Abstract:
The present invention relates to an adjustable resistor embedded in a circuit board and a method of fabricating the same. The adjustable resistor comprises a resistor with a number of connection terminals, and a number of via holes extending to contact with the resistor. The resistive value of the resistor is variable depending on the size of the via holes, the number of the via holes, or the distance between the via holes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to circuit boards, and more particularly, to a circuit board with embedded resistors and a method of fabricating the same. 
     2. Background of the Invention 
     The market for communication devices is growing at an amazing pace. Nowadays, the basic requirements for communication devices are small, thin and lightweight. With increasingly stricter communication standards, packing circuit components in a small device while retaining its efficiency becomes a major issue for circuit design. In recent years, system-in-package technology is developed to pack circuit components in multilayer printed circuit boards with a high density to meet market demand. Integration of passive discrete components, such as resistors, capacitors and inductors, into printed circuit boards may miniaturize the system packages as well as reduce assembly time and manufacturing cost. 
     One major concern for embedded resistor technology is control of resistance values because it may affect the yield of manufacturing process. Indeed, resistance values depend on the thickness of resistors after board lamination process, and thus, it is desirable to improve board lamination process for better control of resistance value. In addition, it is also desirable to develop a method to adjust resistance values of embedded resistors after completion of the lamination process to meet various needs. 
     U.S. Pat. No. 4,443,782 describes a method for regulating the resistance value of a thick film resistor by adjusting the length of a slot located in the resistive material region. Variation of the resistance value is a linear function of the slot length. A slot may be formed by use of a grinding machine or a laser beam. U.S. Pat. No. 4,899,126 describes thick film resistor type printed circuit board having a common terminal electrode for connecting multiple resistors. The patent also mentions using laser beam to form slots to adjust resistance value. Both methods described above are for surface-mounted resistors and do not apply to embedded resistors. 
     BRIEF SUMMARY OF THE INVENTION 
     One example consistent with the invention provides a printed circuit board comprises a dielectric substrate, a resistive layer formed on first portions of the dielectric substrate, a wiring layer formed on second portion of the dielectric substrate for providing electrical connection, and a via hole formed in a thickness direction of dielectric substrate and extending to contact with the resistive layer. The resistance value of the resistive layer is variable depending on the size of the via hole. 
     Another example consistent with the invention provides a printed circuit board which comprises a dielectric substrate, a resistive layer formed on first portions of the dielectric substrate, a wiring layer formed on second portions of the dielectric substrate for providing electrical connection, a number of via holes formed in a thickness direction of dielectric substrate and extending to contact with the resistive layer. The resistance value of the resistive layer is variable depending on the size of the via holes, the number of the via holes, or the distance between the via holes. 
     Another example consistent with the invention provides an adjustable resistor embedded in a circuit board comprises a resistor with a number of connection terminals, and a via hole extending to contact with the resistor. The resistive value of the resistor is variable depending on the size of the via hole. 
     In another example consistent with the invention, an adjustable resistor embedded in a circuit board comprises a resistor with a number of connection terminals, and a number of via holes extending to contact with the resistor. The resistive value of the resistor is variable depending on the size of the via holes, the number of the via holes, or the distance between the via holes. 
     In another example consistent with the invention, a method of fabricating a printed circuit board comprises providing a dielectric layer, forming a wiring layer, forming a resistive layer, and forming a via hole extending through the dielectric layer and extending to contact with the resistive layer. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended, exemplary drawings. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
       In the drawings: 
         FIG. 1A  is a cross-sectional view of a conventional printed circuit board with an embedded resistor; 
         FIG. 1B  is a top view of a conventional printed circuit board with an embedded resistor; 
         FIG. 1C  is a cross-sectional view of a conventional multilayer printed circuit board. 
         FIG. 2A  is a cross-sectional view of a printed circuit board with an embedded resistor in examples consistent with the present invention; 
         FIG. 2B  is a top view of a printed circuit board with an embedded resistor in examples consistent with the present invention; 
       FIGS.  2 C(a) and (b) are cross-sectional views of a printed circuit board with an embedded resistor in examples consistent with the present invention; 
         FIG. 2D  is a cross-sectional view of a multilayer printed circuit board with embedded resistors in examples consistent with the present invention; 
         FIG. 2E  shows the experimental results in an exemplary implementation; 
         FIG. 2F  shows the experimental results in an exemplary implementation; 
         FIG. 3A  is a cross-sectional view of a printed circuit board with an embedded resistor in examples consistent with the present invention; 
         FIG. 3B  is a top view of a printed circuit board with an embedded resistor in examples consistent with the present invention; 
       FIGS.  3 C(a) and (b) are cross-sectional views of a printed circuit board with an embedded resistor in examples consistent with the present invention; 
         FIG. 3D  is a cross-sectional view of a multilayer printed circuit board with embedded resistors in examples consistent with the present invention; 
         FIG. 3E  shows the experimental results in an exemplary implementation; 
         FIG. 3F  shows the experimental results in an exemplary implementation; 
         FIGS. 4A-4B  are top views of exemplary printed circuit boards in examples consistent with the present invention; 
         FIGS. 5A-5D  are top views of exemplary printed circuit boards in examples consistent with the present invention; 
         FIGS. 6A-6H  are top views of exemplary printed circuit boards in examples consistent with the present invention; 
         FIGS. 7A-7D  are top views of exemplary printed circuit boards in examples consistent with the present invention; 
         FIGS. 8A-8D  are top views of exemplary printed circuit boards in examples consistent with the present invention. 
         FIG. 9A  is a cross-sectional view of a printed circuit board with an embedded resistor in examples consistent with the present invention; 
         FIG. 9B  is a top view of a printed circuit board with an embedded resistor in examples consistent with the present invention; 
         FIG. 9C  is a cross-sectional view of a printed circuit board with an embedded resistor in examples consistent with the present invention; 
         FIG. 9D  is a simplified electrical circuit equivalent for the resistor in  FIG. 9C ; and 
         FIG. 9E  shows the experimental results in an exemplary implementation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1A  illustrates an exemplary structure of a conventional circuit board  10  with an embedded resistor. Referring to  FIG. 1A , the circuit board  10  may include dielectric substrate  100 , a resistive layer  102 , wiring layers  104   a  and  104   b , and copper-plated via holes  106  connecting wire layers  104 . The fabrication process may start with providing a layer of a polymeric dielectric material to form the dielectric substrate  100  having a top and bottom surfaces. The dielectric substrate  100  may be made of as polyimide, BT resin polymer, glass fiber, or high dielectric constant (DK) material with DK ranging from 15-80. For example, the dielectric substrate  100  may be an aluminum oxide substrate, a low-temperature cofired ceramic substrate or a ceramic substrate. The dielectric substrate  100  typically has a thickness or dimension between its top and bottom surfaces of 150 um or less. Next, the process continues with providing a conductive material such as copper on the top surface of the dielectric substrate  100  and patterning it to form the wiring layer  104   a . Thereafter, a resistive material is provide on the exposed dielectric substrate  100  and the wiring layer  104   a  and then is patterned to form the resistive layer  102 . The resistive material may be provided as a paste that includes a highly conductive material such as a metal powder, metal oxides, graphite or other form of carbon or semiconductor materials in a nonconductive matrix such as a polymer. Another dielectric layer  120  as the substrate  100  is subsequently formed on the exposed wiring layer  104   a  and the resistive layer  102 . Next, another wiring layer  104   b  is formed on the dielectric layer  120 . Via holes  106  are subsequently formed in the thickness direction of the substrate  100  and extend through the dielectric layer  120 . The via holes  106  may be form by, for example, laser ablation, plasma etching, punching and drilling. The via holes  106  are then filled with, plated with, or coated with a conductive paste to form conductors. In one example, via holes  106  are coated with copper. With copper-plated via holes  106 , the wiring layers  104   a  and  104   b  are electrically connected to each other.  FIG. 1B  shows the top view of  FIG. 1A . 
       FIG. 1C  illustrates an exemplary structure of a conventional multilayer circuit board  12  with a number of embedded resistors. The multilayer circuit board  12  of  FIG. 1C  comprises a laminate of at least two printed circuit boards of  FIG. 1A . As shown at  FIG. 1C , resistors may be embedded between any two adjacent layers of the dielectric substrates. 
       FIG. 2A  illustrates an exemplary structure of a printed circuit board  20  in examples consistent with the present invention.  FIG. 2B  shows the top view of  FIG. 2A . The printed circuit board  20  includes a dielectric substrate  100 , a resistive layer  102 , wiring layers  104   a  and  104   b  and via holes  106  as described in connection with  FIG. 1A . In addition, the printed circuit board  20  includes an additional via hole  110  formed in the thickness direction of the dielectric substrate  100 . The via hole  110  may be formed by, for example, laser ablation, plasma etching, punching and drilling. FIGS.  2 C(a) and (b) show that the via holes may pouch through both the dielectric layer  120  and the resistor layer  102  or extend through the dielectric layer  120  to contact the resistor layer  102  without pouching through the resistor layer  102 .  FIG. 2D  illustrates an exemplary structure of a multilayer circuit board  22  with a number of embedded resistors and via holes. 
       FIG. 2E  shows the experimental results in an exemplary implementation. In the experiment, the area size of the resistor is 90*30 mil 2 , the thickness of the resistor is 0.8 mil, and 
             ρ   =     160   ⁢       (     siemens   m     )     .             
The results indicate that the resistance value of the finished resistor depends on the area size of the via hole. Table 1 below shows the experimental results on the resistance value of a printed circuit board of  FIG. 2A  with only one via hole contacting the resistive layer.
 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Original 
                 5 mil 
                 10 mil 
                 13 mil 
               
               
                 @ 10 MHz 
                 resistor 
                 via hole 
                 via hole 
                 via hole 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Resistance (Ohm) 
                 628.87 
                 636.48 
                 669.36 
                 860.87 
               
               
                 Resistance change (%) 
                 — 
                 1.21 
                 6.43 
                 36.89 
               
               
                   
               
             
          
         
       
     
       FIG. 2F  shows experimental results in another exemplary implementation where the printed circuit board has more than one via holes. The results indicate that the resistance value of the printed circuit board increases when the number of the via holes increases. Table 2 below shows the experimental results. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Original 
                 One 
                 Two 
                 Three 
               
               
                 @ 10 MHz 
                 resistor 
                 via hole 
                 via holes 
                 via holes 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Resistance (Ohm) 
                 628.87 
                 669.36 
                 818.15 
                 869.59 
               
               
                 Resistance change (%) 
                 — 
                 6.43 
                 30.09 
                 38.27 
               
               
                   
               
             
          
         
       
     
       FIG. 3A  illustrates an exemplary structure of a printed circuit board  30  in examples consistent to the present invention.  FIG. 3B  shows the top view of  FIG. 3A . The structure of  FIG. 3A  is similar to the structure of  FIG. 2A , except that the via hole  110  is either filled, coated or plated with conductive materials  110   a . In one example, the via hole  110  is coated with copper. FIGS.  3 C(a) and (b) show that via holes  110  plated with conductive metal may either pouch through the resistive layer  102  or make contact with the resistor layer  102  without pouching through the resistive layer  102 .  FIG. 3D  illustrates an exemplary structure of a multilayer circuit board  32  with a number of embedded resistors and via holes plated with conductive materials. 
       FIG. 3E  shows experimental results in an exemplary implementation where the via hole is plated with copper. The results indicate that the resistance value decreases in accordance with the increase of via radius. Table 3 below shows the experimental results. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Original 
                 5 mil 
                 10 mil 
                 15 mil 
               
               
                 @ 10 MHz 
                 resistor 
                 via hole 
                 via hole 
                 via hole 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Resistance (Ohm) 
                 628.87 
                 601.18 
                 575.69 
                 575.69 
               
               
                 Resistance change (%) 
                 — 
                 −4.4 
                 −8.45 
                 −13.08 
               
               
                   
               
             
          
         
       
     
       FIG. 3F  shows experimental results in another exemplary implementation where the printed circuit board has more than one via holes and each via hole is plated with copper. The results indicate that the resistance value of the printed circuit board decreases when the number of the via holes increases. Table 4 below shows the experimental results. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                 Original 
                 One 
                 Two 
                 Three 
               
               
                 @ 10 MHz 
                 resistor 
                 via hole 
                 via holes 
                 via holes 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Resistance (Ohm) 
                 628.87 
                 575.69 
                 525.27 
                 491.09 
               
               
                 Resistance change (%) 
                 — 
                 −8.45 
                 −16.47 
                 −21.90 
               
               
                   
               
             
          
         
       
     
     Referring again to  FIGS. 2C and 3C , the exemplary printed circuit board in consistent with the present invention may include more than one via holes  110 . FIGS.  2 C(a) and (b) illustrate an exemplary structure of a printed circuit board with multiple via holes in examples consistent with the present invention.  FIG. 4(   a ) shows the top view of the printed circuit board of FIGS.  2 C(a) or (b). FIGS.  3 C(a) and (b) illustrate an exemplary structure of a printed circuit board with multiple via holes plated with conductive materials in examples consistent with the present invention.  FIG. 4(   b ) shows the top view of the printed circuit board of FIGS.  3 C(a) or (b). 
       FIGS. 5A-5D  show the top views of exemplary printed circuit boards in examples consistent with the present invention.  FIGS. 5A and 5B  illustrate that the via holes of the exemplary printed circuit boards may be in different sizes.  FIGS. 5C and 5D  illustrate that the via holes plated with conductive materials of the exemplary printed circuit boards may be in different sizes. 
       FIGS. 6A-6H  show the top views of exemplary printed circuit boards in examples consistent with the present invention.  FIGS. 6A-6D  illustrate that the via holes of the exemplary printed circuit boards may have via holes in various shapes.  FIGS. 6E-6H  illustrate that the via holes plated with conductive materials of the exemplary printed circuit boards may be in various shapes. 
       FIGS. 7A-7D  show the top views of exemplary printed circuit boards in examples consistent with the present invention.  FIGS. 7A and 7B  illustrate that the via holes of the exemplary printed circuit boards may be formed in various locations of the resistive layers.  FIGS. 7C and 7D  illustrate that the via holes plated with conductive materials of the exemplary printed circuit boards may be formed in various locations of the resistive layers. 
       FIGS. 8A-8D  show the top views of exemplary printed circuit boards in examples consistent with the present invention.  FIGS. 8A-8D  illustrate that the via holes in an exemplary printed circuit board may be in different size or shape, or in various locations of the resistive layer. In addition, referring to  FIGS. 8A-8D , the via holes in an exemplary printed circuit board are in combination of metal plated via holes and via holes without metal plated. 
     The present invention also provides a multilayer printed circuit board with adjustable embedded resistors where the resistance value is adjusted by reducing the length of the resistors. According to Equation (1) below, the length of a resistor is linear function of the resistance value of the resistor. Therefore, the resistance value can be precisely adjusted by reducing the length of the resistor. 
     
       
         
           
             
               
                 
                   
                     
                       R 
                       = 
                       
                         ρ 
                         ⁢ 
                         
                           L 
                           A 
                         
                       
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     ρ 
                     ⁢ 
                     
                       : 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Resisitivity 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     L 
                     ⁢ 
                     
                       : 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Resistor 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     length 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     A 
                     ⁢ 
                     
                       : 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     resistor 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     section 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     area 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
       FIG. 9A  illustrates an exemplary structure of a printed circuit board  900  in examples consistent with the present invention. Referring to  FIG. 9A , the printed circuit board  900  may include dielectric substrates  100  and  120 , a resistive layer  102 , wiring layers  104   a  and  104   b , and copper-plated via holes  106 ,  110   a  and  110   b  to connect wire layers  104  as described in  FIG. 2A . The via holes  110   a  and  110   b  may be either filled, coated or plated with conductive materials such as copper as described in connection with  FIG. 3A . In addition, the printed circuit board  40  of  FIG. 9A  also includes a dielectric layer  910  between the resistive layer  102  and the dielectric substrate  100 . The dielectric layer  910  may be a material selected from polyimide, BT resin polymer, glass fiber, and high dielectric constant (DK) material with DK ranging from 15-80. On the bottom surface of the dielectric layer  910 , there is a supplement metal layer  920 . The supplement metal layer  920  may be made of any conductive materials, such as copper. As discussed below, variation of the resistance value of the resistor layer  102  depends on the distance D between the two copper-plated via holes  110   a  and  110   b .  FIG. 9B  shows the top view of  FIG. 9A . 
       FIG. 9C  illustrates the DC signal path for the printed circuit board  900  of  FIG. 9A . The DC signal first passes through portions of the resistive layer  102 , then flows through the supplement metal layer  920  due to the low resistance value of the metal layer  920 . Then the DC signal flows through another portions of the resistive layer  102  as indicated.  FIG. 9D  is a simplified electrical equivalent circuit for the resistor of the printed circuit board  900  of  FIG. 9C . Assuming that the original resistance value of the resistance layer  102  is R 1 +R 2 +R 3 , the resistance value decreases to R 1 +R 3  because of the supplement metal layer  920 . 
       FIG. 9E  shows experimental results in another exemplary implementation where the distance between two via holes  110   a  and  110   b  varies. The results indicate that the resistance value of the printed circuit board decreases when the distance between the two via holes increases. Table 5 below shows the experimental results. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                 Original 
                 Distance: 
                 Distance: 
                 Distance: 
                 Distance: 
               
               
                 @ 10 MHz 
                 resistor 
                 40 mil 
                 80 mil 
                 120 mil 
                 160 mil 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Resistance 
                 420.81 
                 371.43 
                 331.58 
                 313.82 
                 277.59 
               
               
                 (Ohm) 
               
               
                 Resistance 
                 NA 
                 11.73 
                 21.20 
                 25.42 
                 34.03 
               
               
                 change(%) 
               
               
                   
               
             
          
         
       
     
     The via holes  110   a  and  110   b  pouching through the resistive layer to contact with the supplemental metal layer  920  may be in different sizes or shapes as discussed above in connection with  FIGS. 5-8 . 
     In addition to multilayer printed circuit boards, the present invention may apply to embedded resistor in integrated circuit load circuit. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.