Patent Publication Number: US-2005139643-A1

Title: Method and system for applying solder

Description:
TECHNICAL FIELD OF THE INVENTION  
      This invention relates in general to the production of electronic devices and more particularly to applying solder to circuit board assemblies.  
     BACKGROUND OF THE INVENTION  
      Soldering is a common technique for mounting integrated circuits and other components to circuit boards. Solder paste is applied to specified points on the circuit board assembly where connections with a mounted component are desired. Then the component is mounted on the solder paste, and the solder paste is heated to liquefy it. After the liquefied solder paste cools, the solder paste forms a binding, conductive connection between the mounted component and the circuit board. If too much solder paste is deposited for connections to a particular component, the connections may run together creating shorts in the resulting circuit board assembly. However, if too little solder paste is deposited, the connections provide poor conductivity between the circuit board and mounted component and are more likely to break. As the size of electronic components continues to decrease, the acceptable margin of error in sizing the connections has fallen sharply. This leads to unnecessarily complex soldering techniques in the construction of circuit board assemblies, particularly where components of multiple different sizes are used.  
      Often solder paste is applied by an applicator dispensing a flow of solder paste while moving across the circuit board. Because differently sized connections require different amounts of solder paste and because conventional solder applicators often can not change dispensing rates within a single pass over the circuit board, applying multiple sizes of connections will often require the solder applicator to make multiple passes over the circuit board. In such a case, solder paste will be applied for the connections of a first size. Then, to avoid any disturbance of these solder deposits during application of a second set of connections, the components corresponding to the first set will be mounted, and the solder paste will be liquefied and cooled. After the first set of components are fully mounted and soldered, solder paste will be applied for connections of a second size, and the process will be repeated. This significantly adds to the production time and cost of circuit board assemblies.  
     SUMMARY OF THE INVENTION  
      From the foregoing, it may be appreciated by those skilled in the art that a need has arisen for a more efficient method of providing multiple sizes of soldering connections on a single circuit board assembly. In accordance with the present invention, a method and system for applying solder paste to circuit boards are provided that substantially eliminate or reduce at least some of the disadvantages and problems associated with the previous techniques and systems.  
      In accordance with a particular embodiment of the present invention, a method for applying solder paste to a circuit board is disclosed which includes covering the circuit board with a first stencil. The first stencil includes a first stencil hole. The method also includes applying a solder paste to a first area of the circuit board through the first stencil and covering the circuit board with a second stencil. The second stencil includes a second stencil hole and a void enclosure. The void enclosure covers the first area of the circuit board. The method additionally includes applying a solder paste to a second area of the circuit board through the second stencil.  
      The present invention provides various technical advantages over conventional circuit board assembly techniques. In particular embodiments, a technical advantage may be more accurate sizing of circuit board assembly connections. Specifically, multiple sizes of connections may be formed with accuracy. Another technical advantage may be the elimination of unnecessary steps in the production process. All solder deposits can be heated at the same time.  
      Embodiments of the present invention may have some, all, or none of the following technical advantages. Other technical advantages of the present invention will be readily apparent to one skilled in the art from the figures, description, and claims included herein.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of the present invention and for further features and advantages thereof, reference is now made to the following description, taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which:  
       FIG. 1  illustrates a cross sectional view of a first step in a method for applying connections to a circuit board;  
       FIG. 2  illustrates a cross sectional view of a circuit board and a first stencil during the first step in the method for applying connections to a circuit board;  
       FIG. 3  illustrates a cross sectional view of a circuit board and a second stencil during the second step in the method;  
       FIG. 4  illustrates a cross sectional view of a circuit board and electronic components during the third step in the method;  
       FIG. 5  illustrates a cross sectional view of a circuit board and a second stencil when a force is applied to the second stencil during the second step;  
       FIG. 6  illustrates a cross sectional view of a circuit board and a second stencil with void supports during the second step;  
       FIG. 7  illustrates various types and configurations of void supports;  
       FIG. 8  illustrates a cross sectional view of a circuit board and a needle array applicator during the second step; and  
       FIG. 9  is a flowchart detailing the method for applying solder paste according to a particular embodiment.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  illustrates a system  10  for applying solder paste  60  to a circuit board  20  to form soldered connections between circuit board  20  and electronic components to be mounted on circuit board  20 . System  10  includes circuit board  20 , a first stencil  30 , a second stencil  40 , and a solder applicator  50 . Once solder paste  60  has been applied to circuit board  20  to form solder deposits, electronic components may be mounted. Solder deposits then may be heated to soften the solder deposits and cooled to harden the solder deposits around pins of the electronic components, forming a conductive bond between circuit board  20  and the electronic components. Additionally, by utilizing first stencil  30  and second stencil  40 , system  10  may produce solder deposits of multiple sizes without heating the solder deposits multiple times.  
      First stencil  30  and second stencil  40  provide for selective application of solder paste  60  to circuit board  20 . First stencil  30  includes one or more first stencil holes  35 . First stencil  30  may include any number of first stencil holes  35  according to the amount and arrangement of soldering desired. When placed over circuit board  20 , first stencil  30  blocks solder paste  60 , preventing solder paste  60  from flowing down to circuit board  20  except through first stencil holes  35 . Thus, by selective placement of first stencil holes  35 , first stencil  30  can be designed to generate a solder connection pattern appropriate for circuit board  20  based on the electronic components to be mounted on circuit board  20 , the type of connection desired, or any other suitable factors. First stencil  30  may be composed of metal, plastic, of any other material suitable for repelling solder paste  60 .  
      Second stencil  40  includes second stencil holes  45 , which operate similarly to first stencil holes  35 , and void enclosure  55 . When placed over circuit board  20 , second stencil  40  blocks solder paste  60 , preventing solder paste  60  from flowing down to circuit board  20  except through second stencil holes  45 . Thus, by selective placement of second stencil holes  45 , second stencil  40  can be designed to generate a connection pattern appropriate for circuit board  20  based on the electronic components to be mounted on circuit board  20 , the type of connection desired, or any other suitable factor. Second stencil  40  may include any number of second stencil holes  45  according to the amount and arrangement of soldering desired.  
      Additionally, void enclosure  55  prevents a portion of second stencil  40  from touching first deposits  37  when second stencil  40  is placed over circuit board  20 . This may prevent second stencil  40  from disturbing solder paste  60  that has been already deposited on circuit board  20 . In general, void enclosure  55  may be designed and configured in any appropriate manner to prevent second stencil  40  from disturbing first deposits  37  made using first stencil  30 . In a particular embodiment, void enclosure  55  has an enclosure height  57  that is greater than a first deposit height  38  of first deposits  37 . Thus, with proper design and placement of first stencil holes  35 , second stencil holes  45 , and void enclosure  55 , system  10  can produce a predetermined pattern of differently-sized solder deposits on circuit board  20  by sequential use of first stencil  30  and second stencil  40 .  
      Solder applicator  50  applies solder paste  60  to circuit board  20 . Solder applicator  50  may be any appropriate component capable of applying solder paste  60  as described below. In a particular embodiment, solder applicator  50  applies a continuous flow of solder paste  60  as solder applicator  50  moves across circuit board  20 . The solder paste apply process can utilize a conventional squeezy printing method.  
      Circuit board  20  serves as a mounting platform for electronic components. Circuit board  20  connects to electronic components through solder connections and then provides electrical connections between the various electronic components through wires or other conductive elements. Circuit board  20  may be composed of plastic, glass, or any other suitable material.  
      Operator  70  represents a human operator or mechanical or electrical components capable of controlling the operation of solder applicator  50  and the placement of first stencil  30  and second stencil  40 . For example, solder applicator  50 , first stencil  30 , and second stencil  40  may be integrated into a single machine into which circuit board  20  may be loaded. In this case, operator  70  may represent components, such as a stencil alignment module, of the machine that are capable of initiating a flow of solder paste  60  from solder applicator  50  and moving first stencil  30  and second stencil  40  into position over circuit board  20 .  
       FIGS. 2-4  illustrate operation of system  10  according to a particular embodiment. Although  FIGS. 2-4  illustrate operation of a system  10  that uses first stencil  30  and second stencil  40  and includes two stages of solder application, system  10  may be configured with appropriate modifications to incorporate any number of stencils and to include any number of solder application stages.  
       FIG. 2  illustrates a first step in the operation of a particular embodiment of system  10 . Operator  70  places first stencil  30  over circuit board  20 . Operator  70  then engages solder applicator  50 . Solder applicator applies solder paste  60  over first stencil  30 . First stencil  30  prevents solder paste  60  from flowing down to circuit board  20  except through first stencil holes  35 . The flow of solder paste  60  through first stencil holes  35  creates solder deposits, first deposits  37 , in a first area  110  of circuit board  20 . In the example system  10 , first deposits  37  share the width of the corresponding first stencil holes  35 , i.e. a first width  115 . In other systems  10 , various factors, such as a tendency of solder paste  60  to cling to the sides of first stencil holes  35 , may cause the width of first deposits  37  to vary from first width  115  of first stencil holes  35 . Operator  70  then removes first stencil  30  from over circuit board  20 .  
      First stencil  30  and first stencil holes  35  may be designed to produce certain characteristics in the resulting solder deposits generated by first stencil  30 . As one example, first width  115  of first stencil holes  35  may be selected to produce first deposits  37  of a particular width. Furthermore, in a particular embodiment first stencil  30  may include first stencil holes  35  of multiple different widths producing first deposits  37  of multiple different widths. As another example, first stencil  30  may include a first height  117  that produces first deposits  37  of a particular height.  
       FIG. 3  illustrates a second step in the operation of system  10 . After removing first stencil  30 , operator  70  places second stencil  40  over circuit board  20 . Second stencil  40  is designed so that void enclosure  55  covers first area  110  and second stencil  40  does not touch first deposits  37 . As a result, second stencil  40  does not disturb first deposits  37  when operator  70  moves second stencil  40  into position over circuit board  20 .  
      Operator  70  then engages solder applicator  50 . Second stencil  40  prevents solder paste  60  from flowing down to circuit board  20  except through second stencil holes  45 . The flow of solder paste  60  through second stencil holes  45  creates second deposits  47  in a second area  210  of circuit board  20 . In the example system  10 , second deposits  47  share the width of the corresponding second stencil holes  45 , i.e. a second width  215 . As noted above with respect to  FIG. 2 , in other systems  10 , various factors may cause the width of second deposit  47  to vary from second width  215  of second stencil holes  45 . Operator  70  then removes second stencil  40  from circuit board  20 .  
      Like first stencil  30  and first stencil holes  35 , second stencil  40  and second stencil holes  45  may be designed to produce certain characteristics in the resulting solder deposits generated by second stencil  40 . As one example, second width  215  may be selected to produce second deposits  47  of a particular width. Moreover, in a particular embodiment, second stencil  40  may include second stencil holes  45  of multiple different widths producing second deposits  47  of multiple different widths. As another example, second stencil  40  may possess a second height  217  that produces second deposits  47  of a particular height.  
      Differences in first stencil  30  and second stencil  40  and/or first stencil holes  35  and second stencil holes  45 , may result in differences in first deposits  37  and second deposits  47 . For example, differences in first width  115  and second width  215  and/or first height  117  and second height  217  may cause differences in first deposits  37  and second deposits  47 . Thus, first stencil  30  and second stencil  40  may be designed to facilitate mounting of components with varying solder requirements.  
       FIG. 4  illustrates a third step of the operation of system  10 . After removing second stencil  40 , operator  70  mounts electrical components on circuit board  20 . As shown, the electrical components include a narrow pitch component  410  and a wide pitch component  420 . In the example system  10 , narrow pitch components  410  are appropriate for mounting on connections having a narrower pitch, such as those formed by first deposits  37 . Similarly, wide pitch components  420  are appropriate for mounting on connections having a wider pitch, such as those formed by second deposits  47 .  
      Operator  70  aligns pins or other connectors of narrow pitch components  410  and wide pitch components  420  with first deposits  37  and second deposits  47 , respectively. Then, operator  70  heats first deposits  37  and second deposits  47  to soften first deposits  37  and second deposits  47 . Operator  70  then cools, or allows to cool, first deposits  37  and second deposits  47 . Operator  70  may execute these steps using conventional soldering techniques. For example, operator  70  may use one-time reflow techniques to complete the soldering process. After cooling, the solder of first deposits  37  form first conductive bonds  39  between narrow pitch components  410  and circuit board  20 . Similarly, the solder of second deposits  47  form second conductive bonds  49  between wide pitch components  420  and circuit board  20 . Thus, void enclosure  55  and first stencil  30  and second stencil  40  may operate to allow components with differing pitches, sizes, or pin heights, or, in general, components that require solder deposits of varying characteristics to be mounted on circuit board  20  without requiring repeated reflow of solder. The print sequence preferably starts from most finest pitch (corresponding to the amount of solder paste desired) and the order is preferably determined by the terminal pitch or solder land pitch. Although  FIGS. 2-4  show a preferred ordering of steps that includes narrow pitch components  410  being mounted prior to wide pitch components  420 , components may be mounted in any appropriate order using this method.  
       FIG. 5  illustrates the second step of operation of system  10  under different circumstances from  FIG. 3 . While in position over circuit board  20 , second stencil  40  may undergo a pressure  510 . Pressure  510  may cause void enclosure  55  of a particular embodiment of second stencil  40  to buckle. The buckling of void enclosure  55  may bring second stencil  40  into contact with first area  110  of circuit board  20 , thereby disturbing first deposits  37 .  
       FIG. 6  illustrates operation of a particular embodiment of second stencil  40  under similar circumstances to those illustrated by  FIG. 5 . Second stencil  40  includes void supports  610  to help support second stencil  40  and prevent buckling of void enclosure  55  as a result of pressure  510 . Void supports  610  may be designed in conjunction with first stencil  30  to ensure void supports  610  are placed within void enclosure  55  in a configuration that prevents any void supports  610  from touching first deposits  37  while second stencil  40  is positioned over circuit board  20 . Void supports  610  may be formed of the same material as second stencil  40  or any suitable alternative. Void supports  610  may be shaped and placed in any manner appropriate to prevent void enclosure  55  or void supports  610  from coming into contact with first deposits  37  as a result of pressure  510 .  
       FIG. 7  illustrates two example configurations of void supports  610 , pocket supports  610   a  and pillar supports  610   b.  Pocket supports  610   a  are formed by removing material from second stencil  40 . Pocket supports  610   a  are sized and spaced so that pockets formed by pocket supports  610  align with first deposits  37  when second stencil  40  is placed in position over circuit board  20 . In an embodiment of second stencil  40  that includes pocket supports  610   a,  void enclosure  55  may simply represent the collective area within the pockets formed by pocket supports  610   a.    
      Pillar supports  610   b  are formed by adding material to void enclosure  55 . Pillars supports  610   b  are sized and spaced so that pillar supports  610   b  rest between first deposits  37  when second stencil  40  is placed in position over circuit board  20 . In an embodiment of second stencil  40  that includes pillar supports  610   b,  void enclosure  55  may represent the collective area surrounding the various pillar supports  610   b.    
       FIG. 8  illustrates operation of a particular embodiment of system  10  that utilizes a needle array applicator  810  to apply solder paste  60  in addition or instead of solder applicator  50 . Needle array applicator  810  is a device capable of dispensing solder paste  60  from any of a plurality of needle heads  820 , each needle head  820  applying solder paste to a single point directly beneath that particular needle head  820 . By locating needle array applicator  810  over a particular section of circuit board  20  and then selectively dispensing solder paste  60  from one or more needle heads  820 , operator  70  can generate a solder paste  60  pattern similar to those described above, as being produced with first stencil  30  and second stencil  40 . Furthermore, as with first stencil holes  35  and second stencil holes  45 , needle heads  820  can be designed to produce certain characteristics, such as a desired width, in the solder deposits that needle array applicator  810  produces.  
      Needle array applicator  810  may eliminate the need for one or both of first stencil  30  and second stencil  40 . System  10  may apply first deposits  37  as described above and then use needle array applicator  810  to apply second deposits  47  as shown in  FIG. 8 . Needle array applicator  810  can then apply second deposits  47  without disturbing first deposits  37 , providing some or all of the benefits of second stencil  40  as described above. Although in  FIG. 8 , needle array applicator  810  is shown applying second deposits  47  that are appropriate for narrow pitch components  410 , needle array applicator  810  may be configured to apply solder deposits of any type appropriate for circuit board  20 .  
       FIG. 9  is a flow chart detailing a method of applying solder paste  60  according to a particular embodiment. At step  910 , operator  70  places first stencil  30  in position over circuit board  20 . Operator  70  applies solder paste  60  to first area  110  of circuit board  20  through first stencil holes  35  creating first deposits  37  at step  920 . At step  930 , operator  70  removes first stencil  30  from position over circuit board  20 .  
      Operator  70  places second stencil  40  over circuit board  20  at step  940 . At step  950 , operator  70  applies solder paste  60  to second area  210  of circuit board  20  through second stencil holes  45  creating second deposits  47 . Operator  70  mounts narrow pitch components  410  and wide pitch components  420  to circuit board  20  at step  960 . At step  970 , operator  70  heats circuit board  20  liquefying both first deposits  37  and second deposits  47 . Operator  70  cools circuit board  20 , at step  980 , to produce first conductive bonds  39  and second conductive bonds  49  between circuit board  20  and narrow pitch components  410  and wide pitch components  420 , respectively.  
      Although the present invention has been described with several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformations, and modifications as fall within the scope of the appended claims.