Abstract:
A three piece housing, designed to house semiconductor chips is molded using a polymer material. The chip sits on a metal base. Electrical leads pass through a molded housing to provide electrical contact between the semiconductor chip and external circuitry.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates, in general, to pressure sensors, and more particularly, to a pressure sensors mounted in polymer housings. 
     2. Description of the Related Art 
     Semiconductor pressure sensors have been around for some time, and are generally packaged in either a one-piece or two-piece housing. In both of these types of packages, the actual transducer device generally is mounted on a base formed from a polymer material. U.S. Pat. No. 4,655,088 entitled, Unibody Pressure Transducer Package, discloses a one piece housing for mounting a semiconductor transducer. The housing is molded from a polymer material. Electrical leads pass through the one piece housing to provide electrical contact between the semiconductor transducer and outside circuitry. The transducer is mounted on a base portion of the polymer housing. 
     One problem with mounting a semiconductor transducer in a polymer housing arises due to a mismatch in thermal coefficients of expansion (TCE). The transducer ordinarily is secured to the polymer base using room temperature vulcanization (RTV). The RTV adhesive acts as a buffer to compensate for significant differences in the TCEs of the polymer housing and the semiconductor transducer. Differences in TCEs can be important. For example, it is conceivable that a packaged semiconductor transducer will be mounted on the inside of an automobile tire. Certain regions of the country such as, Michigan in the winter may be very cold, and other regions of the country, such as Florida in the summer may be very hot. The performance of the semiconductor transducer may be impacted by temperature changes which cause the semiconductor transducer and the polymer to expand or to contract at different rates consistent with their different TCEs. 
     Another problem which may be experienced by a semiconductor transducer is interference from radio frequency (RF) signals. Using the above example, a packaged semiconductor transducer may be secured within an automobile tire in order to monitor pressure within the tire. Pressure readings provided by the sensor may be read using radio frequency signals. For example, an external device may transmit RF signals to and receive RF signals from a circuit board on which the pressure sensor is mounted within the tire. These same RF signals, however, may impact the performance of the transducer. Hence, there may be a need to protect the transducer from the effects of RF signals. 
     A challenge in the manufacture of pressure sensors has been achieving wire bonds between metal leads which pass through a housing and a semiconductor transducer mounted inside the housing. U.S. Pat. No. 4,686,764 entitled. Membrane Protected Pressure Sensor, discloses a semiconductor sensor mounted in a housing and wire bonded to a metal lead. One technique for achieving the wire bonding is to heat a lead frame which bears the metal leads in order to heat the semiconductor transducer sufficiently to affect a bonding between the lead, the wire bond and the transducer. This technique typically requires a transfer of heat from the metal lead to the housing and then to the semiconductor transducer. Unfortunately, there can be difficulties with this mechanism for the transference of heat energy since the housing material may transmit heat less efficiently than the metal lead. 
     Thus, there has been a need for an improved semiconductor sensor in which there is a better match of TCEs of a semiconductor transducer and a base on which the transducer rests, which can better withstand the effects of RF interference, and which can more effectively use wire bonding techniques during production. The present invention needs these. 
     SUMMARY OF THE INVENTION 
     A pressure sensor is provided in which a metal base pad is molded into a polymer housing. A semiconductor transducer is placed on the metal base pad. A cap is attached to the housing to form a protective chamber in which the transducer is mounted on the base pad. A vent hole provides access to a measurand environment. 
     The metal base pad has a thermal co-efficient of expansion closely matched to that of the semiconductor transducer. Moreover. when the metal base pad is electrically grounded it provides a degree of protection from RF signals. Additionally, during wire bonding, the metal pad serves as an effective conductor of heat to the semiconductor transducer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a pressure sensor in accordance with a first embodiment of the invention; 
     FIG. 2 is a top view of the pressure transducer of FIG. 1 with the silicone gel absent and the cap removed; 
     FIG. 3 is a circuit diagram showing the interconnection of transducer circuit elements and electrical leads for the semiconductor transducer FIG. 2; 
     FIG. 4 is a top view of the housing and lead frame assembly of the sensor of FIG. 2; 
     FIG. 5A is a top elevation view of an alternate embodiment of a housing and metal leads similar to the view of in FIG. 3 but without a transducer; 
     FIG. 5B is a side cross-sectional view along lines  5 B— 5 B of FIG. 5A; 
     FIG. 5C is a side cross-sectional view along lines  5 C— 5 C of FIG. 5A; 
     FIG. 6A is a top elevation view of a cap for either of the embodiments of FIGS. 1-4 or the embodiment of FIGS. 5A-5C; 
     FIG. 6B is a side cross-sectional view along lines  6 B— 6 B of FIG. 6A; 
     FIG. 6C is a side cross-sectional view along lines  6 C— 6 C of FIG. 6A; 
     FIG. 7 is a top elevation view of a lead frame structure in accordance with the invention; 
     FIG. 8 is an enlarged portion of the lead frame shown within the dashed line circle of FIG. 7; 
     FIG. 9 is a side view of the portion of FIG. 8 showing the “downset” base; and 
     FIG. 10 is a top elevation view of a lead frame like that of FIG. 7 with housing bodies molded thereon. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention comprises a novel pressure sensor with a transducer mounted on a metal base. The following description is presented to enable any person skilled in the art to make and use the invention. Descriptions of specific applications are provided only as examples. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     Referring to the illustrative drawings of FIG. 1 there is shown a cross-sectional view of a pressure sensor  20  in accordance with a first embodiment of the invention. The pressure sensor  20  includes a housing  22  molded from a polymer material. The polymer material can be any thermal plastic or thermal set material which is compatible with the environment in which the pressure sensor  20  is intended for use. The housing is penetrated by metal leads  24  and  25 . The metal leads are capable of providing electrical contact with external circuitry (not shown). In the presently preferred embodiment the leads are copper. One of the metal leads  25  is “downset” so as to form a base  26  on which a semiconductor transducer  28  is mounted. The transducer  28  is secured to the base by an adherent material which exhibits an acceptable TCE match with the respective TCEs of the downset metal lead  25 / 26  and the semiconductor transducer  28 . In a presently preferred embodiment, the material used to secure the semiconductor  28  to the base  26  is an epoxy cement  30  which firmly bonds the transducer  28  to the base  26 . 
     Wire bonds  32  provide electrical contact between the semiconductor transducer and the certain of the leads  24 . The electrical lead  25  that has been downset to serve as the base  26  does not require a wire bond. In addition, as explained below, one or more of the other metal leads  24  may lack wire bond connections. The housing  22  defines, together with the metal base  26 , a chamber in which the transducer  28  is mounted. The chamber is open at the top to permit insertion and bonding of the transducer. The chamber is substantially filled with a silicone gel  34  which serves to protect the transducer from damage and contamination and also serves as a pressure transfer medium. A cap  36  is secured over the open top of the housing  22  to enclose the chamber. A vent hole  38  extends through the cap into the chamber. The pressure of a measurand environment is applied to the transducer through the vent hole The silicone gel transfers the pressure directly to the transducer. 
     The TCE of the transducer  28  and the metal base  26  are relatively well matched. Moreover the material  30  selected to secure them together also is chosen to be well matched to the TCEs of the transducer and the metal base. Hence, the transducer  28  can operate over a relatively wide range of temperatures without a significant risk of physical stress due to mismatch of the TCEs of the semiconductor transducer and the metal base. Furthermore, by electrically grounding the metal lead  25  which has a downset portion that serves as the base  26 , the transducer can be shielded somewhat from the effects of RF signals. Finally, as explained below, by mounting the semiconductor transducer directly on a metal base, a better transfer of heat energy from the metal lead  25 / 26  to the semiconductor transducer  28  can be achieved during wire bonding. 
     Referring to the illustrative drawing of FIG. 2, there is shown a top view of the pressure transducer  20  of FIG. 1 without the silicone gel  34  and with the cap  36  removed. The transducer  28  is mounted on the metal base  26  within the housing  22 . Lead line  25  is downset to form the base  26 . The electrical leads  24  are shown to penetrate the housing  22  so as to form contact regions  40  for the wire bonds  32 . 
     The circuit diagram of FIG. 3 represents the circuit formed on the transducer  28  plus the metal leads wire bonded to the transducer. The resistors shown in the circuit diagram of FIG. 3 are circuit elements on the transducer  28 . The operation of this circuit will be readily understood by those skilled in the art and need not be explained herein. The nodes labeled +IN, and −IN, +OUT and −OUT represent the identically labeled metal leads in FIG.  2 . The metal leads of FIG. 2 labeled N/C are not interconnected with the transducer  28  in the current implementation. However, it will be appreciated that the number of leads and their interconnections to the transducer is not important to the invention. 
     Referring to the illustrative drawing of FIG. 4, there is shown a top view of the pressure sensor housing  22  superimposed on a top view of a lead frame assembly  42  similar to the view of FIG.  2 . The housing  22  is illustrated with solid lines, and the lead frame assembly  42  is illustrated with dashed lines. It can be seen that the base  26  nearly is an extension of one of the lead frame members. 
     Referring to the illustrative drawing of FIG. 5A, there is shown a top view of alternate embodiment of a pressure sensor  20 ′ in accordance with the invention. The view in FIG. 5A is similar to the view of FIG. 2 but with the transducer  28  removed. Elements in FIGS. 5A-5C that are the same as those of the first embodiment are labelled with primed reference numerals identical to those used to reference corresponding elements in the first embodiment. The housing  22 ′ is shown together with external portions  24 - 1 ′ of the metal leads that are external to the housing  22 ′ and internal portions  24 - 2 ′ that are internal to the housing  22 ′. The base  26 ′ is shown within the housing as well. 
     Referring to the illustrative drawing of FIG. 5B, there is shown a cross-sectional view of the alternative housing and lead frame taken generally along lines  5 B— 5 B of FIG.  5 A. The housing  22 ′ is similar in all respects to the housing  22  of the first embodiment described above, except that the alternative polymer housing  22 ′ extends fully beneath the metal base  26 ′. Thus, the base  26 ′ is embedded within the housing  22 ′ which extends beneath it. In essentially all other respects, the housings  22  and  22 ′ of the first embodiment and the alternative embodiment are the same. Thus, the following description of the alternative housing  22 ′ applies to the housing  22  of the first embodiment as well. 
     The housing comprises  22 ′ walls  42 ′ which upstand from the region of the metal base  26 ′. The walls  42 ′ are internally contoured to define a shelf region  44 ′ which extends above the base. As shown in FIG. 5B, the internal portions  24 - 2 ′ of the metal leads are exposed in the shelf region so as to be available for wire bonding with the semiconductor transducer. It will be appreciated that the internal sides of the walls  42 ′ are inclined slightly so as to facilitate insertion of the semiconductor transducer. 
     Referring to the illustrative drawing of FIG. 5C, there is shown a cross-sectional view of a housing and lead frame taken along lines  5 C— 5 C of FIG.  5 A. Note that in this view, there are two exposed internal portions  24 - 2 ′ of metal leads in the shelf region  44 ′. Also, note the rim portion  46 ′ that follows the top perimeter of the walls  42 ′ and that is contoured to interfit with a channel of the cap  36  as described below. 
     Referring to FIG. 6A, there is shown a top view of the cap  36 . The same cap is used for both embodiments discussed herein. The cap  36  includes a vent hole  38 . The two concentric lines surrounding vent hole  38  represent a circular grove  48  which can be used to secure a gasket, for example. The dashed lines about the perimeter of the cap  36  represent the path of the channel  50  described below. 
     Referring to the illustrative drawings of FIGS. 6B and 6C, there are shown cross-sectional views of the cap generally along lines  6 B— 6 B and  6 C— 6 C. A channel  50  follows the perimeter of the cap. It interfits with the rim portion  46 ′ of the housing  22 . It will be appreciated, that the rim portion is the same for the first housing embodiment and for the alternative housing embodiment. During assembly of a pressure sensor  20  or  20 ′, the channel is filled with an adherent material in order to secure the cap  36  to the rim portion of a housing. 
     Referring to FIG. 7, there is shown a lead frame structure  52 . FIG. 8 shows details of one of the lead frame units of the structure. FIG. 9 shows the downset of the base  26  relative to the other metal leads  24  and  25 . FIG. 10 shows housings  22  molded over two lead frame units. 
     During manufacture, the lead frame of FIG. 7 is formed. The base portions are downset as shown in FIG.  9 . The packages are molded onto the lead frame as shown in FIG.  10 . The sensors are epoxied to the bases. The epoxy is thermally cured to set it. The transducers are wire bonded to the metal leads. The housings are filled with silicone. The silicone is heated to set it as gel. Adherent material is deposited into the cap channels, and the caps are put in place. The enclosed package is heated to set the adherent material. The leads are trimmed from the frame and are formed into a gull winged shape. 
     While two embodiments of the invention have, been described and illustrated, it will be appreciated that modification can be made to these embodiments without departing from the spirit of the invention. Thus, the invention is intended to be defined in terms of the following claims.