Abstract:
A method of packaging a pressure sensor die begins with patterning and etching a metal strip and forming metal traces on the strip. Further build-up is performed to transform the metal strip into a layered substrate. Cavity walls are formed on one side of the strip with a molding process and then the metal on the back side of the strip is removed. Next semiconductor dies are attached to the strip within the cavities and electrically connected to pads formed on the surface of the strip and/or to pads on other ones of the dies. A gel coating is deposited over the dies and then a metal lid is secured over the cavity. The strip is then singulated along ones of the cavity walls to form multiple sensor devices.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to pressure sensor devices, and more particularly to a method of assembling pressure sensor devices. 
         [0002]    Various types of sensors now are widely used in electronic devices such as mobile phones, mobile computing devices, and automotive electronics. For example, pressure sensors and accelerometers may be packaged together with a microcontroller that processes the sensor data to provide useful output data. Such sensor packages require a small form factor, low power, and competitive pricing. Thus, it is important to be able to assemble such devices using a process that is cost efficient and reliable. 
         [0003]    The pressure sensor die typically has a thin differential pressure-sensing membrane that is susceptible to mechanical damage during handling and packaging. One way of packaging the pressure sensor die is mounting the die to a premolded lead frame and encapsulating the package with a mold compound. However, such pre-molded lead frames are expensive. Further, dies such as piezo resistive transducer (PRT) dies do not allow full encapsulation because that would impede their functionality. As a result, the premolded lead frame requires a metal lid or cap be placed over the die to protect it from the outside environment. 
         [0004]    It would be advantageous to be able to efficiently package pressure sensor dies in which the risk of environmental damage to the pressure sensor die is substantially reduced or eliminated while reducing the overall packaging costs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the thicknesses of layers and regions may be exaggerated for clarity. 
           [0006]      FIG. 1  is a top, cross-sectional view of a pressure sensor package in accordance with one embodiment of the present invention; 
           [0007]      FIG. 2  is a side, cross-sectional view of the pressure sensor package of  FIG. 1 ; and 
           [0008]      FIGS. 3A-3V  are side cross-sectional views illustrating a various steps of a method for assembling the pressure sensor package shown in  FIGS. 1 and 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    Detailed illustrative embodiments of the present invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention. The present invention may be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It further will be understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” specify the presence of stated features, steps, or components, but do not preclude the presence or addition of one or more other features, steps, or components. It also should be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. 
         [0010]    This present invention provides a pre-molded, coreless package for a pressure sensor die and other dies such as a microcontroller (MCU) and other sensor dies like a G-cell or gyro. Some of the main features of the package include a coreless carrier, pre-molded side walls, and a metal lid. Wires for interconnection for the pressure sensor die and/or combination of other dies such as MCU, G-cell, and Gyro, and silicone gel are some of the main internal features of the package. 
         [0011]    In one embodiment, the present invention provides a method of packaging a pressure sensor die. The method includes forming a coreless substrate on a sheet of metal foil. The forming includes forming one or more die attach areas and a plurality of electrical contacts on an upper surface of the substrate. The method also includes forming side walls at predetermined locations on the upper substrate upper surface. A first die such as a micro-control unit or control die is attached to a first one of the die attach areas with a die attach adhesive and a pressure sensor die is attached to a second one of the die attach areas, also with a die attach adhesive. The die attach adhesive is then cured and bond pads of first die are electrically connected to the plurality of electrical contacts on the substrate upper surface, and bond pads of the pressure sensor die are electrically connected to the first die. A gel is dispensed onto a top surface of at least the pressure sensor die. The gel is cured and a lid is attached to the side walls such that the lid covers the first die and the pressure sensor die. The lid attach adhesive then is cured. In an alternative embodiment, an accelerometer also is packaged within the side walls and covered with the lid. 
         [0012]    In another embodiment, the present invention is a packaged pressure sensor die formed in accordance with the above-described method. 
         [0013]    Referring now to  FIGS. 1 and 2 , a top, cross-sectional view and a side cross-sectional view of a packaged pressure sensor die  10  according to an embodiment of the present invention is shown. The packaged die  10  includes a coreless substrate  12  having a lower surface  14  with exposed electrical contacts  16  and an upper surface  18  with exposed substrate contact pads  20  and one or more die attach areas. The substrate  12  will be described in more detail with reference to  FIG. 3 . 
         [0014]    A first die  22  is attached to a first die attach area and electrically connected to the substrate contact pads  20 . In one embodiment, the first die is a microcontroller or MCU die. However, the first die  22  also could be an Application Specific Integrated Circuit (ASIC). The first die  22  is in electrical communication with the substrate  12 . More specifically, bonding pads on an active surface of the first die  22  are electrically connected to the contact pads  20  on the upper surface  18  of the substrate  12  with first bond wires  24 . A pressure sensor die  26  is attached to a second die attach area on the upper surface  18  of the substrate  12  and electrically connected to the first die  22  with second bond wires  28 . The pressure sensor die  26  also may be electrically connected to the exposed contact pads  20  on the upper surface  18  of the substrate  12  with third bond wires  30 . As can be seen in  FIG. 1 , a second sensor die  32  is attached to a third die attach area on the upper surface  18  of the substrate  12  and electrically connected to the first die  22  with fourth bond wires  34 . The second sensor die  32  also may be electrically connected to the exposed contact pads  20  on the upper surface  18  of the substrate  12  with fourth bond wires  36 . The first through fifth bond wires  24 ,  28 ,  30 ,  34 ,  36  may be formed of gold, aluminium, silver or copper and are well known and commercially available, and may be attached using conventional wire bonding equipment. The first die  22 , pressure sensor die  26 , and second sensor die  32  may be attached to the substrate  12  using conventional die attach adhesive, die attach film or even a double sided tape, also as is known in the art. 
         [0015]    The pressure sensor die  22  may take various forms, such as a piezo resistive transducer (PRT) or a pressure sensor cell (P-cell) and the second sensor die  32  may be an accelerometer. 
         [0016]    A side wall  38  is formed on the upper surface  18  of the substrate  12 . The side wall  38  surrounds the dies  22 ,  26  and  32 . The side wall  28  may be formed of a molding compound, plastic material, epoxy, silica-filled resin, ceramic, halide-free material, and the like, or combinations thereof, as is known in the art, and may be formed with a conventional molding process. 
         [0017]    A gel  40  is dispensed over at least the pressure sensor die  26  and in the embodiment shown, the gel  40  covers each of the dies  22 ,  26  and  32 . The gel  40  comprises a pressure-sensitive gel material, such as a silicon-based gel. The pressure-sensitive gel  40  enables the pressure of the ambient atmosphere to reach the pressure-sensitive active region of pressure sensor die  26 , while protecting the die  26  and the bond wires  24 ,  28 ,  30 ,  34  and  36  from mechanical damage during packaging and environmental damage (e.g., contamination and/or corrosion) when in use. Examples of a suitable pressure-sensitive gel  40  are available from Dow Corning Corporation of Midland, Michigan. 
         [0018]    A lid  42  is supported by the side walls  38  and covers the gel covered control die  22 , the pressure sensor die  26  and the accelerometer  32 . The sidewalls  38  may include notches  46  that receive ends of the lid  42 . The lid  42  may be secured within the notches  46  and to the side walls  38  with an adhesive. The lid  42  preferably is formed of metal and includes a through hole  44 . The hole  44  allows the ambient atmospheric pressure immediately outside the sensor device  10  to reach the pressure-sensitive gel  40  and therethrough the active region of pressure sensor die  26 . The hole  44  can be located anywhere within the area of the lid  42 . The hole  44  may be (pre-)formed in the lid  42  by a known fabrication process such as drilling or punching. 
         [0019]    Referring now to  FIGS. 3A-3V , a process for assembling the pressure sensor die package  10  will be described.  FIG. 3A  is a side view showing a strip or sheet of copper foil  50  that is used as a starting point for forming the substrate  12  ( FIG. 1 ). The copper foil  50  preferably comprises a bare copper foil sheet. The foil  50  is sized and shaped for the formation of the substrate  12  upon which a plurality of the packaged dies  10  may be simultaneously formed. For example, a 75 mm×240 mm sheet of bare copper may be used. Next, as shown in  FIG. 3B , the foil  50  is coated with a resist coating or a dry film lamination  52 . The coating is required for subsequent patterning processes to define metal traces to establish interconnection between the dies and the pads  16  on the bottom side of the substrate  12 . The resist coating  52  is developed ( FIG. 3C ), which forms channels  54  in the coating  52 . At  FIG. 3D , the copper foil  50  is etched at the channels  54 , which increases the depth of the channels  54 . For example, the channels  54  may be etched to a depth of between 25 um to 100 um depending on the final thickness of the substrate  12 .  FIG. 3E  shows the resist coating  52  being stripped off of the foil  50 , such as by machine or manual process. At  FIG. 3F , the top surface of the foil  50  is patterned and a trace metal  56  is deposited on the surface of the foil  50  such as by electroplating. The trace metal  56  comprises a conductive metal or metal alloy such as gold or nickel or nickel palidium gold. The pattern is determined by the interconnection path between the lower and upper surfaces  14 ,  18  of the substrate  12 . 
         [0020]    Next, as illustrated in  FIG. 3G , a solder resist film  58  is formed on the upper surface of the foil  50 , i.e., over the trace metal  56 . Then, as illustrated in  FIG. 3H , the patterned and laminated foil  50  is exposed, developed, and thermally cured, which forms a pattern  60  on the upper surface of the foil  50 .  FIG. 3I  illustrates an electrolytic plating process being performed in which a conductive metal  62  such as gold is plated over the pattern  60  on the upper surface of the foil  50 . Then, as shown in  FIG. 3J , another solder resist film  64  is formed of laminated onto the plated pattern on the upper surface of the foil  50 .  FIG. 3K  illustrates another step of exposure, development and thermal curing (similar to as shown in  FIG. 3H ), which forms yet a further pattern  66  in the upper surface of the foil  50 .  FIG. 3L  illustrates the patterned upper surface of the foil  50  undergoing a seeding and electrolytic plating process, in which a conductive metal  68  such as gold is formed over the top surface of the foil  50 . Alternatively, an electroless plating process could be performed to deposit the conductive metal  68  on the patterned surface of the foil  50 . 
         [0021]    In  FIG. 3M , a step of forming or laminating yet another solder resist film  70  over the upper surface of the foil  50  is shown, and in  FIG. 3N , another step of exposure, development and thermally curing is performed to further develop a pattern  72  on the surface of the foil  50 , and  FIG. 3O  illustrates a further conductive metal  74 , such as gold, being plated onto the surface of the pattern  72 . The build-up layers formed on the surface of the foil  50  illustrated in  FIGS. 3A-3O  illustrate how the substrate  12  is formed. Thus, in  FIG. 3O , the substrate  12  is shown as formed on the upper surface of the foil  50 . 
         [0022]      FIG. 3P  illustrates the formation of the side walls  38  formed on the upper surface of the foil  50  over the plated pattern  72 . The side walls  38  form housing areas  76 , in which packaged devices will be assembled. As previously discussed, the side walls  38  may be formed using a conventional molding process. The mold tool used to form the side walls has several inserts. The inserts contact the substrate upper surface  18  forming a path to form the four sided wall during the molding process. Although only two housing areas  76  are shown, it will be appreciated by those of skill in the art that either a strip or an array of housing areas  76  may be simultaneously formed on the surface of the foil  50 . 
         [0023]      FIG. 3Q  illustrates the removal of the foil  50  from the now formed substrate  12 . The foil  50  may be removed by etching. In one alternative embodiment of the invention, the foil  50  is removed before the formation of the side walls  38 , and in another embodiment of the invention, the foil  50  is not removed until after the dies  22 ,  26  and  32  have been attached to the substrate  12 . 
         [0024]    Referring now to  FIG. 3R , the first die  22  and the pressure sensor die  26  are attached to the surface of the substrate  12  within the housing areas  76  using known die bonding techniques. For example, the dies  22  and  26  (and  32 ) may be attached to the surface of the substrate  12  using a die attach adhesive or a double sided tape (not shown). The die attach adhesive is subsequently cured in an oven to harden the die attach adhesive.  FIG. 3S  illustrates the dies  22  and  26  (and  32 ) being electrically connected to each other and to the substrate  12  with bond wires  24 ,  28  and  30 , via a standard wire bonding process and using conventional wire bonding equipment. Although not shown, another way of electrically connecting the first die  22  to the substrate  12  would be a flip-chip with bumps and then have the sensor dies  26  and  32  connect to the first die  22  by way of a wiring pattern in the substrate  12 , with the sensor dies  26  and/or  32  having bond wires from their contact pads to contact pads of the substrate  12 . 
         [0025]      FIG. 3T  illustrates the gel  40  being deposited or dispensed over the dies  22 ,  26  (and  32 ) and the bond wires  24 ,  28 ,  30  (and  36 ), while  FIG. 3U  illustrates the lids  42  being secured over the housing areas  76  by attaching ends of the lids to the side walls  38 . The gel  40  may be dispensed with a nozzle of a conventional dispensing machine, as is known in the art. Subsequently, the gel  40  is cured in an oven. The lids  38  may be secured within the notches  46  in the side walls  38  using a conventional lid attach adhesive. Finally,  FIG. 3V  illustrates adjacent devices  10  being separated by a singulation process, such as with a saw, laser cutting, or scribing and cutting, etc. 
         [0026]    The present invention, as described above, allows for packaging a pressure sensor die without requiring lead frames or especially pre-molded lead frames to package the die. The pressure sensor die packaged using the process described above is protected from moisture by the gel material, the lid and the substrate. It is noted that, because there is no lead frame and especially no pre-molded lead frame, the sensor device of the present invention exhibits improved reliability even in view of a rapid decompression event (RDE). For a lead frame or pre-molded lead frame type device that undergoes an RDE, air bubbles may travel up the path between the mold compound and the metal lead frame. Yet another benefit of the present invention is that the due to the relatively soft nature of the solder mask included in the substrate, then a device in accordance with the present invention is better able to absorb stress without cracking or breaking. 
         [0027]    By now it should be appreciated that there has been provided an improved packaged pressure sensor die and a method of forming the packaged pressure sensor die. Circuit details are not disclosed because knowledge thereof is not required for a complete understanding of the invention. Although the invention has been described using relative terms such as “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, such terms are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. 
         [0028]    Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims.