Abstract:
An integrated circuit packaging structure comprises at least one Micro Electrical Mechanical Systems (MEMS) gyroscope die mounted directly on a multi-layer flexible substrate having at least one metal layer and wire-bonded to the flexible substrate and a lid or die coating protecting the MEMS die and wire bonds.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to processes for packaging integrated circuit dies, and more particularly, to a method of packaging gyroscope sensor modules. 
     (2) Description of the Related Art 
     The miniaturization of electronics drives the expansion of product functionality and application. However, the application of the gyroscope (‘gyro’) sensor module has been limited due to the size and weight of the MEMS (Micro Electrical Mechanical Systems) gyro package. Currently, MEMS gyros are available commercially in the form of a package within which the MEMS die  24  is mounted by wire bonding  28 , as shown in  FIG. 1 . For protection the die is contained in an enclosure  20  with a lid  22  which is normally ceramic and/or metallic. On one side of the package, solder ball grid array (BGA)  15  is present to allow the package to be mounted on the main circuit board  10  by solder reflow. Such package design provides a rigid structure to hold the MEMS die, as well as the overall shape of the package allowing it to be handled easily by standard surface mount technology (SMT) processes. 
     To reduce the spatial requirement of the main circuit board, high density circuitry on multilayer printed circuit boards (PCBs) are available. However, the size of the gyro packages has become a key factor which limits its application in small portable devices, especially when up to three MEMS gyroscopes are required to sense and respond to X-, Y- and Z-axis movements. As demonstrated by J. Barton et al, “Design, Fabrication and Testing of Miniaturized Wireless Inertial Measurement Units (IMU),” IEEE, 2007, a 10 mm SMD IMU sensor could be produced but with little room left for further size reduction due to the large footprint of the gyro packages. 
     Therefore, the design of gyro sensor modules has been optimized for applications in small, portable devices. By redesigning the MEMS enclosure, the size and thus the weight of the MEMS gyro could be reduced. Consequently, the main circuit board can also reduce in size through the use of a multi-layer flexible substrate and increasing the circuit density. Such smaller, lighter assemblies can be incorporated into a greater variety of portable and mobile devices which opens up opportunities for new applications. 
     U.S. Pat. No. 6,784,020 (Lee et al), U.S. Pat. No. 8,804,993 (Shulka), U.S. Pat. No. 8,653,642 (Sutanto et al), and U.S. Pat. No. 8,368,154 (Trusov et al) and U.S. Patent Application 2012/0318059 (Otsuki) disclose MEMS devices mounted on flexible substrates. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide a cost-effective and very manufacturable method of packaging a MEMS die. 
     Another object of the invention is to provide method of mounting a MEMS die (s) directly on a multi-layer flexible substrate. 
     Yet another object of the invention is to provide a package wherein a MEMS die (s) is mounted directly on a multi-layer flexible substrate. 
     According to the objects of the invention, a method of packaging a MEMS die is achieved. At least one MEMS die is mounted directly on a multi-layer flexible substrate having at least one metal layer. Electrical connections are made between the MEMS die and the flexible substrate. The MEMS die and electrical connections are protected with a lid or die coating. 
     Also according to the objects of the invention, a MEMS die (s) mounted directly on a multi-layer flexible substrate having at least one metal layer is achieved. An integrated circuit packaging structure comprises a MEMS die mounted directly on a flexible substrate and wire-bonded to the flexible substrate and a lid or die coating protecting the MEMS die and wire bonds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings forming a material part of this description, there is shown: 
         FIG. 1  schematically illustrates in cross-sectional representation a MEMS package of the prior art. 
         FIGS. 2-9  schematically illustrate in cross-sectional representation MEMS packages of the present disclosure. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present disclosure details a method for and a device comprising a gyro MEMS sensor package in integrated circuit packages and modules. The method and device of the present disclosure hold a number of advantages over the traditional gyro sensor ceramic package sensing technologies as the MEMS die(s) is mounted directly on the flex surface of a multi-layer flexible substrate which offers a significant area reduction on the package size, lower profile, and lighter weight, as compared to traditional ceramic packages. 
     The MEMS gyro package is redesigned to achieve a smaller sensor footprint. Currently, the commercially available MEMS gyro package is approximately 7×7×3.3 mm which contains the MEMS die of only approximately 140×140×110 μm within a fully enclosed environment, as shown, for example, in  FIG. 1 . 
     In this disclosure, to reduce the size of the gyro assembly, the design of the enclosure is modified to eliminate the thickness contributed by the bottom side of the ceramic enclosure  20 . As shown in  FIG. 2 , the MEMS dies are bonded to an interposer flex circuit or multi-layer flexible substrate  18  by standard die attach, such as conductive or non-conductive epoxy  32 . 
     The flexible substrate technology can include finer line width than the conventional package and spacing of less than 10 μm. The flexible substrate technology can offer small vias of less than 10 μm to connect the different conductive metal layers. 
     The flexible substrate has at least one metal layer and can be one conductive metal layer or more than one conductive metal layer. Additionally, the flexible substrate can have double sided conductive metal layers or more than two stack-up conductive metal layers. The dielectric material in the flexible substrate may be polyimide (PI), liquid crystal polymer (LCP), Polyester (PET), polyethylene-naphthalate (PEN), poly tetra fluoro ethylene, or a laminate substrate such as epoxies and BT, or Teflon or modified Teflon. 
     Wire bonding  28 ,  30  through, for example, gold fingers  29  completes the electrical connections. Wire bonds may be copper, gold, paladium, aluminum, or any other conductive metals or alloys. 
     A single piece of metallic or liquid crystal polymer (LCP) lid  19 , attached by epoxy  32 , is used to protect the MEMS die from foreign particles. This gyro MEMS die assembly could then be handled as a component for mounting onto the main circuit (PCB or flex printed circuit board (FPCB))  12  by methods such as soldering  15 . The soldering material comprises eutectic PbSn or any lead-free Sn Ag or Sn Ag Cu, or by a combination of a conductive adhesive or solder with a non-conductive adhesive. It is estimated that using this design, the MEMS package size could be reduced to 6×6×2 mm, which is equivalent to a volumetric reduction of approximately 55%. 
     To further reduce the spatial requirement of the MEMS gyro package, the enclosure is eliminated with wire connections  28 ,  30  and MEMS die fully protected by die coating  34 , as shown in  FIG. 3 . This design could result in a volumetric reduction of approximately 80% for each set of the MEMS gyro. 
     To even further minimize the gyro footprint, the MEMS die is placed directly onto the sensor multi-layer flexible circuit  18 , in  FIGS. 4 and 5 , which eliminates the need for the flex interposer. Using conductive/non-conductive paste or film epoxy  32 , a solder reflow process is not needed which further reduces the die profile upon the main circuitry, in addition to the possible benefits gained from a simplified manufacturing procedure. In  FIG. 4 , metal or LCP lid  19  is attached to the flex substrate  12  using epoxy  32 . In  FIG. 5 , die coating  34  protects the MEMS die. 
     In parallel to the modification to the gyro package design, the main circuit is also modified to enhance the size and weight reduction. In the existing sensor design, the main PCB circuit is based on a 4 metal layer (ML) rigid flex stackup which comprises a 2ML PCB board and a 2ML flex joined by soldering. 
     To reduce the overall substrate thickness and weight, a piece of multi metal layer (e.g. 4 metal layer) flex  18  is used as the main circuit substrate. The multi metal layer flex could be based on a multi core or single core design. Such design eliminates the soldering process as required by the established rigid flex design. The use of a multi-layer flexible substrate also allows fine pitch circuitry with via sizes of less than 10 μm diameter and trace width and spacing of less than 10 μm. 
     The MEMS die (s) mounted directly on a multi-layer flexible substrate allows for a smaller, thinner, and lighter gyro MEMS sensor package in integrated circuit packages and modules. The MEMS die  24  with MEMS cap  26  can be directly mounted onto the flexible substrate  18  using non-conductive or conductive epoxy  32 . This results in an ultra small package size, eliminating the BGA pads as seen on the ceramic package. Because of the thinner overall package thickness, the wire bond  28  from the MEMS die to flex can be done with lower loop height. 
     For structural rigidity, as shown in  FIGS. 6-9 , stainless steel stiffeners  14  could be used as in the existing design whilst the flexibility of the flex  16  is maintained for folding after the MEMS gyro attachment.  FIGS. 6 and 7  show the at least one MEMS die attached to the flexible substrate  16  using conductive or non-conductive epoxy paste or film  32 . In  FIGS. 8 and 9 , the at least one MEMS die is die attached directly onto the stiffener  14 . In all of these figures, the multi-layer flexible substrate  16  is wrapped around the stiffener  14 . The stiffener material can be copper, aluminum, stainless steel, ceramic, zinc, or tungsten, for example, or any other metal. The flexible substrate  16  can be folded around the stiffener either before or after placement of the dies. 
     As in the previous embodiments, the flexible substrate  16  has at least one metal layer and can be one conductive metal layer or more than one conductive metal layer. Additionally, the flexible substrate can have double sided conductive metal layers or more than two stack-up conductive metal layers. The dielectric material in the flexible substrate may be polyimide (PI), liquid crystal polymer (LCP), Polyester (PET), polyethylene-naphthalate (PEN), poly tetra fluoro ethylene, or a laminate substrate such as epoxies and BT, or Teflon or modified Teflon. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.