Patent Publication Number: US-6659335-B2

Title: Soldering method and soldering apparatus

Description:
This is a divisional of application Ser. No. 09/761,655 filed Jan. 18, 2001 now U.S. Pat. No. 6,457,634, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a soldering method and soldering apparatus for dropping molten or semi-molten solder (hereinafter referred to simply as molten solder) on a workpiece to effect soldering. 
     BACKGROUND OF THE INVENTION 
     There is proposed a soldering apparatus for automatically applying molten solder onto a desired point or area of a workpiece, such as a printed circuit board, wherein a conical pot is used for melting a solid solder (Japanese Patent Publication (KOKOKU) No. 46-34209 (1971)). A rod-shaped solder is cut and fed to the conical pot to be melted therein and the molten solder is allowed to drop onto a workpiece through a hole provided at the bottom of the pot. However, this type soldering apparatus has a problem that some portion of the solid solder drops through the hole in the unmelted state while the other portion of the solder is dropped in the overheated condition, since it is impossible to retain the whole solid solder for a constant time, resulting in uneven and unreliable soldering. 
     There is also proposed a soldering apparatus having a conical melting pot provided with a needle valve at the bottom of the pot (Unexamined Japanese Utility Model Publication (KOKAI) No. 50-77427 (1975)). In this known apparatus, a molten mass of solder is contained in the conical melting pot, and a constant volume of the molten solder is allowed to drop through the needle valve. However, due to surface tension of the molten solder, it becomes impossible to control the volume of dropping molten solder precisely particularly when the volume of molten solder to be applied onto the workpiece is relatively small, as is the case where the workpiece is a printed circuit board. 
     In order to solve the aforementioned problems of the known apparatuses, the present inventor has previously proposed an improvement in soldering in my earlier Japanese Patent Application No. 61-110505 (1986) which is published as JP-A-62-270272. According to my previous proposal, a solder melting pot is formed by plural trowel members which engage with each other to hold a solid solder tip or piece for a pre-set time to melt the same and then they are separated to allow the molten solder to drop onto a desired point or area of a workpiece. 
     These conventional methods must prevent so-called “knotty or tubercled soldering” by allowing additional heating the soldering point to enable excellent flow of molten solder to the soldering point after supplying molten solder to the soldering point. In this case, the trowel members are separated and raised to allow the molten solder to drop onto the soldering point, and the trowel members are again lowered to come close to the same desired point right after dropping. 
     However, such a process leads to the deteriorated process efficiency when a plurality of desired points are to be soldered. That is, the time duration for supplying the molten solder and the time duration for additional heating are required with respect to one point to be soldered. 
     Further, it is desirable to preheat a point to be soldered before supplying the molten solder. It can be considered to provide a preheat time during which the tip-ends (lower ends) of the trowel members are moved close to the soldering point before the trowel members are opened to drop the molten solder. However, by doing so, a time required for supplying the molten solder, an additional heating time and a preheat time therefor are required with respect to one point to be soldered. Thus, the process efficiency is further deteriorated. 
     SUMMARY OF THE INVENTION 
     In view of the above described problems, it is a first object of the present invention to provide a soldering method by which a plurality of soldering points aligned at predetermined intervals can be efficiently soldered. 
     It is a second object of the present invention to provide a soldering apparatus directly used for realization of such a soldering method. 
     According to the present invention, the first object is attained by a soldering method for soldering plural soldering points aligned at a predetermined interval; comprising the steps of forming a solder melting pot by assembling plural trowel members, each of the trowel members having a surface made of a solder repellent material at least the portion contacting with the solder; supplying a solder tip having a constant volume to said solder melting pot; and sequentially supplying the molten solder to the respective soldering points from the solder melting pot: 
     an improved method wherein an additional heating portion which comes close to said solder melting pot is formed to said trowel members, and wherein the soldering point to which molten solder has been supplied by prior molten solder supply operation is additionally heated during the supply operation of the molten solder to the next soldering point. 
     In the present invention, the trowel members are used to simultaneously perform the molten solder supply operation and the additional heating operation for the soldering point to which the molten solder was supplied by the preceeding molten solder supply operation. 
     In the present specification, the term “soldering point” means a point or portion to be soldered of a workpiece, and includes a point or portion which will be soldered or has been soldered. 
     The solder melting pot may be formed by grooves formed on the opposing surfaces of the trowel members so that a cavity defining the solder melting pot is formed by these grooves when the trowel members engages with each other. Such a solder melting pot may have an opening or hole at the bottom of the pot. In this case, the opening may be closed or plugged by the upper surface or portion of the soldering point when the trowel members are lowered to come close to the soldering point. That is, the solder melting pot is formed by engaging the trowel members and the bottom opening of the pot is closed with the soldering point by lowering the trowel members assembly to come close to the soldering point. The solid solder tip is dropped to the formed and closed solder melting pot to be-molten. The molten solder can be supplied to the soldering point by moving up the trowel members. Otherwise, the trowel members may be opened or separated to supply the molten solder to the soldering point. 
     A plurality of solder melting pots may be formed by one set of the trowel members so that each solder melting pot can simultaneously supply the molten solder to different points to be soldered by separating or raising the trowel members. For example, the solder melting pots may be provided at intervals equal to pitches of the soldering points. In this case, the additional heating portion is also formed between the solder melting pots. Alternatively, the solder melting pots may by provided at intervals as integral multiples of the same pitches. The additional heating portion is formed along the length of the multiple pitches so that it can move close to the plural soldering points ready to be soldered at the same time. 
     It is possible to effect the control so that a workpiece having plural soldering points can continuously move relative to the trowel member in an alignment direction of the soldering points at a fixed speed and the solder melting pot(s) can supply molten solder to predetermined points to be soldered. A workpiece may move intermittently or move while changing the speed. For example, when a plurality of solder melting pots are formed by a plurality of sets of trowel members provided along the alignment direction of the soldering points, it is preferable to periodically change the workpiece transfer speed for each of a predetermined number of soldering processes based on the number of the solder melting pots or intervals. 
     By providing to the trowel members a preheating portion which is placed to the opposed side of the additional heating portion with the solder melting pot therebetween, this preheating portion can be used to preheat a next soldering point to which molten solder is to be supplied. That is, the molten solder supplying operation, the additional heating operation for the soldering point to which the molten solder has been supplied by the preceeding supplying operation, and the preheating operation for a next soldering point can be simultaneously carried out. 
     The second object of the present invention can be attained by the provision of a soldering apparatus for soldering plural soldering points of a workpiece, the plural soldering point being aligned at a predetermined interval, comprising: 
     plural trowel members each having a surface made of a solder repellent material at least the portion contacting with a solder, said plural trowel members being engaged together to form a solder melting pot and separated from each other to supply the molten solder to the soldering point, a solder tip having a constant volume being supplied to said solder melting pot to be molten and then allowed to drop when said plural trowel members are separated from each other; 
     an additional heating portion which is formed to said trowel members in close vicinity to said solder melting pot and additionally heats an adjacent soldering point to which the molten solder has been already supplied during an operation of said solder melting pot for supplying the molten solder to the next soldering point; 
     a workpiece holder for relatively moving the workpiece with respect to said trowel members along an alignment direction of the plural soldering points; and 
     a controller for moving said workpiece holder and controlling said trowel members in synchronism with the relative movement of the workpiece, so that said trowel members are moved up and down and/or opened and closed to supply the molten solder from said solder melting pot to each of the soldering points of the workpiece. 
     The solder melting pot may be formed by grooves formed on the opposing surface of the trowel members. A cavity defining the solder melting pot is formed by engaging the opposing trowel members. The molten solder can be supplied to the soldering point by separating the trowel members. The solder melting pot formed by the grooves of the trowel members may have a bottom opening which can be closed or plugged with the upper portion of the soldering point when the trowel members are lowered to come close the soldering point. In this case, the molten solder can be supplied to the soldering point when the bottom of the solder melting pot is opened by moving up or opening the trowel members. 
     When a plurality of solder melting pots are provided to the assembly of the trowel members at the same pitches as those of the soldering points or pitches of integral multiples, plural points can be soldered all at once, thereby improving the efficiency. 
     When the preheating portion is provided to the trowel members so that the preheating portion is arranged on the opposed side of the additional heating portion with the solder melting pot therebetween, the molten solder supplying operation, the additional heating operation and the preheating operation can be performed all at once. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front end view of a main body A for use in a soldering apparatus according to an embodiment of the present invention; 
     FIG. 2 is a side view of the main body A depicted in FIG. 1; 
     FIG. 3 is an exploded perspective view showing parts of the main body A depicted in FIG. 1; 
     FIG. 4 is a partially exploded perspective view showing parts of the main body A depicted in FIG. 1; 
     FIG. 5 is a further exploded perspective view showing parts of the main body A of FIG. 1; 
     FIG. 6 is a perspective view of a solder tip supply assembly; 
     FIG. 7 is a cross-sectional view of a solder cutter for cutting a solder tip from a wire or rod of a solder; 
     FIG. 8 is an exploded perspective view showing the details of the solder tip guide assembly; 
     FIG. 9 is a sectional side elevation taken along the center line of the solder tip guide assembly; 
     FIGS. 10A to  10 C are views for illustrating the operation of a cradle; 
     FIG. 11 is a view showing the state in which the main body A is combined with a moving device B; 
     FIG. 12 is a partially ruptured perspective view showing a shape of ends of trowel members; 
     FIG. 13 is a sectional side elevation showing the soldering operation; 
     FIG. 14 is a cross-sectional view taken along the line P—P in FIG. 13 (when dropping a solder tip); 
     FIG. 15 is a cross-sectional view taken along the line P—P in FIG. 13 (when melting solder); 
     FIG. 16 is a cross-sectional view taken along the line P—P in FIG. 13 (when the trowel members are opened); 
     FIGS. 17A,  17 B,  17 C and  17 D are explanatory drawings of a continuous operation; 
     FIG. 18 is a perspective view showing the trowel members of another embodiment; and 
     FIG. 19 is a view for illustrating the operation of the embodiment of FIG.  18 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Presently preferred embodiments of the present invention will now be described in detail with reference of the appended drawings. The main body A for use in an embodiment will first be described with reference to FIGS. 1 to  10 . 
     Main Body 
     The main body A is assembled to a carrier plate denoted by reference numeral  10  in FIGS. 1 to  4 . The carrier plate  10  is attached to a moving device B shown in FIG. 11 so as to be capable of vertically and horizontally moving. This carrier plate is elongated, and a linear bearing track  12  (FIGS. 3 and 4) is fixed on one side of the carrier plate  10  to extend vertically, and three movable blocks  12 A,  12 B and  12 C are movably mounted on the track  12 . Each of the movable blocks  12 A to  12 C contains a bearing and is moved smoothly along the track  12  in the vertical direction without rotating around the track  12 . An elongated window or opening  14  is formed through the carrier plate  10  at the right side as seen in FIGS. 3 and 4. 
     A movable plate  20  is fixed to the uppermost movable block  12 A to be moved in the vertical direction. A feed screw  22  extends in the vertical direction and has an upper end fixedly held by a bearing holder  24  which is fixed at an upper right portion of the carrier plate  10 , the feed screw being thrusted into a nut holder  26  fixed to the movable plate  20 . The feed screw  22  is rotated by a stepping motor  28  secured to an upper portion of the carrier plate  10 . As the feed screw  22  rotates, the movable plate  20  which is integrally fixed to the nut holder  26  is moved in the vertical direction. 
     A fan motor  30  is provided at the left side (as viewed in FIGS. 1 and 2) of the motor  28 . The top, front, left and right sides of the main body A of the soldering apparatus are covered by a cover plate  32  (see FIGS.  2  and  11 ), and the carrier plate  10  serves as a backing plate. Fresh air is introduced by the fan motor  30  into the space covered by the cover plate  32 , and the thus introduced air is flown downwards inside the cover plate  32  to remove smokes generating from the solder as it is melted. 
     In FIGS. 1,  3 ,  4  and  5 , a solder guide bracket is denoted by reference numeral  40 . The bracket  40  is fixed to the intermediate movable block  12 B mounted on the linear bearing track  12  and extends forwardly through a cut-out  20 A formed through the upper movable plate  20 . An arm  42  is fixed to the fore end of the bracket  40  and has a lower end extending to a vicinity of the front center portion of the soldering apparatus. A solder guide assembly  80  serving as a solder tip guide means, which will be described in detail hereinafter, is fixed to the lower end of the arm  42 . 
     The bracket  40  is moved in the vertical direction by means of an air cylinder  44  which is fixed to the upper movable plate  20 . In detail, the cylinder of the air cylinder  44  is fixed to the upper movable plate  20 , and a piston rod operatively associated with the air cylinder  44  has an lower end secured to the top face of the bracket  40 . 
     In FIGS. 1,  2 ,  4 ,  5  and  6 , reference numeral  50  designates a solder feeder plate fixed on the upper movable plate  20 . As shown in FIG. 4, the plate  50  is fixedly secured on the right end face of the upper movable plate  20  to extend forwardly. A solder tip supply assembly  52  is mounted on the plate  50 , and the solder tip supply assembly  52  supplies a solder tip  58 A cut to have a constant length to the solder guide assembly  80  which will be described in detail hereinafter. 
     Referring to FIGS. 2 and 4, a reel holding arm is denoted by  54  and has one end fixed to the back side of the upper movable plate  20  and the other end extending rearwards through the window  14  of the carrier plate  10 . A solder reel  56  is held by the reel holding arm  54 , and a continuous rod-shaped or wire solder  58  is fed from the reel  56  to pass through a window  20 B (see FIG. 4) formed aside the cut-out  20 A of the upper movable plate  20  into the solder tip supply assembly  52 . 
     The solder tip supply assembly  52  comprises a feed section  60  and a cutter section  62 , as shown in FIG.  6 . The feed section  60  includes paired roller  64 ,  64  for grasping the solder  58 , and a stepping motor  66  which drives one of the rollers  64 . The motor  66  is controlled by a controller  67  shown in FIG. 2 to rotate the roller  64  at a pre-set timing to feed a predetermined length of the solder  58  to the cutter section  62 . 
     The cutter section  62  includes a holder  68  secured to the feeder plate  50  and having a shape of lying squalish letter U, a cylindrical body  70  secured to the bottom face of the lower leg of the holder  68 , a cutter pin  72  movably inserted into the body  70 , a restoration spring  74  (see FIG. 7) for restoring the cutter pin  72  to the upper position, and an air cylinder  76  mounted on the upper leg of the holder  68  to press the cutter pin  72  downwardly. A solder inlet port  78  is provided through the side wall of the body  70  and faces to the paired rollers  64 ,  64 . 
     As a pre-set length of the rod-shaped solder  58  is fed by the feed section  60 , the fore end of the solder  58  is moved through the solder inlet port  78  into the bore of the body  70 , and then air is fed into the air cylinder  76  by changing over a change-over valve  122  in response to the instruction from the controller  67  so that the cutter pin  72  is pressed downwards. Whereupon, the fore end of the solder  58  is cut, and the cut solder tip  58 A falls downwards through the bore of the body  70 . 
     As shown in FIGS. 8 to  10 , the solder guide assembly  80  has a body  82  through which a guide bore  84  is formed. The guide bore  84  extends in the vertical direction and has a funnel-shaped upper portion and a lower portion connected to a sleeve  86 . In detail, a slot is formed transversely through the lower portion of the body  82 , and the sleeve  86  inserted in the lower portion of the body  82  is fixed by fastening screws  88  (see FIGS.  8  and  9 ). 
     A transverse through-hole  90  extends through the wall of the body  82  at the position where the upper enlarged portion (i.e. the funnel-shaped upper portion) of the guide bore  84  is formed. A cradle  92  is contained in the enlarged portion of the guide bore  84 , and a shaft  94  integrally fixed to the cradle  92  extends through the transverse through-hole  90 . The cradle  92  serves as a receiver for receiving the solder tip  58 A falling from the cutter section  62  and has the inner surface fitted with a cup  96  made of a fluorocarbon resin, such as Teflon (Trade Name) for preventing the solder tip  58  to stick onto the inner surface of the cradle  92 . The inner surface of the cup  96  may be embossed with fine embosses or fine indents to facilitate smoother delivery of the solder tip  58 A. Otherwise, the inner surface of the cradle  92  may be lined with a fluorocarbon resin. 
     The shaft  94  has a fore end to which an arm  98  is connected. A stopper  100  is connected to the body  82  so that the arm  98  swings as the shaft  94  is rotated by about 180 degrees between upper and lower opposing walls  100 A and  100 B of the stopper  100 . When the arm  98  engages with the upper wall  100 A of the stopper  100 , the cradle  92  is held at the upside position where the opening of the cradle  92  faces upwards as shown in FIG.  10 A. When the arm  98  engages with the lower wall  100 B of the stopper  100 , the cradle  92  is inverted to be held at the upside-down position where the opening of the cradle  92  faces downwards as shown in FIG. 10C. A coil spring  102  extends between the bent end of the arm  98  and a pin  100 C fixed to the stopper  100  to retain the cradle  92  stably at the upside position shown in FIG.  10 A and also at the upside-down position shown in FIG.  10 C. 
     Two pins  104  and  106  are mounted to the rear end of the shaft  94  and extend radially from the shaft  94 , the angle between the pin  104  and the pin  106  being about 90 degrees. These pins  104  and  106  extend obliquely as shown in FIG. 10A when the cradle  92  is retained in the upside position. On the other hand, an arm  108  (see FIGS. 2,  4 ,  5  and  8 ) is connected to the feeder plate  50  which is integrally mounted to the upper movable plate  20 , the arm  108  extending downwardly and having the lower end from which a cam follower  110  protrudes to be engaged with the pins  104  and  106 . With such a construction, as the solder guide assembly  80  is moved in the vertical direction by means of the air cylinder  44 , these pins  104  and  106  engage with the cam follower  110  so that the cradle  92  is inverted between the upside position shown in FIG.  10 A and the upside-down position shown in FIG.  10 C. Namely, when the coil spring  102  goes beyond the unstable point of the arm  98  as the solder guide assembly  80  is lowered, the cradle  92  is swiveled to be retained in the upside-down position. 
     A sensor  112  for sensing the falling solder tip  58 A is disposed in the body  82  of the solder guide assembly  80  (see FIGS.  8  and  9 ). In detail, a transverse hole  114  is formed through the body  82  and crosses the solder guide bore  84  at right angle, and a light emitting element  112 A is disposed at one end of the hole  114  and fixedly retained by a sensor carrying plate  116  while a light receiving element  112 B is disposed at the other end of the hole  114  and fixedly retained by another sensor carrying plate  118 . As the light receiving element  112 B senses passage of the solder tip  58 A, the controller  67  actuates clock means contained therein which counts the time from the timing at which the solder tip  58 A is sensed. The time period for retaining and melting the solder tip  58 A within the solder melting pot  157  formed by the trowel members  156 , the details of which will be described hereinafter, is controlled by the controller  67  with reference to the lapse of time counted by the clock means. 
     The body  82  of the solder guide assembly  80  is also provided with an air inlet port  83  (see FIG. 10A) through which air is blown into the guide bore  84 . The air blown through the inlet port  83  prevents the smoke generated from the molten solder in the trowel members  156  from sticking to the inner surface of the guide bore  84  to ensure smooth fall-down of the solder tip  58 A. 
     Air fed to the body  82  and the air cylinders  44  and  76  is supplied from an external air pump  120 , as shown in FIG.  2 , and intermittently supplied by means of a change-over valve  122 . 
     The solder guide assembly  80  is provided with a light emitting element  123  and a light receiving element  124  (FIGS. 2,  5 ,  8  and  10 A). In detail, a horizontal plate  125  is fixed to the body  82 , and a rod  126  extends downwardly from the plate  125  and has a lower end to which the light emitting element  123  is secured (FIG.  8 ). The light emitting element  123  may be LED, semiconductor laser or another type light emitting element and emits a light beam toward a portion of the molten solder contained in the melting pot  172  formed by two opposing trowels  156  ( 156 A,  156 B), as viewed in FIG.  10 A. 
     The light receiving element  124  may be, for example, a photo transistor and is secured on the side wall of the body  82 . On the other hand, a reflector mirror  127  (FIG. 10) is formed on the bottom of the cradle  92 . As shown in FIG. 10A, the mirror  127  has a reflecting surface which extends at an angle of 45 degrees relative to the horizontal plane when the cradle  92  is in the upside condition. As the solder is melted in the solder melting pot  157  formed by the trowel members  156 , the light beam from the light emitting element  123  is reflected by the surface of the molten solder mass and a portion of the reflected light beam passes through a sleeve  86  and the guide bore  84 . The mirror  127  reflects the light beam passing through the sleeve  86  and the guide bore  84  to the light receiving element  124 . 
     Driving Mechanism of Trowel Members 
     The details of the heatable trowel members  156 A,  156 B which come together to form the solder melting pot  172  will now be described. In FIG. 3, reference numeral  130  designates a lower movable plate. The lower movable plate  130  is secured to the movable block  12 C (FIGS. 3 and 4) to be moved in the vertical direction. The left side of the lower or first movable plate  130  is suspended from the upper or second movable plate  20  through a coil spring  132 , and the right side thereof is suspended from the upper movable plate  20  through a long bolt  134 . As a result, the lower movable plate  130  is moved in the vertical direction together with the upper movable plate  20  (FIGS. 1,  2  and  3 ). 
     The bolt  134  is slidingly inserted through a bore formed in the upper movable plate  20  so that the lower movable plate  130  is moved upwards together with the trowel members  156  when the trowel members  156  impinge on the surface of the workpiece to prevent damage of the workpiece upon such impingement by the trowel members  156 . A compression spring  136  is disposed around the bolt  134  to apply a proper compression force on the trowel members  156  when it is desired to press the trowel members  156  onto the surface of the workpiece. 
     A slide rail  138  is secured to the lower movable plate  130  and extends horizontally (FIGS.  2  and  3 ). Movable blocks  140 A and  140 B are slidingly mounted to the rail  138 , and paired trowel member carrier plates  142  ( 142 A,  142 B) are fixed to the movable blocks  140 A and  140 B. A stepping motor  144  is mounted on the back side of the lower movable plate  130  and has a shaft extending forwards through the lower movable plate  130 . An encoder cam  146  and a cam  148  are fixedly mounted on the fore end of the shaft of the motor  144 . 
     The encoder cam  146  detects the rotation angle of the motor  144  by a photo-sensor  150 . The cam  148  has a generally elliptical contour and symmetrically mounted on the shaft of the motor  144 . Cam followers  152  ( 152 A,  152 B) mounted to the aforementioned trowel member carrier plates  142  rotatably engage with the cam  148  from the right and left sides. The trowel member carrier plates  142  are biased by a spring  154  to be applied with a force which acts to allow the carrier plates  142  to come close with each other (see FIG.  3 ). 
     In the illustrated embodiment, paired trowel members (or soldering iron members)  156  ( 156 A,  156 B) are secured to the trowel member carrier plates  142 A,  142 B, the angle and position of each trowel member  156  being adjustable. These trowel members  156  extend obliquely from the carrier plates  142 A,  142 B to have their lower ends separably engaged with each other just beneath the sleeve  86  connected to the aforementioned solder tip guide assembly  80 . When these trowel members  156 A,  156 B come close with each other, they form the solder melting pot  157  having a V-shaped section when viewed from the front side. Each trowel member  156  contains a Nichrome wire heater and has a surface made of a material which repels molten solder, such as ceramics, fluorocarbon resin or stainless steel. A silicon nitride ceramic material is particularly preferred, since it is highly resistive to permeation of solder and to chemicals, such as hydrochloric acid, contained in the flux of the solder and also has improved durability. 
     Moving Device 
     The main body A is combined with the moving device B shown in FIG. 11 for use. The moving device B includes a base board  160 , a pair of column supports  162  erected on the base board  160 , and a horizontal rail  164  held by the column supports  162 . A movable box  166  capable of moving in a horizontal direction (X direction) is held by the horizontal rail  164 . 
     The main body A is held by this movable box  166  so as to be capable of vertically moving. The main body A vertically moves by a driving unit  168  provided on the upper part of the movable box  166  to perform positioning in a Y direction (vertical direction). The main body A is held by the movable box  166  so that the trowel members  156  move close to or away from each other along the direction parallel to the horizontal rail  164 . In addition, a workpiece support or holder  170  capable of moving in the front-and-back direction (Z direction) is disposed on the upper part of the base board  160 . 
     A print wiring board is mounted to the workpiece holder  170  and moves the movable box  166  and the workpiece support board  170  in directions orthogonal to each other (X and Z directions) so that the workpiece is positioned below the assembly of the trowel members  156 . Soldering is effected by moving the main body A in the vertical direction (Y direction). 
     Shape of Trowel Members 
     FIG. 12 is a partially ruptured perspective view showing the shape of the tip-end portion of the trowel members  156 . Grooves  172 A are formed on the opposing surfaces of these trowel members  156  (only one groove being shown in FIG. 12) so that a cavity defining a solder melting pot  172  is formed by grooves in the vicinity of the central portion when the both trowel members  156 A and  156 B are assembled. The tip-end of the assembled trowel members  156  has preheating/additional heating portions  174  ( 174 A and  174 B) positioned on the both sides of the solder melting pot  172 . 
     In the embodiment, one of the preheating/additional heating portions  174  on the movement side serves as a preheating portion  174 A (or  174 B) in association with the actual moving direction of the workpiece, and the other portion  174  on the side opposed to the movement side functions as the additional heating portion  174 B (or  174 A). It is to be noted that positions of the preheating portion and the additional heating portions are naturally changed over when the moving direction of the workpiece is reversed. 
     In this embodiment, the solder melting pot  172  formed by the grooves  172 A has a bore piercing in the vertical direction. When the trowel members  156  move down and the bottom faces thereof come close to the soldering point  184 , the bottom opening of the solder melting pot  172  is closed by the soldering point  184 . As a result, a solder melting pot  172  having the bottom being opened/closed by the soldering point is formed. 
     Operation 
     This soldering apparatus is used when a plurality of points to be soldered are aligned at predetermined intervals. For example, it is suitable when soldering a connector socket to a print wiring board. 
     FIG. 13 is a sectional side elevation showing the soldering operation for a connector socket. FIGS. 14,  15  and  16  are cross-sectional views taken along the line P—P in FIG.  13 . FIG. 14 shows the state when a solder tip is dropped; FIG. 15 shows the state when the solder is molten; and FIG. 16 shows the state when the trowel members are opened. In addition, FIG. 17 is an explanatory drawing of the continuous operation. 
     Referring to FIGS. 13 to  16 , reference numeral  180  denotes a print wiring board as a workpiece to be processed and  182  designates a connector socket. The connector socket  182  has a plurality of pin-type lead wires linearly aligned, i.e., soldering point  184 . The connector socket  182  also has a connector insertion hole  186  elongated in the lengthwise direction of the socket  182  and provided on the surface opposed to these lead wires  184  (FIG.  14 ). When a non-illustrated connector is inserted into the connector insertion hole  186 , each lead wire  184  can be connected to the connector. 
     The lead wires  184  are thrusted into a plurality of through holes  188  provided on the print wiring board  180 . In this state, the connector socket  182  is temporarily fixed to the print wiring board  180  by, an adhesive. The print wiring board  180  to which the connector socket  182  is temporarily fixed is held on the workpiece holder  170  of the moving device B. At this time, the alignment directions of the lead wires  184  are matched with the moving direction (Z-axial direction) of the workpiece holder  170 . 
     After fixing the workpiece, i.e., the print wiring board  180  in this manner, the movable box  166  of the moving device B is moved in the horizontal direction (X direction) so that the solder melting pot  172  formed by the trowel members  156  is positioned above the lead wires  184  potioned on one end of the printing wiring board  180 . The driving unit  168  of the moving device B is driven to position the main body A at a predetermined height (Y direction), and the continuous soldering operation is then started as follows. 
     On reception of a start signal, the controller  134  actuates the trowel members  156  to start heating and to be separated by a small gap by rotation of the motor  144 . The motor  28  is actuated to lower the upper movable plate  20 . When the trowel members  156  are positioned to a height at which the lead wire  184  can be grasped, the motor  144  is actuated to close the trowel members so that the solder melting pot  172  is formed. The solder melting pot  172  is then moved close to the lead wire  184  (the state shown in FIG. 14, FIG.  17 A). That is, the bottom of the solder melting pot  172  is closed by the lead wire  184 . In this state, the solder tip is dropped into the solder melting port  172  (FIG.  14 ). 
     In order to drop the solder tip  58 A, the solder tip  58 A is put in the cradle  92 . More specifically, a pre-set length of the rod-shaped solder  58  is supplied by the solder tip supply assembly  52 , and then the cutter pin  72  is lowered by the air cylinder  76  to cut a solder tip having a predetermined length. The thus cut solder tip  58 A is allowed to fall down into the cradle  92 . Solder supply and cutting operations are repeated until a desired number of solder tip  58 A is contained in the cradle  92  of the solder tip guide assembly  80 . The cradle  92  is inverted to drop the solder tip. 
     The cradle  92  is inverted as follows. When a start signal is fed, air is supplied into the air cylinder  44  to lower the bracket  40  together with the solder tip guide assembly  80 . The pin  104  of the shaft  94  engages with the cam follower  110  to rotate the shaft  94  as shown in FIG. 10B as the assembly  80  is lowered. 
     When the spring  102  connected to the fore end of the stopper  100  moves beyond the unstable point at which the spring  102  passes the center axis of the shaft  94 , the shaft  94  is rotated promptly to the position shown in FIG. 10C by the action of the spring  102 . The cradle  92  is thus inverted, and the solder tips  58 A leave from the cradle  92  while being applied with a shock of prompt inversion of the cradle  92  to fall through the guide bore  84  in the solder tip guide assembly  80  and then through the sleeve  86 . 
     As the solder tip guide assembly  80  is moved to the lower position, the lower end of the sleeve  86  comes close to the solder melting pot  172  formed by the assembled trowel members  156  (FIG.  10 C). The solder tips  58 A fall through the sleeve  86  to be received in the solder melting pot  172 . As the solder guide assembly  80  is lowered, the end of the lead wire  184  is inserted in-between the gap formed by the opposing ends of the trowel members. 
     The solder guide assembly  80  is returned back to the raised stand-by position shown in FIG.  10 A. The trowel members  156  come closer with each other to grasp the end of the lead wire  184  by the ends thereof and the end of the lead wire  184  is heated during the time when the solder guide assembly  156  is raised. The trowel members  156  are opened and then closed repeatedly to ensure smooth contact between the solder tips  58 A and the surface of the solder melting pot  172  and to realize uniform melting of the solder. 
     After the solder guide assembly  80  is returned back to the raised position, the cradle  92  is in the position as shown in FIG. 10A, and a light beam is emitted from the light emitting element  123  toward the solder melting pot  157 . As the solder tips  58 A are melted, the solder is liquefied (FIG. 15) to form a bead-shaped molten solder mass having a surface which reflects the light beam. The reflectivity of the molten solder is higher than that of the unmelted solid solder tip, so that the intensity of the reflected light beam is abruptly increased. A portion of the reflected light beam passes through the sleeve  86  and reflected by the mirror  127  provided on the outer surface of the bottom of the cradle  92  to be received by the light receiving element  124  (FIG.  10 A). 
     On the other hand, the light receiving element  112 B of the solder sensor senses the solder tips  58 A falling through the guide bore  84 . The signals generated from the light receiving element  112 B are fed to the controller  67  which instructs the timer to begin counting of the lapse of time. When melting of the solder is detected by the light receiving element  112 B, the controller  67  instructs another timer to begin counting of the lapse of time. Reference numeral  58 B denotes molten solder in FIG.  15 . 
     When the time duration goes beyond a predetermined time duration required for additional heating the molten solder to the desired temperature, the motor  144  is actuated to rotate the cam  148 , whereby the trowel member carrier plates  142  are pushed outwardly to separate the trowel members  156 . Whereupon, the molten solder  58 B contained in the solder melting pot  172  drops onto the soldering point of the workpiece to effect soldering (FIG.  16 ). By opening and closing the trowel members  156  repeatedly, dropping of the molten solder  58 B is ensured. 
     The molten solder  58 B falling from the solder melting pot  172  is absorbed into a gap between the lead wire  184  as a soldering point and the through hole  188  by capillary phenomenon, and the state shown in FIG. 16 is obtained. While the trowel members  156  are supplying the molten solder to one soldering point in this manner, the preheating portion  174 A adjacent to the solder melting pot  172  is positioned close to the next lead wire  184  (to which solder is subsequently supplied), and this adjacent lead wire  184  is preheated (FIG.  17 B). 
     Upon completion of supply of the molten solder, the trowel members  156  move up together with the upper movable plate  20  by the motor  28 . At this time, the workpiece support  170  carrying the workpiece  182  moves in the Z-axial direction. Specifically, the workpiece support  170  moves in one direction by a distance corresponding to the pitches or intervals of the lead wires  184  of the connector socket  182 . Therefore, the lead wire  184  preheated by the preheating portion  174  is positioned below the assembled trowel members  156  as described above. In this state, the trowel members  156  are lowered and the solder tip  58 A is supplied to the solder melting pot  172 . The solder tip is melted and the molten solder is supplied to the next soldering point (FIG.  17 C). 
     Here, the additional heating portion  174 B of the trowel members  156  are placed close to the lead wire  184  to which the molten solder  58 B was most recently supplied by the preceeding solder supplying operation and additionally heats this lead wire  184 . Accordingly, additional heating can avoid “knotty or tubercled soldering” caused due to insufficient heat, thereby improving the reliability of soldering. At this time, the preheating portion  174 A comes to a position close to a next soldering point and preheats this point. Therefore, flow of the molten solder is enhanced during supply of the molten solder, which leads to improvement in the reliability of soldering. 
     As described above, soldering is repeated with predetermined cycles while sequentially feeding the workpiece support  170  in accordance with pitches of the lead wire  184  (FIG.  17 D). With such an arrangement, the time for maintaining the trowel members to perform additional heating after supply of the molten solder (additional heating time) can be omitted. Thus, a plurality of points to be soldered can be efficiently soldered. Further, a soldering point to which solder is subsequently supplied can be preheated by the preheating portion  174 A while supplying solder to the adjacent soldering point, thereby enabling the efficient soldering. 
     Although the workpiece support  170  can be intermittently moved, it may be continuously moved at a fixed speed. In such a case, it is preferable that the groove(s)  172 A of the solder melting pot  172  is formed so that a sufficient gap can be provided in the moving direction of the workpiece in order to allow the lead wire  184  to move in the opening of the solder melting pot  172 . 
     Another Embodiment 
     FIG. 18 is a perspective view showing trowel members of another embodiment, and FIG. 19 is an explanatory drawing of the operation thereof. This embodiment uses the apparatus illustrated in FIGS. 1 to  17  to solder cable terminals of a flat cable to a print wiring board. 
     In these figures, reference numeral  200  denotes a print wiring board and  202  designates lands formed on the print wiring board  200 . Reference numeral  204  represents a flat cable having one side edge from which cable terminals  206  aligned at predetermined intervals (pitches) protrude. The lands  202  are formed at the same pitches as that of the cable terminals  206 . Each cable terminal  206  is superimposed on an associated land  202  to form a point to be soldered. 
     Trowel members  208  ( 208 A and  208 B) have a shape which is somewhat different from that of the elements illustrated in FIGS. 1 to  17 . That is, a solder passage or groove  210  piercing in the vertical direction is formed between assemblies of the trowel members  208 , and the cavity defining a solder melting pot  210 A is formed by the solder passage  210 , the cable terminal  206  and the land  202  at a point to be soldered. 
     The lower surfaces of the trowel members  208  are so formed as to be wide in the alignment direction of the soldering points, i.e., the alignment direction of the lands  202 , and these lower surfaces serves as preheating/additional heating portions  212  ( 212 A and  212 B). In FIG. 19, since a workpiece (the print wiring board  200 ) is fed toward the left hand side,  212 A functions as an additional heating portion and  212 B acts as a preheating portion. 
     According to this embodiment, the trowel members  208 A and  208 B are assembled on the land  202  as shown in FIG. 19 in order to form the solder melting pot  210 A. A solder tip (not shown) is dropped in the pot  210 A to be molten. By raising the trowel member  208 , the molten solder falls onto the land  202  and the cable terminal  206 . Since the cable terminal  206  and the land  202  are preheated by the preheating portion  212 B, the molten solder flows into a gap between the cable terminal  206  and the land  202  by capillary phenomenon. 
     If the workpiece  200  moves in a direction indicated by an arrow in FIG. 19, the soldering point ( 206 ,  202 ) to which the molten solder was supplied gets under the additional heating portion  212 A to be heated. The molten solder, therefore, assuredly flows between the cable terminal  206  and the land  202 . When the workpiece  200  further moves, this soldering point gets out of the additional heating portion  212 A to be cooled down. 
     Although the above has explained the embodiments in which the lead wire  184  or the cable terminal  206  is soldered to the through hole  188  or the land  202 , the present invention is not restricted thereto. When trowel members having a shape associated with a point to be soldered are used, the present invention can be applied to soldering with respect any other member than the lead component or the flat cable. For example, the present invention can be applied to soldering for a surface mounting type IC having a gull-wing lead wire. 
     As described above, according to the present invention, the additional heating portion which comes close to the solder melting pot is formed to the trowel member(s). The additional heating portion heats a soldering point to which molten solder has already been supplied by the prior molten solder supplying operation, while the solder melting pot supplies solder to the next soldering point. Accordingly, the trowel members do not have to be held at the soldering point after supply of solder for additionally heating. Since an independent additional heating time does not have to be provided, the processing speed can be increased to improve the throughput. 
     Moreover, according to the present invention, since the preheating portion is formed to the trowel member(s) on the opposed side of the additional heating portion with the solder melting pot therebetween, it is possible to preheat the next soldering point while the current soldering point is soldered and the prior soldering point is additionally heated. The preheating time, therefore, does not have to be additionally provided, which leads to the improved throughput.