PATENT DOCUMENT

Publication Number: US-9624093-B2
Application Number: US-201414543468-A
Country: US
Kind Code: B2

Title: Method and apparatus of making MEMS packages

Abstract:
MEMS packages and modules are described. In an embodiment, a module includes a package mounted within an opening in a module board. The package includes a flexible wiring board mounted to a back surface of the module board and spanning across the opening in the module board. A die is mounted on the flexible wiring board and is encapsulated within an overmold. An air gap exists laterally between the overmold and side surface of the opening in the module board.

Claims:
What is claimed is: 
     
       1. A module comprising:
 a module board; 
 an opening extending through and laterally surrounded by the module board; 
 a package within the opening, wherein the package comprises:
 a flexible wiring board mounted on a back surface of the module board and spanning across the opening; and 
 a micro-electro-mechanical systems (MEMS) die mounted on the flexible wiring board, wherein the MEMS die is encapsulated within an overmold and an air gap exists laterally between the overmold and side surfaces of the opening in the module board. 
 
 
     
     
       2. The module of  claim 1 , further comprising a stiffener layer bonded to a back side of the flexible wiring board opposite the MEMS die. 
     
     
       3. The module of  claim 2 , wherein the stiffener layer is more flexible than the module board. 
     
     
       4. The module of  claim 1 , wherein the MEMS die is bonded to an underlying substrate with a die attach film including an elastomeric material. 
     
     
       5. The module of  claim 4 , wherein the underlying substrate is an integrated circuit (IC) die. 
     
     
       6. The module of  claim 5 , wherein the MEMS die is wire bonded to the IC die. 
     
     
       7. The module of  claim 6 , wherein the IC die is wire bonded to the flexible wiring board. 
     
     
       8. The module of  claim 4 , wherein the underlying substrate is the flexible wiring board. 
     
     
       9. The module of  claim 1 , wherein the overmold is formed directly on the flexible wiring board. 
     
     
       10. The module of  claim 1 , further comprising a stiffener layer bonded to a back side of the flexible wiring board opposite the MEMS die, wherein the stiffener layer is partially contained within the opening. 
     
     
       11. The module of  claim 1 , further comprising an additional package surface mounted onto a front surface of the module board. 
     
     
       12. The module of  claim 11 , wherein a profile height of the additional package is raised above a profile height of the package comprising the MEMS die. 
     
     
       13. The module of  claim 1 , further comprising a second die mounted on the flexible wiring board laterally adjacent the MEMS die. 
     
     
       14. The module of  claim 13 , wherein the second die and the MEMS die are encapsulated within the overmold. 
     
     
       15. The module of  claim 13 , further comprising a second overmold, and the second die is encapsulated within the second overmold. 
     
     
       16. The module of  claim 1 , further comprising a wire bond connected to the MEMS die, wherein the wire bond is encapsulated within the overmold. 
     
     
       17. The module of  claim 16 , wherein the overmold is formed directly on the flexible wiring board. 
     
     
       18. The module of  claim 16 , wherein the wire bond connects the MEMS die directly to the flexible wiring board. 
     
     
       19. The module of  claim 16 , wherein the MEMS die is attached to an IC die, and the wire bond connects the MEMS die to the IC die. 
     
     
       20. The module of  claim 19 , further comprising a second wire bond that connects the IC die to the flexible wiring board, and the second wire bond is encapsulated within the overmold.

Description:
BACKGROUND 
     Field 
     Embodiments described herein relate to MEMS packaging. 
     Background Information 
     Micro-electro-mechanical systems (MEMS) die can be formed from customized integrated circuits. MEMS are often used to sense environmental characteristics or act as a user input for electronic products. MEMS, unlike some general purpose integrated circuits, can have unique packaging and mounting requirements since MEMS often require exposure to an ambient external environment, such as an ambient environment of a user using the electronic product having the sensor. MEMS have become a significant growth area in consumer space. Gyrometers, accelerometers, microphones, pressure sensors, and magnetometers are all sensitive to strain induced performance drift. As consumer MEMS packages and modules continue to see footprint and profile reduction, price reduction, and higher level of integration, it becomes more difficult to manage strain induced drift. In addition, stress impact to MEMS output change may become more serious when package thickness and form factor are further reduced to meet market needs. 
     SUMMARY 
     In an embodiment, a module includes a package mounted within an opening in a module board. The package may be a flexible package. For example, the package can include a flexible wiring board mounted to a back surface of the module board and spanning across the opening in the module board. A die is mounted on the flexible wiring board and is encapsulated within an overmold. An air gap exists laterally between the overmold and side surface of the opening in the module board. In an embodiment, the overmold is formed directly on the flexible wiring board. In an embodiment, the die is at least partially contained inside the opening. In an embodiment, the die is a MEMS die, and the package is a MEMS package. Additional die or die stacks may be mounted on the flexible wiring board adjacent the MEMS die. For example, the additional die or die stacks may be encapsulated within the same overmold as the MEMS die, or separate overmolds. 
     A stiffener layer may be bonded to a back side of the flexible wiring board opposite the MEMS die. The stiffener layer may be more flexible than the module board so that the stiffener layer provides an amount of structural support. In an embodiment, the stiffener layer is partially contained within the opening in the wiring board. 
     The MEMS package may include a variety of die configurations. For example, the die may include a MEMS die and IC die. Each of the die may additionally be bonded to an underlying substrate with a die attach film including an elastomeric material. For example, a MEMS die may be bonded to an IC die with a die attach film, and the IC die may in turn be bonded to the flexible wiring board with a die attach film. Wire bonding may be used to provide electrical connections to the MEMS die and IC die. 
     Additional packages may be surface mounted onto the front or back surfaces of the wiring board. In an embodiment, profile heights of the other packages are raised above the profile height of the package including the MEMS die. 
     In an embodiment, a method of forming a module includes forming an opening through a module board, and laminating a flexible wiring board of a MEMS package onto a back surface of the module board such that an air gap exists laterally between an overmold encapsulating the die on the flexible wiring board and side surfaces of the opening in the module board. Laminating may include attaching the flexible wiring board to the back surface of the module board with a z-axis film. The z-axis film may be laminated on the module board or the flexible wiring board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view illustration of a MEMS package surface mounted onto a module board. 
         FIGS. 2A-2C  are cross-sectional side view illustrations of a MEMS package mounted in an opening of a module board in accordance with embodiments. 
         FIG. 2D  is a cross-sectional side view illustration of front and back MEMS packages mounted in an opening of a module board in accordance with an embodiment. 
         FIGS. 3-4  are cross-sectional side view illustrations of a MEMS package including multiple die mounted in an opening of a module board in accordance with embodiments. 
         FIG. 5  is an illustration a first level package flow in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view illustration of stacked die attached to a flexible wiring board in accordance with an embodiment. 
         FIG. 7  is a cross-sectional side view illustration of wired bonded stacked die in accordance with an embodiment. 
         FIG. 8  is a cross-sectional side view illustration of molded stacked die in accordance with an embodiment. 
         FIG. 9  is a cross-sectional side view illustration of a singulated MEMS package in accordance with an embodiment. 
         FIG. 10  is an illustration a second level package flow in accordance with an embodiment. 
         FIG. 11  is a cross-sectional side view illustration of an opening formed in a module board in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view illustration of a z-axis film attached to a module board in accordance with an embodiment. 
         FIG. 13  is a cross-sectional side view illustration of mounting a MEMS package in an opening of a module board in accordance with an embodiment. 
         FIG. 14  is a schematic top view illustration of a module board including a screw and MEMS package mounted in an opening of the module board in accordance with an embodiment. 
         FIG. 15  is a cross-sectional side view illustration of a MEMS package including a flexible wiring board and a z-axis film in accordance with an embodiment. 
         FIGS. 16-17  are schematic top view illustrations of a interconnections between a module board and a MEMS package including a flexible wiring board and a z-axis film in accordance with embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments describe MEMS packages, modules, and methods of fabrication. In various embodiments, description is made with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the embodiments. In other instances, well-known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the embodiments. Reference throughout this specification to “one embodiment” means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The terms “above”, “over”, “to”, “between”, “spanning” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “above”, “over”, “spanning” or “on” another layer or bonded “to” or in “contact” with another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers. 
     In one aspect, embodiments describe MEMS packages, modules and manners of fabrication which may address MEMS device stress and strain induced performance drift, particularly for high sensitivity applications. For example, thermo-mechanical stress from a MEMS package or underlying module board can cause MEMS device output shift, particularly as temperature changes. In an embodiment, a MEMS package is molded onto a flexible wiring board, also commonly known as a flexible printed circuit board or flex circuit. The flexible wiring board is mounted on a module board such that the MEMS package is at least partially contained inside an opening in the module board. The module board may be a rigid substrate, such as a rigid printed circuit board (PCB). In this manner, in plane strain from the module board may be diverted away from the MEMS die by the flexible wiring board. Thus, stress propagation from the module board to the MEMS die may be reduced, such as mechanical stress from screws, bending, twisting, stress from other high strain components on the module board, or hygroscopic stress from the module board. In another aspect, the flexible wiring board may enable better shock and drop performance of the device incorporating the MEMS module. In another aspect, the recessed MEMS package design may enable reduced overall module thickness. In yet another aspect, the recessed MEMS package design may allow for an increased MEMS die or package thickness for increased immunity to strain. 
     Referring now to  FIG. 1  a cross-sectional side view illustration is provided of a MEMS package surface mounted onto a module board. As illustrated, the exemplary MEMS package  100  includes a surface mount substrate  102 , an integrated circuit (IC) die  104 , such as an application specific integrated circuit (ASIC) die, attached to the surface mount substrate  102  with a die attach film  106 , and a MEMS die  108  attached to the IC die  104  with a die attach film  110 . Wire bonds  112  are used to provide electrical connection between the MEMS die  108 , IC die  104 , and surface mount substrate  102 . An overmold  114  of a molding compound encapsulates the die and wire bonds on the surface mount substrate  102 . 
     As shown in  FIG. 1 , a module substrate  120  may be a rigid PCB for example. MEMS package  100  is mounted onto a front surface  130  (or back surface  128 ) the module substrate  120  with conductive bumps  116 , such as solder bumps. Module substrate  120  may have one or more additional packages  140  mounted on the front surface  130  or back surface  128 . It has been observed that MEMS package  100  may be sensitive to stress  122  from the module substrate  120 . For example, stress can be attributed to the module substrate  120  material properties, thickness, and metal layout. For example, this stress can be transferred to the MEMS package  100  from screws  124  or other high stress packages  140  mounted on the module substrate, mechanical bending stress from thermal expansion mismatch of substrates or components, and hygroscopic stress associated with moisture ingression. When the MEMS package  100  is rigidly mounted onto the module substrate  120 , as illustrated in  FIG. 1 , these stresses can be directly transferred to the MEMS package, resulting in strain induced performance drift. 
     Still referring to  FIG. 1 , the profile height of the MEMS package  100  is illustrated as being greater than a profile height of the other packages  140  on the same surface of the module board  120 . This height difference may also be attributed to mitigating strain induced performance drift of the MEMS die  108 , with a thicker MEMS die  108  being less sensitive to strain. Thus, as consumer devices, particularly mobile and wearable devices, continue to become thinner with increased functionality, further reduction in MEMS die  108  and MEMS package  100  thickness may be met with a tradeoff of increased strain induced performance drift. 
     Referring now to  FIG. 2A , in accordance with embodiments described herein a MEMS package  200  including a flexible wiring board  202  is mounted on a back surface  128  of the module board  120  spanning across an opening  126  in the module board  120  (see also  FIGS. 11-14 ). As shown, the opening  126  extends through and is laterally surrounded by the module board  120 . A die is mounted on the flexible wiring board and at least partially contained inside the opening. The die is encapsulated within an overmold  114  and an air gap  132  exists laterally between the overmold  114  and side surfaces of the opening  126  in the module board. In an embodiment, the flexible wiring board  202  is mounted to the back surface  128  of the module board  120  with a z-axis film  204 . A z-axis film is a film with anisotropic electrical conductivity. For example, the film includes an adhesive material (e.g. epoxy/acrylate blend) filled with conductive particles enabling electrical interconnection through a thickness of the film (the z-axis) between substrates. A stiffener layer  206  may optionally be bonded to the backside of the flexible wiring board  202  to mechanically secure the die within the opening  126  in the module board  120 . For example, the stiffener layer  206  may be formed of a material such as polyimide, or stainless steel. In an embodiment, the stiffener layer  206  is more flexible than the module board  120 . For example, this may be obtained by modulation of thickness and material. In the embodiment illustrated in  FIG. 2A , the flexible wiring board  202  and optional stiffener layer  206  are bent toward the opening  126  so that they are at least partially contained within the opening. 
     In one aspect, embodiments describe a MEMS module configuration including a flexible MEMS package which mitigates in plane strain propagation from the module board  120  to the MEMS package  200 . Integration of a flexible MEMS package into a recessed module board design, creates a physical discontinuity in the module board allowing for mitigation of in-plane related strain propagation from the module board to the MEMS die, as well as the transfer of external stress resulting from shock and drop of the electronic device into which the MEMS module is integrated. 
     In another aspect, embodiments describe a MEMS module configuration in which a flexible MEMS package is coupled to the module board  120  such that motion including acceleration and rotation of the MEMS package and module board are coupled. A stiffener layer may additionally be bonded to the flexible MEMS package to allow x/y/z motion and rotation sensing that is in sync with the module board, and overall system. 
     In another aspect, embodiments describe a MEMS module configuration in which a recessed module board design may allow for MEMS package profile height, and therefore MEMS die  108  thickness, to be maintained or increased. In a typical fabrication process, MEMS die are often thinned during fabrication in order to meet packaging specifications. In an embodiment, a total module thickness of the module board and attached components (e.g. surface mounted packages) may be reduced while MEMS package  200  and MEMS die  108  thickness may be maintained or increased for improved MEMS die  108  performance and reduced strain induced performance drift. Thus, embodiments may enable continued reduction in footprint and profile of a MEMS module, and increased functionality in a given footprint on the module board  120 , without requiring further reduction in MEMS package  200  and MEMS die  108  thickness. 
     In the particular embodiment illustrated in  FIG. 2A , a die stack arrangement is illustrated as similar to that in  FIG. 1 . Specifically, the die stack arrangement includes an IC die  104  attached to a flexible wiring board  202  with a die attach film  106 , and a MEMS die  108  attached to the IC die  104  with a die attach film  110 . Wire bonds  112  are used to provide electrical connection between the MEMS die  108 , IC die  104 , and flexible wiring board  202 . An overmold  114  of a molding compound encapsulates the die and wire bonds on the flexible wiring board  202 . In an embodiment, the MEMS die  108  is not attached to the flexible wiring board  202  with a rigid solder bond. For example, in the embodiment illustrated in  FIG. 2A , the MEMS die  108  is attached to the IC die  104  with die attach film  110 , and the IC die  104  in turn is attached to the flexible wiring board  202  with a die attach film  106 . Die attach films  110 ,  106  may be formed of an elastomeric material, such as silicone, which can be characterized as a high flexibility and low stress adhesive material employed in packaging. Overmold  114  may be formed directly on the flexible wiring board  202 . In the embodiment illustrated in  FIG. 2A , both the MEMS die  108  and IC die  104  are at least partially contained within the opening  126 . In an embodiment, either or both of the MEMS die  108  and IC die  104  are completely contained within the opening  126 . In the embodiment illustrated in  FIG. 2A , a profile height of the MEMS package  200  (e.g. top surface of the overmold  114 ) is raised above the front surface  130  of the module board  120 . 
     It is to be appreciated that the particular die stack arrangement provided in  FIG. 2A  is exemplary, and that embodiments are compatible with a variety of alternative MEMS die  108  arrangements that include MEMS die  108  encapsulated on a flexible wiring board  202  with an overmold  114  of a molding compound. For example, a MEMS die  108  or IC die  104  can be mounted directly on the flexible wiring board  202  with a die attach film. 
     Referring now to  FIGS. 2B-2C , a MEMS package  200  including a flexible wiring board  202  is mounted on a back surface  128  of the module board  120  spanning across an opening  126  in the module board  120  in accordance with embodiments.  FIG. 2B  is similar to the embodiment illustrated in  FIG. 2A , with one difference being that the flexible wiring board  202  and optional stiffener layer  206  are bent away from the opening  126 , such that they are not contained within the opening  126 . In an embodiment, a bent flexible wiring board  202  and optional stiffener layer  206  may allow for additional decoupling of in plane strain from the mounting board  120  such that when the mounting board is stretched, the flexible wiring board  202  and optional stiffener layer  206  in turn may bend, rather than stretch.  FIG. 2C  is similar to the embodiment illustrated in  FIG. 2A , with one difference being that the flexible wiring board  202  and optional stiffener layer  206  are not bent, and are flat across the opening  126 . 
       FIG. 2D  is a cross-sectional side view illustration of front and back MEMS packages mounted in an opening in a module board in accordance with an embodiment. As illustrated in  FIGS. 2A-2C , the location of a flexible MEMS package within the opening in the mounting board  120  can be adjusted to achieve different profile heights (or elevations) of the MEMS packages  200 ,  201  relative to other packages  140 ,  141  mounted on the module board  120 . In the embodiment illustrated in  FIG. 2D , a second flexible package  201  is mounted on a back surface of the stiffener layer  206  where the stiffener layer is secured in the opening  126 . Flexible package  201  may be a single die or multiple die stack, similar to MEMS package  200 . Furthermore, additional packages  141  may be mounted on the back surface  128  of the module board  120 . In an embodiment, flexible package  201  is thicker than package  141 , yet it is not elevated beyond the bottom height of package  141  due to being recessed within the opening  126 . In the embodiment, illustrated in  FIG. 2D , flexible package  201  includes a MED die  104  or IC die  108 , though this is meant to be exemplary, and other die or die stack configurations are contemplated. 
       FIGS. 3-4  are cross-sectional side view illustrations of MEMS packages  200  including multiple die connected to a flexible wiring board mounted across an opening in a module board in accordance with embodiments. In such configurations, a group of strain sensitive components can be mounted within the opening  126  of the module board  120 . In the embodiment illustrated in  FIG. 3 , multiple die or die stacks are arranged side-by-side on the flexible wiring board  202 , with each laterally separate die or die stack encapsulated in a separate overmold  114  on the flexible wiring board  202 . For example, such a configuration may be suitable where each laterally separate die or die stack is an individually tested component prior to molding. In the embodiment illustrated in  FIG. 4 , multiple die or die stacks are arranged side-by-side on the flexible wiring board  202 , with each laterally separate die or die stack encapsulated in the same overmold  114 . For example, such a configuration may be suitable where the laterally separate die or die stacks are a multifunctional component. In the embodiments illustrated in  FIGS. 3-4 , the multiple die or die stacks include multiple MEMS die  104  arranged side-by-side. As illustrated, an air gap  132  exists laterally between the overmold(s)  114  and side surfaces of the opening  126  in the module board. 
     Referring now to  FIG. 5  in combination with  FIGS. 6-9  a first level package flow is provided in accordance with an embodiment. At operation  510  laterally separate die or die stacks are attached on a flexible wiring board  202  with a die attach film, followed by cure of the die attach film. In the particular embodiment illustrated in  FIG. 6 , die stacks including an IC die  104  and MEMS die  108  are attached with die attach films  106 ,  110  and cured. Following the die attach, the die are wire bonded at operation  520 . In the exemplary embodiment illustrated in  FIG. 7 , wire bonds  112  are used to provide electrical connection of the MEMS die  108  to the IC die  104 , and electrical connection of the IC die  104  to the flexible wiring board  202 . This particular configuration is meant to be exemplary, and other wire bonding configurations are contemplated. At operation  530  the flexible wiring board  202  is molded and cured to form overmolds  114  over one or more die or die stacks as illustrated in  FIG. 8 . At operation  540  individual flexible MEMS packages  200  are singulated as illustrated in  FIG. 9 . While the process flow illustrated in  FIGS. 6-9  is made with regard to a single stacked die arrangement, this process is exemplary, and may be used to form a variety of other MEMS package configurations including, but not limited to, those illustrated and described with regard to  FIGS. 2A-4 . 
     Referring now to  FIG. 10  in combination with  FIGS. 11-14  a second level package flow is provided in accordance with an embodiment. At operation  1010  a module board  120  is fabricated with one or more openings  126 , as illustrated in  FIG. 11 . At operation  1020  a z-axis film  204  may be attached to the module board  120  adjacent the one or more openings  126  as illustrated in  FIG. 12 . Alternatively, the z-axis film  204  may attached to the flexible wiring board  202  rather than the module board  120 . At operation  1030  a flexible MEMS package  200  is laminated in an opening  126  in the module board  120  as illustrated in  FIG. 13 . In an embodiment, the flexible MEMS package  200  is laminated in the opening  126  after other packages  140  have been mounted on the module board  120 .  FIG. 14  is a schematic top view illustration of a module board including a screw  124  and MEMS package  200  mounted in an opening  126  in the module board in accordance with an embodiment. As shown, an air gap  132  is formed around a periphery of the overmold  114  inside the opening  126 . 
     As described above, z-axis film  204  may be applied to either the module board  120  or the flexible wiring board  202  for laminating the flexible MEMS package  200  onto the module board  120 .  FIG. 15  is a cross-sectional side view illustration of a MEMS package  200  including a flexible wiring board  202  and a z-axis film  204  in accordance with an embodiment.  FIGS. 16-17  is a schematic top view illustrations of a interconnections between a module board and a MEMS package including a flexible wiring board and a z-axis film in accordance with embodiments. In accordance with embodiments, the module board  120  may include a plurality of contact pads  150  for making electrical contact with the flexible wiring board  202 . In the embodiment illustrated in  FIG. 16 , an arrangement of contact pads  150  is formed around the opening  126 . In such an embodiment, the z-axis film  204  can surround the overmold  114 , for example, in the shape of a ring (which may be rectangular). In the embodiment illustrated in  FIG. 17 , an arrangement of contact pads  150  is formed on opposite sides of the opening  126 . In such an embodiment, the z-axis film  204  can be formed as two strips on opposite sides of the overmold  114 . 
     In utilizing the various aspects of the embodiments, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for forming a MEMS module with a MEMS package mounted in an opening of a module board. Although the embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that the appended claims are not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as embodiments of the claims useful for illustration.

Metadata:
Filing Date: 20141117
Publication Date: 20170418
Grant Date: 20170418
Priority Date: 20141117
Inventors: JIANG TONGBI
ZHAI JUN
Assignee: APPLE INC
CPC Classifications: [{"code": "B81C2203/0154", "inventive": false, "first": false, "tree": "[]"}, {"code": "B81C1/00325", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81B7/0048", "inventive": true, "first": true, "tree": "[]"}, {"code": "B81B2207/012", "inventive": false, "first": false, "tree": "[]"}, {"code": "B81B2207/012", "inventive": false, "first": false, "tree": "[]"}, {"code": "B81B7/0048", "inventive": true, "first": true, "tree": "[]"}, {"code": "B81C1/00325", "inventive": true, "first": false, "tree": "[]"}, {"code": "B81B7/0048", "inventive": true, "first": true, "tree": "[]"}, {"code": "B81C2203/0154", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 55961070