Abstract:
A method for fabricating the embedded thin film resistors of a printed circuit board is provided. The embedded thin film resistors are formed using a resistor layer built in the printed circuit board. In comparison with conventional discrete resistors, embedded thin film resistors contribute to a smaller printed circuit board as the space for installing conventional resistors is saved, and better signal transmission speed and quality as the parasitic capacitive reactance effect caused by two contact ends of the conventional resistors is also avoided. The method for fabricating the embedded thin film resistors provided by the invention can be conducted using the process and equipment for conventional printed circuit boards and thereby saving the investment on new types of equipment. The method can be applied in the mass production of printed circuit boards and thereby reduce the manufacturing cost significantly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a method for fabricating embedded thin film resistors of a printed circuit board, and more specifically to a method of fabricating one or more embedded resistors in a printed circuit board. 
     2. The Prior Arts 
     In general, besides using conventional discrete passive elements, a printed circuit board can also use a thick film or a thin film process to develop the embedded resistors required. In the thick film process, the resistors of the printed circuit board are made of carbon paste printed on the printed circuit board. Then the resistances of the resistors are fine-tuned by the laser trimming after drying. In the thin film process, on the other hand, a nickel-silicide-plated copper foil is used to replace the pure copper foil. The nickel-silicide-plated copper foil and the epoxy resin of the printed circuit board are pressed together during the fabricating process of the printed circuit board. The nickel-silicide-plated side of the copper foil faces toward the laminate of the printed circuit board and the non-plated side of the copper foil faces outward. Then, in a subsequent photolithography process, an acid etching solution is first used to etch both the copper and nickel-silicide layers, and then an alkaline etching solution is used to etch away the copper layer above the embedded resistors. One or more nickel-silicide blocks with the required dimensions are thereby formed. Laser is then used to trim each of the nickel-silicide blocks to achieve the precise resistance required. 
     In addition, U.S. company, Macdermid, developed a process to fabricate embedded resistors by adopting the nickel-phosphorus or palladium-phosphorus deposition technology conventionally used in industry. The process includes circuitry fabrication, activation, forming photosensitive etching resist, lithographic etching to expose resistor locations, immersing in an electroless dip solution, removing photosensitive etching resist, and fine-tuning resistance with laser trimming to accomplish the fabrication of embedded thin film resistors. 
     SUMMARY OF THE INVENTION 
     In aforementioned conventional thick film resistor fabricating methods, using high curing temperature carbon paste for the resistors is rather simple, and the technology is mature and less costly. However, because the laminate of the printed circuit board is susceptible to high temperature, only a low curing temperature carbon paste can be used. The macromolecular polymer contained in the low curing temperature carbon paste will remain in the formed resistors even after the curing and solidification processes of the resistors. The hydrophilic property of the macromolecular polymer is the major factor causing the resistances of the resistors to vary along with the environmental change. Therefore, resistors having constant and precise resistances are difficult to achieve. On the other hand, the technology for embedded resistors using nickel-silicide-plated copper foil use the same technological process as the conventional printed circuit board fabrication methods. The fabricated embedded resistors also have better stability and accuracy than those made by thick film methods. However, because the nickel-silicide-plated copper foil is difficult to manufacture, there are only limited supply sources and therefore the price is high. As the activation technology of the method using the electroless deposition technology developed by Macdermid uses Palladium or other precise metal, and the current industrial activation technology is immersion, the immersion of the entire printed circuit board in the Palladium solution is very costly and considered as a waste. Furthermore, the thickness of the activation layer is deposited to only the micro meter scale. When pressing the photosensitive etching resist, the activation layer will be attached to the etching resist film, which will be washed away in the subsequent photolithography step. Also, the alkaline solution for photolithography will dissolve some part of the activation layer, which leads to the damage of the activation function and reduces the reliability of the embedded resistors. This may be the reason that the method is not widely accepted by the market since the publication of the method in 2001. While the present invention is similar to the method for fabrication embedded thin film resistors of printed circuit board developed by Macdermid, the present invention overcomes the interferences to the sensitization caused by attaching the photosensitive etching resist and photolithography technique. To overcomes the aforementioned interferences, the present invention changes the activation technique into the sensitization step and the activation step, as well as move the attachment of the photosensitive etching resist and photolithography steps after the steps of sensitization and activation. By increasing the density of the sensitization solution, the aforementioned interferences to the sensitization caused by attaching the photosensitive etching resist and photolithography technique can be overcome. Furthermore, the sensitization solution, such as stannous chloride, is inexpensive; therefore, the cost increase incurred by increasing the density is limited. Because the Palladium-based activation solution is expensive, the present invention is cost effective by arranging the activation step after the photolithography step so as to ensure the activation function of the Palladium as well as only depositing the expensive Palladium on the embedded resistors. 
     The present invention provides a method for fabricating embedded thin film resistors of printed circuit boards to overcome the problems and the disadvantages of one or more conventional methods. 
     The method for fabricating embedded thin film resistors of printed circuit boards and the printed circuit board structure achieved disclosed in the following are only for illustrating purpose. The method provided by the present invention can be applied to single-sided, double-sided, multi-layered, and build-up printed circuit boards to fabricate the embedded resistors required by the circuitry design through the use of electroless dip technique. 
     In comparison with the conventional methods, the present invention of a method for fabricating embedded thin film resistors of printed circuit boards has the following advantages:
         1. The embedded thin film resistors made by the present invention are embedded within the printed circuit board to replace the bulky conventional discrete resistors. The printed circuit board can therefore have finer circuit layout and much smaller size.   2. The embedded thin film resistors made by the present invention are embedded within the printed circuit board to reduce, or even eliminate, the parasitic capacitive reactance effect usually found at the connectors of conventional discrete resistors. The signal transmission speed and quality of the printed circuit board is therefore significantly enhanced, especially for high frequency applications.   3. The process for forming the resistor layer provided by the present invention is very similar to that used for ordinary printed circuit boards and can be carried out using the same equipment. Therefore there is no significant investment on new equipment. The process for forming the resistor layer provided by the present invention, just like the process for ordinary printed circuit boards, is applicable in mass production and contributes to a significant lower manufacturing cost.       

     The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart showing the steps of forming embedded thin film resistors on a printed circuit board according to a first embodiment of the present invention. 
         FIGS. 2(   a )- 2 ( e ) are schematic diagrams showing the various steps of  FIG. 1  respectively. 
         FIG. 3  is a flow chart showing the steps of forming embedded thin film resistors on a printed circuit board according to a second embodiment of the present invention. 
         FIGS. 4(   a )- 4 ( j ) are schematic diagrams showing the various steps of  FIG. 3  respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a flow chart showing the steps of forming embedded thin film resistors on a printed circuit board according to a first embodiment of the present invention. These steps are described sequentially as follows. 
     In step  101 , as shown in  FIG. 2(   a ), a conductive circuitry ( 21 ) is formed on the copper foil layer ( 2 ), which is on the surface of the laminate ( 1 ) made of an insulating material, and the circuitry ( 21 ) also includes one or more corresponding resistor windows ( 22 ) according to the locations and the sizes of resistors required by said circuitry ( 21 ). 
     The foregoing conductive circuitry ( 21 ) and resistor windows ( 22 ) on the copper foil ( 2 ) can be formed using the electroplating, image transfer and etching techniques of an ordinary printed circuit board fabrication process such as the conventional subtractive, additive, or semi-additive process. 
     In step  102 , as shown in  FIG. 2(   b ), a sensitized layer ( 3 ) is coated on the printed circuit board of step  101 , where the sensitized layer ( 3 ) has a strong reduction capability. The most common sensitization solution is stannous chloride. The sensitized layer ( 3 ) must be coated to the thickness to retain the sensitization capability after the subsequent attaching of photosensitive etching resist and photolithography steps. 
     In step  103 , as shown in  FIG. 2(   c ), a photosensitive etching resist layer ( 4 ) is painted or attached to the surface of the sensitized copper foil layer ( 2 ), and the photolithography technique is used to expose each resistor frame ( 23 ). The resistor frames ( 23 ) expose the sensitized layer ( 3 ) on top of the copper foil layer ( 2 ). Then, the activation technique is performed on the printed circuit board. The activation solution will only reduce the surface of the resistor frames ( 23 ) covered with sensitized layer ( 3 ) to the activated layer ( 5 ). The length of resistor frames ( 23 ) is the same as or slightly longer than the resistor windows ( 22 ) in the circuitry ( 21 ) direction to ensure the good contact condition of the contact endpoints. 
     In step  104 , as shown in  FIG. 2(   d ), the printed circuit board is immersed in an electroless dip solution. The electroless dip solution only coats a resistor layer ( 6 ) on the activation layer ( 5 ) inside the resistor frames ( 23 ), until the thickness of the resistor layer ( 6 ) to a pre-defined thickness. The deposited resistor layer ( 6 ) forms the resistor elements ( 61 ). 
     The aforementioned electroless dip solution for the resistor layer ( 6 ) can be nickel-phosphorus or palladium-phosphorus dip solution or other electroless dip solutions with resistance. 
     In step  105 , as shown in  FIG. 2(   e ), the photosensitive etching resist layer ( 4 ) on the copper foil layer ( 2 ) is stripped away. 
     In Step  106 , a laser trimming technique is used to perform precise trimming on the resistor layer ( 6 ) so as to fine-tuning the locations, sizes, and resistances of resistor elements ( 61 ). The resistor layer ( 6 ), after the trimming, forms the resistor elements ( 61 ). 
     In the foregoing step of laser trimming, each resistor element ( 61 ) of the resistor layer ( 6 ) can be coated with a protective layer of ink. The protective ink is then heated and solidified so that subsequent fabrication steps of the printed circuit board will not affect the resistance of each resistor element ( 61 ). The coating and solidification of the protective ink layer can also be conducted before the laser trimming. In this way, undesirable influence of the ink coating and solidification on the resistances of the resistor elements ( 61 ) can be avoided after their resistances are adjusted by laser trimming. 
       FIG. 3  is a flow chart showing the steps of forming embedded thin film resistors on a printed circuit board according to a second embodiment of the present invention. These steps are described sequentially as follows. 
     In step  201 , as shown in  FIG. 4(   a ), a photosensitive etching resist layer ( 4 ) is painted or attached on the copper foil layer ( 2 ), which is on the surface of the laminate ( 1 ) made of an insulating material. The photosensitive etching resist layer ( 4 ) is then etched with a photolithography technique to form one or more resistor windows ( 22 ) according to the locations and the sizes of the resistors required in a circuitry. The photosensitive etching resist layer ( 4 ) is then stripped away. 
     In step  202 , as shown in  FIG. 4(   b ), a sensitization solution is used to coat a sensitized layer on the surface of the resistor windows ( 22 ) to sensitize chemical polymer insulation interface exposed by the resistor windows ( 22 ) on the laminate ( 1 ), where the sensitized layer ( 3 ) has a strong reduction capability. The most common sensitization solution is stannous chloride. The sensitized layer ( 3 ) must be coated to the thickness to retain the sensitization capability after the subsequent attaching of photosensitive etching resist and photolithography steps. 
     In step  203 , as shown in  FIG. 4(   c ), a photosensitive etching resist layer ( 4 ) is painted or attached and the photolithography technique is then to expose each resistor frame ( 23 ). The length of resistor frames ( 23 ) is the same as or slightly longer than the resistor windows ( 22 ) in the circuitry ( 21 ) direction to ensure the good contact condition of the contact endpoints. The activation step is then performed on the printed circuit board. The activation solution only reduces the surface of the resistor frames ( 23 ) covered with sensitized layer ( 3 ) to the activated layer ( 5 ). 
     In step  204 , as shown in  FIG. 4(   d ), the printed circuit board is immersed in an electroless dip solution to coat a resistor layer ( 6 ) on the activation layer ( 5 ) inside the resistor frames ( 23 ), until the thickness of the resistor layer ( 6 ) to a pre-defined thickness. The deposited resistor layer ( 6 ) forms the resistor elements ( 61 ), with the two contact endpoints ( 62 ). Then, the photosensitive etching resist layer ( 4 ) is stripped away. 
     The aforementioned electroless dip solution for the resistor layer ( 6 ) can be nickel-phosphorus or palladium-phosphorus dip solution or other electroless dip solutions with resistance. 
     In step  205 , as shown in  FIG. 4(   e ), with the photosensitive etching resist layer painting or attaching technique and the photolithography technique, an etching resist thin film ( 7 ) with a pattern of a circuitry ( 21 ) and resistor elements ( 61 ) is formed on the copper foil layer ( 2 ) and the resistor layer ( 6 ) according to the required circuitry. 
     In step  206 , as shown in  FIG. 4(   f ), an etching technique is applied to etch the copper foil layer ( 2 ) so that the copper foil layer ( 2 ) forms the circuitry ( 21 ) corresponding to the pattern of the etching resist thin film ( 7 ). 
     In step  207 , as shown in  FIG. 4(   g ), the etching resist thin film ( 7 ) on the copper foil layer ( 2 ) is stripped away. 
     Step  205  to step  207  are the steps of positive film etching technique. 
     On the other hand, if a negative film electroplating etching technique, the following steps (starting right after  FIG. 4(   d )) can accomplish the fabrication of the circuitry and the embedded resistors. 
     Following step  204  ( FIG. 4(   d )), in step  208 , as shown in  FIG. 4(   h ), with the photosensitive etching resist layer painting or attaching technique and the photolithography technique, an electroplating resist thin film ( 7 ′) with a pattern of a circuitry ( 21 ) and resistor elements ( 61 ) is formed on the copper foil layer ( 2 ) and the resistor layer ( 6 ) according to the required circuitry. 
     In step  209 , as shown in  FIG. 4(   i ), a layer of copper (Cu II) and etching resist metal ( 8 ), such as, tin or tin-lead, is electroplated to a pre-defined thickness. The electroplating resist thin film ( 7 ′) is then stripped away. The photosensitive etching resist layer painting or attaching technique and the photolithography technique are used to form an etching resist thin film ( 7 ″) on the resistor elements ( 61 ), and then the etching is performed. 
     In step  210 , as shown in  FIG. 4(   j ), the etching resist metal layer ( 8 ) on the circuitry ( 21 ) and the etching resist thin film ( 7 ″) on the resistor elements ( 61 ) are stripped away. 
     Step  208  to step  210  are the steps of negative film electroplating etching technique. 
     In step  211 , a laser trimming technique is used to perform precise trimming on the resistor layer ( 6 ) so as to fine-tuning the locations, sizes, and resistances of resistor elements ( 61 ). The resistor layer ( 6 ), after the trimming, forms the resistor elements ( 61 ). 
     In the foregoing step of laser trimming, each resistor element ( 61 ) of the resistor layer ( 6 ) can be coated with a protective layer of ink. The protective ink is then heated and solidified so that subsequent fabrication steps of the printed circuit board will not affect the resistance of each resistor element ( 61 ). The coating and solidification of the protective ink layer can also be conducted before the laser trimming. In this way, undesirable influence of the ink coating and solidification on the resistances of the resistor elements ( 61 ) can be avoided after their resistances are adjusted by laser trimming. 
     As shown in  FIG. 2(   e ) and  FIG. 4(   j ), the printed circuit board manufactured by the aforementioned fabrication process will include a laminate ( 1 ), a copper foil layer ( 2 ) formed on top of the laminate ( 1 ), and then a resistor layer ( 6 ) on top of copper foil layer ( 2 ). The resistor layer ( 6 ) includes a plurality of resistor elements ( 61 ), and each resistor element ( 61 ) can be of a specific resistance according to the requirement of the circuit design on the printed circuit board. Each aforementioned resistor element ( 61 ) has two contact endpoints ( 62 ), with each contact endpoint ( 62 ) forming electrical connection with the corresponding circuitry ( 21 ) of the copper foil layer ( 2 ). 
     Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.