Patent Publication Number: US-2005139389-A1

Title: Method of mounting electronic component on substrate without generation of voids in bonding material

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a method of mounting an electronic component, such as a ball grid array (BGA) semiconductor package. In particular, the invention relates to a method of mounting an electronic component on a substrate by melting a conductive bonding material on a terminal pad on the substrate.  
      2. Description of the Prior Art  
      Some methods of mounting an electronic component on a substrate utilize a solder paste. The solder paste is previously printed on terminal pads disposed on the surface of the substrate. Terminal conductors, such as solder balls, of the electronic component are placed on the corresponding terminal pads. After the electronic component has been set on the substrate in this manner, the substrate is passed through a reflow oven. Solder particles dispersed within the solder paste are caused to melt in the reflow oven. Subsequent cooling treatment serves to solidify the solder on the terminal pads on the substrate. The terminal conductors of the electronic component are thus bonded to the corresponding terminal pads on the substrate.  
      The solder paste usually includes an organic solvent. When the solder particles melt in the solder paste, the organic solvent is forced to vaporize in the melting solder. In this case, the vaporized or gaseous organic solvent is locked within the melting solder in the aforementioned conventional method, since the surface of the solder paste is covered with the terminal conductors of the electronic component. Voids remain within the solidified solder. The voids may cause a failure in electric contact between the electronic component and the substrate. The voids are supposed to reduce the strength of bonding between the electronic component and the substrate.  
     SUMMARY OF THE INVENTION  
      It is accordingly an object of the present invention to provide a method of mounting an electronic component on a substrate, which is capable of enhancing the reliability of bonding between the electronic component and the substrate.  
      According to a first aspect of the present invention, there is provided a method of mounting an electronic component on a substrate, comprising: placing the electronic component on the substrate with a solid support interposed between the electronic component and the substrate so as to space a terminal conductor of the electronic component from a corresponding terminal pad on the substrate; melting a conductive bonding material on the terminal pad; and thereafter melting the solid support so as to move down the electronic component toward the substrate, thereby contacting the terminal conductor with the conductive bonding material melting on the corresponding terminal pad.  
      The method enables a reliable prevention of contact between the conductive bonding material on the terminal pad and the terminal conductor when the conductive bonding material gets melted. The melted conductive bonding material gets exposed to the peripheral atmosphere over a larger area. Even if a bubble is generated within the melted conductive bonding material, the bubble is allowed to easily get out of the melted conductive bonding material. Removal of the gas is promoted in the melted conductive bonding material. Removal of the gas in this manner leads to improvement in the strength of bonding between the substrate and the electronic component. A solder paste may be supplied to the surface of the terminal pad in this method. The solder paste may comprise a flux including an organic solvent, for example, and solder particles as the conductive bonding materials dispersed in the flux. When the solder particles get melted, the organic solvent in the solder paste may get vaporized in the melted solder.  
      In this case, a solid support is employed to lift the electronic component above the substrate. The solid support of this type can simply be interposed between the substrate and the electronic component. The terminal conductor of the electronic component can easily be prevented from contacting the terminal pad on the substrate. Moreover, the terminal conductor is caused to fall or drop toward the terminal pad in response to melting of the solid support. Contact can be established between the terminal conductor and the terminal pad with a simple structure. The solid support may be made of a thermoplastic resin material having the melting point higher than that of the conductive bonding material. The solid support of this type provides an electronic circuit board comprising: a substrate; an electronic component mounted on a surface of the substrate and having a terminal conductor received on a terminal pad on the substrate; and a thermoplastic resin material interposed between the substrate and the electronic component.  
      According to a second aspect of the present invention, there is provided a method of mounting an electronic component on a substrate, comprising: melting a conductive bonding material on a terminal pad on the substrate under a high temperature atmosphere; and contacting a terminal conductor of the electronic component on the conductive bonding material on the terminal pad continuously under the high temperature atmosphere.  
      The conductive bonding material is allowed to get exposed to the peripheral atmosphere over a larger area. Even if a bubble is generated within the melted conductive bonding material, the bubble easily gets out of the melted conductive bonding material. The gas is reliably removed from the melted conductive bonding material. The terminal conductor of the electronic component can be placed on the terminal pad on the substrate after removal of the gas out of the melted conductive bonding material. Removal of the gas in this manner leads to improvement in the strength of bonding between the substrate and the electronic component. A solder paste may be supplied to the surface of the terminal pad in this method. The solder paste may comprise a flux including an organic solvent, for example, and solder particles as the conductive bonding materials dispersed in the flux. When the solder particles get melted, the organic solvent in the solder paste may get vaporized in the melted solder.  
      Moreover, the conductive bonding material is simply exposed under the high temperature atmosphere when the conductive bonding material is to be melted. Groups of the electronic components can simultaneously be mounted on one or more substrates. The productivity can be improved as compared with the case where the electronic components are separately or individually mounted on the substrate, or the mounting of the electronic components is separately or individually effected on the individual substrates.  
      The method may further comprise: placing the electronic component on the substrate, prior to melting of the conductive bonding material, with a solid support interposed between the electronic component and the substrate so as to space the terminal conductor from the terminal pad; and melting the solid support so as to move down the electronic component toward the substrate, thereby contacting the terminal conductor with the conductive bonding material on the corresponding terminal pad, when the terminal conductor is contacted on the terminal pad. In this case, the solid support may be made of a thermoplastic resin material having the melting point higher than that of the conductive bonding material.  
      According to a third aspect of the present invention, there is provided a method of mounting an electronic component on a substrate, comprising: melting a solder paste on a terminal pad on the substrate; and placing a terminal conductor of the electronic component on the solder paste on the terminal pad, said solder paste being kept melted.  
      The solder particles in the solder paste get melted in the melted solder paste. The solvent in the solder paste simultaneously gets vaporized. Since the terminal conductor of the electronic component is prevented from contacting the terminal pad on the substrate, the melted solder is allowed to get exposed to the peripheral atmosphere over a larger area. The gaseous or vaporized organic solvent easily gets out of the melted solder. The terminal conductor of the electronic component can be placed on the terminal pad on the substrate after removal of the gas out of the melted conductive bonding material. Generation of voids can be prevented in the solidified solder. Removal of the gas or the voids in this manner leads to improvement in the strength of bonding between the substrate and the electronic component.  
      The method may further comprise in the aforementioned manner: setting the electronic component on the substrate, prior to melting of the solder paste, with a solid support interposed between the electronic component and the substrate so as to space the terminal conductor from the terminal pad; and melting the solid support so as to move down the electronic component toward the substrate, thereby contacting the terminal conductor with the solder paste on the corresponding terminal pad, said solder paste being kept melted. In this case, the solid support may be made of a thermoplastic resin material having a melting point higher than that of the solder paste. The method likewise serves to provide an electronic circuit board comprising: a substrate; an electronic component mounted on a surface of the substrate and having a terminal conductor received on a terminal pad on the substrate; and a thermoplastic resin material interposed between the substrate and the electronic component.  
      In any event, the solid support may have an adherent property on its surface. The adherent property of the solid support may be utilized to stick the electronic component on the substrate. The electronic component can thus stably be positioned on the substrate even during transportation or displacement. The adherent property may be established based on the natural property of the material for the solid support or based on an adhesive provided on the surface of the solid support.  
      The solid support also may have a high heat conductivity. The solid support of this type leads to a promoted radiation of heat from the electronic component mounted on the substrate. The solid support may contain alumina (Al 2 O 3 ) particles dispersed in a resin material.  
      In any of the aforementioned methods, there may be provided an electronic component unit comprising: a terminal conductor of a predetermined height standing on a surface opposed to a substrate; and a solid support standing on the surface, said solid support having a height larger than the predetermined height.  
      When the electronic component unit is set on the substrate, the solid support is naturally interposed between the electronic component and the substrate. The operator is not required to additionally insert the solid support before or after setting the electronic component on the substrate. The productivity can thus be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiment in con junction with the accompanying drawings, wherein:  
       FIG. 1  is a perspective view illustrating an electronic circuit board;  
       FIG. 2  is an enlarged vertical sectional view of the electronic circuit board for illustrating the structure of a ball grid array (BGA) semiconductor package in detail;  
       FIG. 3  is a plan view of the BGA semiconductor package for illustrating the array of solder balls;  
       FIG. 4  is an enlarged partial section view of a printed wiring board for illustrating the process of placing the BGA semiconductor package on the printed wiring board;  
       FIG. 5  is an enlarged partial section view of the printed wiring board for illustrating the process of generating melted solder on terminal pads on the printed wiring board;  
       FIG. 6  is an enlarged partial section view of the printed wiring board for illustrating the process of melting a solid support between the printed wiring board and the BGA semiconductor package;  
       FIG. 7  is a perspective view schematically illustrating the structure of an electronic component unit;  
       FIG. 8  is an enlarged partial section view of a printed wiring board for illustrating the process of placing a full matrix BGA semiconductor package on the printed wiring board; and  
       FIG. 9  is an enlarged partial section view of a printed wiring board for illustrating the process of placing a quad flat package (QFP) on the printed wiring board. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       FIG. 1  schematically illustrates the structure of a electronic circuit board  11 . The electronic circuit board  11  includes a printed wiring board or substrate  12  of a resin material, for example, and one or more electronic components  13 , such as ball grid array (BGA) semiconductor packages, mounted on the surface of the printed wiring board  12 . An electrically conductive wiring pattern, not shown, spreads over the surface or/and interior of the printed wiring board  12  so as to establish electric connections between the BGA semiconductor packages  13 , for example.  
      As shown in  FIG. 2 , the BGA semiconductor package  13  includes a semiconductor chip  15  mounted on the upper surface of a small-sized printed wiring board or substrate  14  of a ceramic material, for example. A plurality of connection terminals  16  are attached to the lower surface of the small-sized printed wiring board  14 . The connection terminals  16  are received on corresponding terminal pads  17  on the printed wiring board  12 . In this manner, electric connection can be established between terminal pads, not shown, of the small-sized printed wiring board  14  and the terminal pads  17  on the printed wiring board  12 .  
      A so-called underfill  18  is interposed between the small-sized printed wiring board  14  of the BGA semiconductor package  13  and the printed wiring board  12 . The underfill  18  may be made of a thermoplastic resin material, for example. The connection terminals  16  are embedded within the underfill  18 . The underfill  18  serves to reinforce the strength of bonding between the BGA semiconductor package  13  and the printed wiring board  12 . In addition, the underfill  18  reliably prevents the individual connection terminals  16  from getting exposed to the atmosphere, for example, so that corrosion or deterioration can be prevented in the connection terminals  16 .  
      Next, a detailed description will be made on a method of producing the electronic circuit board  11 . The electronic components, namely, the BGA semiconductor packages  13  are first prepared. As shown in  FIG. 3 , a plurality of bumps or solder balls  19  of a predetermined array are attached on the back surface of the small-sized printed wiring board  14 . It should be noted that a naked space  21  without any solder balls  19  is defined at a central area of the back surface of the small-sized printed wiring board  14 . The back surface of the small-sized printed wiring board  14  is allowed to fully get exposed at the naked space  21  in the BGA semiconductor package  13 .  
      As shown in  FIG. 4 , the BGA semiconductor package  13  is placed on the upper surface of the printed wiring board  12 . The terminal pads  17  are formed on the upper surface of the printed wiring board  12 . The terminal pads  17  are arranged in the corresponding array for the individual BGA semiconductor packages  13 . A solder paste  22  is previously printed over the individual terminal pads  17 . The solder paste  22  consists of a flux including an organic solvent, and a conductive bonding material, namely, solder particles dispersed in the flux.  
      A solid support  23  is interposed between the printed wiring board  12  and the BGA semiconductor package  13 . The BGA semiconductor package  13  is received on the top surface of the solid support  23  at the naked space  21  of the small-sized printed wiring board  14 . The solid support  23  serves to lift the BGA semiconductor package  13  above the surface of the printed wiring board  12 . Specifically, the solid support  23  serves to space the solder balls  19  of the BGA semiconductor package  13  from the surface of the terminal pads  17 . The individual solder balls  19  are prevented from contacting the solder paste  22  on the corresponding terminal pads  17 .  
      Here, the melting point of the solid support  23  is set higher than that of the solder paste  22  or the solder particles. If the solder particles are made of an eutectic solder having the melting point of 183 degrees Celsius, for example, the solid support  23  may be made of a material having the melting point of approximately 200 degrees Celsius. The solid support  23  may be made of a thermoplastic resin material such as a polyurethane-based resin, a polyester-based resin, an acrylic-based resin, rosin, a polyamide-based resin, or the like. Alternatively, the solid support  23  may be made of a material other than the aforementioned resin material.  
      The solid support  23  preferably has an adherent property on its surface, for example. The adherent property of the solid support  23  may be utilized to stick the BGA semiconductor package  13  on the printed wiring board  12 . The BGA semiconductor package  13  can thus stably be positioned on the printed wiring board  12  even during transportation or displacement. The adherent property may be established based on the natural property of the material for the solid support  23  or based on an adhesive provided on the surface of the solid support  23 .  
      Thereafter, a relowing process is effected. The printed wiring board  12  is inserted into a reflow oven. A high temperature atmosphere of 220 degrees Celsius is maintained in the reflow oven. As shown in  FIG. 5 , the solder particles in the solder paste  22  are first allowed to melt on the surface of the terminal pads  17 . The organic solvent simultaneously gets vaporized. The melted solder  24  remains on the surface of the terminal pads  17 . The solder balls  19  of the BGA semiconductor package  13  also gets melted. The individual solder ball  19  maintains its spherical shape based on the surface tension.  
      In this situation, the temperature of the solid support  23  does not yet reach the melting point. Specifically, the solid support  23  keeps the solder balls  19  of the BGA semiconductor package  13  spaced from the melted solder  24  on the terminal pads  17  by a predetermined space w. The surface of the melted solder  24  gets exposed to the peripheral atmosphere. The vaporized or gaseous organic solvent in the solder paste  22  is allowed to easily get out of the melted solder  24 . Removal of the gas can be promoted in the melted solder  24 .  
      When the printed wiring board  12  is further maintained in the reflow oven, the temperature of the solid support  23  reaches the melting temperature. The solid support  23  gets melted. Specifically, the solid support  23  gets fluidized. The solid support  23  is thus removed below the BGA semiconductor package  13 . The BGA semiconductor package  13  is allowed to drop toward the printed wiring board  12  based on its own weight. As shown in  FIG. 6 , the solder balls  19  are thus received on the corresponding terminal pads  17 . The solder ball  19  gets unified with the melted solder  24  on the terminal pad  17 . In this manner, the connection terminals  16  of the melted state can be established. When the gravity acting from the small-sized printed wiring board  14  to the connection terminals  16  is balanced with the overall surface tension of the connection terminals  16 , the small-sized printed wiring board  14  stops falling or dropping.  
      The fluid of the solid support  23  spreads over the surface to the printed wiring board  12  between the connection terminals  16 . The fluid fills the space defined between the printed wiring board  12  and the small-sized printed wiring board  14 . The fluid remain within the space between the printed wiring board  12  and the small-sized printed wiring board  14  based on its own surface tension.  
      Thereafter, the printed wiring board  12  is taken out of the reflow oven. The printed wiring board  12  is subjected to a cooling treatment in the normal atmosphere or at a room temperature. The fluid gets solidified so as to provide the underfill  18  between the printed wiring board  12  and the small-sized printed wiring board  14 . The connection terminals  16  subsequently get solidified. Electric connection can thus be obtained between the printed wiring board  12  and the small-sized printed wiring board  14 . The mounting of the BGA semiconductor package  13  has been completed.  
      In particular, in the case where the BGA semiconductor packages  13  are to be mounted on the single printed wiring board  12 , the printed wiring board  12  needs be exposed only once to the high temperature atmosphere in the aforementioned method. The BGA semiconductor packages  13  are simultaneously mounted on the printed wiring board  12 . The productivity can be improved as compared with the case where the BGA semiconductor packages  13  are separately or individually mounted on the printed wiring board  12 . The production time is also shortened. Moreover, a group of the printed wiring boards  12  may simultaneously be subjected to the high temperature atmosphere in the aforementioned method. The productivity can be improved as compared with the case where the mounting of the BGA semiconductor packages  13  is separately or individually effected on the individual printed wiring boards  12 . The production time is still further shortened.  
      As shown in  FIG. 7 , when the aforementioned method is to be realized, an electronic component unit  26  may be prepared. The electronic component unit  26  includes the BGA semiconductor package  13  and the solid support  23 . The solid support  23  is previously attached to the small-sized printed wiring board  14  at the naked space  21 . In this case, the solid support  23  should have the height H larger than the height h of the terminal conductors or solder balls  19  on the back surface of the small-sized printed wiring board  14 . When the electronic component unit  26  of this type is placed on the printed wiring board  12 , the solid support  23  can be interposed between the printed wiring board  12  and the small-sized printed wiring board  14  so as to space the solder balls  19  of the BGA semiconductor package  13  from the corresponding terminal pads  17  on the printed wiring board  12 , as is apparent from reference to  FIG. 4 , for example.  
      The volume of the solid support  23  may be determined as follows:  
      [Expression 1]
 
 As·H≧=Ac·g−Vt·n   (1) 
 
 Here, the constant As represents the area of the bottom of the solid support  23 . The constant Ac represents the area of the back surface of the small-sized printed wiring board  14 . The constant g represents the space between the front surface of the printed wiring board  12  and the back surface of the small-sized printed wiring board  14  in the final electronic circuit board  11 . The constant Vt represents the volume of the single solder ball  19 . The constant n represents the number of the solder balls  19  interposed between the printed wiring board  12  and the small-sized printed wiring board  14 . The space g may be set at approximately 60% of the height h of the solder balls  19 , for example. The setting in this manner enables establishment of the underfill  18  based on the solid support  23  without any supplement of materials. It should be noted that the volume of the solid support  23  may be set smaller than the volume of the underfill  18 . 
 
      In addition, the height H of the solid support  23  should be set as follows:  
      [Expression 2]
 
 H&gt;h+t+p   (2) 
 
 Here, the constant t represents the height of the solder paste on the surface of the terminal pad  17 . The constant p represents the thickness of the terminal pad  17  superposed on the surface of the printed wiring board  12 . The height t of the solder paste  22  may be determined as follows: 
 
 [Expression 3] 
             t   =             3   ⁢   Vm     π     +           (       3   ⁢   Vm     π     )     2     +     r   6           3     +           3   ⁢   Vm     π     -           (       3   ⁢   Vm     π     )     2     +     r   6           3               (   3   )             
 
 Here, the constant Vm represents the volume of the melted solder  24  existing on the terminal pad  17 . The constant r represents the radius of the terminal pad  17 . The volume Vm of the melted solder  24  may be determined as follows: 
 
 [Expression 4]
 
 Vm=Vp·m   (4) 
 
 Here, the constant Vp represents the volume of the solder paste  22  when printed. The constant m represents the content of the solder particles in the solder paste in the form of ratio in volume. The content m may be set at approximately 0.5, for example. The volume Vp of the solder paste  22  may be determined as follows: 
 
 [Expression 5] 
             Vp   =           D   2     ⁢   π     4     ·   tm   ·   k             (   5   )             
 
 Here, the constant tm represents the thickness of a mask employed to print the solder paste  22 . The constant D represents the diameter of the opening defined in the mask at a position corresponding to the terminal pad  17 . The constant k represents the rate or percentage of filling of the solder paste  22  within the opening. The rate k of filling may be set approximately in a range between 60% and 80%, for example. In general, the height t of the solder paste  22  gets smaller than the thickness m of the mask. It should be noted that the height t of the solder paste  22  may be replaced with the thickness tm of the mask in the aforementioned [Expression 2]. 
 
      The aforementioned method may be employed to mount a BGA semiconductor package  13   a  of a full matrix type. The solder balls  19  of the full matrix are arranged on the back surface of the small-sized printed wiring board in the full matrix BGA semiconductor package  13   a . In order to realize the mounting of the full matrix BGA semiconductor package  13   a  on the printed wiring board  12 , the solid support  23  may be provided in the form of a stud, as shown in  FIG. 8 , for example. The solid support  23  may be located on the individual corners of the small-sized printed wiring board  14 .  
      The solder particles in the solder paste  22  is allowed to melt on the terminal pads  17  prior to melting of the solid supports  23 . The organic solvent simultaneously gets vaporized. The melted solder is allowed to get exposed to the peripheral atmosphere over a larger area. Accordingly, the vaporized or gaseous organic solvent in the solder paste  22  is allowed to easily get out of the melted solder. Removal of the gas can be promoted in the melted solder. After solidification of the connection terminals  16 , a thermosetting resin material underfill may be injected within the space between the printed wiring board  12  and the small-sized printed wiring board  14 .  
      Otherwise, the aforementioned method may be employed to mount a quad flat package (QFP)  13   b . As shown in  FIG. 9 , the terminal conductors or terminal leads  28  extend outward from the side surfaces of a package body  27  in the QFP  13   b . The individual terminal leads  28  are received on the corresponding terminal pads  17  on the printed wiring board  12 . As is apparent from  FIG. 9 , the solid support  23  may be employed to mount the QFP  13   b  on the printed wiring board  12  in the aforementioned manner. The solid support  23  may be located at the central area of the package body  27 .  
      The solder particles in the solder paste  22  is allowed to melt on the terminal pads  17  prior to melting of the solid support  23 . The organic solvent simultaneously gets vaporized. The melted solder is allowed to get exposed to the peripheral atmosphere over a larger area. Accordingly, the vaporized or gaseous organic solvent in the solder paste  22  is allowed to easily get out of the melted solder. Removal of the gas can be promoted in the melted solder.  
      The solid support  23  may be made of a thermoplastic resin material having a high heat conductivity. The thermoplastic resin of this type may comprise alumina (Al 2 O 3 ) particles dispersed in a resin material. Employment of the thermoplastic resin material having a high heat conductivity leads to a promoted radiation of heat from the BGA semiconductor package  13 , the full matrix BGA semiconductor package  13   a , and the QFP  13   b . As long as the solid support  23  after melting and solidification is prevented from contacting the connection terminals  16  and the terminal leads  28 , the solid support  23  may have a property of electrically conductivity.  
      It should be noted that any types of terminal conductors, other than the aforementioned solder balls  19 , may be employed in the BGA semiconductor package  13  and the full matrix BGA semiconductor package  13   a.