FIELD OF THE INVENTION 
     This invention relates to ultra-reliable pressure sensor assemblies for use under adverse conditions including high vibration and corrosive fluids. 
     BACKGROUND OF THE INVENTION 
     As disclosed in U.S. Pat. No. 5,935,189, for example, pressure sensors may be employed to sense the exhaust pressure of internal combustion engines. These exhaust gases contain moisture and highly corrosive gases. It has previously been proposed to use semiconductor pressure sensors with a very thin layer of glass passivation on the surface thereof. In addition, gel has been employed to further protect against corrosion, see U.S. patent application Ser. No. 09/204,326. However, as discussed below, these sensors have been subject to corrosion degradation despite the glass coating, with or without the additional gel. 
     Incidentally, sensors per se are disclosed in U.S. Pat. Nos. 5,929,498 and 5,578,843, as well as in U.S. patent application Ser. No. 09/704,376, filed Nov. 2, 2000 (Docket 410005-144), all of which are assigned to the assignee of the present invention. The specification and drawings of these prior patents and patent application are hereby incorporated into this application by reference. It is also noted that vacuum deposited polymeric coatings have previously been used to protect electronic semi-conductive chip assemblies. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, it has been discovered that, in the adverse environment of automobile exhaust, gases, semi-conductor pressure transducers, even with glass passivation and gel protection, are subject to significant degradation due to the penetration of moisture through the gel, over time. More specifically, when the leads are sonically bonded to the conductive pads on the chips, tiny cracks may be formed which permits entry of corrosive gases and moisture. Also, pin hole openings may be present in thin glass layers. 
     To overcome this problem it has been determined that additional effective anti-corrosion protection may be obtained through the use of an additional vacuum deposited polymeric coating in addition to a glass layer and gel. Accordingly, a sensitive, reliable, corrosion proof pressure sensor system for the adverse vibration and corrosive intensive environment of internal combustion engines may be realized by the use of a semi-conductive pressure diaphragm and associated electronics mounted on a substrate, with conductive wire bonding pads and sonically welded leads, and with glass passivation over the substrate, an additional gel, and a vapor deposited polymeric coating being provided to insure complete protection of the semi-conductive components and the lead bonding areas. Preferably, the semi-conductive material is protected against oxidation by the thin glass layer, the vacuum formed polymeric layer is deposited, and finally a layer of gel overlies the subassembly. 
     In accordance with a feature of the invention, a corrosion proof pressure transducer system has a pressure transducer including a chip having a variable capacitor including a diaphragm, preferably a semi-conductive diaphragm, and conductive pads, thereon, together with additional electronic circuitry coupled to the variable capacitor and pads, with the chip having glass passivation thereon, wires sonically bonded to the pads, and a vacuum deposited polymeric coating extending over the chip including the pads, wire connections and leads; and with gel being optionally provided to overly the chip, either before or after the deposition of the polymeric coating. 
     To summarize, it would normally be expected that the glass passivation layer and a layer of gel would be adequate to preclude corrosion degradation of the pressure sensor. However, it has been determined that, with (1) cracks produced by wire bonding, (2) the vibration present in an automobile exhaust system, together with (3) the very corrosive exhaust gases and moisture, the further protection as outlined above is desirable to provide a long life, reliable and dependable pressure sensor. 
     Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic showing of an engine with pressure transducers coupled to the exhaust system of the internal combustion engine; 
     FIG. 2 is a diagrammatic showing of a semi-conductive pressure sensitive diaphragm; 
     FIG. 3 is a showing of a chip, including a ceramic substrate and semi-conductive components thereon including wires bonded to conductive pads on the chip; 
     FIG. 4 is an enlarged showing of a lead sonically bonded to a pad on an electronic chip; 
     FIG. 5 is a diagrammatic showing of a chip including a semi-conductive diaphragm protected with glass passivation, vacuum deposited polymeric coating and gel. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring more particularly to the drawings, FIG. 1 shows an internal combustion engine  12  with dual exhaust pipes  14  and  16 , and with capacitive sensors  18  and  20  mounted to sense the pressure in the exhaust pipes  14  and  16 . The exhaust pipes  14  and  16  are joined and lead to the catalytic converter  22  on route to the final exhaust from the system which may of course be mounted in an automobile. 
     Referring now to FIG. 2 of the drawings, this figure is the same as FIG. 7 of U.S. Pat. No. 5,929,498 granted Jul. 27, 1999, and assigned to the assignee of the present invention. In FIG. 2, the diaphragm  32  has variable pressure applied to it, and in accordance with this variable pressure, it flexes inwardly toward the cavity  34  in the semi-conductive structure  36 . The flexing of the diaphragm  32  varies the electrical conditions of the semi-conductive unit shown in FIG.  2  and provides output signals corresponding to the changes in pressure applied to diaphragm  32 . For further details of the mode of operation of the assembly of FIG. 2, reference is made to U.S. Pat. No. 5,929,498, as mentioned above. 
     FIG. 3 shows an electronic chip  42  with the diaphragm  32  of FIG. 2 appearing as a circle in this embodiment. The nearby smaller circle  44  is a reference capacitor. A number of additional semi-conductive components are mounted on the chip  42 , and the conductive pads  46  are shown with leads  48  sonically bonded thereto. The leads  48  provide power to the electronic components on the chip  42 , and also provide the output from the chip with signals corresponding to the changes in pressure applied to the diaphragm  32 . 
     FIG. 4 is an enlarged showing of a lead  48  sonically bonded to the conductive pad  46 . In the course of sonically bonding the lead  48  to the pad  46 , note that the pad  46  is subject to cracks and openings  50  resulting from the sonic bonding step. When the cracks are created, the area becomes vulnerable to chemical damage due to corrosion resulting from noxious gases and associated moisture or water vapor entering the opening  50 . Where this is repeated on a number of the pads  46  as shown in FIG. 3, the result may be significant degradation of the function of the pressure sensor over time. 
     Referring now to FIG. 5 of the drawings, this is a diagrammatic showing of a pressure sensor assembly  62  including the chip  64  having a diaphragm  66 . With pressure being applied as indicated by the arrow  68 , the diaphragm  66  will flex, and an indication of the applied pressure will be transmitted over the leads  70 , which correspond schematically to the leads  48  as shown in FIG. 3, for example. On the active upper surface of the chip  64  is a very thin glass passivation layer which is so thin as to be essentially represented by the upper line indicating the upper surface or boundary of the chip  64 . Following sonic bonding of the leads  70  to the pads on the chip  64 , a vacuum formed polymeric coating  72  is applied to the chip  66  and the conductive leads  70 . Overlying the upper surface of the chip  64  is a layer of gel  74  which will conduct or transmit applied pressure as indicated by arrow  68  to the diaphragm  66 . Overlying the gel layer  74  is shown an optional additional layer  76  of vacuum formed coating of a polymeric dielectric material. 
     In the assembly of FIG. 5 two vacuum deposited polymeric dielectric coating layers  72  and  76  may be provided, or only one of them may be employed. This vacuum deposited polymeric dielectric coating may be a product known as Parylene™ available from Speedline Coating Systems, 4435 East Airport Drive, Suite 100, Ontario, Calif., 91761-8157. 
     In conclusion, in the foregoing detailed description and in the accompanying drawings, preferred embodiments of the invention have been disclosed. Various changes and modifications may be made without departing from the spirit and scope of the invention. Thus, by way of example and not of limitations, the vacuum deposited coating may be applied only as indicated reference numerals  72  and on the leads  70  and the additional coating  76  may be omitted. Alternatively, the vacuum deposited coating  72  may be omitted and the coating  76  may be applied, or both coatings may be employed. In addition, other coatings which form thin integral coatings may be employed instead of the specific vacuum formed coating identified hereinabove. Further, instead of the specific capacitive semi-conductive diaphragm assembly as shown in FIG. 2 of the drawings, other pressure sensors using flexible diaphragms may be employed. Accordingly, the present invention is not limited to the precise embodiments described in detail hereinabove and shown in the drawings.
A corrosion proof pressure transducer for measuring exhaust gas pressure includes a chip with a semiconductive diaphragm, electronics, and conductive pads thereon; with leads sonically bonded to the pads. To preclude degradation of the transducer by internal combustion engine exhaust gases, the chip may have (1) a thin glass passivation layer, (2) a vacuum deposited polymeric coating and (3) a layer of gel, thereon.