Patent Publication Number: US-2011072655-A1

Title: Method of soldering an electronic component

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2009-224660, filed on Sep. 29, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein relate to a method of soldering an electronic component. 
     BACKGROUND 
     A pin-type electronic component, such as a connector and a large-capacitance capacitor, is mounted on a printed circuit board in such a manner that pins of the electronic component are inserted into and soldered to through-holes of the printed circuit board. On the other hand, a chip-type electronic component does not require through-holes when mounted on the printed circuit board. 
     There is a known method of soldering a chip-type electronic component onto a printed circuit board called reflow soldering in which the chip-type electronic component is placed on solder paste applied on the printed circuit board and the solder paste is then melted by heating (see for example Japanese Patent No. 3974525 or Japanese Laid-open Patent Application No. 2007-208176). On the other hand, flow soldering method is known, which soldering pins of a pin-type electronic component into through-holes provided in a printed circuit board in which the back surface of the printed circuit board opposite the surface having the electronic component is dipped in molten solder contained in a solder bath (see for example Japanese Laid-open Patent Application No. 5-92259). 
     In the flow soldering method, if the back surface of the printed circuit board is dipped in the molten solder for an unnecessarily long time, an increased amount of copper may be eluted from the inner walls of the through-holes and through-hole pads provided at the through-holes into the molten solder, damaging the inner walls of the through-holes and the through-hole pads. 
     First, a soldering method according to a first comparative example will be described. 
     In the soldering method according to the first comparative example, soldering of pins of a component into through-holes of a printed circuit board is performed by simply dipping a region of the printed circuit board having the through-holes into which the pins have been inserted in molten solder contained in a solder bath. 
       FIG. 6  illustrates the soldering method according to the first comparative example. 
     In the soldering method according to the first comparative example shown in  FIG. 6 , soldering is performed by simply dipping a region of a printed circuit board  510  having through-holes  511  into which pins  521  of a component  520  have been inserted in molten solder  611  contained in a solder bath  610 . The molten solder  611  is melted by a heater  612  provided in the solder bath  610 . 
     In such a soldering method, it is preferable that the region of the printed circuit board having the through-holes be dipped in the molten solder for a shorter time. If the region of the printed circuit board having the through-holes is dipped in the molten solder for an unnecessarily long time, copper may be eluted from the inner walls of the through-holes and through-hole pads provided at the through-holes into the molten solder, damaging the inner walls and the through-hole pads. 
     In such a soldering method, it is preferable that the region of the printed circuit board having the through-holes be dipped in the molten solder for a shorter time. If the region of the printed circuit board having the through-holes is dipped in the molten solder for an unnecessarily long time, copper may be eluted from the inner walls of the through-holes and through-hole pads provided at the through-holes into the molten solder, damaging the inner walls and the through-hole pads. 
     Next, a soldering method according to a second comparative example obtained by adding to the soldering method according to the first comparative example a technique that minimizes the difficulty in short-time soldering due to the slow rise of temperature inside the through-holes will be described. 
     The soldering method according to the second comparative example differs from the soldering method according to the first comparative example in that soldering is performed by dipping the region of a printed circuit board having through-holes in a solder bath while feeding hot air toward a component on the printed circuit board from above. The second comparative example will now be described, focusing on the difference from the first comparative example. 
       FIG. 7  illustrates the soldering method according to the second comparative example. 
     In  FIG. 7 , elements the same as those shown in  FIG. 6  are denoted by reference numerals the same as those in  FIG. 6 , and redundant descriptions thereof are omitted. 
     In the second comparative example, when soldering is performed, a cover  620  is placed over the top surface of the mounting region of the printed circuit board  510  having the component  520 . The cover  620  is provided with a gas supply port  621  at a position thereof above the component  520 . When soldering is performed, hot air  622  is supplied through the supply port  621 . The hot air  622  is fed from above toward the component  520  and is exhausted from a plurality of exhaust ports  623  provided in sidewalls of the cover  620 . 
     In the soldering method according to the second comparative example, since the surfaces of the component  520  and the printed circuit board  510  are heated with the hot air  622 , the temperature inside the through-holes  511  rises quicker than in the soldering method according to the first comparative example. Nevertheless, in the soldering method according to the second comparative example, it is difficult to introduce a sufficient amount of hot air  622  into the through-holes  511  that are behind the body of the component  520  and are substantially stopped by the pins  521 . Therefore, in most cases in the second comparative example, the original temperature of the hot air  622  is set to a high level so that the temperature inside the through-holes  511  may be raised to a sufficiently high level even with such an insufficient amount of hot air  622 . If, however, the temperature of the hot air  622  is too high, the component  520  directly receiving the hot air  622  may be heated beyond the temperature that the component  520  may resist. Consequently, the component  520  may be damaged. 
     Because of factors such as increases in the density of components  520  mounted on the printed circuit board  510  and in the number of wires provided in the printed circuit board  510  in recent years, the number of layers included in the printed circuit board  510  has been increasing. Accordingly, the thickness of the printed circuit board  510  has also been increasing. To cause the molten solder  611  to sufficiently spread into the through-holes  511  in the soldering method according to the first comparative example, the temperature inside the through-holes  511  may be raised to above the melting point of the solder. In the soldering method according to the first comparative example, the interiors of the through-holes  511  are heated with the heat from the molten solder  611 . Therefore, if the printed circuit board  510  is thick as described above, it takes a long time for the heat to be sufficiently transferred over the entirety of the through-holes  511 , making it difficult to complete soldering in a short time. Moreover, depending on the type of the component  520 , the lengths of the pins  521  may be too short to protrude from the through-holes  511 . In such a case, it takes more time for the heat to be transferred to the interiors of the through-holes  511 , making it more difficult to complete soldering in a short time. If the printed circuit board  510  is thicker than 2 mm, the through-holes  511  may extend through four or more layers including solid pattern layers such as a power-source layer and a grounding layer. Since such solid pattern layers draw much heat from the through-holes  511 , the difficulty in completing soldering in a short time is increased. 
     In addition, due to environmental considerations in recent years, the use of lead-free solder, such as Sn—Ag—Cu (SAC) solder, has been promoted. The lead-free solder has a higher melting point than the solder containing lead, such as Sn—Pb eutectic solder. The SAC solder has a melting point 30° C. or more higher than that of the Sn—Pb eutectic solder. In a case where the lead-free solder is used, before the temperature inside the through-holes  511  is raised to a level required for sufficient spreading of the molten solder  611 , the component  520  may be adversely affected in terms of heat resistance and the like. This also increases the difficulty in completing soldering in a short time. 
     SUMMARY 
     According to an aspect of an embodiment, a soldering method of an electronic component includes inserting pins of the electronic component into through-holes provided in a printed circuit board; preheating interiors of the through-holes with an inert fluid; and bonding the pins to the through-holes by exposing molten solder to the preheated through-holes. 
     The object and advantages of the invention will be realized and attained by means of at least the features, elements, and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a flowchart showing an example of an embodiment of a soldering method; 
         FIG. 2  schematically illustrates a preheating step shown in  FIG. 1 ; 
         FIG. 3  schematically illustrates a bath-replacing step shown in  FIG. 1 ; 
         FIG. 4  schematically illustrates how molten solder spreads into through-holes; 
         FIG. 5  schematically illustrates a discharging step and a board-removing step shown in  FIG. 1 ; 
         FIG. 6  illustrates a soldering method according to a first comparative example from the Related Art; and 
         FIG. 7  illustrates a soldering method according to a second comparative example from the Related Art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Specific embodiments of the soldering method according to the basic embodiment described above will now be described with reference to the accompanying drawings. 
     In the figures, dimensions and/or proportions may be exaggerated for clarity of illustration. It will also be understood that when an element is referred to as being “connected to” another element, it may be directly connected or indirectly connected, i.e., intervening elements may also be present. Further, it will be understood that when an element is referred to as being “between” two elements, it may be the only element layer between the two elements, or one or more intervening elements may also be present. 
       FIG. 1  is a flowchart showing an example of an embodiment of the soldering method. 
     In a soldering method shown by the flowchart in  FIG. 1 , soldering is performed after the interiors of through-holes into which pins of a component have been inserted is preheated to a specific preheating temperature. The soldering method includes a preheating step (Step S 101 ), a bath-replacing step (Step S 102 ), a discharging step (Step S 103 ), and a board-removing step (Step S 104 ). 
     The individual steps of the soldering method shown by the flowchart in  FIG. 1  will now be described in detail. 
     In the preheating step (Step S 101 ) of  FIG. 1 , 
     after pins of a pin-type electronic component are inserted into through-holes of a printed circuit board, the preheating step is performed as described below. 
       FIG. 2  schematically illustrates the preheating step shown in  FIG. 1 . 
     In  FIG. 2 , through-holes  111  of a printed circuit board  110  and pins  121  of an electronic component  120  are schematically shown in an enlarged scale. 
     In the preheating step, a first medium bath  210  is placed over the electronic component  120  mounted on the top surface of the printed circuit board  110 , and a second medium bath  220  is placed over the back surface of the printed circuit board  110 . The two medium baths  210  and  220  are filled with a heating medium  230  described below. 
     The heating medium  230  is an electrically insulating, stable (inert) fluid (solvent), and is resistant to corrosion and/or deterioration with respect to the solder and the printed circuit board  110 . 
     The heating medium  230  may be an inert fluid having good electrical-insulation having a dielectric permittivity in the range of 2.0 to 3.0 and heat-transfer characteristics in the range of 0.05 to 0.1 W/m° C., and is not limited to a specific type. The heating medium  230  used in the present embodiment is, for example, a fluorine-containing oil such as perfluoropolyether (PEPE) oil, chlorotrifluoroethylene (CTFE) oil, or fluorocarbon-based fluid. If PEPE oil is used, Galden® (a registered trademark of Solvay Solexis) HT200 or HT270 having a boiling point of 270° C. may be used. In use of Galden® as a heating medium, the dielectric permittivity may be 2.1, and the heat-transfer characteristic may be 0.07 W/m° C. 
     The first medium bath  210  is provided at a position thereof above the electronic component  120  with a supply port  211  through which the heating medium  230  is supplied. The supply port  211  is not limited to be in the first medium bath  210  and may be provided in the second medium bath  220 . The first medium bath  210  is also provided in a sidewall thereof with a discharge port  212  through which the heating medium  230  is discharged in the discharging step described below. The discharge port  212  is closed during the preheating step. The discharge port  212  is not limited to be in the sidewall of the first medium bath  210 . 
     The present embodiment employs a fluid circulator  240  described below. The two medium baths  210  and  220  communicate with each other through a circulation pipe  250  guiding the heating medium  230  from the second medium bath  220  to the first medium bath  210 . The fluid circulator  240  circulates the heating medium  230  along a circulation path starting from the second medium bath  220  through the circulation pipe  250  to the first medium bath  210  and returning from the first medium bath  210  through the through-holes  111  to the second medium bath  220 . The fluid circulator  240  includes a heater. In the present embodiment, the heating medium  230  that is being circulated is heated by the heater while flowing through the circulation pipe  250 . 
     In the present embodiment, the preheating step is performed as follows, for example. 
     First, one end of the circulation pipe  250  is attached to the second medium bath  220 . 
     Subsequently, the second medium bath  220  is filled with the heating medium  230  that is preheated to a preheating temperature of 150 to 180° C., which is lower than the above-mentioned boiling point, in such a manner that the heating medium  230  does not flow out of the other end of the circulation pipe  250 . The preheating temperature to which the heating medium  230  is preheated corresponds to a preheating temperature suitable for soldering to be performed later. The preheating temperature to which the heating medium  230  is preheated is sufficiently lower than the maximum temperature that the electronic component  120  can resist. 
     Subsequently, the printed circuit board  110  is placed over the second medium bath  220  in such a manner that the back surface of a mounting region of the printed circuit board  110  having the electronic component  120  touches the heating medium  230  in the second medium bath  220 , and then the first medium bath  210 , which is empty, is placed over the mounting region having the electronic component  120 . 
     Subsequently, the other end of the circulation pipe  250  is attached to the first medium bath  210 . Thus, the first medium bath  210  and the second medium bath  220  are made to communicate with each other through the circulation pipe  250  guiding the heating medium  230 . 
     Subsequently, the heating medium  230  that is preheated to the above preheating temperature is supplied into the first medium bath  210  through the supply port  211 . After the first medium bath  210  is filled with the heating medium  230 , the supply port  211  is closed. 
     Since the heating medium  230  supplied into the first medium bath  210  is fluid, the heating medium  230  spreads into the through-holes  111  that are behind the body of the electronic component  120  and are substantially stopped by the pins  121  under gravity and by capillary action. Consequently, the interiors of the through-holes  111  are exposed to the heating medium  230  having a temperature of 150 to 180° C. 
     Subsequently, the heating medium  230  that has spread as described above is circulated by the fluid circulator  240  along the circulation path. During the circulation, the temperature of the heating medium  230  that is being circulated is maintained at the preheating temperature with the heat from the heater. Because of the circulation, the interiors of the through-holes  111  continue to be exposed to the heating medium  230  having the preheating temperature of 150 to 180° C. 
     The interiors of the through-holes  111  are exposed to the heating medium  230  for a preheating time described below. 
     In the present embodiment, how long the through-holes  111  are to be exposed to the heating medium  230  before the temperature inside reaches the preheating temperature is determined in advance using samples. In the present embodiment, the mounting region having the electronic component  120  is exposed to the heating medium  230  for the preheating time determined in advance, whereby the temperature inside the through-holes  111  is raised to the preheating temperature. 
     Next, the bath-replacing step (Step S 102 ) shown in  FIG. 1  will be described. 
       FIG. 3  schematically illustrates the bath-replacing step shown in  FIG. 1 . 
     In the present embodiment, the bath-replacing step is performed as follows, for example. 
     First, after the fluid circulator  240  is stopped and the circulation of the heating medium  230  is thus stopped, the circulation pipe  250  connecting the fluid circulator  240  and the two medium baths  210  and  220  is detached. Subsequently, the second medium bath  220  is retracted away from the mounting region of the printed circuit board  110  having the electronic component  120 . In this state, because of factors including that of the supply port  211  of the first medium bath  210  being closed and that of the through-holes  111  being substantially stopped by the pins  121 , incidences of the heating medium  230  leaking from the through-holes  111  are reduced or prevented. 
     Subsequently, a solder bath  260  filled with molten solder  261  is placed below the mounting region. Located within the solder bath  260 , is a heater  262  that heats solder to its melting point or higher, whereby the solder is maintained in a melted state. In the present embodiment, the molten solder  261  is lead-free, for example. 
     By the bath-replacing step, a region of the printed circuit board  110  having the through-holes  111  and the pins  121  that are to be soldered to each other are dipped in the molten solder  261  that is maintained in a melted state in the solder bath  260  as described above. In this state, the molten solder  261  spreads into the through-holes  111 . 
       FIG. 4  schematically illustrates how the molten solder  261  spreads into the through-holes  111 . 
     The through-holes  111  and the pins  121  that have been preheated as described above are further heated by the molten solder  261  to a temperature higher than or substantially equal to the melting point of the solder. The molten solder  261  in a wet state moves upward into the through-holes  111  that have been heated to a high temperature. Thus, the molten solder  261  spreads in the through-holes  111 . Here, since the through-holes  111  and the pins  121  are preheated, the heating with the molten solder  261  performed thereon is completed in a short time. 
     In the present embodiment, as shown in  FIG. 4 , the molten solder  261  spreads in a state where the top surface of the mounting region of the printed circuit board  110  having the electronic component  120  is covered by the first medium bath  210  filled with the heating medium  230 . Therefore, the temperature of the through-holes  111  and the pins  121  that have been preheated is substantially prevented from being lowered. Since the mounting region is kept covered by the first medium bath  210  as described above, the molten solder  261  in the wet state moves upward in the through-holes  111  that are filled with the heating medium  230 . Here, the temperature of the molten solder  261  is higher than the boiling point of the heating medium  230 . Therefore, when the molten solder  261  spreading upward touches the heating medium  230  in the through-holes  111 , the heating medium  230  is volatized. Thus, the heating medium  230  is removed from the through-holes  111 . 
     In the present embodiment, the solder bath  260  also has inside an agitator  263  that agitates the molten solder  261  upward from the bottom of the solder bath  260  as indicated by arrows A. The agitator  263  produces upward flows of the molten solder  261  near the center of the solder bath  260 . With the upward flows, the molten solder  261  is forcibly fed toward the through-holes  111 . Consequently, the molten solder  261  easily spreads into the through-holes  111   
     Next, the discharging step (Step S 103 ) and the board-removing step (Step S 104 ) shown in  FIG. 1  will be described. 
       FIG. 5  schematically illustrates the discharging step and the board-removing step shown in  FIG. 1 . 
     In the present embodiment, the two steps are performed as follows, for example. 
     In the discharging step (Step S 103 ), the temperature of the heating medium  230  is first lowered from the preheating temperature to about the normal temperature. Subsequently, the discharge port  212  is opened and the heating medium  230  is discharged to the outside of the first medium bath  210 . In the present embodiment, the lowering of the temperature of the heating medium  230  in the discharging step (Step S 103 ) provides safety and contributes to the hardening of the molten solder  261  in the through-holes  111 . 
     Furthermore, in the discharging step (Step S 103 ), the upward flows of the molten solder  261  are continuously produced in the solder bath  260 . Since the temperature of the heating medium  230  is lowered, incidences of unnecessary solder adhesion to the back surface around the through-holes  111  and to the pins  121  are reduced or prevented. 
     After the discharging step (Step S 103 ), the board-removing step (Step S 104 ) is performed. In the board-removing step (Step S 104 ), the first medium bath  210  is removed from the printed circuit board  110 , and the printed circuit board  110  that have undergone soldering is removed from the solder bath  260 . With the removal of the two baths  210  and  260 , the hardening of the molten solder  261  in the through-holes  111  is further promoted. 
     As described above with reference to  FIG. 5 , in the present embodiment, the temperature of the heating medium  230  is first lowered, the heating medium  230  is then discharged from the first medium bath  210 , and the printed circuit board  110  that has undergone soldering is subsequently removed. That is, in the present embodiment, the provision of safety and the hardening of the molten solder  261  in the through-holes  111  are performed substantially simultaneous, and the printed circuit board  110  that has undergone soldering is subsequently removed. 
     Through the two steps described with reference to  FIG. 5 , the soldering according to the present embodiment ends. If there are any other components to be mounted by soldering of pins thereof into other through-holes, the process shown by the flowchart in  FIG. 1  is repeated for the number of mounting regions for those components. 
     In contrast to the soldering methods according to the comparative examples of Related Art  FIGS. 6-7 , in the embodiment described with reference to  FIGS. 1 to 5 , soldering is performed after the interiors of the through-holes  111  are preheated with the heating medium  230  in a fluid state. Therefore, when soldering is performed, the time required for raising the temperature inside the through-holes  111  to a level higher than the melting point of the solder is shortened by the preheating time with the use of the heating medium  230 . Furthermore, the preheating is performed by dipping the printed circuit board  110  in the heating medium  230  in a fluid state. Hence, the heating medium  230  easily spreads into a space below the electronic component  120  and into the through-holes  111  under gravity and by capillary action. Therefore, the heat of the heating medium  230  is easily transferred to the interiors of the through-holes  111 . Consequently, in the soldering method according to the above embodiment, the pins  121  of the electronic component  120  are soldered to the through-holes  111  in a short time and in a good manner. 
     In the soldering method according to the embodiment, the heat of the heating medium  230  is easily transferred to the interiors of the through-holes  111  as described above. Therefore, even if the temperature of the heating medium  230  is set to a little low level, the interiors of the through-holes  111  is preheated to a sufficient level. Consequently, the temperature of the heating medium  230  can be set to be lower than the temperature that the electronic component  120  can resist, whereby prevention of damage on the electronic component  120  and performance of short-time soldering are realized simultaneously. 
     In the soldering method according to the above embodiment, as shown in  FIG. 2 , the supply port  211  through which the heating medium  230  is supplied is provided in the first medium bath  210 , covering the top surface of the mounting region of the printed circuit board  110  having the electronic component  120 , at a position above the electronic component  120 . Therefore, in the preheating step, the heating medium  230  is supplied through the supply port  211  in a direction in which the electronic component  120  is pressed against the printed circuit board  110 . Consequently, the printed circuit board  110  is dipped in the heating medium  230  while the electronic component  120  is substantially prevented from, for example, unintentionally floating during the preheating step. 
     That is, in the present embodiment, a first modification is preferable in which the first fluid bath has a supply port through which the fluid is supplied at a position thereof higher than the mounting region. 
     In the soldering method according to the embodiment, preheating is performed while the heating medium  230  is circulated between the two medium baths  210  and  220  by the fluid circulator  240  along the circulation path including the through-holes  111 . Thus, the heating medium  230  is actively introduced into the through-holes  111 . Consequently, the interiors of the through-holes  111  are preheated efficiently. 
     That is, in the present embodiment, an example of a second modification in which the preheating step may include a communicating sub-step and a circulating sub-step, the communicating sub-step being a step of making the first fluid bath and the second fluid bath communicate with each other through a fluid passage guiding the fluid, the circulating sub-step being a step of circulating the fluid along a circulation path connecting the first fluid bath, the second fluid bath, and the through-holes. 
     As described with reference to  FIG. 2 , the circulation of the heating medium  230  is performed while the heating medium  230  that is being circulated is heated by the heater provided in the fluid circulator  240 . Thus, the temperature of the heating medium  230  in the two medium baths  210  and  220  is substantially prevented from being lowered. Consequently, the interiors of the through-holes  111  are assuredly preheated. 
     That is, in the second modification in which the preheating step includes the communicating sub-step and the circulating sub-step, a further modification is preferable in which the circulating sub-step is a step of heating the fluid being circulated along the circulation path to the preheating temperature at any position on the circulation path. 
     In the soldering method according to the embodiment, as shown in  FIG. 4 , upward flows of the molten solder  261  are produced near the center of the solder bath  260  while soldering is performed. With such upward flows, the molten solder  261  is forcibly fed toward the through-holes  111 , as described above. Consequently, the molten solder  261  easily spreads into the through-holes  111 . 
     That is, in the present embodiment, an example of a third modification in which upward flows of the molten solder toward the printed circuit board may be produced in the solder bath. 
     In the embodiment, the provision of safety and the hardening of the molten solder  261  in the through-holes  111  are performed simultaneously, and the printed circuit board  110  that has undergone soldering is subsequently removed, as described above. The embodiment provides an efficient method because the provision of safety and the hardening of the molten solder  261  are performed simultaneously when the printed circuit board  110  that has undergone soldering is removed. 
     That is, in the present embodiment, an example of a fourth modification in which the hardening step may be performed such that the temperature of the fluid in the first fluid bath is first lowered in a state where the back surface of the mounting region is dipped in the molten solder in the solder bath, the fluid is then discharged from the first fluid bath, and the solder bath and the first fluid bath are subsequently removed from the printed circuit board. 
     The specific embodiment of the soldering method according to the basic embodiment concerns a case where pins of a single component are soldered into through-holes. The soldering method according to the basic embodiment, however, is not limited to such a case and may be applied to another case where pins of a plurality of components are soldered into through-holes. In that case, corresponding adjustments to the method would including using medium baths and a solder bath having wide openings that cover all of mounting regions for those components. 
     All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.