Patent Publication Number: US-7213323-B2

Title: Method of forming an electronic pressure sensitive transducer on a printed circuit board

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional application Ser. No. 60/267,455 filed Feb. 8, 2001 which is incorporated herein by reference. This application is a divisional of U.S. patent application Ser. No. 10/067,952, filed Feb. 5, 2002, now U.S. Pat. No. 6,909,354. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The Present invention relates to transducer systems for sensing pressure. 
   2. Background Art 
   Pressure sensitive transducers generate a signal indicative of the amount of pressure applied to a flexible membrane. Such transducers may also generate a signal based on the location of pressure applied to the flexible membrane. Such pressure sensitive transducers provide inputs for a wide variety of applications such as remote controls, game controllers, mouse pads, tactile sensors, and the like. Pressure sensitive transducers are typically coupled with electronics that condition and amplify pressure signals. 
   Various constructions for pressure sensitive transducers are possible. One type includes one or more force sensing resisters (FSRs). Various FSRs have been disclosed, such as those described in commonly assigned U.S. Pat. Nos. 4,314,227; 4,314,228; and 4,489,302; each of which is hereby incorporated by reference in its entirety. Typically, an FSR is composed of three parts, a rigid base, a spacer, and a resistive membrane. Conductive traces are typically arranged in separated interdigitated sets on the base. These traces may be configured in a single zone or in multiple zones to allow, for example, pointing devices as described in commonly assigned U.S. Pat. Nos. 5,659,334 and 5,828,363, each of which is hereby incorporated by reference in its entirety. The flexible resistive membrane is spaced apart from the base layer by a spacer, which is typically a ring of material around the outer edge of the conductive traces. The spacer is also typically coated with adhesive to hold the device together. The flexible top membrane may be made of a polymer coated on its inner face with semi-conductive or resistive ink, giving the FSR force sensing properties. This ink is described in commonly owned U.S. Pat. Nos. 5,296,837 and 5,302,936, each of which is hereby incorporated by reference in its entirety. 
   In most practical applications, the FSR must be connected to sensing and conditioning electronics in order to effectively operate. One way this may be accomplished is by connecting the FSR to a printed circuit board containing the electronics with a multi-conductor cable. Another way of connecting the FSR to support electronics is to adhere the FSR base directly to the circuit board containing the electronics. Electrical connection may be made between traces on the FSR and corresponding traces on the printed circuit board using z-tape, which only conducts in a direction perpendicular to the tape surface. While either method is effective, both have unnecessary manufacturing steps and require unnecessary components, thus increasing the cost of a pressure sensitive transducer system as well as increasing the likelihood of system failure. What is needed is a pressure sensitive transducer and a method for making such a transducer that requires fewer components and fewer manufacturing steps without sacrificing transducer performance. 
   SUMMARY OF THE INVENTION 
   The present invention decreases the cost and complexity of an electronic pressure sensitive transducer by constructing such a transducer directly on a printed circuit board containing support electronics. 
   An electronic pressure sensitive transducer producing an electrical signal indicative of applied pressure is provided. The transducer includes a printed circuit board accepting a plurality of electronic elements for processing the transducer electrical signal. Conductive traces are formed on the printed circuit board to define a contact area. A flexible substrate having an inner surface is positioned over the contact area. An adhesive spacer substantially surrounds the contact area. The adhesive spacer attaches the flexible substrate to the printed circuit board. At least one resistive layer is deposited on the flexible substrate inner surface. The resistive layer contacts at least two of the traces in response to pressure applied to the flexible substrate to produce the electrical signal indicative of applied pressure. 
   In an embodiment of the present invention, at least one resistive layer is made with resistive ink. 
   In another embodiment of the present invention, a pedestal is formed on the printed circuit board substantially around the contact area. The pedestal receives the adhesive spacer for attaching the flexible substrate. The pedestal increases the space between resistive layers on the substrate and conductive traces on the printed circuit board. The pedestal may be formed by coating traces on the printed circuit board with a non-conductive material such as soldermask. 
   In yet another embodiment of the present invention, the conductive traces include a plurality of sets of traces. Each set of traces is interconnected within a zone of the contact area. An interconnected set of contact traces extends into each zone. At least one of the interconnected set of traces may be connected to the electronic elements for processing the transducer signal via a through-hole in the printed circuit board. The through-hole may be within the contact area. 
   In still another embodiment of the present invention, conductive traces are arranged in sets of interconnected traces. At least two sets of traces are interdigitated. 
   In a further embodiment of the present invention, conductive traces comprise copper traces covered with an oxidation preventing conductive material. 
   In a still further embodiment of the present invention, conductive traces comprise screen printed carbon ink. 
   A method of forming an electronic pressure sensitive transducer on a printed circuit board is also provided. The printed circuit board accepts electronic components for producing signals generated by the pressure sensitive transducer. A plurality of conductive traces are formed on the printed circuit board to form a contact area. At least one resistive layer is deposited on an inner side of a flexible substrate. The flexible substrate is assembled on the printed circuit board such that the flexible substrate resistive layer is facing the printed circuit board conductive traces. The flexible substrate is held to the printed circuit board by an adhesive substantially surrounding at least a portion of the contact area. 
   A transducer system is also provided. The system includes a printed circuit board having a plurality of conductive traces. At least two of these traces define contact areas. The printed circuit board is constructed to accept electronic elements for processing electrical signals produced by a plurality of transducers. Each of these signals is indicative of pressure applied to at least one of the transducers. The system also includes at least one flexible substrate having an inner surface facing each contact area. At least one adhesive spacer substantially surrounds each contact layer to attach at least one flexible substrate to the printed circuit board. At least one resistive layer is deposited on a flexible substrate inner surface. Each resistive layer contacts at least two of the traces in response to pressure applied to the flexible substrate to produce an electrical signal indicative of applied pressure. The contact areas, at least one flexible substrate, at least one adhesive spacer and at least one resistive layer form the plurality of transducers, each transducer constructed on the printed circuit board. 
   A method of forming an electronic pressure sensitive transducer on a printed circuit board supporting electronic elements is also provided. A portion of conductive material on a printed circuit board is selectively removed to define traces in a contact area, traces connecting the electronic elements to the contact area, and at least a portion of a pedestal. 
   The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded cross-sectional view illustrating a conceptualized pressure sensitive transducer according to an embodiment of the present invention; 
       FIG. 2  is a top view of a printed circuit board illustrating a four-zone contact area according to an embodiment of the present invention; 
       FIG. 3  is a top view of a printed circuit board illustrating carbon traces according to an embodiment of the present invention; 
       FIG. 4  is a top view of a pressure sensitive transducer constructed using the printed circuit board of  FIG. 2  or  3  according to an embodiment of the present invention; 
       FIG. 5  is an exploded view illustrating conceptualized multiple single-zone FSRs in a pressure sensitive transducer according to an embodiment of the present invention; and 
       FIG. 6  is a top view of a printed circuit board implementing multiple single-zone FSRs in a pressure sensitive transducer according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
   Referring to  FIG. 1 , an exploded cross-sectional view illustrating a conceptualized pressure sensitive transducer according to an embodiment of the present invention is shown. An electronic pressure sensitive transducer, shown generally by  20 , produces an electrical signal indicative of applied pressure. Pressure transducer  20  includes printed circuit board  22  accepting a plurality of electronic elements, not shown for clarity, for processing the transducer electrical signal. Conductive traces  24  are formed on printed circuit board  22  to define contact area  26 . Flexible substrate  28  has inner surface  30  which faces contact area  26  when pressure transducer  20  is assembled. At least one resistive layer  32  is deposited on inner surface  30 . Adhesive spacer  34  substantially surrounds contact area  26 . Adhesive spacer  34  attaches flexible substrate  28  to printed circuit board  22 . When assembled, resistive layer  32  contacts at least two traces  24  in response to pressure applied to flexible substrate  28  to produce electrical signals indicative of applied pressure. 
   Flexible substrate  28  with resistive layer  32  together with traces  24  on printed circuit board  22  implement a force sensing resistor (FSR), shown generally by  36 . Integrating FSR  36  directly onto printed circuit board  22  creates a pressure sensor transducer which may be referred to as “sensor on board” (SOB). Such a construction eliminates some of the materials and manufacturing steps previously required to manufacture a pressure sensitive transducer. 
   Virtually any printed circuit board  22  can be adapted to receive FSR  36  according to the present invention, providing that sufficient surface space is available for FSR  36 . This includes both rigid and flexible circuit boards. Conductive traces  24  on printed circuit board  22  may be formed by any suitable means known in the art. For example, traces  24  may be formed by depositing conductive material on printed circuit board  22  then selectively removing a portion of the conductive material to define traces  24 . Dimensions for conductive traces  24  depend on the dimensions of FSR  36 , material and construction for flexible substrate  28 , material and construction for resistive layers  32 , and the like. Typical line thicknesses for conductive traces  24  range between 0.010 inches (0.25 mm) and 0.060 inches (1.5 mm). Typical line spacing between conductive traces  24  ranges between 0.010 inches (0.25 mm) and 0.060 inches (1.5 mm). Positional tolerance of FSR  36  on printed circuit board  22  varies as well, with ±0.015 inches (±0.4 mm) typical. 
   Two sets of interconnected traces  24  may be used to form a single zone within contact area  26 . Multiple zones within contact area  26  permit location of pressure on flexible substrate  28  to be determined. Multiple zones may be obtained by using a plurality of sets of interconnected traces  24  and one interconnected set of common traces  24  extending into each zone. Alternatively, each zone may be defined by two or more separate sets of traces  24 . Preferably, the sets of traces  24  in each zone are interdigitated. 
   Flexible substrate  28  may be constructed from any suitably flexible material such as, for example, Mylar. The thickness of substrate  28  varies depending on the application and dimensions of FSR  36 . Preparing substrate  28  for resistive layers  32  entails cutting material for substrate  28  into sheets or other shapes suitable for printing or other methods of depositing resistive layers  32 . 
   One or more resistive layers  32  are deposited on substrate  28  by conventional means such as, for example, by screen printing resistive ink onto substrate  28 . Resistive layers  32  may be printed over the entire substrate  28  or only over that portion which will cover operative traces  24  on printed circuit board  22 . Resistive layers  32  may also be deposited such that several distinct regions are formed over that portion of substrate  28  which will cover operative traces  24  on printed circuit board  22 . 
   Adhesive  34  is used to attach flexible substrate  28  to printed circuit board  22 . Adhesive  34  also provides spacing between resistive layers  32  on substrate  28  and traces  24  on printed circuit board  22 . Adhesive  34  may be applied to printed circuit board  22 , to flexible substrate  28 , or to both as suits manufacturability of FSR  36 . Adhesive  34  may be applied to either surface in a conventional manner such as, for example, by depositing a bead of adhesive  34  around some or all of the perimeter of contact area  26 . In a preferred embodiment, adhesive layer  34  comprises an adhesive ink screen printed onto substrate  28 . Adhesive inks that may be used include product numbers SP-7533 from 3M or ML25184 from Acheson Industries, Inc. of Port Huron, Mich. 
   If adhesive  34  is applied to substrate  28  in advance of final assembly, a protective release liner may be cut or positioned over adhesive  34  to prevent inadvertent adhesion to other surfaces or airborne materials. 
   Substrate  28  may be formed into strips and kiss cut, or partially cut, into the desired final shape. Individual substrates  28  may then be easily separated by hand or machine immediately prior to assembly onto printed circuit board  22 . Preferably, this final assembly step occurs after electronic components have been mounted on printed circuit board  22  to prevent heat damage to substrate  28  from soldering operations. 
   If additional space is required between printed circuit board  22  and substrate  28 , a pedestal, shown generally by  38 , may be formed on printed circuit board  22  in regions where substrate  28  is adhered to printed circuit board  22 . Pedestal  38  increases the distance that substrate  28  must be depressed prior to contact between resistive layer  22  and traces  24  on printed circuit board  22 . Pedestal  38  may be formed in a variety of manners. First, printed circuit board  22  may be manufactured with additional thickness to form pedestal  38 . Second, one or more layers of conductive traces  40  may be built on printed circuit board  22  to form pedestal  38 . Pedestal  38  is then covered with a non-conducting material  42 , such as soldermask, to prevent inadvertent short circuits between traces  40  and resistive layer  32 . Third, a polymer thick film may be deposited on printed circuit board  22  to form pedestal  38 . Fourth, a sheet of material may be adhered to printed circuit board  22  to form pedestal  38 . As will be recognized by one of ordinary skill in the art, many constructions for pedestal  38  are possible. 
   Referring now to  FIG. 2 , a top view of a printed circuit board illustrating a four-zone contact area according to an embodiment of the present invention is shown. Printed circuit board  22  includes traces  24  within contact area  26  forming four zones, each shown generally by  50 . Each zone  50  includes one set of traces  24  interconnected by line  52  which extends out of contact area  26  to through-hole  54 . Through-hole  54  permits traces  24  to be connected to electronics mounted on the bottom side of circuit board  22  by soldering processes known in the art. Traces  52  may also connect traces  24  to electronic elements on the same side of printed circuit board  22  as traces  24 . Each zone  50  may also share a common set of interconnected traces joined to electronics on the back side of circuit board  22  via through-hole  56  in the center of contact area  26 . 
   Traces  24  may be formed by any means which presents a conductive surface to resistive layer  32 . Since air typically fills the gap between printed circuit board  22  and flexible substrate  28 , traces  30  should resist corrosion. Traces  24  may be constructed, for example, by plating or coating copper traces with an oxidation preventing conductive material such as gold, silver, solder, carbon ink, and the like. Alternatively, traces  24  may be constructed by screen printing carbon, silver, or other conductive inks onto printed circuit board  22 . 
   Traces  24  and at least a portion of pedestal  38  may be fabricated at the same time and of the same materials. This may result in traces  24  and the base of pedestal  38  extending the same height above board  22 . The height of pedestal  38  may be increased by the addition of solder mask or similar material and adhesive spacer  34 . This typically will result in a distance between traces  24  and flexible substrate  28  sufficient to prevent inadvertent contact between resistive layer  32  and conductive traces  24 . Simultaneous cofabrication of pedestal  38  and traces  24  together with the remainder of circuit board  22  results in very low incremental cost to circuit board  22 . 
   Referring now to  FIG. 3 , a top view of a printed circuit board with carbon ink traces according to an embodiment of the present invention is shown. Printed circuit board  22  includes screen printed carbon ink traces  24  within contact area  26  forming four zones  50 . Due to the high resistivity of screen printed carbon ink, traces  24  are printed over copper pads  60  located just outside of contact area  26 . Copper traces  62  connect pads  60  to through holes  54  or to electrical components on the same side of printed circuit board  22  as traces  24 . 
   Referring now to  FIG. 4 , a top view of a pressure sensitive transducer constructed using the printed circuit board of  FIGS. 2  or  3  according to an embodiment of the present invention is shown. Flexible substrate  28  is shown adhered to pedestal  38  on printed circuit board  22 . This results in FSR  36  directly constructed on printed circuit board  22 . 
   Referring now to  FIG. 5 , an exploded view illustrating multiple single-zone FSRs in a pressure sensitive transducer according to an embodiment of the present invention is shown. Six FSRs are shown. Each contact area  26  includes two sets of interdigitated contacts  24 . Spacer  38 , in this example a thin plastic sheet coated with adhesive on both sides, is adhered to printed circuit board  22 . Spacer  38  includes an opening for each contact area  26 . Flexible substrate  28  is then adhered to spacer  38 . The entire inner surface  30  of flexible substrate  28  may be coated with one or more resistive layers  32 . Alternatively, separate disconnected resistive layers  32  corresponding with each contact area  26  may be formed on flexible substrate  28 . 
   Referring now to  FIG. 6 , a top view of a printed circuit board implementing multiple single-zone FSRs in a pressure sensitive transducer according to an embodiment of the present invention is shown. Printed circuit board  22  includes six contact areas  26  with traces  24  formed by screen printing carbon ink onto printed circuit board  22 . Copper pedestal  38  surrounds contact areas  26 . Pedestal  38  may have a wide variety of shapes, but typically mirrors the outline of flexible substrate  28 , not shown for clarity. Soldermask  42  covers much of circuit board  22  while leaving contact areas  26  exposed to contact resistive layer  32  on flexible substrate  28 . Printed circuit board  22  also contains electronic components, shown generally by  70 , soldered to printed circuit board  22  for receiving electrical signals from traces  24  indicative of pressure applied to the FSRs. 
   While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.