Patent Publication Number: US-2011073252-A1

Title: Conductive paste and method of manufacturing printed circuit board using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2009-0093189 filed with the Korea Intellectual Property Office on Sep. 30, 2009, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a conductive paste and a method of manufacturing a printed circuit board using the same, and more particularly, to a conductive paste including two kinds of low melting point metals having different melting points, and a method of manufacturing a printed circuit board using the same. 
     2. Description of the Related Art 
     Currently, a greater number of passive elements and high density and multilayer packages are mounted on a printed circuit board (PCB) due to thickness reduction and functionalization of electronic products, and this trend will continue in the future. 
     Basically, the PCB plays a role of connecting various electronic components to a PCB substrate according to a circuit design of electric wiring or supporting the components. And a drilling and plating method is most widely used as a method of transmitting interlayer signals and power of a substrate. However, in case that the number of holes used for connecting the interlayer signals is increased and a thin PCB is manufactured, a method of transmitting an interlayer signal by forming a bump has a great advantage in comparison with the drilling and plating method. 
     A mixture of conductive powder such as Ag and an epoxy resin as a binder is mainly used as a material of a conductive paste generally used for forming the bump. 
     Among conventional PCB manufacturing methods, a method of manufacturing a high density PCB by forming a bump with a conductive paste made of the above material on copper foil, stacking an insulating layer passing through the bump, stacking copper foil on the insulating layer, and performing heating and pressing is used. This is called a B2it method, and by using this method, it is possible to improve manufacturing efficiency by simply forming a via for interlayer connection. 
     The interlayer connection using this conductive paste is formed by mechanical connection between the copper foil and Ag powder particles. 
     That is, in case of applying force to the copper foil with high temperature and pressure, the Ag powder are inserted along a surface profile of the copper foil. There generates a difference in movement of the Ag powder according to the degree of pressure, and contact force is affected by the amount of the binder. 
     However, this conventional PCB manufacturing method using the conductive paste has problems on a resistance change rate at the time of thermal shock due to the contact force between the copper foil and the bump, collapse of the bump, and an interlayer insulation distance. 
     That is, there is a possibility that resistance is increased and electrical connection is not formed at the time of thermal shock since the contact force between the conductive paste and the copper foil is not secured. And in case of increasing the amount of the powder to improve resistivity at the time of penetration of the insulating layer, the collapse of the bump is generated, and in case of implementing a multilayer substrate by collective stacking, it is not possible to secure the interlayer insulation distance. In order to reduce the resistivity, the amount of the powder should be increased, and in that case, on the contrary, since the bump is not easily pressed at the time of pressing, the interlayer insulation distance is increased and a bump crack may be generated due to excessive pressure. 
     SUMMARY OF THE INVENTION 
     The present invention has been proposed in order to solve the above-described problems, and it is, therefore, an object of the present invention to provide a conductive paste and a method of manufacturing a printed circuit board using the same capable of improving contact force and electrical conductivity of a bump and securing an insulation distance by forming the bump with a conductive paste including two kinds of low melting point metals having different melting points. 
     In accordance with an aspect of the present invention to achieve the object, there is provided a conductive paste including: a conductive powder particle including a polymer powder and a first low melting point metal and a second low melting point metal which are sequentially provided on a surface of the polymer powder and have different melting points; and a binder mixed in the conductive powder particle. 
     Here, the polymer powder may be made of a thermoplastic polymer. 
     Further, the polymer powder may have a melting point of 180° C. to 200° C. 
     Further, the polymer powder may have a shape of one of a sphere, an oval, a plate, ∞, and a polyhedron. 
     Further, the first low melting point metal may have a melting point of 160° C. to 220° C. 
     Further, the first low melting point metal may be made of one of SnZn 9 , SnZn 8 Bi 3 , and SnIn 8-0 Ag 3-5 Bi 0-5 . 
     Further, the second low melting point metal may have a melting point lower than that of the first low melting point metal. 
     Further, the second low melting point metal may have a melting point of 85° C. to 150° C. 
     Further, the second low melting point metal may be made of one of SnBi 57 Ag 1 , SnBi 58 , and SnIn 52 . 
     Further, the binder may include an epoxy resin or a phenol resin. 
     In accordance with another aspect of the present invention to achieve the object, there is provided a method of manufacturing a printed circuit board including the steps of: printing a conductive paste on a first substrate, wherein the conductive paste includes a conductive powder particle having a polymer powder and a first low melting point metal and a second low melting point metal, which are sequentially provided on a surface of the polymer powder and have different melting points, and a binder mixed in the conductive powder particle; drying the conductive paste; forming an insulating layer on the first substrate through the conductive paste; and stacking and pressing a second substrate on the insulating layer. 
     Here, the conductive powder particle may be formed by sequentially coating the first low melting point metal and the second low melting point metal on the surface of the polymer powder. 
     Further, the polymer powder may be made of a thermoplastic polymer. 
     Further, the polymer powder may have a melting point of 180° C. to 200° C. 
     Further, the polymer powder may have a shape of one of a sphere, an oval, a plate, ∞, and a polyhedron. 
     Further, the first low melting point metal may have a melting point of 160° C. to 220° C. 
     Further, the first low melting point metal may be made of one of SnZn 9 , SnZn 8 Bi 3 , and SnIn 8-0 Ag 3-5 Bi 0-5 . 
     Further, the second low melting point metal may have a melting point lower than that of the first low melting point metal. 
     Further, the second low melting point metal may have a melting point of 85° C. to 150° C. 
     Further, the second low melting point metal may be made of one of SnBi 57 Ag 1 , SnBi 58 , and SnIn 52 . 
     Further, the first low melting point metal may have a thickness larger than that of the second low melting point metal. 
     Further, in the step of printing the conductive paste, the conductive paste may be printed in a conical shape. 
     Further, in the step of drying the conductive paste, the drying process may be performed at a melting temperature of the second low melting point metal to melt the second low melting point metal. 
     Further, in the step of stacking and pressing the second substrate, the press process may be performed at a melting temperature of the first low melting point metal to melt the first low melting point metal. 
     Further, the first substrate and the second substrate may be made of copper foil. 
     Further, in the step of forming the insulating layer, the insulating layer may have a thickness smaller than that of the conductive paste. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a view schematically showing a conductive paste in accordance with an embodiment of the present invention; 
         FIG. 2  is a view showing a reaction when the conductive paste in accordance with an embodiment of the present invention is dried on a substrate; 
         FIG. 3  is a view showing a reaction when the conductive paste in accordance with an embodiment of the present invention is pressed on the substrate; and 
         FIGS. 4 to 6  are cross-sectional views showing a method of manufacturing a printed circuit board using a conductive paste in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The functions, effects, and technical constitution of a conductive paste and a method of manufacturing a printed circuit board using the same in accordance with the present invention to achieve the object will be clearly understood from the following detailed description with reference to the accompanying drawings in which the preferable embodiments of the present invention are shown. 
     Hereinafter, a conductive paste and a method of manufacturing a printed circuit board using the same in accordance with an embodiment of the present invention will be described in detail with reference to  FIGS. 1 to 6 . 
     First,  FIG. 1  is a view schematically showing a conductive paste in accordance with an embodiment of the present invention. 
     As shown in  FIG. 1 , a conductive paste  60  in accordance with an embodiment of the present invention includes a conductive powder particle  40  having a polymer powder  10  and a first low melting point metal  20  and a second low melting point metal  50 , which are sequentially provided on a surface of the polymer powder  10  and have different melting points, and a binder  50  mixed in the conductive powder particle  40 . 
     The conductive powder particle  40  is formed by sequentially coating the first low melting point metal  20  and the second low melting point metal  30  on the surface of the polymer powder  10 . 
     A high temperature spray method and so on may be used as a coating method of the first and second low melting point metals  20  and  30 . 
     The conductive paste  60  in accordance with an embodiment of the present invention is manufactured by uniformly mixing the binder  50  in the conductive powder particle  40  including the polymer powder  10  and the first and second low melting point metals  20  and  30  in a predetermined ratio. 
     Here, the polymer powder  10  is made of a thermoplastic polymer having a melting point of 180° C. to 200° C. 
     Further, the polymer powder  10  has a size of 0.1 μm to 5 μm and a spherical shape. The shape of the polymer powder  10  is not limited to the spherical shape, and the polymer powder  10  may have various shapes such as oval, plate, ∞, and polyhedron. 
     The first low melting point metal  20  coated on the surface of the polymer powder  10  is a metal having a melting point of 160° C. to 220° C. 
     At this time, the first low melting point metal  20  may be, for example, SnZn 9 , SnZn 8 Bi 3 , or SnIn 8-0 Ag 3-5 Bi 0-5 . 
     Among the above materials of the first low melting point metal  20 , SnZn 9  has a melting point of 199° C., SnZn 8 Bi 3  has a melting point of 191° C. to 198° C., and SnIn 8-0 Ag 3-5 Bi 0-5  has a melting point of 197° C. to 208° C. 
     The first low melting point metal  20  has a thickness of 0.1 μm to 10 μm. 
     The second low melting point metal  30  coated on a surface of the first low melting point metal  20  is a metal having a melting point lower than that of the first low melting point metal  20 , that is, a metal having a melting point of 85° C. to 150° C. 
     The second low melting point metal  30  may be, for example, SnBi 57 Ag 1  SnBi 58 , or SnIn 52 . 
     Among the above materials of the second low melting point metal  30 , SnBi 57 Ag 1  has a melting point of 137° C. to 139° C., SnBi 58  has a melting point of 138° C., and SnIn 52  has a melting point of 118° C. 
     The second low melting point metal  30  has a thickness of 0.1 μm to 10 μm. 
     The reason for using the first and second low melting point metals  20  and  30  having the different melting points as shown above is that drying and press process temperatures of a bump formed of the conductive paste  60  are different from each other and to optimally respond to an external temperature applied to the conductive paste  60  in the drying and press processes. 
     That is, the drying process of the bump is performed at a temperature of 85° C. to 150° C., which is the melting point of the second low melting point metal  30 , and the press process is performed at a temperature of 160° C. to 220° C., which is the melting point of the first low melting point metal  20 . 
     Further, it is possible to reduce material cost of the conductive powder particle  40  by using the polymer powder  10  inside the conductive powder particle  40 , and it is possible to secure an interlayer insulation distance required for a substrate by applying the polymer powder  10  having the melting point of approximately 200° C. 
     The binder  50  mixed with the conductive powder particle  40  in which the first and second low melting point metals  20  and  30  having the different melting points are sequentially coated on the surface of the polymer powder includes an epoxy resin or a phenol resin. 
     A conventional conductive paste including Ag powder depends on a binder and secures electrical conductivity and contact force through mechanical junction, but the conductive paste  60  in accordance with the above-described embodiment of the present invention is a paste capable of forming an intermetallic compound as well as the binder  50 . 
     Especially, the conductive paste  60  in accordance with an embodiment of the present invention includes two kinds of the first and second low melting point metals  20  and  30  having the different melting points, so that the second and first low melting point metals  30  and  20  and the polymer powder  10  sequentially react in the drying and press processes of the bump, thereby improving contact force and electrical conductivity of the bump and securing an insulation distance. 
     Next,  FIG. 2  is a view showing a reaction when the conductive paste in accordance with an embodiment of the present invention is dried on a substrate, and  FIG. 3  is a view showing a reaction when the conductive paste in accordance with an embodiment of the present invention is pressed on the substrate. 
     First, referring to  FIG. 2 , after the conductive paste  60  is printed on a first substrate  100  which is made of copper foil and so on, the conductive paste  60  is dried at a temperature of 85° C. to 150° C. 
     As the drying process is performed under the above temperature condition, the second low melting point metal  30  disposed on the outermost layer of the conductive powder particle  40  is melted to be combined with the adjacent second low melting point metal  30 , and the binder  50  is partially cured. 
     And the first substrate  100 , which is made of copper foil and so on, and the second low melting point metal  30  are combined with each other, so that it is possible to secure contact force between the first substrate  100  and the conductive powder particle  40  and improve electrical conductivity. 
     As the drying process is finished, an intermetallic compound having strong bonding force is formed through mutual combination between the adjacent second low melting point metals  30  as well as hardness generated by the partially cured binder  50 , thereby obtaining the bump having high hardness. 
     Next, referring to  FIG. 3 , the conductive paste  60  passing through the drying process is pressed at a temperature of 160° C. to 200° C. 
     As the press process is performed, the first low melting point metal  20  is melted to be combined with the adjacent first low melting point metal  20 , and the polymer powder  10  inside the first low melting point metal  20  is also melted to secure an optimal interlayer distance. At this time, the polymer powder  10  is an insulating material, and thus it is possible to secure an insulation distance even though the bump is lowered to a desired height. Therefore, it is possible to collectively stack the substrates. 
     The first and second low melting point metals  20  and  30  before reaction are completely melted at a temperature near the melting point, but they show completely different phases after reaction and are not easily melted even at a high temperature. 
       FIGS. 4 to 6  are cross-sectional views showing a method of manufacturing a printed circuit board using a conductive paste in accordance with an embodiment of the present invention. 
     A method of manufacturing a printed circuit board using a conductive paste in accordance with an embodiment of the present invention, first, as shown in  FIG. 4 , forms a conical bump by printing a conductive plate  60  on a first substrate  100 . 
     The first substrate  100  is made of copper foil and so on. 
     The conductive paste  60  includes a conductive powder particle  40  having a polymer powder  10  and a first low melting point metal  20  and a second low melting point metal  30 , which are sequentially provided on a surface of the polymer powder  10  and have different melting points, and a binder  50  mixed in the conductive powder particle  40 . 
     The conductive powder particle  40  is formed by sequentially coating the first low melting point metal  20  and the second low melting point metal  30  on the surface of the polymer powder  10 . 
     A high temperature spray method and so on may be used as a coating method of the first and second low melting point metals  20  and  30 . 
     The conductive paste  60  in accordance with an embodiment of the present invention is manufactured by uniformly mixing the binder  50  in the conductive powder particle  40  including the polymer powder  10  and the first and second low melting point metals  20  and  30 . The binder  50  includes an epoxy resin or a phenol resin. 
     The polymer powder  10  is made of a thermoplastic polymer having a melting point of 180° C. to 200° C. 
     Further, the polymer powder  10  has a size of 0.1 μm to 5 μm and a spherical shape. The shape of the polymer powder  10  is not limited to the spherical shape, and the polymer powder  10  may have various shapes such as oval, plate, ∞, and polyhedron. 
     The first low melting point metal  20  coated on the surface of the polymer powder  10  is a metal having a melting point of 160° C. to 220° C. 
     At this time, the first low melting point metal  20  may be, for example, SnZn 9 , SnZn 8 Bi 3 , or SnIn 8-0 Ag 3-5 Bi 0-5 . 
     Among the above materials of the first low melting point metal  20 , SnZn 9  has a melting point of 199° C., SnZn 8 Bi 3  has a melting point of 191° C. to 198° C., and SnIn 8-0 Ag 3-8 Bi 0-5  has a melting point of 197° C. to 208° C. 
     The first low melting point metal  20  has a thickness of 0.1 μm to 10 μm. 
     The second low melting point metal  30  coated on a surface of the first low melting point metal  20  is a metal having a melting point lower than that of the first low melting point metal  20 , that is, a metal having a melting point of 85° C. to 150° C. 
     The second low melting point metal  30  may be, for example, SnBi 57 Ag 1 , SnBi 58 , or SnIn 52 . 
     Among the above materials of the second low melting point metal  30 , SnBi 57 Ag 1  has a melting point of 137° C. to 139° C., SnBi 58  has a melting point of 138° C., and SnIn 52  has a melting point of 118° C. 
     The second low melting point metal  30  has a thickness of 0.1 μm to 10 μm. 
     Then, the conductive paste  60  is dried to give hardness to the bump. The drying process of the conductive paste  60  is performed at a temperature of 85° C. to 150° C., which is the melting point of the second low melting point metal  30 . 
     As the drying process is performed under the above temperature condition, the second low melting point metal  30  disposed on the outermost layer of the conductive powder particle  40  is melted to be combined with the adjacent second low melting point metal  30 , and the binder  50  is partially cured. 
     And the first substrate  100 , which is made of copper foil and so on, and the second low melting point metal  30  are combined with each other, so that it is possible to secure contact force between the first substrate  100  and the conductive powder particle  40  and improve electrical conductivity. 
     As the drying process is finished, an intermetallic compound having strong bonding force is formed through mutual combination between the adjacent second low melting point metals  30  as well as hardness generated by the partially cured binder  50 , thereby obtaining the bump having high hardness. 
     Next, as shown in  FIG. 5 , an insulating layer  200  is formed on the first substrate  100  through the conductive paste  60 . The insulating layer  200  is a means for implementing interlayer electrical insulation, for example, prepreg. The insulating layer  200  has a thickness smaller than that of the conductive paste  60 . 
     At this time, since the hardness of the bump is improved by the intermetallic compound formed by the combination between the second low melting point metals  30  as well as the cured binder  50 , there is an advantage that the insulating layer  200  easily passes through the conductive paste  60 . 
     Meanwhile, if the melting points of the first and second low melting point metals  20  and  30  are the same, the insulating layer  200  may not easily pass through the conductive paste  60  since the height of the bump is dramatically reduced due to the metal melted in the drying process. Therefore, the melting points of the first and second low melting point metals  20  and  30  should be different, and it is advantageous that a coating thickness of the first low melting point metal  20  is larger than that of the second low melting point metal  30 . 
     After that, as shown in  FIG. 6 , a second substrate  300  is stacked and pressed on the insulating layer  200 . The second substrate  300  is made of copper foil and so on, like the first substrate  100 . 
     The press process is performed at a temperature of 160° C. to 200° C., which is the melting point of the first low melting point metal  20 . Accordingly, the first low melting point metal  20  is melted to be combined with the first low melting point metal  20 . At this time, the polymer powder  10  inside the first low melting point metal  20  is also melted to secure an optimal interlayer distance. 
     The polymer powder  10  is an insulating material, and thus it is possible to secure an insulation distance even though the bump is lowered to a desired height. Therefore, it is possible to collectively stack the substrates. 
     Further, since the contact force between the first substrate  100  and the conductive powder particle  40  is further improved in the press process, it is possible to further improve electrical characteristics. 
     Then, although it is not shown in the drawings, in case of a double-sided substrate, after forming a circuit pattern by patterning the first and second substrates  100  and  300 , processes of forming PSR and a gold plating layer may be performed on the circuit pattern. 
     Further, in case of a multilayer substrate, after forming a circuit pattern by patterning the second substrate  300 , processes of printing the conductive paste  60  on the circuit pattern and stacking the insulating layer  200  may be repeated. 
     As described above, according to the conductive paste and the method of manufacturing the printed circuit board using the same in accordance with the present invention, by forming the bump using the conductive paste in which the first and second low melting point metals having the different melting points are sequentially coated on the surface of the polymer powder, the second and first low melting point metals and the polymer powder sequentially react in the drying and press processes of the bump, so that it is possible to improve the contact force and electrical conductivity of the bump and secure the interlayer insulation distance. 
     Further, since the intermetallic compound is formed inside the conductive paste of the present invention, it is possible to obtain the bump having high hardness, and due to this, there is an advantage that the insulating layer easily passes through the bump. 
     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.