Abstract:
A microphone assembly includes a base, a cover, and a microelectromechanical system (MEMS) die. The cover extends at least partially over and is coupled to the base. The cover and the base form a cavity. The MEMS die is coupled to the base and disposed within the cavity. At least a portion of the cover is constructed of a copper-nickel-zinc alloy that is effective in preventing solder from moving from a first portion of the cover to a second portion of the cover.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent claims benefit under 35 U.S.C. §119 (e) to U.S. Provisional Application No. 61/804,087 entitled “Cover for a MEMS Microphone” filed Mar. 21, 2013, and Application No. 61/804,004 entitled “Cover for a MEMS Microphone” filed Mar. 21, 2013, the contents of which are incorporated herein by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to MicroElectroMechanical components and, more specifically, the covers of these devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    MicroElectroMechanical System (MEMS) devices include microphones and speakers to mention two examples. In the case of a MEMS microphone, sound energy enters through a sound port and vibrates a diaphragm and this action creates a corresponding change in electrical potential (voltage) between the diaphragm and a back plate disposed near the diaphragm. This voltage represents the sound energy that has been received. Typically, the voltage is then transmitted to an electric circuit (e.g., an integrated circuit such as an application specific integrated circuit (ASIC)). Further processing of the signal may be performed on the electrical circuit. For instance, amplification or filtering functions may be performed on the voltage signal at the integrated circuit. 
         [0004]    The internal devices (e.g., integrated circuit, MEMS device) of a microphone are disposed within an assembly. For example, these devices may be attached to a base and covered with a cover. In other words, a cavity is formed by the cover and the internal devices (e.g., an integrated circuit, MEMS device) are disposed on the base within the cavity. 
         [0005]    In many examples, the cover is coupled to the base of the acoustic device with solder. In fact, the cover may be attached to the base by solder on both on the inside portion (in the cavity and exposed to the MEMS device and the integrated circuit) and on the exterior of the device (exposed to the external environment). 
         [0006]    In order that the solder can be attached to the cover (and thus make the connection to the base), the entire cover of previous systems is typically plated in gold (or some other appropriate metal) and then the attachment between the cover and base is made. However, in many situations after the attachment is made the device (including the solder) is re-heated. In these circumstances, the solder will melt and continues to interact with the plating, and “creeps” or flows up the cover. As this happens within the cavity, some solder can be discharged from the cover and cause failure to the MEMS device or integrated circuit within the cavity. On the outside of the cover, the “creeping” solder can continue to flow up the cover onto the surface of the cover and this will interfere with subsequent gasketing of the microphone. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein: 
           [0008]      FIG. 1  comprises a perspective view of a MEMS bottom port microphone according to various embodiments of the present invention; 
           [0009]      FIG. 2  comprises a top view of the MEMS microphone of  FIG. 1  according to various embodiments of the present invention; 
           [0010]      FIG. 3  comprises a cross sectional view of the MEMS microphone taken along line A-A of  FIG. 2  according to various embodiments of the present invention; 
           [0011]      FIG. 4  comprises a cross sectional view of the MEMS microphone taken along line B-B of  FIG. 2  according to various embodiments of the present invention; 
           [0012]      FIG. 5  comprises a perspective view of a MEMS top port microphone according to various embodiments of the present invention; 
           [0013]      FIG. 6  comprises a top view of the MEMS microphone of  FIG. 5  according to various embodiments of the present invention; 
           [0014]      FIG. 7  comprises a cross sectional view of the MEMS microphone taken along line G-G of  FIG. 6  according to various embodiments of the present invention; 
           [0015]      FIG. 8  comprises a cross sectional view of the MEMS microphone taken along line F-F of  FIG. 6  according to various embodiments of the present invention; and 
           [0016]      FIG. 9  comprises a cross sectional view of the MEMS microphone taken in area G of  FIG. 6  according to various embodiments of the present invention. 
       
    
    
       [0017]    Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. 
       DETAILED DESCRIPTION 
       [0018]    The present approaches provide a metal cover for a MicroElectroMechanical System (MEMS) microphone that eliminates or substantially reduces solder creep and provides other advantages described herein. In one aspect, a copper nickel zinc alloy that is approximately 55% copper, approximately 18% nickel, and approximately 27% zinc is used. In another aspect, this alloy has a material designation of C77000 under the Unified Numbering System (UNS). In one example, the alloy used may be the C77000 alloy produced by Wieland Metals, Inc. 
         [0019]    In other aspects, the covers constructed of the copper nickel zinc alloys described herein are resistant to corrosion and have a good shelf life. Additionally, the covers constructed of the copper nickel zinc alloys provide for a good solderability and can be joined readily with soft solders. Further, the covers constructed of the copper nickel zinc alloys described herein do not need gold plating (or plating of any kind) or any surface finish. Being non-plated, these covers do not exhibit solder creeping on the cover surface during post assembly reflow processes. This eliminates customer returns due to visual defects. RF performance of the covers constructed of the copper nickel zinc alloys described herein is also adequate and comparable to previous approaches. 
         [0020]    In still further aspects, since the covers constructed of the copper nickel zinc alloys described herein also include copper, the mechanical properties of the covers so-provided are similar to brass. The covers constructed of the copper nickel zinc alloys described herein can be constructed with existing tooling during the metal cover stamping process. Still further, the covers constructed of the copper nickel zinc alloys can be used as replacement on any microphone assembly with similar dimensions. In yet another advantage, the covers constructed as described herein can be used with existing production solder pastes and reflow techniques. 
         [0021]    Referring now to  FIGS. 1-4 , one example of a microphone assembly  100  is described. The assembly  100  includes a base  102 , a MicroElectroMechanical system (MEMS) device  104 , an integrated circuit  106 , wires  108 , a port  110 , and a cover  112 . The port  110  extends through the base  102  making this a bottom port device. All dimensions shown in the drawings are in millimeters. However, it will be understood that other dimensions for the components and their placement may be used. 
         [0022]    The base  102  is a substrate on which the cover and other components rest. In one example, the base  102  is constructed of an FR-4 material. Other examples of materials may also be used. The MEMS device  104  and the integrated circuit  106  are attached to the base  102  by, for example, an epoxy attachment. 
         [0023]    The MEMS device  104  receives sound energy and converts the sound energy into electrical energy. In that respect, the MEMS device  104  may include a diaphragm  114  and a back plate  116 . Sound energy enters the port  110  causes movement of the diaphragm  114  and this varies the electrical potential between the diaphragm  114  and the back plate  116 . The current or voltage that is produced represents the sound energy that has been received by the MEMS apparatus  104 . 
         [0024]    The integrated circuit  106  is any kind of integrated circuit that performs any kind of processing function. In one example, the integrated circuit  106  is a buffer or an amplifier. Other examples of integrated circuits are possible. The wires  108  are connections that couple electrical components together. Internal conductive connections (not shown) are provided through the base  102  to allow the integrated circuit  106  to communicate with pads (not shown) on the base and thereby with external electrical devices. 
         [0025]    Solder holds the cover and the base together. The solder, in one aspect, is on the inside and outside of the cover. Other examples of conductive fusion materials (e.g., conductive epoxy) are possible. 
         [0026]    In one aspect, the entire cover  112  is constructed of a copper nickel zinc alloy that is approximately 55% copper, approximately 18% nickel, and approximately 27% zinc. In other cases, only a lip  122  of the cover is constructed of the alloy (and the remainder constructed of brass to mention one example). In another example, approximately one-half (the half nearest the base  102 ) of the cover  112  is constructed of the alloy. In these regards, it will be appreciated that the amount or section of the cover  112  actually constructed of the material may vary. In some aspects, the amount of copper can vary by +/−1.5% (from 53.5-56.5%); the amount of nickel can vary by +/−1% (from 17-19%). The amount of zinc can vary based upon the amount of variance of the other two materials. As will be readily appreciated, no plating or coating is used or needed for the cover  112 . 
         [0027]    The cover  112  can be stamped out from a piece of the alloy. Once formed, the cover  112  may be placed on the substrate and soldered into place using conventional soldering approaches. 
         [0028]    Referring now to  FIGS. 5-9  one example of a microphone assembly  500  is described. The assembly  500  includes a base  502 , a MicroElectroMechanical system (MEMS) device  504 , an integrated circuit  506 , wires  508 , a port  510 , and a cover  512 . The port  510  extends through the cover  512  making this a top port device. All dimensions shown in the drawings are in millimeters. However, it will be understood that other dimensions for the components and their placement may be used. 
         [0029]    The base  502  is a substrate on which the cover and other components rest. In one example, the base  502  is constructed of an FR-4 material. Other examples of materials may also be used. The MEMS device  504  and the integrated circuit  506  are attached to the base  502  by, for example, an epoxy attachment. 
         [0030]    The MEMS device  504  receives sound energy and converts the sound energy into electrical energy. In that respect, the MEMS device  504  may include a diaphragm  514  and a back plate  516 . Sound energy enters the port  510  causes movement of the diaphragm  514  and this varies the electrical potential between the diaphragm  514  and the back plate  516 . The current or voltage that is produced represents the sound energy that has been received by the MEMS apparatus  504 . 
         [0031]    The integrated circuit  506  is any kind of integrated circuit that performs any kind of processing function. In one example, the integrated circuit  506  is a buffer or an amplifier. Other examples of integrated circuits are possible. The wires  508  are connections that couple electrical components together. Internal conductive connections (not shown) are provided through the base  502  to allow the integrated circuit  506  to communicate with pads (not shown) on the base and thereby with external electrical devices. 
         [0032]    Solder holds the cover and the base together. The solder, in one aspect, is on the inside and outside of the cover. Other examples of conductive fusion materials (e.g., conductive epoxy) are possible. 
         [0033]    In one aspect, the entire cover  512  is constructed of a copper nickel zinc alloy that is approximately 55% copper, approximately 18% nickel, and approximately 27% zinc. In other cases, only a lip  522  of the cover is constructed of the alloy (and the remainder constructed of brass to mention one example). In another example, approximately one-half (the half nearest the base  502 ) of the cover  512  is constructed of the alloy. In these regards, it will be appreciated that the amount or section of the cover  512  actually constructed of the material may vary. In some aspects, the amount of copper can vary by +/−1.5% (from 53.5-56.5%); the amount of nickel can vary by +/−1% (from 17-19%). The amount of zinc can vary based upon the amount of variance of the other two materials. As will be readily appreciated, no plating or coating is used or needed for the cover  512 . 
         [0034]    The cover  512  can be stamped out from a piece of the alloy. Once formed, the cover  512  may be placed on the substrate and soldered into place using conventional soldering approaches. 
         [0035]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the invention.