Abstract:
A piezoresistive pressure sensor that uses a protective gel to protect the piezoresistive device is susceptible to lead wire failure by vibration-induced waves in the protective gel. Such waves can be reduced and the device made more robust by the use of three-dimensional structures in the gel, which are configured to reduce and/or re-direct vibration-induced pressure waves in the gel. The structures are referred to as “breakwaters” in that they protect lead wires and lead wire connections from wave fronts and the damage that wave-induced pressure on the lead wires causes.

Description:
BACKGROUND 
       [0001]      FIG. 1  is a perspective view of a pressure sensor  2  used to measure a pressurized liquid or gas. The sensor  2  is comprised of an injection-molded plastic housing  4  having two attachment flanges  6 . Through-holes  8  in the flanges  6  receive attachment screws, not shown, which allow the pressure sensor  6  to be attached to a surface, such as an engine manifold. A cover  10  is ultrasonically welded or slygard bonded over a cavity  16 , which encloses a diaphragm-type pressure sensor element  14  for the pressure sensing. For differential pressure sensing, pressurized gases and/or liquids flow through a hollow port  51  on the backside of the housing  4  (See  FIG. 9 ) and reach the backside of the diaphragm of the pressure sensor element  14  while ambient gases, typically air surrounding the sensor  2 , flow through the opening  9  in the cover  10  and reach the topside of the diaphragm of the pressure sensor element  14 . The two pressures, i.e., in the hollow port  51  and ambient pressure, exert forces on the diaphragm of the pressure sensor element  14  mounted in the cavity  16  and cause diaphragm stresses and diaphragm deformation. 
         [0002]    The diaphragm-type pressure sensor element  14  includes a piezoresistive transducer, the resistance of which changes in response to diaphragm deflection caused by pressure applied to the diaphragm. The piezoresistive element&#39;s resistance changes are converted into measurable electrical signals by circuitry in an integrated circuit (IC)  18  co-located within the cavity  16  and which is connected to the sensor  14  via lead wires  20  that extend between the sensor  14  and the IC  18 . Electrical signals generated by the IC  18 , and which represent a pressure applied to the sensor  14  are carried from the IC  18  to a lead frame not shown in the figure via lead wires  20  that extend from the IC  18  to a lead frame  21  inside the cavity but not visible in  FIG. 1 . 
         [0003]    The lead wires  20  used to connect the sensor  14  to the IC  18 , and which are used to connect the IC  18  to the lead frame  21 , are thin. Regardless of how the lead wires  20  are connected to the lead frame  21 , the attachment of the lead wires  20  to the lead frame  21  is susceptible to failure if the lead wires  20  are subjected to mechanical stresses. 
         [0004]    Inasmuch as the pressure sensor  2  is used to measure pressures of liquids and gases that are known to have corrosive chemicals in them, the cavity  16  is substantially filled with a gel  22 , not visible in  FIG. 1  and which covers the sensor  14 , the IC  18 , the lead wires  20  and the lead frame  21 . The gel  22  acts to protect the devices and connections inside the cavity  16  from corrosive chemicals in liquids and gases, the differential pressure of which in port  51  is being measured. 
         [0005]    While the gel  22  is effective in protecting electronic devices and connections from chemicals, the gel  22  is also effective in transmitting throughout the cavity  16 , vibration and shock waves that the sensor  2  might be subjected to, especially when the sensor  2  is used to sense the various pressures commonly found in motor vehicles. Harmonic vibration and random vibration are usually present in a motor vehicle under standard operating conditions. The high frequency vibration of a motor vehicle causes the gel  22  to vibrate within the cavity  16 . In some circumstances, sudden loading or impact within, or to the vehicle, causes vibration waves of much greater amplitude than those present during normal operation. In other words, relatively low amplitude harmonic vibration waves occur in a vehicle under normal operating conditions, while high amplitude shock waves are a more random occurrence. Road surface irregularities, engine vibration and door closures are just three sources of impacts and vibrations that can create waves in the gel  22  that cause connection failure between the lead wires and IC  18 , sensor  14  and lead frame  21 . Wave fronts that are induced within the cavity propagate through the gel  22  and strike the lead wires causing them to break. These waves apply normal or near normal forces on the lead wires causing a combination of tensile and bending stress on the lead wires and their bond to the substrate. Two modes of failure occur in the bond between the lead wires and substrate as well as the lead wires themselves. Harmonic vibration and random vibration create completely reversed cyclic loading thereby fatiguing the bonding material between the lead wires and the substrate. Under fatigue loading the bond between the lead wires and the substrate exceeds bonding material&#39;s endurance limit and failure of the connection occurs. Failure of the bond between the lead wires and the substrate is also caused by shock loading. In the case of shock loading, fracture stress of the bonding material is exceeded and causes connection failure to occur. 
         [0006]      FIG. 2  is a top view of the cavity  16 . Reference numeral  30  identifies wave fronts that are induced by either shock, vibration or both. Since a shock wave is in many respects the same as a vibration wave, for purposes of brevity, such wave fronts are considered hereinafter to be vibration-induced wave fronts. The vibration-induced wave fronts are depicted in  FIG. 2  as originating from the right-hand side of the cavity  16  and as traveling toward the left-hand side and thereafter, back and forth. Reference numerals  32  and  32 B, hereafter collectively referred to as “ 32 ” identify reflected and/or refracted wave fronts. As set forth above, reference numeral  20  identifies lead wires that connect the sensor element  14  to the integrated circuit  18  and which also connect the integrated circuit  18  to lead frame  21 . The lead frame  21  passes through the sidewall  28  of the cavity  16  where connections are made to the socket, not shown in the figures. 
         [0007]    The reflected/refracted wave fronts  32  are shown in  FIG. 2  as impinging upon the lead wires  20 - 1  orthogonal to, or nearly orthogonal to the wires  20 - 1 . When wave fronts ( 30  or  32 ) in the gel  22  impact the wires, they exert a lateral force on the wire that is proportional to the gel pressure at the wave front. A lateral force is thus exerted on the wires, the magnitude of which is equal to the product of the wave front pressure and the area of the wire to which the wave front pressure is applied. As set forth below, the wave fronts  30  and  32  that strike the lead wires  20  and  20 - 1  at right angles or approximate right angles therefore tend to exert lateral forces, which over time, fracture the lead wires and/or their attachment to the integrated circuit  18 , the sensor  14  and/or lead frame  21 . 
         [0008]      FIGS. 3A and 3B  are graphical depictions of the forces exerted on a “long” lead wire  20 A and a “short” lead wire  20 B. Lateral forces from the wave fronts ( 30  or  32 ) are distributed over the length of the wire and represented in the figures by the arrows identified by “F.” While the wave fronts ( 30  or  32 ) can strike the wires at any angle, the force that is orthogonal to the wire&#39;s axial length is the force that tends to break the wire and/or its bond due to the lateral displacements D 1  and D 2  that a force normal to the wire&#39;s axis tends to cause. 
         [0009]    As is well known, the total force F exerted on a surface of area A, by a pressure of magnitude P acting uniformly over the entire area, is the product of P and A. In other words, 
         [0000]    
       
      
       F=P×A  
      
     
         [0000]    where F is the force on an area A under a uniform pressure P. 
         [0010]    In the cavity  16 , since the gel edge is considered herein to be essentially “anchored” to the sidewall, the wave front pressure P is proportional to the gel acceleration a multiplied by the gel density ρ times the “length” of the gel, which is the width of the cavity  16 . Stated another way, 
         [0000]    
       
      
       P∝ρ×a×L  
      
     
         [0000]    where ρ is the density of the gel  22 , a is the acceleration of the gel and L is the “length” of the gel, (i.e., the length or width of the cavity  16 ). 
         [0011]      FIGS. 3A and 3B  show that for a given wave-front pressure, the total force exerted on a “long” lead wire will be greater than the total force exerted on a “short” lead wire. The wave fronts  30  and  32  that strike long lead wires thus tend to cause such wires and/or their connections to fail. A method and/or apparatus to reduce vibration-induced wave fronts in a cavity  16  containing gel  22  and/or redirect such wave fronts would be an improvement over the prior art in that reducing or redirecting wave fronts would tend to reduce lead wire failure as well as reduce lead wire connection failure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of a pressure sensor; 
           [0013]      FIG. 2  is a top view of a cavity used in a pressure sensor of  FIG. 1 ; 
           [0014]      FIGS. 3A and 3B  depict forces on lead wires from vibration-induced wave fronts; 
           [0015]      FIGS. 4A and 4B  are graphical depictions of the acceleration amplitude or power spectral density impressed on lead wires by random vibration and harmonic vibration respectively; 
           [0016]      FIG. 5  is a perspective view of a cavity  16  in a pressure sensor  2 , which is provided with breakwaters to control vibration-induced wave fronts; 
           [0017]      FIG. 6  is a top view of the cavity shown in  FIG. 5 ; 
           [0018]      FIG. 7  is a top view of an alternate embodiment of a breakwater; 
           [0019]      FIG. 8  is a top view of a cavity showing yet another embodiment of a breakwater; and 
           [0020]      FIG. 9  is a side view of a preferred embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 4A  is a plot of power spectral density applied to a lead wire as a function of the frequency of wave fronts caused by random vibrations.  FIG. 4A  shows that as the frequency increases above a critical value f 1 , the power spectral density of a lead wire increases linearly to a second frequency f 2 . At frequencies above f 2  random-vibration-induced power spectral density gradually decreases linearly. 
         [0022]      FIG. 4B  is a plot of acceleration applied to lead wire as a function of the frequency of wave fronts caused by harmonic vibration. Acceleration increases rapidly at frequencies above f 3 , but stays relatively constant at frequencies above a fourth frequency f 4 . 
         [0023]      FIG. 5  is a perspective view of the cavity  16 , provided with a first embodiment of a structure for reducing and/or re-directing vibration-induced pressure waves. In  FIG. 5 , two parallelepipeds  24 A extend outwardly from the sidewall  28  at an angle, and upwardly from the floor or bottom  26  of the cavity  16  and into the gel  22 . Two other rectangular parallelepipeds that are identified by  24 B also extend outwardly from the sidewalls  28  and upwardly from the floor or bottom  26 . In  FIG. 5 , the second parallelepiped extend into the cavity  16  and thus into the gel  22 , toward the lead wires  20  extending from the pressure sensor  14  to the integrated circuit  18 . 
         [0024]    As shown in the figure, both pairs of parallelepipeds  24 A and  24 B extend upwardly from the floor or bottom  26  of the cavity  16 . They are also considered herein to extend laterally or sideways from the sidewall  28  into the cavity  16 . The parallelepipeds  24 A and  24 B are also considered to be “protrusions” or “protuberances” into the cavity  16  and thus into gel  22  that is within the cavity  16 . They are also considered to be “in” the gel  22 . 
         [0025]      FIG. 6  is a top view of the cavity  16  shown in  FIG. 5 . When viewed from the top as shown in  FIG. 6 , the first pair of parallelepiped-shaped protuberances  24 A appears to define or depict the cross section of a funnel, the opening of which is adjacent to the lead frame  21 , the narrow portion of which is adjacent to the IC  18 . Vibration-induced wave fronts are identified by reference numeral  30 . Reflected and/or refracted wave fronts are identified by reference numeral  32 . 
         [0026]    When a vibration-induced wave front  30  strikes the narrow end of the “funnel” formed by the two structures identified by reference numerals  24 A. The protuberances  24 A in effect, create a point source for another spherical wave front that propagates from the opening between the protuberances  24 A. The width and location of the opening relative to the lead wires causes the tangent of the spherical wave front to be perpendicular to the direction of the lead wires. The force normal to the lead wires is thereby minimized preventing fatigue or sudden fracture of the lead wires to occur. The area adjacent to  24 A and opposite the lead wires also reflects the on-coming wave  30 . The reflected waves are 180 degrees out of phase of the on-coming wave front  30  thus causing destructive interference. This interference serves to reduce the amplitude of wave  30  thereby reducing impact force and preventing fatigue failure and sudden fracture of the lead wires to occur. The three-dimensional structures identified by reference numerals  24 A and  24 B thus effectively reduce vibration-induced pressure waves in the gel  22 , with respect to the lead wires  20 . With respect to the lead wires  20 , they also re-direct pressure waves, specifically including vibration-induced pressure waves. 
         [0027]    Merriam-Webster&#39;s 11 th  Collegiate Dictionary defines “breakwater” as an offshore structure, such as a wall, which protects a harbor or beach from the force of waves. A breakwater thus protects objects within the breakwater or behind the breakwater, from forces caused by wave fronts that strike the breakwater. As used herein and for purposes of claim construction, a breakwater is a structure in a gel  22  in a cavity  16  of a pressure sensor  2 , which either reduces or eliminates vibration-induced pressure waves in the gel  22 . A breakwater may also re-direct waves and/or wave fronts in the gel  22 . In  FIG. 5  and  FIG. 6 , the structures identified by reference numerals  24 A and  24 B are considered to be “breakwaters” because they either reduce the magnitude of vibration-induced waves or re-direct vibration-induced waves or they do both. In so doing, they protect lead wires  20  from the force of vibration-induced waves in the gel  22 , which would otherwise be applied to the lead wires  20 . 
         [0028]    In  FIG. 6 , the breakwaters  24 A and  24 B cause reflected and/or refracted waves  32  to be created from the original, vibration-induced waves  30 . Waves  30  impact the break waters  24 A and  24 B. Reflected waves are 180 degrees out of phase of incoming wave front  30  and thus interfere with on-coming waves destructively. The destructive interference serves to reduce the amplitude of on-coming wave  30  thereby reducing the force of impact on the lead wires. In the situation of fluid flow past a stationary wall, the wall also serves to decrease force of wave impact on the lead wires. In at least one embodiment, taller breakwaters might prevent gel from overflowing. 
         [0029]    As is well known to those of ordinary skill in the mechanical arts, the velocity of a viscous fluid flow at the solid boundary of a surface is zero. The velocity of the gel  22  at the surfaces of the breakwaters  24 A and  24 B will therefore also be zero relative to that solid boundary. In fluid mechanics, this phenomenon is commonly referred to as the no slip boundary condition. The velocity profile of the on-coming fluid flow of wave  30  increases as the distance from the stationary wall increases. In other words the close proximity of break waters  24 A and  24 B will also effectuate a decrease in fluid flow velocity, thereby decreasing the amount of force the waves can apply to the lead wires, preventing connection failure. 
         [0030]    The directions of reflected waves  32  are also different because the angle at which the on-coming incident wave  30  strikes the surface of the breakwaters is equal to the angle at which the incident waves  30  are reflected. The reflected waves, therefore, interfere with and divert the direction of the on-coming wave  30 . The angle of the breakwater relative to the sidewall and incident wave has significant influence on the vector and thus impact force of the on-coming wave. Stated another way, the breakwaters  24 A and  24 B are configured to reduce vibration-induced pressure waves  30  and  32  in the gel  22 , at least where those waves  30  and  32  would otherwise impinge on the lead wires  20  tending to cause them to break. 
         [0031]      FIG. 7  is a top view of an alternate embodiment of a third style of breakwater  24 C. The breakwaters  24 A shown in  FIGS. 5 and 6  is supplemented with a breakwater structure  24 C having a non-rectilinear shape that extends outwardly from the sidewall  28  and upwardly from the floor  26 , narrowing the cavity  16  and directing or focusing the vibration-induced wave fronts  30  into the first breakwater structures  24 A that extends from the sidewall  28  at an angle. The addition of  24 C into the cavity adjacent the IC  14  and  18  also serves to reduce velocity and thus impact force of vibration wave  30 . The no slip boundary condition described above, provides that the velocity of viscous fluid flow at a solid boundary is zero relative to the boundary. When vibration wave  30 , i.e. fluid flow occurs, the no slip boundary condition applies to the flow across the surface of  24 C. The velocity of fluid flow will increase as the distance from  24 C increases. Since the overall width of the cavity is decreased, however, the overall velocity of wave  30  is reduced. The decrease in velocity will decrease the amount of force that is applied to the lead wires thus preventing fatigue failure or sudden fracture of lead wires and/or bonding material. 
         [0032]      FIG. 8  shows a top view of yet another embodiment of a breakwater  34 . The breakwater  34  shown in  FIG. 8  is a substantially trapezoidal-shaped structure having sidewalls  36  inclined at angles, relative to the sidewalls  28 . As can be seen in this figure, the reflected/refracted wave fronts  32  from the sidewalls  36  are of a reduced magnitude and hence less likely to fracture lead wires  20 . Wave fronts are reflected off the inclined sidewalls and thus tend to reduce and re-direct vibration-induced pressure waves. 
         [0033]    Referring now to  FIG. 9  there is shown a side view of a pressure sensor  2 , similar to the one shown in  FIGS. 1 and 2 , albeit with the addition of breakwaters  24  shown in  FIGS. 4 and 5 . As can be seen in this figure, the “depth” of the gel  22  in the cavity  16  is lower than the height of the breakwaters  24  such that the gel  22  and breakwaters  24  do not interfere with the attachment of a cover  10  over the cavity  16 . 
         [0034]    Those of ordinary skill will recognize that the breakwater  34  depicted in  FIG. 8  as well as the breakwaters  24 A,  24 B and  24 C are just some examples of structures that will reduce and/or re-direct vibration-induced pressure waves in a gel  22 . Those of ordinary skill will also recognize that other structures can also function as breakwaters. Such structures include, but are not limited to, a cube, a parallelepiped, a pyramid, a frustum of a pyramid, segments of a sphere, a hemisphere, a truncated cylinder, a cone and a frustum of a cone. For purposes of claim construction, protrusions from any surface within the cavity  16  and which extends into or is inside the gel  22  are considered to be structures equivalent to each other in that they all would reduce and/or re-direct vibration-induced pressure waves in the gel. 
         [0035]    In some embodiments, the breakwater structures are formed as part of an injection molding process used to form the sensor housing  4 . In other embodiments, the breakwater structure can be applied using adhesive or ultrasonic welding. They can also be formed by machining. The breakwater structures can be attached to either both the sidewall  28  and the bottom  26 , or to only one of the sidewall  28  and the bottom  26 . 
         [0036]    In a preferred embodiment 2, the vibration-induced pressure waves have a frequency range from about 5 Hz up to about 2000 Hz, with a vibration amplitude up to about 300 m/s 2  and power spectral density up to 20 (m/s 2 ) 2  per hertz. The protective gel  22  has a dynamic viscosity between about 100 cP (centipoise) and about 100 kcP. 
         [0037]    In the foregoing examples, a method for reducing vibration-induced pressure waves in a pressure sensor  2  includes of course reducing and re-directing vibration-induced pressure waves in the gel  22  by using a breakwater in the gel  22 . The breakwater can be any sort of three-dimensional structure that extends into gel from a surface of the sensor housing that encloses the gel. 
         [0038]    The foregoing description is for purposes of illustration only. The true scope of the invention is set forth in the appurtenant claims.