Abstract:
A surface mount package for a multi-chip device has a leadframe formed with first and second die pads and readouts from the respective die pads. An environmentally responsive sensor chip is secured to the first die pad and an environmentally isolated chip is secured to the second die pad. The chips are electrically coupled through the lead frame. A body formed with an over molded portion encases the isolated chip and an open molded portion formed with a recess receives the environmentally sensitive chip. An apertured cover is secured in the recess to form a protective covering over the sensor chip and for allowing communication of the sensor chip externally of the package.

Description:
BACKGROUND OF INVENTION  
       [0001]     The invention pertains to sensor packages, and particularly to a tire pressure sensor employing semiconductor chips in a protective housing.  
         [0002]     Tire pressure monitoring systems (TPMS) monitor the pressure in automotive tires. TMPS provide a safety warning to alert the driver of low tire pressure. In addition, tire pressure monitoring provides information to the driver/owner, which encourages maintenance of even tire pressure. This results in better fuel economy and extended tire life.  
         [0003]     TMPS employ a number of components to sense and transmit data to the driver. The components are typically separated by function. One such component comprises a pressure sensor for sensing tire pressure and providing data outputs indicative thereof. An application specific integrated circuit (ASIC) responsive to temperature and system battery voltage processes the data outputs and communicates the data to a receiver for display on the instrument panel.  
         [0004]     Known systems have been packaged in a variety of ways. One such system over-molds or encapsulates the pressure sensor and ASIC in a single sealed package. Another system locates the pressure sensor and ASIC in an open chamber on a circuit board. These arrangements have shortcomings which interfere with performance or increase cost or both.  
         [0005]     Available TMPS have sensor packages with existing leadout arrangements. These systems require a compatible or uniform package configuration. Also, new leadout arrangements are desired for alternative arrangements.  
       SUMMARY OF INVENTION  
       [0006]     The invention is based on the discovery that components for a tire pressure monitoring system (TMPS) may be packaged on a common leadframe within an integrated circuit package in which the ASIC is over-molded and rendered insensitive or isolated to the local environment, and the sensor is interfaced with the ASIC in open cavity having a cover and pressure transfer aperture. The package has readouts which are compatible with existing standardized TMPS.  
         [0007]     In a particular embodiment, the invention is a surface mount package for a multichip device having at least one environmentally sensitive device or sensor chip, and at least one environmentally insensitive device or ASIC chip secured on a common leadframe in a molded body. The molded body includes an over-molded portion for encapsulating the ASIC and isolating it from environmental effects; and an open molded recess portion or open cavity for receiving the sensor chip therein. The leadframe, having a tie bar and lateral dam bars, is secured in the package between the chips and has a die pad for receiving the ASIC for mounting and a die pad for receiving the sensor for mounting. The ASIC and pressure sensor are wirebonded to selected leads on the leadframe. Certain leadframe leads are commonly wirebonded to the sensor and ASIC thereby interconnecting the chips. These leadframe leads are not directed to the dam bar side, but to the tie bar side, and then trimmed out. The arrangement allows for a reduced package size and a number of leadouts compatible with available TMPS. The leadframe and sensor chip are overmolded and the sensor is encapsulated in a flexible material which enables the sensor to be responsive to pressure. The recess has an aperture cover disposed over the opening to close the chamber. The leadframe is singulated by severing a tiebar and dam bar between untrimmed leadframes after molding the body. In a particular embodiment the die pad for the sensor is located in a plane offset from the die pad of the ASIC. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]      FIG. 1  is a perspective view of the overall arrangement of the sensor package according to the invention.  
         [0009]      FIG. 2  is a perspective view with the sensor cover removed.  
         [0010]      FIG. 3  is a fragmentary top plan view of the sensor package shown in  FIG. 1 .  
         [0011]      FIG. 4  is a sectional view of the package taken along line  4 - 4  of  FIG. 3 .  
         [0012]      FIG. 5  is a sectional view of the package taken along line  5 - 5  of  FIG. 3 .  
         [0013]      FIG. 6  is a button view of the sensor package shown in  FIG. 1 .  
         [0014]      FIG. 7  is a side elevation of the sensor package shown in  FIG. 1 .  
         [0015]      FIG. 8  is a sectional drawing illustrating an alternative arrangement for mounting the ASIC on a lower surface of the leadframe.  
         [0016]      FIG. 9  is a plan view of the leadframe employed in the package according to the invention.  
         [0017]      FIG. 10  is a plan view of the leadframe as it appears prior to trimming.  
         [0018]      FIG. 11  is a plan view of an alternative arrangement showing a lead frame having additional leadouts after trimming. 
     
    
     DETAILED DESCRIPTION  
       [0019]     As illustrated in  FIG. 1 , the invention comprises a sensor package  10  for use in a tire pressure measurement system (TMPS), not shown. The package  10  includes a body  12 , having an upper surface  14  and an opening  16  formed therein. The opening has an apertured cover  18  located therein. The apertured cover  18  has a hole  20  formed therein. The package  10  has lead-outs  22  which extend outwardly of the body  12  from the lateral sides as shown. The configuration of the lead-outs  22  is a standardized arrangement.  
         [0020]     As further illustrated in  FIGS. 2-7 , the body  12  of package  10  includes an over-molded or closed portion  40  and open portion  42 . A leadframe  44  is over molded within the body. The leadframe  44  has respective upper and lower surfaces  46  and  48 ; a die pad  50  for supporting a pressure sensor chip  52  on the upper surface  46 ; and a die pad  54  for supporting an application specific integrated circuit (ASIC)  56  on the upper surface  48 . The die pads  50  and  54  are commonly connected by lead element  58  ( FIG. 9 ). As illustrated in  FIG. 80 ., the ASIC  56  may be optionally mounted on the lower surface  46  if desired.  
         [0021]     Trimmed the leadframe  44  is shown in  FIG. 9 . The untrimmed leadframe  44 , shown in  FIG. 10 , has tie bar  60 ; laterally disposed dam bars  62 , and inboard contact leads  64  connected to the readouts  22 , as shown. The sensor  52  and the ASIC  56  have respective contacts  66  and  68  which are wirebonded by respective wirebonds  70  and  72  to selected ones of the contact leads  64  as shown. The contacts  66  of the sensor  52  are commonly coupled to selected ones of the contacts  68  on the ASIC by wirebonds  72  and interconnected portions  74  of the contact leads  64 , as shown. In this way, the sensor  52  and ASIC  56  are electrically interconnected.  
         [0022]     In some conventional overmolded systems, the sensor contacts  66  and ASIC contacts  68  are directly connected by wirebonds. The arrangement results in the number of readouts  22  shown. In order to conform the package of the invention to the size and lead out configuration of at least one conventional system, selected ones of the inboard contact leads  64  are used to interconnect the ASIC and sensor chips and at the same time allow for isolation of the ASIC and sensor.  
         [0023]     As illustrated in  FIGS. 2-5 , the body  12  has a recess  78  formed therein which extends from the opening  16  in the upper surface  14  to a lower wall  80  having an aperture  82  formed therein, centered on the die pad  50 . The sensor chip  52 , secured to the die pad  50  is located in the aperture  82  as shown. Apertures  84  are formed in the body which are discussed hereinafter.  
         [0024]     A flexible encapsulating gel  86  is deposited over the sensor  52  to protect the device. In the exemplary embodiment discussed herein, the sensor  52  is responsive to ambient pressure. The encapsulating gel  86 , which may be a conventional silicone material, is sufficiently flexible to allow the sensor  52  to be responsive to changes in ambient pressure.  
         [0025]     The opening  16  is formed with an offset  90  having an upstanding ridge  92  forming a channel  94 . The cover  18  has an outer edge  96  which mates with the channel  94 . An appropriate adhesive  98 , e.g. epoxy sealant, is disposed in the channel  94  to secure the cover  18  in the opening  16 . The aperture  20  in the cover  18  is sized for allowing the sensor  52  to communicate with the environment exterior of the package  10  and at the same time to prevent infiltration of contaminants into the chamber  78 .  
         [0026]      FIGS. 9 and 10  illustrate the leadframe  44 . It should be understood that the leadframe  44  is typically formed from a metal ribbon wherein a plurality of leadframes are joined end-to-end along the tie bar  60  and laterally along the dam bar  62 . When the package of the invention is formed, as discussed below, the selected portions of the leadframes are removed and the packages are singulated.  
         [0027]     The ASIC  56  is responsive to temperature and system battery voltage. The ASIC  56  is secured to the die pad  54  of the leadframe  44  and is over-molded therewith during fabrication of the package. The over-molded or closed portion  40  of the package  10  isolates the ASIC from the environment, yet allows it to be responsive to temperature excursions. As noted above, the inboard contact leads  64  of leadframe  44 ; the lead element  54 ; and the wirebonds  70  and  72  electrically couple the sensor  52  and the ASIC  56  to each other and to the readouts  22 .  
         [0028]     The leadframe  44  is generally planar in configuration and lies in a plane  100  within the body  12  near a mid-plane thereof. The die pad  50 , supporting the sensor chip  52  is offset from the plane  100 . The die pad  50  is attached to the leadframe  44  by integral lead portions  102  which are deformed as shown to position the die pad  50  in a plane  100 ′ offset from the plane  100  of the leadframe  44 . As a result, the package height is minimized, and the package weight is likewise minimized. Such minimization of the package size and weight is advantageous in the environment of an automobile tire where the TPMS is located.  
         [0029]     Fabrication of the package  10  is accomplished as follows. The ASIC chip  56  may be secured to the respective die pad  54  of the leadframe  44 . Wirebonds  72  are attached from the ASIC contracts  68  to corresponding inboard contact leads  64 .  
         [0030]     Thereafter, the package body  12  may then be formed by over molding the ASIC  56  and leadframe  44 , thereby producing the over moulded portion  40  and open portion  42 . The sensor chip  52  is then mounted to the die pad  50  inside the open chamber  78  through the opening  16  and recess  82 . The contacts  66  of the sensor  52  are wire bonded by wirebonds  70  to common leads  74  as shown. Silicone gel  86  is deposited in the chamber  78  over the sensor  52 , and the cover  18  is secured in the opening  16  using epoxy sealant  98 .  
         [0031]     After the body is assembled, the sensor package is singulated, In untrimmed form, the leadframe  44 , shown in  FIG. 10 , has tiebar  60  and dam bars  62  which hold the leadframe and various metal contact surfaces in alignment prior to assembly of the package. After the leadframe  44  is assembled in the body  12 , bodies are singulated in a known manner.  
         [0032]     The readouts  22  may then be deformed to produce distal ends  94  for connection to external circuitry. The distal ends  94  are arranged so as to be positioned immediately adjacent to the under side  106  of the body  12  generally coplanar therewith as shown.  
         [0033]     In the exemplary embodiment, tie bar  60  is trimmed along line  110 , and a selected number of fourteen readouts  22  are formed on the lateral sides of the body by trimming between the lead outs  22 , as shown, whereby the package of the invention has pin compatibility with known systems.  
         [0034]     An alternative arrangement is shown in  FIG. 11 , wherein the leadframe  120  has additional leadouts  122  extending from the end  124  of the leadframe and directed to the dam bar side. This arrangement increases the lead count from fourteen to eighteen with an increase in package size.  
         [0035]     The dotted lines indicate the trimming process. The process includes (1) lead length cut and dam bar removal, (2) singulating; trimming the tie bar and the 4 sensor readouts, and (3) lead forming. As noted, the additional readouts may be optionally formed as well.  
         [0036]     The process for manufacturing the package according to the invention employs conventional molding techniques wherein a plurality of leadframes are provided. The die pad  50  is offset from the plane  100  of the leadframe  44 . Thereafter the chips are attached to the corresponding die pads as discussed above. The wirebonds are then attached to the chips and contacts. Thereafter, the package is secured in a mold cavity, not shown, and held in position during transfer molding of the body. During the molding process, the leadframe is held in position by contact devices, not shown, and which contact devices result in the formation of ejector pin apertures  82 ,  84  and  130  in the body discussed below. These apertures, necessitated by the molding process, are positioned so as to have no effect on the integrity of the package  10 , or to adversely effect the sensitivity of the sensor.  
         [0037]     In  FIG. 6  the large opening  130  to the left is a bottom ejector pin mark, to be used for releasing out the packaged body from the mold die after completion of the molding process. The four holes  84  are fixture holes which are clamped/pinched by opposed pinching pins, not shown, which maintains cleanliness of the sensor wirebond leads. The hole  82  is a fixture hole as well formed by pinching pins, (not shown) in the same way as the holes. The hole  82  is for clamping the sensor die pad which is regressed (i.e. downset) from the main leadframe line.  
         [0038]     After molding, the encapsulating gel sealant  86  is deposited in the chamber  78  over the sensor  50  and the apertured cover  18  is installed in the opening, the package is then singulated. It should be understood that if an additional ASIC or additional sensor is desired, that the arrangement of the invention may be employed to incorporate multiple chips within a single package.  
         [0039]     While there has been described what at present is considered to be an exemplary embodiment of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is intended in the accompanying claims to cover such changes and modifications as fall within the true spirit and scope of the invention.