies generally to a component soldered to a substrate having a power contact to which a power conductor is to be soldered, such as a diode array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a solar cell module installed in a cowling according to some embodiments. 
         FIG. 1B  is a sectional view of a solar cell module installed in a cowling according to some embodiments. 
         FIG. 2  illustrates a side cross-sectional view of a system for soldering a plurality of power conductors to a plurality of power contacts of a solar cell module according to some embodiments. 
         FIG. 3  illustrates a side cross-sectional view of a sub-system for soldering a power conductor to a power contact of a solar cell module according to some embodiments. 
         FIG. 4  illustrates the steps of a method of soldering a power conductor to a power contact on a substrate according to some embodiments. 
         FIG. 5  illustrates a control system for controlling a hot bar soldering system implementing a method of soldering a plurality of power conductors to a plurality of power contacts on a substrate according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the detailed description of the drawings that follows, the embodiments described are intended to illustrate features of the presently-claimed invention. Similar labels refer to similar or identical elements. 
       FIG. 1A  illustrates a mounted solar cell module  150  comprising a solar cell module  100  installed in a cowling  125  according to some embodiments. A conductor  140  enters the cowling  125  from each of two opposing sides. Each conductor  140  is soldered  145  to a contact on the solar cell module  100 . The cowling  125  receives a solar cell die  100  that is soldered to the substrate  120 , electrically coupling the solar cell die  100  to the electrical traces on the first surface of the substrate  120 . The electrical traces terminate in a power contact  145  located on the first surface of the substrate  120 . A power conductor  140  is electrically coupled to the power contact  145  by a soldering process as described below. 
       FIG. 1B  is a sectional view of a mounted solar cell module  150  according to some embodiments. The cowling  125  is coupled to the substrate  120  using a room temperature vulcanization (RTV) sealant (not shown). The solar cell die  100  is soldered onto the substrate  120 . The power conductor  140  is soldered to the power contact  145  on the substrate  120 . 
       FIG. 2  illustrates a side cross-sectional view of a system  200  for soldering the power conductor  140  to the power contact  145  of the mounted solar cell module  150  according to some embodiments. As shown in  FIGS. 1A and 1B , the mounted solar cell module  150  comprises the solar cell die  100  soldered  130  to the substrate  120 . The substrate  120  comprises a plurality of electrical traces (not shown) that terminate in one or more power contacts  145 . The cowling  125  is coupled to the substrate  120  using an RTV sealant (not shown). The mounted solar cell module  150  is received by a solar cell module soldering mount  210 . The soldering mount  210  comprises a recessed area  212  into which the mounted solar cell module  150  is placed. A pre-heat module  200  comprises a pre-heat source  240  that is thermally coupled to the substrate  120  of the mounted solar cell module  150 . 
     A pre-heat module  200  comprises a pre-heat source  240  coupled to a pre-heat actuator  241  for bringing the pre-heat source  240  into thermal contact with the substrate  120 . The pre-heat source  240  spans the location of the one or more power contacts  145  on the substrate  120  such that heat is applied to the substrate  120  and heat is thereby also applied to the one or more power contacts  145 . The pre-heat source  240  comprises a plurality of pre-heat source modules  245 , each independently controllable and each comprising a temperature sensor  242  for interfacing to a control system as shown in  FIG. 5  and as described below. A hot bar solder module  300 , shown in  FIG. 3  and described below, solders the power conductor  140  to the power contact  145 . As shown in  FIG. 2 , two hot bar solder modules  300  are in a side-by-side, mirror image configuration. 
       FIG. 3  illustrates a side cross-sectional view of a hot bar solder module  300  for soldering a power conductor  140  to a power contact  145  as shown in  FIG. 2 . A heating element  315  is installed in a high temperature portion  310  of the hot bar solder module  300 . The heating element  315  is controllable over its operating temperature range by a control system  600  as shown in  FIG. 5  and as described below. The high temperature portion  310  further comprises a heat sensor  340  for use in closed-loop feedback control of the heating element  315 . The high temperature portion  310  is isolated from a high temperature actuation module  360  by a thermal insulation portion  320 . The high temperature actuation module  360  comprises a high temperature actuator plate  325 , a high temperature positioning actuator  330 , and a high temperature actuator guide  335 . The high temperature positioning actuator  330  and guide  335  position the high temperature portion  310  such that a hot bar soldering surface  305  is in thermal contact with a power conductor  140 , a power contact  145 , solder, or any combination thereof. The high temperature portion  310  comprises the hot bar soldering surface  305  and a recessed area  350 . The recessed area  350  permits the hot bar soldering surface  305  to heat the power contact  145  without the high temperature portion  310  contacting the solar cell die  100 . 
       FIG. 4  illustrates the steps  500  of a method of soldering a power conductor  140  to a power contact  145  on a substrate  120  according to some embodiments. At step  510 , a pre-heat module  240  is pre-heated to a first temperature at a first area of the substrate  120  and brought into thermal contact with the substrate  120 . Preferably, the first temperature is in a range 90° C. to 120° C. and the first area thermally encompasses an area of the substrate  120  that comprises a solder joint  130  of a solar cell die  100  soldered to the substrate  120  and a power contact  145  on the substrate  120  adjacent to the solar cell die  100 . At an optional step  520 , the pre-heat temperature is increased, preferably to 150° C., for a duration of time that overlaps with the following step  530 . At step  530 , a hot bar solder module  300  is heated to a second temperature and brought into thermal contact with the power contact  145 , the power conductor  140 , solder, or any combination thereof, to carry out soldering of the power conductor  140  to the power contact  145 . At an optional step  540 , a controlled cooling step is performed to optimize the strength of the solder joint and to prevent high thermal gradients that may crack the substrate or result in a solder cold-joint. The controlled cooling step is accomplished by a controlled reduction of the pre-heat module  240  temperature, controlled cooling such as with a Peltier cooler, application of a controlled-temperature air flow, or any combination thereof. 
       FIG. 5  illustrates a control system  600  for implementing a method of soldering a plurality of power conductors  140  to a plurality of power contacts  145  according to some embodiments. The control system  600  comprises a controller  610 , a memory  620 , storage  630 , a user interface I/O port  640 , a network interface  650 , other I/O  660 , and an expansion I/O module  670 , all communicatively coupled by a system bus  680 . The controller  610  executes instructions programmed into the storage  630  and read into the memory  620 . The programmed instructions carry out the method steps  500  for soldering a power conductor  140  to a power contact  145 . In one embodiment, other I/O  660  comprises interrupt lines, timer/counter inputs and outputs, communications lines such as Clocked Serial I/O, I 2 C, USB, RS232, RS485, and other communications protocols. The expansion I/O module  670  comprises analog inputs (AI), analog outputs (AO), digital inputs (DI) and digital outputs (DO). Analog outputs control the pre-heat modules  245 , the hot bar heating elements  315 , the hot bar solder position actuators  330 , and the pre-heat module position actuator  241 . Analog inputs include the hot bar temperature sensors  340  and the pre-heat module sensors  245 . One skilled in the art would recognize other similar control schemes exemplified by the control system shown in  FIG. 5 . A user interface for obtaining user inputs and for providing information to the user can be coupled to the control system via the User Interface I/O port  640 . 
     Heating Means 
     In one embodiment, a pre-heat module (e.g., element  200  of  FIG. 2 ) of the solar cell module substrate  120  comprises a resistance element and a current driver. Alternatively, a high-powered Peltier thermo-electric cooler is used. Pre-heating can also be achieved by pulsed or continuous wave laser. Other pre-heating means include heat from a heat source fluidly coupled to the substrate  120 , such as a heated atmosphere, a plasma, or a flame. The pre-heat module  200  is preferably controllable over a temperature range from ambient to 200° C. The pre-heat module  200  can further comprise a heat sensor for each pre-heat module, the heat sensor providing closed loop feedback for accurate control of the corresponding pre-heat module. A pre-heat module can be controlled using a servo algorithm such as a proportional, integral, derivative (PID) servo algorithm or a time proportional control servo algorithm. One skilled in the art will recognize other heat control algorithms that can be used in accordance with the embodiments. 
     A hot bar solder module is able to comprise any of the above heat sources described for the pre-heat module, controlled in a like manner. Temperature ranges for the hot bar solder module depend upon the type of solder used in the solder process. Typical temperatures can range from 200 C.° to 400 C.°. 
     Methods of soldering have been described herein as comprising discrete heating steps in relation to a step of soldering a power conductor to a power contact. One skilled in the art would recognize that in a production setting, the heat modules described herein can be maintained at certain predetermined temperatures and the heat modules are able to be brought into contact with a specific workpiece, then withdrawn from the workpiece, to reduce the application of heat to the workpiece. For example, a pre-heat module can be maintained at 150° C. and a hot bar solder surface can be maintained at 330° C. When a substrate is to be heated, the pre-heat module is brought into thermal contact with the substrate for a first duration of time. The hot bar solder surface is then brought into thermal contact with a power conductor, a power contact, solder, or any combination thereof, then removed from thermal contact after a second duration of time. Similarly, the pre-heat source can be removed from the substrate when heat is no longer needed. 
     Programmed Cycle Operation 
     In one embodiment, a soldering operation is performed using a pre-programmed cycle based upon variable inputs, some of which can be pre-programmed into the controller  610 . Variables include the characteristics of the power conductor  140 , such as wire gauge and material type, the material type and thickness of a power contact  145 , the thickness and material type of the substrate  120 , and the material and thickness of the solder joint between a solar cell die and the substrate  120 . Some of these variables will be relatively constant based upon the specifications of a particular solar cell module, power conductor, and power contact. An operator of a soldering system can select a particular solar cell module type with pre-programmed specifications for soldering. Other parameters include specifying a heat source ramp rate, cycle duration, heat source ramp down rate, cooling rate for Peltier heat sources, and cycle duration. 
     Actuator Means 
     As described above, the heater module  240  can be positioned under, and retracted from, the underside of the substrate  120  of amounted solar cell module  150  in the lower assembly mount  210  by means of an actuator  241 . As also described above, the upper vacuum cover  275  can be positioned onto, and retracted from, the cowling  125  of a mounted solar cell module  150  by means of actuators  270 . In  FIGS. 3-5 , the actuators  241  and  270  are shown as pneumatic actuators, driven by air sources  260  and  262 . One skilled in the art will recognize that the actuators could alternatively be hydraulically operated, electrically operated, or manually operated. 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the invention. It will be readily apparent to one skilled in the art that other various modifications are able to be made to the embodiments chosen for illustration without departing from the spirit and scope of the invention as defined by the appended claims.