Abstract:
A base assembly for an acoustic transducer includes a first substrate with an acoustic port and a dielectric layer having a substantially uniform initial density. One surface of the dielectric layer contacts the first substrate. An opposite surface of the dielectric layer contacts a second substrate having an acoustic port. The respective acoustic ports of the first and second substrates are aligned with each other. The first substrate, the dielectric layer, and the second substrate are laminated together. A substantial portion of the dielectric layer laminated to the first and second substrates has a higher final density due to compaction than does a portion of the dielectric layer disposed in the respective acoustic ports of the first and second substrates.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional application of prior U.S. application Ser. No. 14/354,020, entitled “Embedded Dielectric As A Barrier In An Acoustic Device And Method Of Manufacture,” filed Apr. 24, 2014, which is a national stage entry of PCT/US2011/059318, entitled “Embedded Dielectric As A Barrier In An Acoustic Device And Method Of Manufacture,” filed Nov. 4, 2011, the content of all of which are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to acoustic devices and, more specifically, to the construction of these devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    Various types of acoustic devices (e.g., microphones and receivers) have been used through the years. In these devices, different electrical components are housed together within a housing or assembly. For example, a microphone typically includes a diaphragm and a back plate (among other components) and these components are disposed together within a housing. Other types of acoustic devices such as receivers may include other types of components. 
         [0004]    Acoustic devices typically have ports that allow sound to enter into the interior (or exit from the interior) of the housing. For example, a microphone may have a port that allows sound from the exterior to enter and be amplified. In another example, a speaker typically includes a port that allows sound to exit from the interior of the housing. Regardless of the direction of sound travel, one problem associated with the ports is that while they allow the sound to enter (or exit) the device, they also potentially allow debris inside the interior of the acoustic device. For example, if used in a hearing aid, ear wax or other type of debris may be allowed to enter the device by a port. Besides solid debris, various types of liquids can also be allowed to enter into the interior of the device. All of these types of materials can potentially damage the acoustic device and/or adversely impact its operation. 
         [0005]    Previous systems have sometimes used barriers over the port to prevent debris or other foreign materials from entering the interiors of acoustic devices via the ports. In one previous example, a metal screen was disposed over the port. Unfortunately, the metal screen added air flow resistance to the acoustic device. The adding of the air flow resistance degraded the performance of the device creating user dissatisfaction with these previous systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein: 
           [0007]      FIG. 1  comprises a bottom view of a dielectric membrane layer according to various embodiments of the present invention; 
           [0008]      FIG. 2  comprises a side cut-away view of an acoustic device with the dielectric membrane layer of  FIG. 1  along the line denoted A-A according to various embodiments of the present invention; 
           [0009]      FIG. 3  comprises a diagram showing insertion loss of one type of expanded polytetrafluoroethylene (ePTFE) material according to various embodiments of the present invention; 
           [0010]      FIG. 4  comprises a table showing the various layers of a microphone base according to various embodiments of the present invention; 
           [0011]      FIG. 5  comprises a flowchart for creating a microphone base according to various embodiments of the present invention. 
       
    
    
       [0012]    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 
       [0013]    Acoustic devices having a membrane that acts as both a dielectric layer and an ingress barrier are provided. The devices so configured are manufacturable and prevent outside materials from entering the device while at the same time not increasing or substantially increasing acoustic resistance of the device. Additionally, the devices described herein provide a dielectric layer for passive electric components such as capacitors 
         [0014]    In one aspect, the base (or circuit board or substrate) of an acoustic device such as a microphone includes multiple layers of various materials. The layers include at least some metal layers and at least some core layers. A dielectric membrane is sandwiched between some of these layers. A port extends through the layers, but the dielectric membrane extends across the port. A portion of the dielectric membrane is compressed and this compressed portion can be used as a capacitor. The non-compressed across the port acts as a barrier. The dielectric membrane is constructed of any suitable material such as expanded polytetrafluoroethylene (ePTFE) or other polymeric membrane. Other examples of materials are possible. The dielectric membrane provides little acoustic resistance to sound entering (or exiting) the device provided an appropriately sized (e.g., greater than approximately 1.0 mm diameter) port is used. 
         [0015]    The compressed part of the dielectric membrane has an increased dielectric constant that is sufficient for this portion to act as a capacitor. Consequently, a single dielectric membrane acts as both a barrier and a passive electric component. Further, there is enough acoustic/sound flow so that the dielectric membrane does not act as an acoustic resistor. 
         [0016]    In many of these embodiments, a microphone base includes a plurality of metal layers and a plurality of core layers. Each of the plurality of core layers is disposed between selected ones of the metal layers. A dielectric membrane is disposed between other selected ones of the plurality of metal layers. A port extends through the metal layers and the core layers but not through the dielectric membrane. The dielectric membrane has a compressed portion and an uncompressed portion. The uncompressed portion extends across the port and the compressed portion is in contact with the other selected ones of the metal layers. The compressed portion of the membrane is effective to operate as a passive electronic component and the uncompressed portion is effective to act as a barrier to prevent at least some external debris from traversing through the port. 
         [0017]    Referring now to  FIG. 1  and  FIG. 2 , one example of an acoustic device  100  with a dielectric membrane  102  is described. The device  100  includes a cover or can  104 , a dielectric membrane  102 , a first core layer  106 , a second core layer  108 , a first metal layer  110 , a second metal layer  112 , a third metal layer  114 , a fourth metal layer  116 , a transducer  118 , an integrated circuit  120 , a solder mask  122 , solder pads  124 , and an acoustic port  126 . 
         [0018]    The cover or can  104  is constructed of any appropriate material such as a metal or hard plastic. The dielectric membrane  102  includes a non-compressed area  132  and a compressed area  134 . The compressed area  134  is generally located where the dielectric layer  102  is directly sandwiched between other adjoining layers (i.e., is in contact with these adjoining layers). The uncompressed area  132  is generally located where the dielectric layer  102  extends across the port  126  (i.e., where the layer  102  is not in direct contact with adjoining layers). The compression of the layer  102  (by having it squeezed and held between adjoining layers) changes the dielectric constant value of the compressed area, increasing it as compared with the uncompressed area and thereby makes the dielectric suitable for use as a capacitor. The electrodes of the capacitor are electrically connected vertically to the appropriate metal layers and traces by plated through hole vias. 
         [0019]    In one example, the layer  102  is an expanded Teflon expanded polytetrafluoroethylene (ePTFE) film. The film is relatively acoustically transparent due to its low density (e.g., &gt;70 percent volume is air). Referring now to  FIG. 3 , the insertion loss (permeability of sound) of two types of ePTFE is shown for two ePTFE films (NTF 1026 and NTF 1033). 
         [0020]    The first core layer  106  and a second core layer  108  are constructed from glass reinforced laminate in one example. The purpose of the core layers  106  and  108  provide mechanical rigidity and electrical insulation between the metal layers. 
         [0021]    The first metal layer  110 , second metal layer  112 , third metal layer  114 , and fourth metal layer  116  are constructed from an appropriate metal such as copper. The purpose of these layers is to provide conductive paths and routing functions. For instance, the layer  110  may be a routing and wire bond layer. The layer  112  may be a common capacitive ground. The layer  114  may be used for signal electrodes. The layer  116  may be used for customer pads. As shown in  FIG. 4 , one example of a proposed microphone base metal stack up is shown. There are two core layers (C1 and C2) and four metal layers (1, 2, 3, and 4) and the dielectric layer (D). The composition (description), thickness, tolerance, and other information of these layers is included. As shown, the metal layers are constructed of sub-portions. The layers together may be referred to as the microphone base (or circuit board). It will be appreciated that the example base described herein is only one example and that the number, type, configuration, materials, and other aspects of the base can be changed depending upon the needs of a particular user or the requirements of a particular system. 
         [0022]    As a capacitor, the inherent low dielectric constant (k=1.3) of the layer  102  constructed of uncompressed ePTFE would likely present problems for obtaining a useful capacitor. In some aspects, uncompressed ePTFE provides approximately only 10 percent of the capacitance per unit area as embedded dielectric films. However, because of the compressive nature of ePTFE, it can be compacted to near bulk density during the lamination process (except for material at the acoustic port hole which remains uncompressed) to yield a capacitance per unit area of approximately 20 percent of current materials. 
         [0023]    The transducer  118  includes various components such as a MEMS die, diaphragm, charge plate and so forth. The transducer acts to convert sound energy into an electrical signal that is sent to the integrated circuit  120 . The function of the solder mask  122  is to protect and insulate the underlying metal traces and to prevent solder migration. The solder pads  116  provide an electrical and mechanical connection between the base and the final PCB assembly. 
         [0024]    In one example of the operation of the system of  FIG. 1  and  FIG. 2 , the compressed part  132  of the dielectric membrane  102  has an increased dielectric constant that is sufficient for this portion to act as a capacitor while the uncompressed part  134  acts as a barrier in the port  126 . Consequently, the single dielectric membrane  102  acts as both a barrier and a passive electric component. Further, there is enough acoustic/sound flow so that the dielectric membrane  102  does not act as an acoustic resistor and therefore there is little or no adverse impact upon the operation of the device  100 . 
         [0025]    Referring now to  FIG. 5 , one example of a process for creating the microphone base is described. At step  502 , metallization layers are etched onto both the top and bottom cores. At step  504 , a mechanical drill is used to bore an acoustic port hole through the top and bottom cores. At step  506 , a lamination is performed of the ePTFE film between the top and bottom cores. This results in a multi-layer PCB with an embedded dielectric and barrier. 
         [0026]    At step  508 , a mechanical drill is used to bore a plated through hole (PTH) vias, which are used to electrically connect the metal traces and layers. At step  510 , a copper barrel plate is applied to the board. The purpose of copper barrel plating is to coat the inner walls of the vias in order to make them electrically conductive in the axial direction. At step  512 , the top and bottom solder masks are printed and then cured. At step  514 , ENIG finish is applied to provide a corrosion resistant and wire bondable and solderable surface. 
         [0027]    Accordingly, approaches are provided using ePTFE or other similar materials as both ingress barriers and passive electrical components in acoustic devices such as microphones. Fluoropolymer-based films (such as ePTFE) can be embedded according to the present approaches into a multilayer base directly contrary to previous approaches where this aspect was avoided. Additionally, counter intuitively, and in stark contrast to previous approaches, ePTFE materials of the present approaches are used in passive electrical components to provide the various advantages described herein. 
         [0028]    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.