Patent Publication Number: US-2016220955-A1

Title: Formic acid decomposition apparatus and formic acid decomposition method

Description:
TECHNICAL FIELD 
     The present invention relates to a formic acid decomposition apparatus and a formic acid decomposition method. 
     BACKGROUND ART 
     When a solder bump is formed on a semiconductor chip, a solder is deposited on a pad and then a shape of the solder bump is changed from a mush room shape to a hemisphere shape, followed by reflowing to join by soldering. According to a conventional soldering method, in order to form a uniform solder bump, a surface oxide film of a solder is removed with a flux to cleanse the surface of the solder bump. 
     However, according to a soldering process that uses a flux, small cavities (voids) may be formed in the solder bump due to decomposition of the flux. These cavities not only deteriorate electrical and mechanical properties of a formed solder joint but also destroy flatness of a semiconductor with the solder bump and may generate a harmful effect on the following semiconductor joining process. A volatile material of the decomposed flux may contaminate the inside of a reflow processor (soldering apparatus) and as a result a maintenance cost may increase. In addition, a flux residue remains frequently on a semiconductor substrate to cause metal corrosion, and degradation of performance of an assembly. Further, according to a method in which the flux residue is cleansed and removed after reflow step, a time required for soldering increases because of addition of after-cleansing as a new processing step. 
     Therefore, as a soldering method without use of a flux, there is known a method of reducing with formic acid the solder and the joined members such as a substrate and electrode (see Patent Documents 1 to 3 and others). According to such a method of reduction, when a temperature of a substrate on which solder members are mounted reaches a predetermined temperature, the solder members are exposed to a reducing gas including formic acid to perform a reduction treatment in which a surface oxide film is removed and are then molten to be processed. 
     However, formic acid sometimes corrodes a chamber material, a corrosion product may become a contamination source as a metallic foreign matter in the chamber, and fly and adhere to the substrate, electronic components mounted on the substrate and the like. Therefore, it is necessary to remove the formic acid from the inside of the chamber after completion of the reduction process. Further, it is desirable to safely dispose of the formic acid recovered from the chamber because of irritating property thereof. 
     In a soldering apparatus described in Patent Document 1, formic acid is vaporized by heating at a temperature between 150° C. and 200° C. in a heating chamber and decomposed as shown below to generate a hydrogen gas and a carbon monoxide gas, the generated gases are supplied to the soldering apparatus to reduce an oxide film. However, the apparatus is an apparatus that uses a reducing gas (hydrogen and carbon monoxide) generated by heating and decomposing formic acid and is not an apparatus that removes the oxide film with formic acid. 
       HCOOH→H 2 O+CO
 
       HCOOH→H 2 +CO 2  
 
     A soldering apparatus disclosed in Patent Document 2 includes a processing chamber and a formic acid introduction unit that introduces an atmosphere gas containing formic acid into the processing chamber, and further includes a formic acid decomposition means (heater) for heating and decomposing formic acid that is disposed between a reflow processing part of the processing chamber and an inner wall of the processing chamber. After completion of reflow process, the heater installed so as to cover an inner wall surface heats a formic acid atmosphere gas adhering to a shield material and a chamber material at 200° C. or more to be decomposed. 
     A soldering apparatus disclosed in Patent Document 3 includes an exhaust port at which an exhaust pump and a formic acid recovering unit are provided for dissolving the exhausted formic acid into water or alcohol to recover it. Further, at the exhaust port of the soldering apparatus, a formic acid decomposition unit is attached, and the formic acid gas exhausted from a chamber is decomposed by heating at a temperature between 200° C. and 300° C. by the heater. 
     Further, Patent Document 4 discloses a soldering apparatus that includes a formic acid feed unit for feeding formic acid to a heating chamber and an exhaust unit for exhausting formic acid outside of the heating chamber. A formic acid decomposition unit or a formic acid recovery unit is connected to an exhaust port of the exhaust unit, and the formic acid decomposition unit is a heating part for heating an exhaust gas exhausted from the exhaust unit at 400° C. or more. 
     Further, Patent Document 5 discloses a soldering apparatus including a formic acid feed means for feeding formic acid into a heating and melting region and an exhaust unit for exhausting a gas in the heating and melting region of formic acid. The formic acid fed to the heating and melting region is introduced into a formic acid decomposition unit or a formic acid recovery unit by the exhaust unit. Thus, an environment is prevented from being destroyed due to formic acid. Since, in the formic acid decomposition unit, a heater heats an inside of a through-hole at 200° C. to 300° C., formic acid is decomposed into water and carbon during travel through the through-hole and then exhausted. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: JP 2011-060856 A 
     Patent Document 2: JP 2007-125578 A 
     Patent Document 3: JP 2001-244618 A 
     Patent Document 4: JP 2002-361472 A 
     Patent Document 5: JP 2002-210555 A 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, there is a problem in the method described in Patent Document 1 that a reduction process cannot be applied to a lead-free solder having a melting point equal to or lower than a reduction start temperature of hydrogen (about 270° C.) before melting of the solder because the method reduces an oxide film with hydrogen and carbon monoxide. 
     Regarding this point, the methods described in Patent Documents 2 and 3 have an advantage that they can be widely applied also to solders having a relatively low melting point because formic acid (reduction start temperature of about 150° C.) is used. However, according to the method described in Patent Document 2, it is necessary to remove water remaining in the soldering apparatus after decomposition of formic acid. According to the method described in Patent Document 3, it is necessary to process the recovered formic acid with alkali. Both methods are therefore cumbersome in the step of processing formic acid. 
     The present invention was invented in view of the above-described circumstances and intends to provide a formic acid decomposition apparatus, a decomposition method of formic acid and a soldering apparatus, capable of safely and rapidly processing formic acid. 
     Means for Solving the Problem 
     As a result of study to solve the above-described problems, the inventor has found that when a gas containing formic acid is allowed to react with oxygen or air in the presence of a formic acid decomposition catalyst, formic acid can be safely and rapidly detoxified, and the present invention was achieved. 
     That is, the present invention is as follows. 
     (1) A formic acid decomposition apparatus for decomposing formic acid into water and carbon dioxide, the formic acid being contained in an exhaust gas exhausted from a soldering apparatus, the soldering apparatus reducing a surface oxide film with formic acid, the formic acid decomposition apparatus including: 
     a formic acid decomposition part in which a catalyst having a platinum group metal carried on a carrier (excluding a carbon material) is filled; 
     a gas introduction pipe for introducing the exhaust gas containing formic acid from the soldering apparatus into the formic acid decomposition part; and 
     a gas introduction unit for introducing an oxygen or a gas containing oxygen into the formic acid decomposition part, 
     wherein the formic acid decomposition apparatus is connected with the soldering apparatus. 
     (2) The formic acid decomposition apparatus according to (1), wherein the formic acid decomposition part has a heater for heating the catalyst. 
     (3) The formic acid decomposition apparatus according to (1), wherein the formic acid decomposition part has a thermocouple for measuring a catalyst temperature at least at one position on an upstream part, 
     wherein the thermocouple is for measuring an amount of change of the catalyst temperature (ΔT) increased by heat generation during a decomposition reaction of formic acid when the exhaust gas containing formic acid and the oxygen or the gas containing oxygen are introduced into the formic acid decomposition part to determine activity of the catalyst, 
     wherein when the amount of change of the catalyst temperature (ΔT) measured by the thermocouple is not smaller than a setting value, the activity of the catalyst is determined to be maintained, and when the amount of change of the catalyst temperature (ΔT) measured by the thermocouple is smaller than the setting value, the activity of the catalyst is determined to be deteriorated. 
     (4) The formic acid decomposition apparatus according to (3), wherein when the amount of change of the catalyst temperature (ΔT) measured by the thermocouple is smaller than the setting value, an apparatus alarm is operated. 
     (5) The formic acid decomposition apparatus according to (1), wherein the formic acid decomposition part has a thermocouple for measuring a catalyst temperature at least at two positions of an upstream part and a downstream part, 
     wherein the thermocouples at two positions each measures amounts of change of the catalyst temperature (ΔT 1 ,ΔT 2 ) increased by heat generation during a decomposition reaction of formic acid when the exhaust gas containing formic acid and the oxygen or the gas containing oxygen are introduced into the formic acid gas decomposition part to determine activity of the catalyst, 
     wherein, by comparing the amount of change of the catalyst temperature (ΔT 1 ) measured by the thermocouple on the upstream part and the amount of change of the catalyst temperature (ΔT 2 ) measured by the thermocouple on the downstream part, when ΔT 1  is smaller than a setting value, the activity of the catalyst is determined to be deteriorated. 
     (6) The formic acid decomposition apparatus according to (5), wherein when the amount of change of the catalyst temperature (ΔT 1 ) is smaller than the setting value, an alarm of the apparatus is activated. 
     (7) The formic acid decomposition apparatus according to (1), wherein a change of a flow rate of a gas passing through a catalyst layer (a total amount of the gas containing formic acid and the oxygen or the gas containing oxygen) is within 0.2 L/min when an amount of change of a catalyst temperature is measured. 
     (8) The formic acid decomposition apparatus according to (1), wherein the exhaust gas containing formic acid is an inert gas containing formic acid, carbon dioxide and water. 
     (9) The formic acid decomposition apparatus according to (1), wherein the gas containing oxygen is air. 
     (10) A soldering apparatus including the formic acid decomposition apparatus according to any one of (1) to (9). 
     (11) A formic acid decomposition method including the steps of: mixing an exhaust gas containing formic acid exhausted from a soldering apparatus in which a surface oxide film is reduced with formic acid, and an oxygen or a gas containing oxygen such that a molar ratio of formic acid and oxygen (oxygen/formic acid) is 1.5 or more, and allowing the mixed gas to react in the presence of a catalyst having a platinum group metal carried on a carrier (excluding a carbon material) to decompose formic acid into water and carbon dioxide. 
     (12) The formic acid decomposition method according to (11), wherein the mixed gas is reacted by passing through a catalyst layer in which the catalyst having the platinum group metal carried on the carrier (excluding carbon material) is filled. 
     (13) The formic acid decomposition method according to (11), wherein a reaction temperature is between 20° C. and 200° C. 
     (14) The formic acid decomposition method according to (11), wherein the exhaust gas containing formic acid is an inert gas containing formic acid, carbon dioxide and water. 
     (15) The formic acid decomposition method according to (11), wherein the formic acid decomposition method uses the formic acid decomposition apparatus according to (1). 
     EFFECT OF THE INVENTION 
     The formic acid decomposition apparatus and formic acid decomposition method of the present invention can rapidly decompose formic acid into water and carbon dioxide. Therefore, when the formic acid decomposition apparatus is attached to the soldering apparatus using formic acid for reduction process, formic acid remaining in the soldering apparatus (chamber) after the reduction process can be safely processed. Further, by heating the catalyst in the formic acid decomposition apparatus, formic acid can be surely decomposed, and by evaporating water generated as a by-product, water droplets can be prevented from remaining in the apparatus. 
     Undecomposed formic acid can be prevented from being emitted into atmosphere because the formic acid decomposition apparatus of an embodiment of the present invention can determine the degree of activity of the catalyst on a real time basis. A lifetime of the catalyst can be estimated when thermocouples are disposed at two or more positions and the difference of the degree of activity of the catalyst between an upstream part and a downstream part is determined. Further, production efficiency of products accompanying the soldering process can be improved because a time for replacement of the catalyst can be estimated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a formic acid decomposition apparatus and an example applied to a soldering apparatus. 
         FIG. 2  is a diagram, illustrating catalyst activity during a decomposition reaction of formic acid. 
         FIG. 3  is a schematic block diagram of a formic acid decomposition part. 
         FIG. 4  is a conceptual diagram, illustrating a change of a temperature during continuous operation of the formic acid decomposition apparatus (during catalyst activation) and a determination method of the catalyst activity. 
         FIG. 5  is a diagram, showing a result of Example 1 in which a change of a temperature during continuous operation of the formic acid decomposition apparatus is measured. 
         FIG. 6  is a graph, summarizing the change of the temperature measured in Example 1. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A formic acid decomposition apparatus and a formic acid decomposition method according to the present invention are now described in detail as follows. 
       FIG. 1  is a schematic diagram, showing an example of a formic acid decomposition apparatus of the present invention, and also showing an example in which the formic acid decomposition apparatus is applied to a soldering apparatus. In  FIG. 1 , reference numeral  1  denotes a formic acid decomposition apparatus, reference numeral  2  denotes a formic acid decomposition part that is filled with a formic acid decomposition catalyst, reference numeral  3  denotes a gas introduction pipe for introducing gas containing formic acid into the formic acid decomposition part, reference numeral  4  denotes a gas introduction unit for introducing oxygen or gas containing oxygen for decomposing formic acid into the formic acid decomposition part, reference numeral  5  denotes an introduction inlet of the gas containing formic acid, and reference numeral  6  denotes an exhaust outlet of a formic acid decomposition material. The gas introduction pipe  3  preferably has a flowmeter  8  and a flow regulation valve. 
     According to the formic acid decomposition apparatus  1  shown in  FIG. 1 , in the formic acid decomposition part  2 , there is formed a catalyst layer  7  that is filled with the formic acid decomposition catalyst, and there is provided, in the catalyst layer  7 , a thermocouple T for measuring a temperature. The gas containing formic acid introduced from the gas introduction pipe  3  is introduced into the formic acid decomposition part  2  together with oxygen or the gas containing oxygen introduced from the gas introduction unit  4 , then reacts when coming into contact with the catalyst, and is decomposed into water and carbon dioxide. A reaction formula is shown below and the reaction is an exothermic reaction. 
       HCOOH+½O 2 →H 2 O+CO 2  
 
     Because of the exothermic reaction, as shown in  FIG. 2 , when a reaction condition other than the catalyst is set equal, in the case where the activity of the catalyst is high, a reaction rate becomes larger and a heat of reaction generated per unit time increases to raise the catalyst temperature, while in the case where the activity of the catalyst is low, a degree of increase of the catalyst temperature becomes smaller because the heat of reaction generated per unit time is small. 
     As the catalyst filled in the formic acid decomposition part  2 , any known catalysts can be used without particular restriction as long as the catalyst can decompose formic acid, and the types thereof are not particularly limited. For example, platinum group catalysts such as palladium, platinum, ruthenium, rhodium, and iridium; metal catalysts such as copper and nickel; oxide catalysts such as molybdenum, vanadium, iron, and chromium can be used. Among the platinum group catalysts, platinum is preferred. 
     As the platinum group catalyst, catalysts in which platinum group metal such as palladium, platinum, ruthenium, rhodium, or iridium is carried by a carrier such as silica, alumina, silica/alumina, or zeolite (excluding carbon material) can be also used. In particular, a catalyst carrying 1.8 g/L to 3 g/L of the platinum group catalyst with respect to those carriers is preferable. 
     A catalyst layer can be formed by filling a catalyst having an arbitrary form of powder, particle, or granule, or by filling a catalyst in which a platinum group catalyst is carried on a molded body obtained by molding the above-described carrier into an arbitrary shape such as a honeycomb shape and a corrugate shape. The molded body has, in addition to excellent decomposition efficiency and large reaction rate, advantages such as excellent durability and easy replacement of the catalyst. The molded body is preferably formed into a porous body having a large specific surface area, and, in order to rapidly and surely decompose formic acid, cell density of the molded body is preferably 200 cells/in 2  or more. 
     Although a temperature of the catalyst before formic acid begins a decomposition reaction is not particularly limited, the higher the temperature is, the faster the decomposition rate becomes. However, a high temperature necessitates a large apparatus and when the temperature exceeds 500° C. the catalyst may be deteriorated. Therefore, the temperature is set preferably in a range between 20° C. and 200° C., more preferably between 50° C. and 100° C., and should be controlled at a constant temperature. 
     Further, a reaction between formic acid and oxygen or the gas containing oxygen is sufficiently performed under normal pressure, but may be performed under increased pressure or reduced pressure. A molar ratio of formic acid and oxygen (oxygen/formic acid) is preferably 1.5 or more, appropriately selected by considering the types of the catalyst to be used, a reaction condition, cost, and the like, and does not have an upper limit. It should be noted that the reaction between formic acid and oxygen or the gas containing oxygen may be performed according to any one of a batch method, a semi-batch method or a continuous method. 
     The formic acid decomposition apparatus of the present invention can be preferably used as a formic acid processing apparatus in the soldering apparatus (hereinafter, referred to as a chamber)  10  in which formic acid is used as a reducing agent. In this case, as shown in  FIG. 1 , an exhaust gas (a gas mixture containing HCOOH, CO 2  and H 2 O) exhausted after a surface oxide film of the solder or joined members are reduced with formic acid in the chamber  10  is introduced into the gas introduction pipe  3  from the gas introduction inlet  5  of the formic acid decomposition apparatus; then introduced into the formic acid decomposition part  2  together with oxygen or the gas containing oxygen; then allowed to react while passing through the formic acid decomposition part  2 ; and therefore may be merely exhausted from the exhaust outlet  6 . The exhaust gas exhausted from the chamber  10  is preferably an inert gas containing formic acid, carbon dioxide and water. The inert gas includes nitrogen gas, argon gas, helium gas and the like, but nitrogen gas is preferable in view of availability. 
     A temperature of the exhaust gas is different depending on a reduction process condition such as the type of the solder used in the reduction process and is generally not lower than 150° C. and not higher than a solder melting point. In the present invention, it is better to introduce the exhaust gas into the formic acid decomposition part  2  without particularly heating. The solder may be any one of a lead solder and a lead-free solder. 
     The gas introduction unit  4  including a gas introduction inlet of oxygen or the gas containing oxygen (air is preferable from the viewpoint of cost and supply), a flow regulation valve, a flowmeter and the like is preferably used. It is better to control a molar ratio of oxygen, by using the flow regulation valve and the flowmeter, depending on an amount of exhaust gas introduced into the formic acid decomposition apparatus  1  via the vacuum pump  11  from the chamber  10 . 
     According to the formic acid decomposition method of the present invention, formic acid can be decomposed into water and carbon dioxide by allowing a gas containing formic acid to react with oxygen or the gas containing oxygen in the presence of the formic acid decomposition catalyst. The gas containing formic acid and oxygen or the gas containing oxygen for decomposing formic acid (air or the like) may be separately introduced into the formic acid decomposition part, but, are preferably introduced as a gas mixture after mixing both into the formic acid decomposition part because uniformity of the reaction can be improved and flow rate can be regulated easily. 
     Further, an unheated catalyst can promote the reaction but a heated catalyst is preferably used. By heating the catalyst, formic acid can be surely decomposed into carbon dioxide and water, while by-produced water can be vaporized, gasified and surely exhausted outside of the apparatus via the exhaust outlet  6 . 
     Introduction amounts of the gas containing formic acid and oxygen or the gas containing oxygen may be appropriately selected by considering a concentration of formic acid used in the decomposition reaction, the type of the catalyst used, a reaction temperature or the like, but not particularly limited to them. In order to maintain the decomposition rate in a favorable state, it is desirable to vary an amount of the catalyst in accordance with the introduction amounts of these gases. Further, the amount of formic acid in the gas containing formic acid is not particularly limited. 
     The formic acid decomposition apparatus of the present invention can be used in connection with the soldering apparatus that uses formic acid as a reducing agent, and a formic acid decomposition process in this case may be any one of a batch method, a semi-batch method and a continuous method. However, the catalyst activity may degrade with time. 
     Therefore, according to the formic acid decomposition apparatus of the present invention, in the formic acid decomposition apparatus including the formic acid decomposition part filled with the formic acid decomposition catalyst, the gas introduction pipe for introducing the gas containing formic acid into the formic acid decomposition part, and the gas introduction unit for introducing oxygen or the gas containing oxygen into the formic acid decomposition part, the exhaust gas containing formic acid and oxygen or the gas containing oxygen are introduced into the formic acid decomposition part, a change of a catalyst temperature due to heat generation during the decomposition reaction of formic acid is measured with the thermocouple installed inside of the catalyst layer, and the degree of activity of the catalyst is determined from a magnitude of an amount of change of an increased temperature. That is, as shown in  FIG. 2 , the catalyst temperature is measured continuously from a start of introduction to an end of introduction of the gas containing formic acid, and the amount of change of the catalyst temperature (ΔT) increased with heat generation during the decomposition reaction of formic acid is obtained, thereby to determine the degree of activity of the catalyst. 
     Accordingly, when the thermocouple is disposed at least at one position in the upstream part of the catalyst layer and the amount of change of the catalyst temperature (ΔT) is obtained, the degree of activity of the catalyst can be determined. Further, when the thermocouple is disposed at each of at least two positions in the upstream part and downstream part of the catalyst layer and the amounts of change of the catalyst temperatures (ΔT) measured by the respective thermocouples are compared, the degree of activity of the catalyst in the entire catalyst layer can be determined. 
     The amount of change of the catalyst temperature (ΔT) tends to be larger as the content of formic acid in the introduced gas increases and it tends to be larger as the catalyst activity is higher because the amount of change of the catalyst temperature (ΔT) correlates with the amount of the introduced formic acid and the catalyst activity. Even when the content of formic acid in the gas is constant, the amount of change of the catalyst temperature (ΔT) is influenced by a flow rate of the gas. Therefore, in order to make determination accuracy of the degree of activity of the catalyst higher, a change of the flow rate of the gas passing through the catalyst layer (total amount of the gas containing formic acid and oxygen or the gas containing oxygen) is preferably regulated to be within 0.2 L/min. 
     Then, by setting an amount of change of the catalyst temperature (ΔT) to be a criterion of determination, when the amount of change of the catalyst temperature (ΔT) is larger than the setting value, the catalyst activity is determined to be maintained, and when the amount of change of the catalyst temperature (ΔT) is smaller than the setting value, the catalyst activity is determined to be deteriorated. By setting such that an alarm of the apparatus is activated when the catalyst activity is determined to be deteriorated, it can be known that the catalyst should be replaced without always observing the amount of change of the catalyst temperature (ΔT). 
     It is preferable to apply a catalyst activity diagnostic method of the present invention to the formic acid decomposition apparatus of the present invention.  FIG. 3  is a schematic block diagram of a periphery of the formic acid decomposition part  2  of the formic acid decomposition apparatus suitable for applying the catalyst activity diagnostic method of the present invention. In  FIGS. 3 , T 1 , T 2  and T 3  denote thermocouples that are inserted into the catalyst layer  7  in the formic acid decomposition part  2 . T 1  is disposed in the upstream part of the catalyst layer  7 , T 2  is disposed in the middle part of the catalyst layer  7 , and T 3  is disposed in the downstream part of the catalyst layer  7 . 
     In the case where the thermocouple T is located at least at two positions on the upstream part and the downstream part of the catalyst layer, the gas containing formic acid and oxygen or the gas containing oxygen, when flowing in the formic acid decomposition part  2 , first, come into contact with the catalyst in the upstream part, and, thereafter, come into contact with the catalyst in the downstream part. Therefore, the temperature of the catalyst on the upstream side rises due to a heat of reaction between formic acid and oxygen. Then, as the gas flows from the catalyst disposed in the middle part to the catalyst disposed in the downstream part, an amount of formic acid decreases. As a result, the heat of reaction becomes smaller, and a rise width of the catalyst temperature decreases. Thus, it can be readily determined such that, for example, even when the catalyst activity on the upstream side deteriorates, the catalyst activity on the downstream side does not deteriorated because the catalyst activity of the catalyst on the upstream side and the catalyst activity of the catalyst on the downstream side can be compared. The amount of change of the catalyst temperature (ΔT 1 ) measured by the thermocouple on the upstream side and the amount of change of the catalyst temperature (ΔT 2 ) measured by the thermocouple on the downstream side are compared. When either one of the amounts of change is larger than the setting value, it can be determined that the activity of the catalyst is maintained. The thermocouple T 2  is not essential. However, by providing it, the number of measurement points of the catalyst activity increases, and the catalyst activity can be more precisely determined. Three or more thermocouples may be disposed as required. 
     After the temperature values measured by the thermocouple are sent to a data processor as input values, the amount of change of the catalyst temperature (ΔT) at each of measurement points is calculated in the data processor. When the amount of change of the catalyst temperature (ΔT) is not smaller than the setting value, the catalyst activity is determined to be maintained, and when the amount of change of the catalyst temperature (ΔT) is smaller than the setting value, the catalyst activity is determined to be deteriorated (alarm of the apparatus). The setting value can be appropriately set by considering the concentration of formic acid in the gas to be processed, the flow rate of the gas, cost, the flow rate of oxygen, a heating temperature of the catalyst layer, a structure of the catalyst layer, the type of the catalyst and the like. 
     Although an example of a diagnostic method for determining the amount of change of the catalyst temperature by the thermocouple has been described, it is also possible to determine the degree of activity of the catalyst by using the thermocouple to measure an amount of reaction heat. Further, the gas containing formic acid may contain any gas, such as aldehyde, other than inert gas such as nitrogen gas. 
     EXAMPLES 
     Hereinafter, the present invention will be described in more detail with reference to=examples. However, the present invention is not limited only to the following examples. 
     (Test Method) 
     A test was performed using the apparatus as shown in  FIG. 1 . In the apparatus, the chamber  10  has a capacity of 20 L and the formic acid decomposition part  2  was filled with a catalyst of 1 L. In the formic acid decomposition part  2 , as shown in  FIG. 3 , thermocouples T 1 , T 2  and T 3  were disposed at three positions in an upstream part, a middle part and a downstream part of the catalyst layer, respectively. These thermocouples were positioned at the center part of the catalyst layer. As a catalyst, a Pt/alumina catalyst in which Pt is carried on particulate alumina at 1.8 g/L was used. In order to determine the catalyst activity, the test was performed partially using a catalyst of which activity was degraded. 
     A test method is as follows. With a valve  20  and a valve  21  closed and with the vacuum pump  11  operated, by flowing decomposing air via the introduction unit  4  and by heating the catalyst in the formic acid decomposition part  2  by a heater (not shown) to 100° C., a test preparation comes to completion. Then, the valve  20  is opened, via a flowmeter  22 , the gas containing formic acid is introduced into the chamber  10  at a flow speed of 20 L/min. When the gas containing formic acid is introduced into the chamber  10 , the valve  20  is closed, the valve  21  is opened, the gas containing formic acid in the chamber  10  is exhausted by the vacuum pump  11 , the gas containing formic acid and decomposing air are passed through the formic acid decomposition part  2  to decompose formic acid, and, the temperatures at the upstream part, the middle part and the downstream part of the catalyst layer during this period are measured with thermocouples T 1 , T 2  and T 3 , respectively. When the gas in the chamber  10  is entirely exhausted, the valve  21  is closed, and one cycle of test comes to completion. By repeating the cycle, the temperatures are measured with the thermocouples T 1 , T 2  and T 3 . 
     As the gas containing formic acid, a mixed gas obtained by mixing formic acid gas and nitrogen gas and by controlling the concentration of formic acid in the mixed gas to 3% was used. Air was used as the decomposing air. 
     The test was performed by setting an evacuation capacity of the vacuum pump  11  at 120 L/min and a flow rate of the decomposing air at 30 L/min. Therefore, an amount of formic acid passing through the formic acid decomposition part  2  was 3.6 L/min, an amount of oxygen in the decomposing air was 6.3 L/min (as an oxygen concentration in air is 21%), and a molar ratio of oxygen and formic acid was oxygen/formic acid=1.75. 
     (Method for Determining Catalyst Activity) 
     As shown in  FIG. 4 , the catalyst activity is determined from a situation of the catalyst temperatures measured by the thermocouples T 1 , T 2  and T 3 .  FIG. 4  is a conceptual diagram showing a change of the catalyst temperature, and a large part of formic acid introduced in the formic acid decomposition part  2  is decomposed in the upstream part of the catalyst layer, the catalyst temperature measured by the thermocouple T 1  on the upstream part largely rises when formic acid gas is introduced, and is cooled by the decomposing air when the passage of formic acid gas ends. Since a large part of formic acid is decomposed in the upstream part of the catalyst layer and an amount of formic acid reaching the middle part of the catalyst layer is less than that in the upstream part, a degree of increase of the catalyst temperature measured by the thermocouple T 2  is small. Then, since formic acid hardly reaches the downstream part of the catalyst layer, it is found that the temperature of the catalyst measured by the thermocouple T 3  does hardly change. Further, when an amount of change of the catalyst temperature (ΔT in  FIG. 4 ) falls below a preset value, it is determined that the catalyst activity deteriorated. 
     Example 1 
     As a catalyst, a Pt/alumina catalyst in which Pt is carried at 1.8 g/L on particulate alumina was used. A catalyst layer was prepared by filling 1 L of newly prepared catalyst in the formic acid decomposition part  2  and a test was performed according to the above-described test method (refers to as test (a)). 
     Next, 330 ml of a catalyst that is partially deactivated was prepared and filled in a portion of upper one third of the formic acid decomposition part  2  (in lower two thirds, the newly prepared catalyst that was used in the test (a) was filled), the test was performed according to the above-described test method (refers to as test (b)). 
     Further, 330 ml of a catalyst that is completely deactivated was prepared and filled in upper one third of the formic acid decomposition part  2  (in the lower two thirds, the newly prepared catalyst that was used in the test (a) was filled), and the test was performed according to the above-described test method (refers to as test (c)). 
     Changes of temperature of the upstream part, the middle part and the downstream part of the catalyst layer measured by the thermocouples T 1 , T 2  and T 3  in the test (a), test (b) and test (c) are shown in  FIG. 5 . In  FIG. 5 , a test result from first to fourth cycle is illustrated. 
     Further, values of the amounts of change of the catalyst temperatures (ΔT) obtained from the measurement results of the temperatures shown in  FIG. 5  are shown together in  FIG. 6 . In  FIG. 6 , a graph denoted as T 1  shows an amount of change of the catalyst temperature in each of the tests (a), (b) and (c) of the upstream part of the catalyst layer. In the same manner, a graph denoted as T 2  and a graph denoted as T 3  each shows an amount of change of the catalyst temperature in each of the tests (a), (b) and (c) of the middle part and the downstream part of the catalyst layer, respectively. 
     From  FIG. 5  or  FIG. 6 , it is found that in the case of the test (a) where the new catalyst having sufficient catalyst activity was used, almost all of formic acid introduced into the formic acid decomposition part  2  was decomposed in the upstream part of the catalyst layer and did not reach the middle part and the succeeding part of the catalyst layer. In the case of the test (b) where the partially deactivated catalyst was used in the upstream part of the catalyst layer, it is found that formic acid could not be completely decomposed on the upstream part of the catalyst layer and part of formic acid reached the middle part. And, in the case of the test (c) where a completely deactivated catalyst was used in the upstream part of the catalyst layer, it is found that the decomposition of formic acid hardly occurred in the upstream part of the catalyst layer and formic acid was decomposed in the middle part of the catalyst layer. 
     INDUSTRIAL APPLICABILITY 
     A formic acid decomposition apparatus and a formic acid decomposition method according to the present invention can be suitably utilized as an apparatus and a method for processing a reducing gas used in a soldering step of a semiconductor device. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
           1  formic acid decomposition apparatus 
           2  formic acid decomposition part 
           3  gas introduction pipe 
           4  gas introduction unit 
           5  gas introduction inlet 
           6  exhaust outlet 
           7  catalyst layer 
           8  flowmeter 
           10  chamber 
           11  vacuum pump 
           20  valve 
           21  valve 
           22  flowmeter