Abstract:
A method of protecting a thick film resistor, including the steps of: providing a substrate having a plurality of conductive elements thereon; applying an electrically resistive material to a surface of the substrate, thereby forming the thick film resistor, the resistive material being electrically connected to at least one corresponding conductive element; curing the resistive material; and applying a coating over at least a substantial portion of the resistive material.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an apparatus and a method for protecting a resistor, and, more particularly, to a method and apparatus for protecting a thick film resistor with a coating.  
         BACKGROUND OF THE INVENTION  
         [0002]    Thick film resistors are employed in numerous hybrid electronic circuits in a wide range of resistor values. Thick film resistors can be formed by printing methods, such as screen-printing, in which a thick film resistive paste or ink is deposited upon a substrate. The substrate may be a printed wiring board, a flexible circuit, a ceramic substrate or a silicon substrate. Thick film inks or pastes typically include an electrically conductive material, various additives to affect the final electrical properties of the resistor, an organic binder and an organic vehicle. After printing a thick film paste on a substrate, the assembly typically is heated to dry the ink and convert it into a suitable film that adheres to the substrate. If polymer thick film ink is used, the organic binder is a polymer matrix material, and the heating serves to remove the organic vehicle and to cure the polymer material.  
           [0003]    Conventional screen-printing techniques generally employ a template with apertures therein. Each aperture is an opening that reflects the size and shape of the resistor to be created. The template, often referred to as a screening mask, is placed in close proximity to the surface of the substrate on which the resistor is to be printed. The mask is then loaded with a polymer thick film ink, and a squeegee blade is drawn across the surface of the mask to press the ink through the apertures and onto the surface of the substrate.  
           [0004]    Copper terminations for the polymer thick film ink are typically formed prior to deposition of the ink by laminating copper foil to a substrate with subtractive etching to remove unwanted copper. Another method of placing conductive paths on a substrate includes screen-printing using a conductive polymer thick film ink, which is typically applied prior to the application of a resistive polymer thick film ink. Conductive ink is separately cured prior to the printing of the polymer thick film resistive ink.  
           [0005]    Applying a polymer thick film resistive ink on a printed wiring board, otherwise known as a printed circuit board, allows resistive elements to be printed directly on the printed circuit board. The porosity of a polymer thick film resistor can result in moisture entrapment when the resistor is exposed to an atmosphere containing moisture. Another problem is that oxidation occurs between the interface of the polymer thick film resistor and the conductor. Still another problem is the interaction of the polymer thick film with any chemicals to which the polymer thick film resistor may be exposed. Each of these problems causes undesirable charges to the electrical characteristics of the thick film resistors.  
           [0006]    What is needed in the art is a method for protecting a thick file resistor, to reduce the environmental sensitivity of the thick film resistor.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method for coating a thick film resistor.  
           [0008]    The invention comprises, in one form thereof, a method of protecting a thick film resistor, including steps of: providing a substrate having a plurality of conductive elements thereon; applying an electrically resistive material to a surface of the substrate, thereby forming the thick film resistor, the resistive material being electrically connected to at least one corresponding conductive element; curing the resistive material; and applying a coating over at least a substantial portion of the resistive material.  
           [0009]    The invention comprises, in yet another form thereof, a circuit board assembly including: a substrate, a plurality of conductive elements applied to the substrate, at least one resistor applied to a surface of the substrate, at least one resistor electrically connected to at least one of the plurality of conductive elements and a coating applied over substantially all of at least one resistor.  
           [0010]    An advantage of the present invention is protecting a thick film resistor from moisture in the environment.  
           [0011]    Another advantage of the present invention is that oxidation between the interface of the thick film resistor and a conductive path is reduced by preventing the proximity of oxygen with the interface.  
           [0012]    Yet another advantage of the present invention is that resistor variability is reduced in that the thick film material that is part of the resistor is isolated from any chemicals in the environment.  
           [0013]    Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a printed circuit board assembly employing an embodiment of the method of the present invention;  
         [0015]    [0015]FIG. 2 is a thick film resistor of the circuit board assembly of FIG. 1;  
         [0016]    [0016]FIG. 3 is a cross-sectional view along section line  3 - 3  of the resistor of FIG. 2; and  
         [0017]    [0017]FIG. 4 represents a flow chart of a method employed in coating the thick film resistor of FIGS.  1 - 3 . 
     
    
       [0018]    Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description, or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein are for the purpose of description, and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof, as well as additional items and equivalents thereof.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    Referring now to the drawings, and more particularly to FIG. 1, there is shown an electronic circuit board assembly  10  embodying the present invention. Circuit board assembly  10  includes a substrate  12 , connective pads  14 , conductive trace  16 , an integrated circuit  18 , a transistor  20 , conductive pads  22 , resistors  24 , a resistor  26 , uncoated areas  28  and  30  and coated area  32 . Circuit board assembly  10  includes various circuit elements and a way of being electrically connecting to other circuit boards and/or wiring harnesses by way of connective pads  14 . Circuit board assembly  10  allows for an electrical connection to printed resistor  26  in the form of a conductive contact (not shown) that runs along the surface of printed resistor  26 .  
         [0020]    Substrate  12  also known as printed circuit board  12  includes a non-conductive element, which may be an epoxy filled fiberglass or a flexible material also known as a flex circuit. Substrate  12  has a non-conductive surface with relatively good adherence properties for the adherence of connective pad  14 , conductive trace  16 , conductive pads  22  and resistors  24  and  26 . Substrate  12  may also be in the form of ceramic, silicon or plastic material.  
         [0021]    Connective pads  14  are a conductive material, such as copper, that is applied to substrate  12  or is a material that remains on circuit board  12  after an etching process. Connective pads  14  are electrically connected to conductive traces  16  and provide for electrical connection, by way of soldering or physical contact, with another electrical circuit element. Connective pads  14  are shown along one side of substrate  12  even though connective pads  14  may be placed at any point on substrate  12  or along multiple edges of substrate  12 .  
         [0022]    Conductive traces  16  are made of copper, or of some other conductive material that is electrically conductive, and are positioned in accordance with a pattern that routes electricity about circuit board assembly  10 . Conductive traces  16  may be formed on multiple layers of printed circuit board  12 , and electrically connect connective pads  14 , conductive pads  22  and various circuit elements in circuit board assembly  10 . Conductive traces  16  are a conductive material that is either selectively deposited or is the remaining material after a chemical etch of circuit board  12 . Alternatively, conductive traces  16  may be made of a conductive thick film ink that is screen printed and cured on substrate  12 .  
         [0023]    Integrated circuit  18  is a multi-functional circuit element that is soldered to conductive traces  16  or to pads provided for the insertion of pins of integrated circuit  18  through printed circuit board  12 . In a like manner, transistor  20  is mounted as a discrete component on printed circuit board  12 , and is electrically connected by way of conductive traces  16  to other electrical components.  
         [0024]    Conductive pads  22  are electrically connected to certain corresponding conductive traces  16  in accordance with the circuit requirements. Conductive pads  22  are made of the same material as conductive trace  16  or alternatively may be made of conductive thick film ink deposited upon a conductive trace  16 . Conductive pads  22  are positioned for receiving and electrically connecting to printed resistors  24  and  26 . Conductive pads  22  are typically wider than the width of printed resistors  24  and  26  to accommodate positioning and manufacturing variances.  
         [0025]    Resistors  24  and  26  are made of a polymer thick film (PTF) material although another resistive material may be used. Polymer thick film resistor material includes electrically conductive material as well as an organic binder and an organic vehicle. Resistors  24  and  26  may be applied to substrate  12  using a screen-printing method, a pad printing method, a spraying technique or another technique so as to deposit PTF material on circuit board  12 . Once applied to substrate  12 , resistors  24  and  26  are in continuous intimate physical contact with substrate  12 . Resistors  24  and  26  may be made of different formulations of material since resistors  24  will be covered by coating  34  and resistor  26  will be left uncoated in order to allow electrical contact with a conductive contact (not shown). Resistor  26  may have some abrasion resistance or lubricity characteristics, which are unnecessary with resistors  24 . Once resistors  24  and  26  are applied to circuit board  12  and are properly cured, a coating  34  is applied to selected portions of circuit board  12  as represented by reference number  32 . Certain sections, such as uncoated areas  28  and  30 , are not coated by coating  34  to allow for subsequent electrical interaction with other electrical components or for interconnection with other electrical conductors.  
         [0026]    Referring to FIGS. 2 and 3, there is shown resistor  24  with coating  34  thereon. Coating  34  may be an encapsulating compound, a solder mask or a dielectric material. Coating  34  is completely or at least substantially impermeable to gases and vapors. The application of coating  34  reduces or eliminates gas and vapor contact with resistors  24 . Moisture and other chemical contaminants can affect the resistivity of resistors  24  and cause a shift in the electrical characteristics. In the prior art, resistors  24  were not coated to allow for heat dissipation. However, testing of uncoated and coated resistors has demonstrated a reduction in the variability of resistors when protected from moisture and chemical elements. Tables 1 and 2 that follow illustrate shifts in resistive values for 16 and 32 Ohm test resistors that were exposed to a humid environment. A set of thick film resistors having coating  34  applied was tested and compared with a similar set of thick film resistors without a coating.  
                                                           TABLE 1                           Resistor value shift with coating            Before   After   Shift   Before   After   Shift       16 Ohm   16 Ohm   16 Ohm   32 Ohm   32 Ohm   32 Ohm               15.2   15.9   0.7   32.8   34.1   1.3       15.0   15.9   0.9   33.0   34.5   1.5       15.0   15.7   0.7   33.7   34.9   1.2       15.8   16.4   0.6   35.0   36.6   1.6       16.1   16.8   0.7   35.8   37.4   1.6       16.2   16.9   0.7   34.6   36.2   1.7       14.9   15.6   0.7   32.6   33.9   1.3       14.7   15.7   1.0   32.6   34.1   1.5       14.7   15.5   0.8   33.0   34.3   1.3       15.7   16.4   0.7   34.3   35.8   1.5       15.7   16.4   0.7   34.9   36.4   1.5       16.2   16.9   0.7   33.9   35.2   1.3       16.4   17.4   1.0   35.5   36.9   1.4       14.5   15.3   0.8   32.8   34.3   1.5       14.5   15.1   0.6   31.9   33.2   1.3       15.6   16.3   0.7   34.8   36.3   1.5       15.9   16.6   0.7   34.3   35.6   1.3       16.4   17.0   0.6   33.5   35.5   2.0       17.0   17.7   0.7   36.3   37.7   1.4       15.6   16.5   0.9   33.8   35.3   1.5       15.9   16.4   0.5   34.2   35.7   1.5       15.4   15.9   0.5   35.5   36.9   1.4       16.3   17.1   0.8   35.8   37.4   1.6       16.3   17.1   0.8   33.1   34.7   1.6       16.9   17.9   1.0   36.6   39.3   2.7       16.5   17.3   0.8   34.9   36.4   1.5       16.2   17.0   0.8   34.5   36.0   1.5       16.4   17.3   0.9   35.7   37.3   1.6       16.7   17.5   0.8   36.4   38.1   1.7       16.1   16.9   0.8   33.8   36.8   2.0            Avg. Deviation   0.753   Avg. Deviation   1.542       Min.   0.5   Min.   1.2       Max.   1.0   Max.   2.7                  
 
         [0027]    [0027]                                                                                     TABLE 2                           Resistor value shift without coating            Before   After   Shift   Before   After   Shift       16 Ohm   16 Ohm   16 Ohm   32 Ohm   32 Ohm   32 Ohm                    17.2   17.0   0.2   35.0   35.3   0.3       18.5   18.0   0.5   35.9   36.4   0.5       17.3   17.6   0.3   35.4   43.6   8.2       18.2   17.7   0.5   36.2   36.8   0.6       18.2   20.5   2.3   36.5   37.8   1.3       18.1   19.7   1.6   37.0   41.0   4.0       18.5   18.3   0.2   36.4   37.0   0.6       18.3   19.5   1.2   35.9   35.5   0.4       17.0   18.1   1.1   35.8   38.5   2.7       18.7   21.4   2.7   36.8   38.5   1.7       18.2   19.1   0.9   36.1   42.0   5.9       18.6   23.2   4.6   39.1   49.4   10.3       19.0   20.2   1.2   38.3   39.0   0.7       18.9   18.9   0.0   36.0   38.1   2.1       17.4   19.8   2.4   35.8   38.7   2.9       18.6   19.7   1.1   36.9   37.1   0.2       18.6   22.3   3.8   38.3   40.7   2.4       19.8   23.1   3.3   39.9   46.6   6.7       19.3   19.2   0.1   38.4   40.6   2.2       19.2   19.1   0.1   38.5   38.9   0.4       19.0   19.4   0.4   37.1   37.7   0.6       18.6   18.3   0.3   37.4   38.3   0.9       19.8   20.3   0.5   39.9   40.6   0.7       20.6   23.9   3.3   40.5   44.3   3.8       20.4   21.3   0.9   40.7   41.2   0.5       21.2   21.6   0.4   41.0   41.6   0.6       20.2   20.4   0.2   40.0   44.4   4.4       20.5   21.4   0.9   41.3   45.2   3.9       20.9   21.3   0.4   42.9   45.4   2.5       21.2   22.7   1.5   42.7   44.1   1.4            Avg. Deviation   1.228   Avg. Deviation   2.447       Min.   0.0   Min.   0.2       Max.   4.6   Max.   10.3                    
         [0028]    The first and fourth columns of each table contain the resistive value for each resistor prior to exposure to the test environment. The second and fifth columns contain the measurements for each respective resistor after exposure to the test environment. The third and sixth columns indicate the shift in resistance value for each respective resistor. Thirty nominal 16 Ohm and thirty nominal 32 Ohm resistors were tested with a coating and another thirty 16 Ohm and thirty 32 Ohm resistors were tested without coating. All resisters were exposed to the same environmental conditions. The resistors were exposed to ninety-six hours of cycling from 25° C. to 65° C. in a humidity of greater than 90%. Samples were subjected to periods of power and no power. Samples also were subjected to a moisture susceptibility environment, where the circuit boards with the sample resistors thereon were placed in a −40° C. environment for thirty minutes. Then the boards were placed in a 65° C. environment with 95% relative humidity for two minutes and tested under power in the 95% relative humidity condition for three minutes. Table 1 illustrates a shift in resistor values for those resistors having a coating. The data indicates a shift in the 16 Ohm resistors of from 0.5 Ohms to 1.0 Ohm with an average deviation of 0.753 Ohms. In contrast 16 Ohm resistors without coating, as illustrated in Table 2, have an average deviation of 1.228 Ohms. In a like manner, the 32 Ohm resistors having a coating show an average deviation of 1.54 Ohms versus 2.45 Ohms for the 32 Ohm resistors. In each case the maximum shift of the resistors without coating is approximately four times the maximum shift of resistors having coating thereon.  
         [0029]    Now, additionally referring to FIG. 4, there is shown a block diagram representing a method of the present invention used to apply a coating to resistors  24  and  26 . The method of FIG. 4 is depicted by a plurality of processing steps hereinafter referred to as process  100 .  
         [0030]    At the point of beginning of process  100 , and specifically at step  102 , circuit board  12  is cleaned using a solvent or other cleaning method to remove all contamination from circuit board  12 . Once circuit board  12  is cleaned, process  100  continues to step  104 .  
         [0031]    At step  104 , printed circuit board  12  is dried either in a room temperature environment or in a drying oven. Once printed circuit board  12  is dry, process  100  continues to step  106 .  
         [0032]    At step  106 , resistors  24  are applied to circuit board  12  such that they are connected to conductive pads  22  on circuit board  12 . Resistors  24  may be applied using a screen-printing method, a pad printing method or by directing a spray of PTF material onto circuit board  12 . Process  100  then continues to step  108 .  
         [0033]    At step  108 , resistors  24  are cured by placing circuit board  12  in an infrared oven for several minutes at a temperature between 200° C. and 300° C. The curing process removes a substantial amount of the organic vehicle from resistors  24 . Once resistors  24  are cured, process  100  then continues to step  110 .  
         [0034]    At step  110 , resistor  26 , also known as potentiometer  26  is applied to circuit board  12  by screen-printing, pad printing or spraying PTF material onto circuit board  12 . Resistor  26  is in the form of a potentiometer, which has a conductive contact (not shown) either exterior to circuit board  12  or connected thereto that traverses the surface of resistor  26  thereby conducting a potential to the conductive contact. Process  100  then continues to step  112 .  
         [0035]    At step  112 , resistor  26  is cured by placing circuit board  12  in an infrared oven at a temperature between 200° C. and 300° C. for the removal of most, if not all, of an organic vehicle that is contained in the PTF material. Process  100  then continues to step  114 .  
         [0036]    At step  114 , printed circuit board  12  is subjected to a cleaning operation, which involves solvents and air-drying to clean circuit board  12 . The cleaning of circuit board  12  removes copper oxidation, which may be present as a result of some of the applications of resistive inks and/or the curing process. Cleaning printed circuit board  12  also enhances the adherence of coating  34  to be applied at step  118 .  
         [0037]    Coating is selectively applied. Selected areas are masked as a result.  
         [0038]    At step  118 , coating  34  is applied to circuit board  12 , particularly to coated area  32 . The application of coating  34  to coated area  32  is implemented by brushing, spraying, screen-printing, dipping or pad printing. Alternatively, coating  34  may be applied to coated area  32  with a numerically controlled device, which selectively coats area  32  on circuit board  12 , leaving uncoated areas  28  and  30  without coating. Coating  34  conforms to the surface and/or component to which it is applied. Where applied, coating  34  is in continuous intimate physical contact with resistor  24 , conductor  16 , conductive pads  22  and substrate  12 . Coating  34  includes at least one of a dielectric material, an encapsulant, a conformal coating or a solder mask type material. Coating  34  adheres to the surface of substrate  12 , resistors  24  and conductors  16  as coating  34  cures.  
         [0039]    At step  122 , circuit board  12  is electrically tested by selectively applying electrical voltages, currents and/or signals to connective pads  14  and to the surface of resistor  26 . Process  100  is then complete, and circuit board assembly  10  is entered into the manufacturing flow, being subject to other manufacturing processes or assembly procedures.  
         [0040]    Coating  34  advantageously reduces the environmental impact due to the atmospheric contact with resistors  24 . Also, advantageously, coating  34  prevents oxygen from reaching the interface of conductive pads  22  and resistors  24 , thereby reducing oxidation at that juncture. The resulting protection by coating  34  limits the variability of resistors  26  due to environmental variations, thereby providing a more stable electrical circuit on circuit board assembly  10 .  
         [0041]    Variations and modifications of the foregoing are within the scope of the present invention. It is understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention. The claims are to be construed to include alternative embodiments to the extent permitted by the prior art.  
         [0042]    Various features of the invention are set forth in the following claims.