Abstract:
A method is provided for a soldering process. The method can include receiving a liquid phase heat transfer medium at a preheating container. The heat transfer medium can be received from an external supply. The method can also include heating the heat transfer medium to or above a predefined temperature that maintains the liquid phase and directing the heated heat transfer medium from the preheating container to an evaporation container. In the evaporation container, the heat transfer medium can be vaporized to convert the heat transfer medium from the liquid phase to a gas phase. Further, the method can include directing the gas phase heat transfer medium from the evaporation container to a solder chamber. The soldering process occurs in the solder chamber.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 11/109,880, filed Apr. 20, 2005, entitled “Vapor-Generated Soldering System and Vapor Generation System for a Soldering System,” which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The present invention generally refers to soldering systems which are operated by means of a heat transfer medium which is present as vapor at least in part during the process. The present invention further refers to a vapor generation system which can be used for such soldering systems, e.g. condensation reflow-soldering systems, repair soldering systems, wave soldering systems, and selective soldering systems. 
         [0004]    2. Related Art 
         [0005]    When subassemblies are soldered by machine, use is made of soldering systems, for instance reflow soldering systems, in which heat is transferred to the individual components of the subassembly by means of radiation and/or convection of a gas or vapor and also by vapor which when applied to the subassembly will condense, thereby releasing latent heat, so that a corresponding solder is then made to melt and a soldered joint is produced. Especially in condensation soldering systems and vapor-phase soldering systems, a transfer medium is manipulated in the liquid phase at least in part during the process and must be converted into the vapor phase prior to introduction into a corresponding process chamber. The condensation soldering system differs in that the boiling point of the heat transfer medium is higher than the corresponding melting point of the used solder, so that the vapor deposited as condensate leads to a more rapid and more uniform heating of the individual components due to the increased condensation temperature and the released latent heat. 
         [0006]    Preferably, polymers based on perfluoropolyether are used as heat transfer media because they are chemically inert, biologically mostly inactive and can be modified by virtue of their molecular composition such that the boiling point can be adjusted within certain limits. For instance, the product Galden LS200 having a boiling point of 200° C., which is preferably used for tin/lead solders, and the product Galden HS240 having a boiling point of 240° C., which is preferably used for lead-free solders, are available and are often employed in condensation soldering systems and vapor-phase soldering systems. To permit a continuous treatment of subassemblies, a corresponding vapor-phase soldering system must comprise corresponding means to provide enough vapor in the process chamber for the soldering process and for possible preheating or cooling processes at all times. To this end the heat transfer medium is normally circulated in the system in the liquid phase and in the vapor phase, and the condensates of the liquid phase arising in the process chamber and possibly in other regions of the soldering system are collected and supplied to a vapor generation system. It has turned out to be advantageous when the volume of the vapor generator is kept as small as possible so that the outer dimensions of the soldering system are not considerably enlarged on the one hand and a shorter response time of the vapor generator is accomplished on the other hand because small volumes of the liquid phase can in principle be evaporated again at a faster pace. 
         [0007]    For instance, EP 1157771 describes a vapor-phase soldering system which is operated with superheated vapor, the vapor being generated in a vapor generator outside the process chamber or process chambers used for soldering and preheating and cooling, respectively, and then introduced into the corresponding chambers. Although the system described therein permits an efficient soldering in combination with a desired temperature profile, this document describes the construction of an evaporation system for any desired heat transfer medium only in a very general manner. However, especially when heat transfer media based on perfluoropolyether are used, further difficulties arise because in the case of superheating above 300° C. said medium is subjected to a decomposition process which may lead to a change in the boiling point and produce toxic substances in addition. For soldering processes which are to be carried out with a lead-free solder, Galden HS240© being e.g. used, there is only a relatively small temperature difference at standard pressure between the boiling temperature (240° C.) and the maximally applicable temperature (300° C.) in a corresponding vapor generator, so that the evaporation of an adequate amount of the liquid phase is thereby made more difficult. Furthermore, the liquid phase of said heat transfer medium shows a low thermal conductivity which is e.g. about 15 times lower than the heat conduction of water, so that the heat transfer from a corresponding heating element into the liquid phase is relatively small. Since heat conduction must mainly take place within the liquid phase, only a small part of the liquid volume is normally in direct contact with a corresponding heating surface in conventional systems. Another difficulty is that when approaching the boiling temperature the liquid phase of the heat transfer medium considerably increases in volume, which is e.g. about 37% at 240° C. in the case of HS240©, so that considerable level variations arise particularly in small-sized vapor generators. Moreover, a precise level monitoring of the liquid reservoir is most of the time only possible very inaccurately due to the permanently boiling and thus very turbulent surface, so that there is always the risk of an excessively low level and thus of a fluctuating vapor generation in a vapor generator having only a very small volume. 
         [0008]    In general, a vapor generation system for a reflow-soldering system should be able to provide the necessary amount of vapor in continuous fashion, so that the liquid condensates of the heat transfer medium which arise in the soldering process and are again supplied to the vapor generator are efficiently evaporated again. The introduction of a cooled liquid phase of the heat transfer medium, which is e.g. supplied from a collection container or from an external source for compensating for possible circulation losses, may however lead to a sudden decrease or “collapse” in the continuous vapor generation due to the previously explained unfavorable thermodynamic properties of heat transfer media, e.g. the above-described perfluoropolyether materials, with the vapor volume and thus also the vapor pressure in the vapor generator decreasing immediately. The accompanying pressure variations may have a negative effect on the vapor supply to the corresponding process chambers, thereby impairing the possibilities of controlling the vapor volume flowing towards the subassembly to be treated in a desired way. 
         [0009]    It is therefore an object of the present invention to provide a vapor phase soldering system and a corresponding vapor generation system which are capable of continuously providing an adequate amount of vapor, with the possibility of observing a small constructional volume of the vapor generation system. 
       SUMMARY 
       [0010]    According to a first aspect of the present invention this object is achieved by providing a vapor generation system for soldering systems, wherein the vapor generation system comprises an evaporation region configured for evaporation of a heat transfer medium. Furthermore, a preheating region is provided which is configured for heating the heat transfer medium to or above a predefined temperature without exceeding an adjustable upper maximum temperature. Furthermore, the vapor generation system comprises a fluid connection between the evaporation region and the preheating region that is configured to conduct a fluid flow from the preheating region to the evaporation region. 
         [0011]    Hence, the construction of the vapor generation system according to the invention provides an evaporation region and a preheating region that are coupled by means of a fluid connection, so that especially the heat transfer medium supplied by a soldering system in the liquid phase can be preheated to a desired temperature before being introduced into the evaporation region. In this process it is possible to reliably prevent an upper maximum temperature incompatible with the heat transfer medium. For instance, the temperature inside the preheating region can be selected such that said temperature is as close as possible to the boiling temperature of the corresponding heat transfer medium, so that upon introduction of the medium preheated in this manner a corresponding collapse in vapor generation in the evaporation region can be substantially avoided. In this context the term “near the boiling temperature” shall characterize a temperature which differs by not more than about 40° C., still more advantageously by not more than about 5° C., from the boiling temperature, the boiling temperature standing for the boiling temperature in the evaporation region. This means that the temperature of the liquid phase in the preheating region may also be above the boiling temperature if a correspondingly high pressure prevails in the preheating region, but the preheating region is configured such that a selected upper maximum temperature is also not exceeded locally in the preheating region. Although the two regions can be fluidically coupled by providing the preheating region, which is coupled by the fluid connection to the evaporation region, the two regions remain mainly decoupled with respect to thermodynamic fluctuations. For instance, the constructional shape and the volume of the preheating region can be chosen such that a high heat transfer from corresponding heaters to the liquid phase of the heat transfer medium is made possible even upon introduction of the liquid at greatly varying temperatures. On the other hand, the evaporation region, for instance, should be configured such that only a relatively small volume of liquid phase must be present because temperature variations can be substantially avoided, thereby permitting an improved controllability in vapor generation due to the reduced fluctuations and the small fluid volume in comparison with conventional vapor generation systems. 
         [0012]    In another configuration, the evaporation region comprises a first container and the preheating region a second container, the containers corresponding with one another by means of the fluid connection at least temporarily. The use of correspondingly configured closed containers for the evaporation region and the preheating region permits a reliable decoupling of the two regions, so that e.g. different pressure conditions may also prevail in the respective containers with respect to the different temperature conditions without any significant mutual interaction. The fluid connection may here be configured such that fluid can be conveyed from the preheating region into the evaporation region only temporarily or also continuously. 
         [0013]    In a further embodiment, the fluid connection comprises a pump for conveying fluid from the preheating region to the evaporation region. The provision of a pump allows for a high degree of flexibility for operating the preheating region and the vapor generation region because e.g. in the evaporation region a higher pressure level may prevail by means of the pump without the conduction of the fluid being restricted thereby. Furthermore, the pump may e.g. be provided as a controllable pump so that the amount of the fluid introduced into the evaporation region may optionally be adapted to a varying demand for evaporated heat transfer medium. 
         [0014]    In a further advantageous embodiment, a valve is provided in the fluid connection for preventing backflow from the evaporation region into the preheating region. With this constructional measure it is possible to maintain a pressure gradient between the evaporation region and the preheating region even upon standstill or failure of a corresponding pump in the fluid connection, so that a desired thermodynamic state in the evaporation region can also be maintained in the absence of the supply line of the liquid phase. Hence, the vapor generation system can be adapted in a very efficient way to strongly varying operating conditions, including a temporary interruption of the vapor supply for a soldering system. 
         [0015]    In a further advantageous configuration, the vapor generation system comprises a pressure generation device in the preheating region that is configured to produce and maintain a predetermined pressure in the preheating region. 
         [0016]    With this measure the pressure level can be adjusted in the preheating region in the desired manner, so that there is e.g. always a higher pressure level than in the evaporation region. In such an assembly, fluid can be introduced from the preheating region into the evaporation region due to the prevailing pressure difference. A pump can thereby be optionally omitted in the fluid connection, resulting in a smaller constructional volume, lower costs and less proneness to failure on the system&#39;s part. Furthermore, the temperature of the liquid phase can be controlled in a flexible manner by actuation with pressure in the preheating region because a higher temperature of the liquid phase in the preheating region as compared with the temperature of the liquid phase in the evaporation region is e.g. also possible. With a corresponding “superheating” of the liquid phase in the preheating region a heater in the evaporation region can possibly be dispensed with because the introduced liquid phase then boils at a lower temperature in the evaporation region. When a heater is provided in the evaporation region, an increased degree of controllability can be achieved through the pressure generation device with respect to vapor generation in the evaporation region because with respect to both the temperature in the preheating region and the pressure, as compared with the corresponding values in the evaporation region, a variation is possible in both directions. In a mode of operation in which the pressure in the preheating region is considerably above the pressure in the evaporation region, the temperature of the liquid phase in the preheating region can thus be kept above the corresponding boiling temperature in the evaporation region, so that due to the possibility of omitting a heater and e.g. due to the provision of a controllable valve means the evaporation region can be kept very small in terms of construction without any restrictions in the controllability of the vapor volume to be discharged. 
         [0017]    In a further embodiment, the pressure generation device comprises a variable gas volume in the preheating region which can be compressed by the fluid in the preheating region. The pressure in the preheating region can be varied by the variable gas volume within wide limits, and it is particularly possible to use, on the one hand, the change in volume caused by the heating of the liquid phase and, on the other hand, a selective introduction or discharge of gas into or from the gas volume to set and maintain the desired pressure in the preheating region. 
         [0018]    In a preferred embodiment, the pressure generation device comprises an elastic partition wall which separates the variable gas volume from the fluid in the preheating region. Hence, it can be ensured through a corresponding elastic partition wall that a gas in the gas volume does not intermix with the heat transfer medium. Hence, any desired inexpensive and easily available gas, e.g. atmospheric air, can be used for setting the desired pressure conditions in the preheating region. 
         [0019]    In a further embodiment, the pressure generation device comprises a connection for communication with an external gas source. Hence, any desired specified gas can be introduced into the gas volume to produce the desired pressure conditions therewith. For instance, nitrogen can be introduced into the preheating region, and the pressure in the preheating region can be kept by means of the connection at a desired level due to communication with the external source of gas. For instance, a compressed air connection may also serve as the gas source, wherein a correspondingly high pressure can be set in the preheating region and a corresponding pressure relief valve means may be provided if the pressure in the preheating region exceeds the pressure predetermined by the compressed air device. 
         [0020]    In a further advantageous embodiment, a device is provided at the inlet side of the preheating region for introducing the fluid in a controllable manner into the preheating region and for maintaining a specified pressure in the preheating region. With a corresponding device, e.g. in the form of a pump, it is possible on the one hand to introduce the fluid in a reliable manner into the preheating region, independently of the pressure conditions prevailing therein, and on the other hand to provide a control possibility of adjusting the pressure in the preheating region to a desired value. Especially in combination with corresponding means for pressure control of the variable gas volume, this yields a high degree of control flexibility under extremely small constructional efforts. 
         [0021]    In a further advantageous embodiment, the vapor generation system comprises a first sensor means detecting the pressure in the preheating region. It is thereby possible to precisely monitor the pressure conditions prevailing in the preheating region. 
         [0022]    In another embodiment, the vapor generation system comprises a second sensor means that senses the pressure in the evaporation region, thereby permitting a precise monitoring of the pressure conditions in the evaporation region. 
         [0023]    In a further advantageous embodiment, the vapor generation system further comprises a control device which is connected to the second sensor means and is configured to control a manipulated variable influencing the pressure in the evaporation region. With such a control the pressure in the evaporation region can thus be set to a value which is required for the further use of the vapor. To be more specific, the heating power of a heater provided in the evaporation region and/or the fluid amount introduced from the preheating region into the evaporation region, and/or the temperature thereof, can be controlled in response to an output signal of the second sensor means. Advantageously, the control device comprises a control circuit so that even under varying operating conditions the vapor pressure can be adjusted in the evaporation region in a reliable and precise manner. With a corresponding control, it is possible to change the desired value for the pressure in the evaporation region and thus to adapt the mode of operation. For instance, if a constant pressure difference is to be maintained between the evaporation region and a process chamber, the pressure in the evaporation region can be readjusted in a reliable manner with a corresponding change in pressure in the process chamber. 
         [0024]    In a further advantageous embodiment, the vapor generation system further comprises a first level-determining means for determining the liquid level in the evaporation region. It is thereby possible to monitor the liquid level, and especially the supply of the preheated fluid from the preheating region has the effect that the strong level variations known in conventional systems can be reduced considerably in small-sized evaporation systems, whereby the liquid level can be determined more precisely. 
         [0025]    In another embodiment, a second level-determining means is provided for determining the liquid level in the preheating region. In this case it is also true that a substantial decoupling of the evaporation region and the preheating region largely avoids extreme level variations that may otherwise occur during introduction of a cold liquid into a conventional evaporator. To be more specific, corresponding compensation lines may be provided between the level determining means and the preheating region or the evaporation region, so that fluctuations in the display of the level can be reduced further. The level determining means may further be configured such that an optical monitoring, e.g. by an operator of the system, is possible at any time, and/or in the way that corresponding output signals can be supplied to a system control which then takes further measures on the basis of the supplied signals. For instance, with a decreasing liquid level in the preheating region, possibly together with an optional output or display of alarm messages, the supply of a heat transfer medium from an external reservoir may be requested for ensuring a still continuous operation of the system. It is also possible to use corresponding signals from the level determining means for other control tasks of the system, e.g. for controlling corresponding heating means. 
         [0026]    In another advantageous embodiment, a heater is provided in the preheating region and/or in the evaporation region together with a first temperature-measuring means which is configured to output a signal corresponding to the temperature of a heater surface getting into contact with the fluid. With this measure it is possible to reliably ensure that the upper maximum temperature, e.g. a temperature critical for the heat transfer medium, is not exceeded so that the risk of a boiling-point temperature change or the creation of toxic substances can be prevented. 
         [0027]    In another embodiment, a second temperature-measuring means is further provided for outputting a signal corresponding to the temperature of the liquid phase in the preheating region and/or in the evaporation region. It is thereby possible to monitor at least in one of the two regions or, in the presence of corresponding sensor means, in both regions, the actual temperature of the liquid phase and possibly to use it for the control of the system. 
         [0028]    In a further embodiment, a third temperature-measuring means is provided for outputting a signal corresponding to the temperature of the vapor phase in the preheating region and/or in the evaporation region. It is thereby possible to monitor the thermodynamic conditions in the preheating region and/or the evaporation region even more precisely, and the corresponding signal output may also be used for controlling the conditions in the respective region. For instance, it is possible on the basis of the signal output to adjust the vapor temperature within certain limits independently of the temperature of the liquid. Furthermore, with the signals in cooperation with the corresponding signals supplied by the temperature measuring means for the temperature measurement of the liquid phase, it is possible to determine the pressure conditions prevailing in the respective region without the need for a corresponding pressure-sensitive sensor device. 
         [0029]    Preferably, a control means is provided which is connected to the heater to receive the signal of the first temperature-measuring means, the control means being configured to keep the temperature of the surface of the heater at a specified value by outputting a first control signal. 
         [0030]    As has already been mentioned, this measure reliably prevents a change in the properties of the heat transfer medium, the first control signal being e.g. used for controlling the heater and/or for controlling a corresponding means for generating an increased convection particularly near the surface of the heater. For instance, in the case of a rapid rise in the surface temperature the temperature of the heating surface and thus the temperature of the medium in direct vicinity of the heating surface can be kept below a critical value by reducing the heating power and/or by generating a corresponding flow on the surface for achieving an improved heat transfer and thus an improved cooling action. 
         [0031]    Preferably, the control means is connected to the second temperature-measuring means to receive the signal of the second temperature-measuring means, the control means being further configured to control, by outputting a second control signal, the temperature of the surface of the heater on the basis of said second control signal. The signal of the second temperature-measuring means represents the temperature of the liquid phase in the preheating region and/or in the evaporation region, so that on the basis of said received signal the control means can produce the second control signal to efficiently control the surface of the heater, e.g. by changing the supplied heating power, changing a fluid flow on the surface of the heater, etc. Especially when the second temperature-measuring means is configured to detect both the temperature of the liquid in the preheating region and the temperature of the liquid in the evaporation region, the system can be controlled efficiently because the temperature-based control in both regions is possibly performed differently. For instance, it may be advantageous to provide several temperature-sensitive sensors in the preheating region, in which a relatively cold medium might be constantly supplied by pumping, so as to identify a temperature profile within the preheating region, and a corresponding control of the heater may then be performed on the basis of the present temperature profile. For instance, a flow inlet of the fluid connection may be provided at a side facing away from an inlet of the preheating region, so that with a corresponding large-area configuration of the heating surfaces, which may also comprise several separately controllable regions, a corresponding temperature profiling can be performed. To this end a corresponding flow path may be created for the fluid which extends along as many heating surfaces as possible, which are optionally also actuable in different ways, so that a high thermal contact with the heating surfaces is established over a very extensive area of the flow path. Hence, even at low levels in the preheating region, in the case of which upon introduction of major amounts of a cold fluid the temperature of the fluid would strongly decrease in all areas of the preheating region during blending, an adequately high minimum temperature of the fluid can be achieved after all at the flow entry of the fluid connection. 
         [0032]    In a further embodiment, the control means is further connected to the heater to receive the signal of the third temperature-measuring means, the control means being configured to control, by outputting a third control signal, the temperature of the surface of the heater on the basis of the third control signal. Since the third temperature- measuring means outputs a signal representative of the gas phase in the preheating region and/or in the evaporation region, it is possible to control the heater accordingly also in consideration of the gas phase temperature. 
         [0033]    In a further advantageous embodiment, the fluid connection has a substantially horizontal flow outlet. This measure accomplishes a flow and thus a convection and enhanced heat transfer in the liquid phase of the medium in the evaporation region through the introduction of the fluid from the preheating region. Advantageously, the flow outlet is positioned deep below the typical liquid level, e.g. near the bottom area of the evaporation region, so that, on the one hand, an improved heat transfer takes place without causing, on the other hand, any major interference on the surface of the liquid. 
         [0034]    In a further configuration, a supply line is provided for the preheating region, the supply line having a substantially horizontal flow outlet, resulting in a flow for improved heat exchange with the already existing liquid in this instance as well. The flow outlet may also be arranged relatively far below the typical liquid level, e.g. near the bottom area, so that the liquid surface is hardly disturbed by the introduction of the medium. Due to this measure the liquid level can be determined with high accuracy, if necessary. 
         [0035]    In a further advantageous embodiment, the vapor generation system is provided at least in the preheating region with a device for generating a convection flow of the fluid. The device may comprise stirrers, pumps, guide elements, or the like. For instance, in an advantageous variant, one or several deflection elements, which may be configured as heatable surfaces, may be arranged between the fluid inlet and the fluid outlet in the preheating region, so that an inflowing fluid volume must travel a relatively long distance to reach the outlet, and the fluid volume is here possibly conducted near a heated surface. This can accomplish a convection without the provision of complicated mechanical devices and, when the deflection elements are configured as heating elements, a very large heating surface is provided together with a small constructional volume at the same time. 
         [0036]    In a further embodiment, the evaporation region is arranged inside the preheating region. This constructional measure can improve the efficiency of the vapor generation system considerably. For instance, the fluid in the preheating region may be used for preheating or insulating the vessel wall of the evaporation region. For instance, a much less expensive insulation, or possibly no special insulation at all, is required for the vessel walls of the evaporation region because the temperature of the fluid surrounding the evaporation region is already close to the temperature prevailing in the evaporation region. In other cases, the evaporation region may be provided over a large surface with corresponding heating surfaces on the outsides and/or on the insides, so that e.g. additional heating surfaces are available with a minimum constructional volume of the total vapor generator. Furthermore, the fluid connection may thereby be made extremely short and thus shows low loss. Especially during operation of the preheating region at a pressure above the pressure in the evaporation region, the fluid connection may be implemented with an opening and with a valve means provided therein, the valve means allowing fluid flow only in the direction of the evaporation region. This helps to further reduce the overall constructional volume because a separate conveying means is not needed. 
         [0037]    According to a further aspect of the present invention a soldering system is provided which comprises at least one process chamber to be actuated with vapor of a heat transfer medium. Furthermore, the soldering system comprises a vapor generation system, as shown in the preceding embodiments and in embodiments to be still described. Furthermore, a vapor supply line is provided which connects the vapor generation system to the process chamber. 
         [0038]    Due to the combination of the vapor generation system of the invention with a soldering system, it is possible to achieve, in particular, the objectives formulated in the object of the present invention, i.e. to provide vapor in adequate amounts in the at least one process chamber at all times. 
         [0039]    In a further advantageous configuration, the soldering system comprises a pressure sensor in the process chamber, so that the corresponding pressure conditions can be monitored at all times and possibly used for control purposes. 
         [0040]    In a further embodiment, a controllable valve means is provided in the vapor supply line. Said controllable valve means permits the controlled supply of the vapor into the process chamber to produce corresponding conditions on the subassembly to be processed. Especially with a pressure gradient from the evaporation region to the process chamber, the vapor amount and thus optionally the temperature in the process chamber can be controlled without the need for a pump. 
         [0041]    To this end the soldering system is advantageously provided with a system control which is operatively connected to the pressure sensor and configured to actuate the controllable valve means on the basis of a signal output from the pressure sensor. A correspondingly configured system control therefore simplifies the means needed for maintaining desired conditions in the process chamber insofar as only a valve means that is hardly prone to mechanical failure is provided, so that in the presence of a pump no complicated control system is needed for the pump, or a pump for conveying vapor can be dispensed with altogether in a special embodiment. 
         [0042]    In a further advantageous embodiment, the system control is connected to the control device in the vapor generation system and configured to instruct said control means to adjust the pressure in the evaporation region including the signal from the pressure sensor. A desired pressure difference can e.g. be maintained in this way between the evaporation region and the process chamber, so that the supply of vapor to the process chamber can be accomplished without any active conveying means. 
         [0043]    Advantageously a return line is also provided which connects the process chamber to the preheating region of the vapor generation system. A permanent backflow of the heat transfer medium takes place in this way, the relatively low temperature of the condensate in the preheating region being then brought to a correspondingly higher desired temperature. The fluctuations which can often be observed in conventional soldering systems are thereby reduced considerably or can be eliminated altogether in vapor generation. In a further embodiment, the return line is connected to the pressure generation device in the vapor generation system. The condensate can thereby be introduced from the process chamber via the return line according to the desired pressure conditions in the preheating region into said region, so that constant conditions can be observed, or with the help of the pressure generation device, the introduction of the condensate can be used for establishing and maintaining a desired pressure condition in the preheating region. Advantageously, a flow outlet of the return line in the preheating region is predominantly arranged in horizontal fashion so that an efficient convection can be achieved in the fluid. Especially when the flow outlet is distinctly below the liquid level of the medium in the preheating region, a negative impact on the surface of the liquid can largely be avoided. 
         [0044]    Under another aspect of the present invention a method is provided for operating a soldering system, the method comprising maintaining a pressure gradient between an evaporation region of a vapor generator and a process chamber to be fed with vapor by monitoring the pressure of the process chamber, and introducing the vapor into the process chamber by exploiting the pressure gradient. 
         [0045]    Hence, this method provides a simple technique which is thus not prone to failure and also an efficient one for conducting the vapor from the evaporation region to the process chamber because due to the pressure monitoring of the process chamber the pressure difference can always be kept at an appropriate value and pumps are thus in principle not needed for conveying the vapor. This achieves a particularly compact construction of a corresponding soldering system, so that this method, in particular, can also be carried out with conventional soldering systems by way of a slight constructional modification. 
         [0046]    In a further advantageous embodiment, the method also comprises determining a first characteristic value representative of the pressure in the process chamber, determining a second characteristic value representative of the pressure in the vapor generation region, and controlling the amount of vapor introduced into the process chamber in response to the first and second characteristic value. This type of process control thus takes into account the conditions prevailing in the process chamber and in the vapor generation region, so that it is possible to react rapidly and efficiently to changes in the process chamber at the evaporator side. Preferably, the introduced amount of vapor is defined by adjusting a controllable valve element. With little technical equipment this accomplishes an extremely precise control, in contrast to conventional devices in which with increased technical equipment fluctuations in the process chamber may typically lead to uncontrolled fluctuations in the vapor generator. 
         [0047]    Under another aspect of the present invention, said invention is directed to a method for operating an evaporation system. In this method, a heat transfer medium to be evaporated is first preheated and the preheated heat transfer medium is then introduced into an evaporation region where it is then converted into the vapor phase. 
         [0048]    This method substantially yields the same advantageous effects as have already been listed with respect to the evaporation system of the invention. Advantageously, the heat transfer medium is preheated to a temperature near the boiling temperature, i.e. the boiling temperature in the evaporation region, so that the introduction of the preheated medium does not lead to any significant interference with the thermodynamic conditions in the evaporation region. 
         [0049]    Furthermore, the pressure in the evaporation region can advantageously be adjusted to a value which is equal to or greater than a specified minimum pressure. It can thus be ensured that e.g. a certain required amount of vapor can be provided at all times. To be more specific, the minimum pressure can be specified such that it is always higher than a corresponding pressure in the process chamber, so that vapor can be supplied by exploiting the resulting pressure gradient. To this end a characteristic value which is representative of the pressure in the evaporation region is suitably sensed and the evaporation process is controlled on the basis of said characteristic value. The evaporation process can e.g. be controlled by controlling the heating power and/or by controlling the supply of the preheated heat transfer medium. 
         [0050]    In a further advantageous embodiment, preheating takes place in a preheating region, and a pressure gradient is produced between the preheating region and the evaporation region, so that the heat transfer medium is introduced into the evaporation region by exploiting said pressure gradient. 
         [0051]    This reduces the technical equipment for providing the preheated fluid in the evaporation region because active conveying means might not be needed. To be more specific, it is possible to use a vapor generation system which has been described before, and a corresponding pressure generation device is e.g. contained therein, so that a corresponding pressure gradient can inter alia be achieved by one of the above-described measures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0052]    Further advantageous embodiments are also indicated in the attached patent claims and in the following detailed description in which reference is made to the following drawings, of which: 
           [0053]      FIG. 1  shows an exemplary embodiment of a vapor generation system according to the present invention; 
           [0054]      FIG. 2  schematically shows a further embodiment of a vapor generation system with a large effective heating surface; and 
           [0055]      FIG. 3  schematically shows a soldering system and the operative principle thereof, wherein a vapor generation system is used with a preheater. 
       
    
    
     DETAILED DESCRIPTION  
       [0056]      FIG. 1  is a schematic view showing a vapor generation system  100 , wherein a preheating region  110  and a vapor generation region  120  are coupled by means of a fluid connection  130 . The preheating region  110  comprises a supply line  111  which comprises a valve  112  at the inlet side and a flow outlet  113  at the outlet side. Preferably, the flow outlet  113  is arranged substantially horizontally in the operative position of the vapor generation system  100  to efficiently effect a convection flow in a liquid contained in the preheating region  110  during operation. The flow outlet  113  is preferably arranged deep below a liquid level  140 . For instance, a typical distance of the flow outlet  113  from the liquid level  140  is within the range of several centimeters in a typical operative phase. To guarantee a correspondingly deep position of the flow outlet  113  for many possible liquid levels during operation, the flow outlet  113  may be arranged in the vicinity of the bottom area of the preheating region  110 , i.e. it may have a distance of a few mm to about 20 mm. In other embodiments (not shown), the supply line  111  may also be introduced e.g. from below into the preheating region  110 . 
         [0057]    Furthermore, the preheating region  110  comprises a heater  114  with one or several heating elements, each having a corresponding surface  114   a  which gets into contact with the liquid phase of the heat transfer medium. Preferably, the heating device  114  is configured such that the total available surface  114   a  is as large as possible, so that despite the relatively disadvantageous thermodynamic properties of the heat transfer medium an efficient heating can be carried out. For instance, the heater  114  may comprise a plurality of individual tubes that are heated, for instance, by means of a corresponding medium or electrically, resulting in a large surface  114   a  on the whole. In other cases corresponding ribs may be provided in the heater  114 . The heater  114  may be configured as a controllable heater, and in some embodiments the heating power is separately adjustable for different regions. Furthermore, in the illustrated embodiment sensor elements  116   a,    116   b  and  116   c  are provided, each being adapted to output signals representative of the temperature. In the illustrated embodiment, sensor  116   a,  for instance, is in thermal communication with the surface  114   a,  so that the temperature thereof can be monitored. Furthermore, sensor  116   b  is arranged such that a thermal contact is established with the liquid phase of the medium under typical process conditions. Likewise, sensor  116   c  may be arranged such that a signal representative of the temperature of the gas phase in the preheating region  110  can be obtained. Furthermore, a means  115  is provided for determining the liquid level  140 , the means being in fluid communication with the preheating region  110 . This has the effect that possibly arising turbulence in the heated liquid surface  140  has no significant influence on the determination of the liquid level. Furthermore, a compensating line  115   a  may be provided between the device  115  and the preheating region  110  to reduce or avoid the formation of pressure differences between the device  115  and the preheating region  110 , if necessary. Moreover, a pressure relief valve  117  may be provided to prevent an excessive rise in pressure in the preheating region  110 , if necessary. Advantageously, the preheating region  110  is designed as a container which can withstand an increased pressure, e.g. in the order of some bars, the pressure relief valve  117  being dimensioned to prevent an inadmissible pressure load on the container. The pressure relief valve  117  also prevents a situation where the maximally admissible temperature of the liquid, e.g. 300° C., is exceeded. Furthermore, an outlet line  118  including a corresponding valve element may be provided, so that the container of the preheating region  110  can be emptied and rinsed, if necessary. 
         [0058]    The evaporation region  120  may have a structure which is in principle similar to the preheating region  110  if this is of advantage for constructional reasons. In other embodiments, however, the evaporation region  120  and the preheating region  110  may considerably differ from one another. For instance, it may be advantageous to make the dimensions of the preheating region  110  much larger than the corresponding dimensions of the evaporation region  120 . In the illustrated embodiment, the evaporation region  120  comprises a heating means  125  which may be constructed as explained above with reference to the heating means  114 . To be more specific, the heating means  124  may comprise corresponding heating surfaces  124   a  which, in turn, are in thermal contact with a corresponding sensor element  126   a  so that the surface temperature of the heating means  124  can be detected at least in one or, optionally, in several positions. To be more specific, the heating means  124  is designed as a controllable heater which is operatively connected to a control device  150 . Furthermore, other temperature-sensing sensor elements  126   b  and  126   c  may be provided in the evaporation region, the sensor elements being arranged to supply a signal corresponding to the temperature of the liquid phase and a signal corresponding to the temperature of the gas phase of the medium contained in the evaporation region  120 . Furthermore, a means  125  for measuring the liquid level may be provided with a corresponding compensation line  125   a.  A pressure relief valve  127  and a corresponding outlet line  128  with a corresponding valve may be provided by analogy with the preheating region  110  to provide the possibility of increased pressure and of cleaning the container of the evaporation region  120 . Furthermore, a sensor  129  which senses the pressure in the evaporation region  120  is provided which is operatively connected to the control device  150  so that a signal representative of the pressure can be supplied to the control device  150 . Furthermore, a vapor line  160  is provided with a corresponding valve element  161  which is advantageously designed as a controllable valve element to supply the vapor e.g. to a process chamber of a soldering system. 
         [0059]    In the illustrated embodiment, the fluid connection  130  comprises a pump  131  and a valve element  132 , which is configured to prevent a backflow of fluid from the evaporation region  120  into the preheating region  110  if there is a corresponding pressure gradient between said two regions. Furthermore, the fluid connection  130  comprises an inlet line  133  with a substantially horizontal flow inlet  135 , and an outlet line  135  with a substantially horizontal flow outlet  136 . It should be noted that the fluid connection  130  in  FIG. 1  is just shown schematically and may have any desired configuration and length as required for a fluid connection between the containers of the preheating region  110  and the evaporation region  120 . Furthermore, the fluid connection  130  has an appropriate insulation if it extends over long distances outside the preheating region  110  and/or the evaporation region  120 , so that a temperature drop between the flow inlet  134  and the flow outlet  136  is insignificant and has substantially no impact on the thermodynamic stability in the evaporation region  120 . 
         [0060]    A suitable insulation can be accomplished with known means, e.g. by a jacket containing insulating materials, such as glass fiber mats or mineral wool, etc. 
         [0061]    During operation of the vapor generation system  120 , the heat transfer medium in liquid phase is introduced via the inlet line  112  and via the valve element  112 , for instance, from a process chamber or another container into the preheating region  110 . The supply of the medium can be controlled by a corresponding means (not shown) such that the liquid level  140  remains within a defined range, which can be monitored or optionally adjusted by the means  115 . With the introduction of the medium by means of the substantially horizontal flow outlet  113 , a thorough blending can already be accomplished within the preheating region  110  because the flow outlet  113  is arranged particularly in the vicinity of the heater  114 , so that convection takes place from the flow outlet  113  to the flow inlet  134  via the large heating surfaces  114   a.  However, other means may also be provided for generating a convection flow in the liquid, e.g. in the form of corresponding stirrers, deflection elements, or the like. A corresponding embodiment with deflection elements will be described hereinafter with reference to  FIG. 2 . 
         [0062]    The temperature of the heater  114 , i.e. especially the temperature of the surface  114   a  getting into contact with the liquid, is sensed by one or a plurality of the sensors  116   a  and is typically limited to a specific value by way of a corresponding control device (not shown), so that a temperature which is maximally admissible for the heat transfer medium is not exceeded. Furthermore, the temperature of the liquid phase and the gas phase of the medium can be monitored and controlled with the corresponding temperature sensors  116   b  and  116   c.  For instance, the heating power supplied to the heater  114  can be controlled in accordance with the output signal of the sensor  116   b  such that the temperature of the liquid in the preheating region  110  is close to the temperature of the liquid in the evaporation region  120 . Depending on the pressure conditions prevailing in the respective regions, the temperature of the liquid in the preheating region  110  may here be lower than, substantially equal to, or greater than the temperature of the liquid phase in the evaporation region  120 . A relatively stable operation in the evaporation region  120  can be achieved if the temperature of the supplied fluid just differs by a few degrees Celsius from the boiling temperature prevailing in the evaporation region  120 . For instance, the preheating region  110  and the evaporation region  120  can be operated under substantially identical pressure conditions, so that the corresponding boiling temperatures are substantially identical as well. In this mode of operation, liquid can then e.g. be supplied at a temperature differing only slightly from the boiling temperature in the evaporation chamber  120 , the insignificant deviations being caused by possible instable conditions in the preheating region  110  during introduction of the relatively cold medium. Since these fluctuations are however substantially offset in the preheating region  110 , a relatively stable operation of the evaporation chamber  120  is ensured. The fluid preheated to a desired temperature is then introduced into the evaporation region  120 , the heater  124  being then controlled in an advantageous embodiment such that the temperature of the surface  124   a,  on the one hand, remains below a required maximum value and the vapor pressure, on the other hand, which is sensed by means of the sensor  129 , is within a predetermined range. In other embodiments, other parameters, such as the amount of preheated fluid introduced via the fluid connection  130 , can be used for keeping the vapor pressure at a desired level. For instance, upon a sudden demand for vapor, which is to be provided via the outlet line  160  in a process chamber for example, the amount of the fluid introduced into the evaporation region  120  can be increased considerably for a short period of time, so that the increase in volume of the liquid compensates for the decrease in vapor volume at least to some degree, and the heating power can here also be raised at the same time for providing the increased vapor amount also for a longer period of time. With the means  125  the liquid level in the evaporation region  120  can be monitored permanently and/or used for controlling the operation of the evaporation region  120 , as described in a similar way before with reference to the preheating region  110 . Especially with the control device  150  which receives at least one signal of the pressure sensor  129  as input, it is thereby possible to accomplish an efficient control of the operation of the evaporation region  120  because with the setting of a desired pressure in a continuous operation an adequate supply of a corresponding process chamber is also ensured. Furthermore, the control device on the basis of the pressure sensor  129  also permits a certain compensation of short-term variations in vapor demand that may e.g. be caused by corresponding pressure variations in the process chambers. 
         [0063]      FIG. 2  schematically shows a further illustrative embodiment of the present invention. A vapor generation system  200  comprises a preheating region  210  and an evaporation region  220 , a fluid connection  230  and a control device  250 . Furthermore, there is provided a pressure generation device  270 , an inlet region  280  at the inlet side of the preheating region  210 , and an outlet  260  at the outlet side of the evaporation region  220 . 
         [0064]    The preheating region  210  may be provided in the form of a container, as is advantageous for constructional reasons for specific uses. To be more specific, as shown in  FIG. 2 , the preheating region may have a larger volume than the evaporation region  220 , the evaporation region  220  being arranged inside the preheating region  210  in the illustrated embodiment. The preheating region comprises a heater  214  with corresponding surfaces  214   a,  and the elements which carry the individual heating surfaces  214   a  and are also just designated with reference numeral  214   a  are here arranged as deflection elements in the illustrated embodiment to produce a convection flow in the preheating region  210 . In the illustrated embodiment, these may e.g. be plates which are alternatingly mounted on the bottom and the ceiling of the preheating region  210 , so that with a corresponding convection flow a relatively long contact is established with the corresponding heating surfaces  214   a.  The heater  214  may comprise further heating elements (not shown), and deflection elements may also be provided that are not configured as heating elements. Furthermore, it should be noted that the configuration shown in the figure is just of an exemplary nature and it is also possible to establish other flow paths by means of corresponding deflection elements. Further provided are corresponding temperature sensor means  216   a  that are e.g. in thermal contact with one or a plurality of the heating surfaces  214 , and one or a plurality of corresponding temperature sensor means  216   b  that are in thermal contact at one or several places with a fluid to be heated in the preheating region  210 . In other embodiments, the outer wall of the evaporation region  220 , which is designated by  220   a,  may e.g. be designed completely or in part as a heatable surface so that a very large and effective heating surface is provided on the whole for heating the fluid in the preheating region  210 . Furthermore, a level meter  215  is provided, which is e.g. configured in a similar way as the means  115  of  FIG. 1 . 
         [0065]    The inlet region  280  serves the introduction of a medium into the preheating region  210 . To this end the inlet region may comprise a pump  281  and/or a corresponding valve element  282  which is designed as a controllable valve in an advantageous embodiment. Furthermore, a reservoir  283  may be provided which is connected by means of a controllable valve  284  to the preheating region  210 , so that in case of need an additional amount of the heat transfer medium can be fed into the preheating region  210 . 
         [0066]    The pressure generation device  270  comprises a gas volume  273  which may e.g. be represented by the spatial region above the liquid phase, a connection  271  being provided so that the gas volume  273  communicates with a source of gas (not shown) by means of a controllable valve  272 . The gas volume  273  may also be separated by an at least partly flexible membrane  274  from the remaining space of the preheating region  210 , but the flexible membrane permits a change in the gas volume  273 . Furthermore, a pressure sensor  219  is provided that may be arranged in the gas volume  273  and/or in the remaining space above the liquid phase. 
         [0067]    In some embodiments, the evaporation region  220  may comprise a heater  224  which is preferably designed as a controllable heater, and it may be also be without a heater if the vapor generation system  200  is configured for a corresponding operation. The evaporation region  220  is connected by means of the fluid connection  230  to the preheating region  210 , a controllable valve element  232  and optionally a pump (not shown) being provided for introducing the fluid into the evaporation region  220 . Furthermore, a pressure sensor  229  is provided in the area of the vapor phase. The outlet line  260  establishes a fluid connection by means of a controllable valve  261 , e.g., to a process chamber of a soldering system. As is also shown in  FIG. 2 , at least the sensor  229  is connected to the control device  250 , and in advantageous embodiments further controllable means and sensor elements, e.g. the temperature sensors  216   a,    216   b,  the heaters  214  and  224 , and the corresponding valve elements  272  in the pressure device  270 , the valve  282  in the inlet region  280 , the valve  232  in the fluid connection  230 , and the valve  261  in the outlet line  260 , are connected to the control device. 
         [0068]    During operation of the evaporation system  200  the fluid is filled up to a desired liquid level, which can be monitored by the device  215 , and is heated by the heating means  214  in an efficient manner to a desired temperature. The pressure prevailing in the preheating region  210  can here be chosen by means of the pressure generation device  270  such that said pressure is smaller than, equal to, or greater than the pressure prevailing in the evaporation region  220 . Especially if the pressure is adjusted by means of the pressure generation device  270  and/or the pump  281  and the valve  282  in such a manner that said pressure is greater than the desired pressure in the evaporation region  220 , the fluid can be introduced into the evaporation region  220  via the fluid line  230 , exclusively by exploiting the pressure gradient, and the amount to be introduced can optionally be controlled via the valve means  232  and/or the magnitude of the pressure in the preheating region  210 . In this mode of operation, the valve element  232  is designed such that it does not allow any backflow of the fluid. For instance, a source of gas can be connected at a constant pressure by means of connection  271 , and the fluid is introduced via the inlet region  280  under pressure, so that a desired level remains substantially constant. The fluid which is supplied in a relatively cold state then travels along the surfaces or the deflection elements  214   a  and is efficiently heated up in this process and is thus present at the input of the fluid line  230  with the final desired temperature, which may be around the boiling temperature in the evaporation region  220  or also above said temperature, and is then introduced via the valve  232  into the evaporation region  220 . Depending on the conditions, the fluid can already be introduced in a vaporous state due to a drop in pressure in the fluid connection  230  or can be introduced as liquid, and the further evaporation may then take place by means of the heater  224 . To be more specific, in the arrangement shown in  FIG. 2 , the evaporation region  220  is permanently surrounded at least in part by the flow of preheated fluid, so that the demands made on a corresponding insulation of the wall of the evaporation region  220  can be correspondingly low. When the wall  220   a  is designed as a heating element, this simultaneously yields increased efficiency for heating the fluid in the evaporation region  220 . The produced amount of vapor can here be controlled again by means of the pressure sensor  229  in such a manner that the heating power of the heater  224 , if such a heater is provided, and/or the amount of the supplied fluid are adjusted by means of the valve  232  and a possibly provided pump and/or via the pressure generation device  270  in combination with the inlet region  280 . In a mode of operation in which the pressure in the preheating region  210  is permanently higher than the pressure in the evaporation region  220 , the technical equipment can thus be kept small, e.g. a pump can be omitted in the fluid line  230 , and a high degree of process stability can here be achieved after all. Furthermore, a very compact constructional volume can be achieved with the illustrated structure. Of course, other modes of operation are also feasible with the system  200 . Furthermore, if required, the components of the systems  100  and  200  can be replaced or supplemented. For instance, the preheating region  110  and  210 , respectively, and/or the evaporation region  120  and  220 , respectively, may be provided as two or more containers, which may be identical in structure or different. With some applications it may be advantageous when two or more preheating containers are operated in “cascade” form or in parallel to achieve an even more efficient decoupling with respect to instabilities. Likewise, several evaporation containers of a very small volume may be provided and controlled in a suitable manner such that the necessary amount of vapor is always available. 
         [0069]      FIG. 3  schematically shows a soldering system  390  which comprises one or several process chambers  301   a,    301   b  which are connected to a vapor generation system  300  via a corresponding line  360  and vapor valve  361 . The vapor generation system  300  comprises a preheating region  310  and an evaporation region  320  with a sensor  329  which can output a signal representative of the pressure in the gas phase of the evaporation region  320  to a control device  391 . The vapor generation system  300  may be configured in any desired way, as described in the previously illustrated embodiments. To be more specific, the vapor generation system  300  may be constructed in accordance with system  100  or system  200  or a combination thereof. The preheating region  310  is connected to the process chambers  301   a,    301   b  via corresponding lines, collection containers, or the like, by means of a pump  381  and a valve  382 . Furthermore, a pressure sensor  302   a,    302   b  which can output a signal representative of the pressure in the corresponding process chamber to the control device  391  is provided in at least one of the process chambers  301   a,    301   b.    
         [0070]    During operation of the soldering system  390  the control device  391  receives corresponding signals from the sensors  302   a,    203   b,  so that the pressure level is known in the individual process chambers. Furthermore, vapor generation in the system  300  is controlled on the basis of the signal of sensor  329  in a way as described previously, for instance, with reference to  FIGS. 1 and 2 . In one embodiment, the control device  391  can control the system  300  such that a desired pressure gradient is always maintained between the vapor generation region  320  and the process chambers  301   a,    301   b.  The vapor can then be introduced into the process chambers by exploiting the pressure difference, and a flow control may here be carried out in addition by means of the valve element  361 . 
         [0071]    In advantageous embodiments, the control device  391  may also take over the function of the control devices  150  and  250 , respectively. 
         [0072]    Hence, the present invention permits a more stable and efficient vapor generation, and a continuous supply of one or more process chambers of a soldering system can here be achieved without any significant influences of the temperature of the medium returned into the vapor generation system. Especially due to the vapor generation control based on the pressure in the vapor phase, in combination with a preheating of the heat transfer medium, a continuous vapor supply can be ensured. Furthermore, the control strategy according to the invention with maintenance of a pressure gradient between the vapor generation system and the process chamber permits the omission of a corresponding active pump in the vapor supply line. Furthermore, the above-illustrated structural and constructional measures make it possible to keep the outer dimensions of the vapor generation system small, so that already known soldering systems can also be retrofitted by being modified only slightly.