Manufacture of a micro electrostatic acoustic device

Abstract

An electrostatic acoustic device includes two components, a first component and a second component. The components each include a cladded printed circuit board (PCB) material including a flat dielectric substrate with an electrically conductive cladding configured to include a first region and a second region of the cladding. The second region fully encloses the first region. A height of the second region is greater than a height of the first regions. The height difference is measurable relative to a reference plane parallel to the cladded surface. The cladding of the PCB material is processed until the height difference between the first region and the second region is within a previously specified height difference tolerance from the previously determined height difference. A portion of the cladding is removed to electrically isolate the first region from the respective second regions. Through-holes are drilled in the first region through the cladding and fully through the dielectric substrate.

Description

[0001] MANUFACTURE OF A MICRO ELECTROSTATIC ACOUSTIC DEVICE

[0002] BACKGROUND

[0003] 1. Technical Field

[0004] The present invention relates to an electrostatic audio device, particularly manufacture of an electrostatic loudspeaker of small dimension.

[0005] 2. Description of Related Art

[0006] In the art of high fidelity sound reproduction, the electrostatic loudspeaker has received attention because of inherent excellent sound quality and smooth response over wide frequency ranges. In such devices, a flexible sound producing membrane is positioned near an electrode, or in the case of a push-pull arrangement, a pair of electrodes, one on either side of the membrane. A direct current polarization potential is applied between the membrane and the electrodes, and an audio signal is superimposed on the electrodes, causing the membrane to move in response to the audio signal. Electrodes are acoustically transmissive so that sound produced by the moving membrane radiates outward through the electrode to the listening area.

[0007] Electrostatic speakers are highly efficient devices both electrically and mechanically. Electrical impedance is high and decreases with increasing acoustic frequency. High electrical impedance results in very low operating currents and minimal electrical losses. Mechanically, there are no moving parts other than the moving membrane which is very light in weight. Electrostatic speakers are therefore inherently more energy efficient than electrodynamic acoustic devices currently used in battery operated electronic devices.

[0008] Thus, there is a need for and it would be advantageous to have a small electrostatic speaker of high efficiency suitable for use in battery operated electronic devices.

[0009] BRIEF SUMMARY

[0010] Various electrostatic acoustic devices, methods of manufacture and assembly thereof are disclosed herein, according to different features of the present invention. The electrostatic acoustic device includes two components, a first component and a second component. The components each include a cladded printed circuit board (PCB) material including a flat dielectric substrate with an electrically conductive cladding configured to include a first region and a second region of the cladding. The second region fully encloses the first region. A previously determined height difference is previously specified between the first and second regions. A height of the second region is greater than a height of the first regions. The height difference is measurable relative to a reference plane parallel to the cladding surface. The cladding of the PCB material is processed until the height difference between the first region and the second region is within a previously specified height difference tolerance from the previously determined height difference. A portion of the cladding is removed to electrically isolate the first region from the respective second regions. Through-holes are drilled in the first region through the cladding and fully through the dielectric substrate. Electrical connections may be provided through the dielectric substrate to the first regions of the first and second components and to at least one of the second regions of the first and second components. A membrane may be tensioned across and attached to the second region of the first component without contacting the first region of the first component at a centre of the membrane to produce a membrane sub-assembly. The second component is attachable to the membrane sub-assembly by contacting the membrane of the sub-assembly to the second region of the second component opposite the second region of the first component, without contacting the first region of the second component to a centre of the membrane. In the assembled electrostatic acoustic device, respective through-holes of the first and second components may or may not be fully opposing. The previously determined height difference between the first and second regions, may be between 17 and 150 microns. The second regions may be circular of outer diameter between five and ten millimetres or elliptical with an outer major axis of length between three and twenty millimetres. A thickness of the dielectric substrate may be between 0.2 and 1.6 millimetres. The through-holes may be of diameter between 0.3 and 1.5 millimetre. Total area of the through- holes may be between thirty and fifty per cent of an area of the first regions.

[0011] Various components and sub-assemblies of electrostatic devices are disclosed herein, according to different features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0013] Figures 1 illustrates schematically in cross section, an acoustic device according to embodiments of the present invention;

[0014] Figure 2 illustrates a conventional cladded printed circuit for use as a raw material for manufacture of a micro-electrostatic acoustic device, according to an embodiment of the present invention;

[0015] Figure 3 illustrates schematically a cross-section of a component for assembly into a microelectrostatic acoustic device manufactured according to features of the present invention;

[0016] Figure 4 illustrates a top view of a the component of Figure 3, for assembly of a microelectrostatic acoustic device, according to features of the present invention;

[0017] Figure 5 schematically illustrates a top view of a cladded printed circuit board during processing to produce multiple components of Figures 3 and 4;

[0018] Figure 6 illustrates an example of a micro electrostatic acoustic device assembled including two components of Figures 3 and 4 and a membrane;

[0019] Figure 7 illustrates in a top view, relative orientations of holes through electrodes through the micro electrostatic acoustic device of Figure 6, according to features of the present invention;

[0020] Figures 8 illustrates a simplified flow diagram of a method, according to features of the present invention.

[0021] The foregoing and / or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures. DETAILED DESCRIPTION

[0022] Reference will now be made in detail to features of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The features are described below to explain the present invention by referring to the figures.

[0023] By way of introduction, aspects of the present invention are directed to manufacture of a small electrostatic speaker of maximum dimension, e.g. diameter D of 20 millimetres or less, and in some embodiments an electrostatic acoustic speaker of dimension D of 10 millimetres or less, and in yet other embodiments for an in-ear earphone application an electrostatic acoustic speaker of dimension D between three and twenty millimetres. For a circular electrostatic speaker, maximum dimension D may be a diameter, and for an elliptical electrostatic speaker, dimension D may be a length of a major axis of the ellipse.

[0024] Referring now to the drawings, Figure 1 illustrates schematically in cross section, an acoustic device or electrostatic speaker 10, according to embodiments of the present invention. A vertical axis Z is shown through a centre of electrostatic speaker 10. A membrane 15 is supported in tension by edges of electrodes 11, but electrically insulated from electrodes 11, in a plane essentially perpendicular to vertical axis Z. Membrane 15 may be impregnated with a conductive, resistive and / or electrostatic material so that membrane 15 responds mechanically to a changing electric field. Two electrodes 11 are shown in Figure 1 which are mounted in parallel to membrane 15, nominally equidistant, at a distance d, e.g. 17-500 microns from membrane 15. Electrodes 11 are illustrated as perforated with apertures 12 transmissive to sound waves emanating from membrane 15 when electrostatic speaker 10 is operating.

[0025] During operation of electrostatic speaker 10, a constant direct current (DC) bias voltage, e.g. + VDc =+1000 volts, may be applied using a conductive contact to membrane 15. Voltage signals ± / gmay be applied to electrodes 11. Voltage signals ± ,gvary at audio frequencies, nominally between 20-20,000 Hertz. A non-inverted voltage signal + Vsigmay be applied to one of electrodes 11 and an identical but inverted voltage signal -Vsigmay be applied to the other electrode 11. Dotted lines 15A illustrate schematically membrane 15 moving in response to a changing electric voltage due to voltage signals

[0026] A force Fsigon membrane 15 responsive to voltage signals ±Vsigmay be approximated or modelled by equation (1): where A is the nominal surface area of electrostatic speaker 10, and £Ois the electrical constant, or permittivity of free space nominally equal to 8.85 • 1012farads / meter.

[0027] The sound pressure level (SPL) may be measured at a particular distance, e.g. 0.5 meter, along axis Z from an electrostatic speaker and is generally proportional to force Fsigon membrane 15 due to the voltage signals ±Vsig, VDc and further dependent on mechanical modes of oscillation.

[0028] According to equation (1) sound pressure level (SPL) is expected to generally decrease with decreasing area of electrostatic speaker 10 and SPL is expected to generally decrease with decreasing voltages VDc and ±Vsig. In order to compensate for the smaller area A, and maintain a specific pressure level (SPL), a larger DC constant bias voltage VDc, a larger absolute value signal voltage ±Vsigand / or smaller distance d between electrodes 11 and membrane 15 may be required to maintain a required sound pressure level (SPL).

[0029] Micro-electrostatic speakers 10 may be manufactured based on a cladded printed circuit board (PCB) used as a raw material, according to features of the present invention. As an example, series R04000™ (Rogers Corporation, Chandler, AZ, USA) are hydrocarbon ceramic laminates compatible with conventional FR-4 PCB fabrication processes.

[0030] Reference is now also made to Figure 2, which illustrates a flat laminate board with a dielectric material 24 cladded with metallic coating 22. The cladding material is typically copper but other conductive materials, e.g. metals may be used. Thickness of cladding 22 may be 100-200 micron. Thickness of dielectric material 24 may be between 0.2- 1.6 millimetre.

[0031] Reference is now also made to Figure 3 which shows a side cross-sectional view of a component 30 for assembly of micro-electrostatic speaker 10, according to features of the present invention. Component 30 may be circular with a diameter D between three and twenty millimetres. Component 30 may be essentially rotationally symmetric about axis Z. Alternatively, component 30 may be elliptical with a length of major axis between three and twenty millimetres. Membrane support 31 is a portion of cladding 22 intended to support membrane 15 at the edge of membrane 15. Membrane 15 is shown in Figure 1, but not shown in Figure 3. The central portion or first region of cladding 22 is configured to be electrode 11. Membrane 15, when assembled and attached to membrane support 31, is designed to be at a nominal or average distance d from a proximal surface of electrode 11. Distance d may be typically between 17-150 microns, e.g. 75 microns with a tolerance within ± 5 microns, and in some cases the tolerance may be ± 3 microns. A portion 33 of the cladding is removed in order to achieve electrical isolation between membrane support 31 and electrode 11. Electrical connections 35 and 37 are configured to membrane support 31 and electrode 11 which may be copper plated through-holes or vias through dielectric material 24.

[0032] Reference is now also made to Figure 4, which illustrates a top view of component 30, for assembly of micro-electrostatic speaker 10, according to features of the present invention. Dielectric substrate 24 (not visible in present view) and electrode 11 includes through-holes 12 of diameter between 0.3 and 1.5 millimetre to allow for acoustic transparency between membrane 15 to the outside. Total area of the through-holes 12 is between thirty and fifty per cent of an area of electrode 11. Membrane support 31 (second region) is shown to fully enclose electrode 11 (first region). In region 33, cladding 24 is removed to electrically isolate electrode 11 (first region) from membrane support 31 (second region).

[0033] Reference is now also made to Figure 5 which schematically illustrates a top view of a cladded printed circuit board 50 during processing to produce component 30. Multiple components 30 may be simultaneously processed to optimise economy of scale during production of component 30 and during subsequent assembly into micro-electrostatic devices 10. In Figure 5, seven instances of component 30 are fit into cladded PCB 50 in a hexagonal close packed lattice, by way of example.

[0034] Reference is now also made to Figure 6 which illustrates an example of micro electrostatic acoustic device 10 including two components 30A and 30B, instances of component 30 (shown in Figure 3) and membrane 15. Membrane 15 may be tensioned across component 30B and attached to membrane support 31 of component 30B with electrically conductive adhesive, by way of example. Component 30A may be attached to or contacted to membrane 15 from the opposite side.

[0035] Reference is now also made to Figure 7 which illustrates in a top view relative orientations of through-holes 12 respectively in components 30A and 30B in device 10 as shown in Figure 6. Specifically, in some embodiments of the present invention, it may not be desirable to have through-holes 12 of component 30A and 30B exactly opposing. In order to avoid perfect opposition of through-holes 12, during assembly of electrostatic acoustic device 10, component 30B may be rotated about its centre by a small angle 3, typically ten to fifteen degrees. Alternatively, components 30A and 30B may be not identical and configured with holes which do not perfectly oppose. In other embodiments of the present invention, through-holes 12 respectively in components 30A and 30B may be configured to oppose.

[0036] Reference is also made to Figure 8 which illustrates a method of manufacture of a microelectrostatic speaker 10, according to features of the present invention. A cladded printed circuit board (PCB) 20 is provided (step 81) including a flat dielectric substrate 24 of uniform thickness with a cladding 22 of a conductive material of previously defined cladding thickness on a surface of the dielectric material. Multiple first regions 11 and second regions 31 are specified (step 83) on the cladded surface of the PCB material 20. Second regions 31 border and fully enclose the respective first regions 11. A previously determined height difference is specified (step 85) between the first and second regions. Height of second regions 31 is greater than a height of the first regions 11. The height difference is measurable relative to a reference plane (example shown in Figure 2) parallel to the cladded surface. The cladded surface 22 of PCB is processed (step 86) until the height difference between the first regions 11 and the second regions 31 is within a previously specified height difference tolerance. A portion 33 of cladding 22 is removed (step 88) to electrically isolate first regions 11 from respective second regions 31. Through-holes 12 are drilled (step 87) in first regions 11, through cladding 22 and fully through the dielectric substrate 24. Electrical connections 37, 35 are respectively provided (step 89) to each of the first and second regions. Electrical connections 37, 35 may be provided by drilling holes through dielectric substrate 24 and electrically coating an interior of the holes with a conductive material.

[0037] Distance d between membrane 15 and electrodes 11 (Figures 1,3) is nominally between 17-150 microns, e.g. 75 microns with a tolerance within ± 5 microns, and in some cases the tolerance may be ± 3 microns. These tolerances may be beyond standard tolerances for machining using CNC (Computer Numerical Control) machines. Standard CNC precision tolerances may be ± 20 microns or at best ± 10 microns.

[0038] Micromachining techniques may be used to manufacture component 30 according to strict tolerances. A review of surface micromachining techniques may be found in:

[0039] Johnstone RW, Parameswaran M, Parmaswaran A. An Introduction to Surface-Micromachining. Springer Science & Business Media; 2004 Jun 3.

[0040] To achieve a mechanical requirement of a step height of 50-100 microns with tight tolerances on a copper layer of a printed circuit board (PCB), processes such as controlled-depth milling or copper plating may be employed.

[0041] 1. Controlled-Depth Milling: a. PCB substrate is provided that has a uniform copper layer covering its surface, (element 20, Figure 2) A precision milling machine equipped with a fine milling bit may be used to remove the unwanted copper in the first region in a controlled and precise manner. The depth of milling may be accurately adjusted to achieve the desired step height.

[0042] 2. Copper Plating: Provide a PCB substrate that has a uniform copper layer covering its surface. Use a photolithographic process to create a patterned photoresist layer on the copper surface. The PCB may be submerged in an electroplating bath containing a copper electrolyte solution. An electrical current may be applied to the PCB, with the copper layer acting as the cathode. Copper ions in the electrolyte deposit onto the exposed copper surface, gradually building up the desired step height. Careful control of the plating parameters, including current density and plating time, allows for precise control of the step height. After plating, the photoresist layer is removed, leaving behind the copper layer with the required step height. The choice between milling and plating depends on factors such as the desired precision, cost, and production capabilities of the PCB manufacturing facility.

[0043] The term “centre” or “central region” as used herein refers to a portion of an acoustic membrane excluding its perimeter and includes between 70% - 99% radially from a centre of the acoustic membrane toward the perimeter of the acoustic membrane.

[0044] The term “opposing” or “fully opposing” as used herein refers to the relative positions of apertures 12 in electrodes 11 with parallel surfaces. Apertures 12 in respective electrodes 11 fully oppose if a line passing through respective centres of apertures 12 is perpendicular to the parallel surfaces of electrodes 11.

[0045] The term “edge” as used herein refers to a portion of an acoustic membrane excluding the centre.

[0046] The term “clad” or “cladding” as used herein refers to a metallic layer, typically copper, applied to the surface of a board substrate.

[0047] The term “dimension” D as used herein refers to the largest dimension. For a polygon of 2n vertices, where n as an integer greater than 1, the dimension D is a diagonal. For a polygon of 2n+l vertices, where n is an integer greater than 0, the term “dimension” as used herein refers to the largest distance along a line which bisects an edge of the polygon to the opposite vertex. The term “dimension” as used herein for an ellipse is the length of the major axis which bisects the ellipse. For a circle, the term “dimension” as used herein is the diameter.

[0048] The term “acoustic device” as used herein and refers to an electrostatic speaker and / or earphone acoustic device.

[0049] The term “acoustic” refers to a mechanical response at audio frequencies, nominally between 20-20,000 Hertz.

[0050] The term “close packed” as used herein refers to a two dimensional lattice a centre point surrounded by six points in a plane. The centres of the six holes may form a regular hexagon.

[0051] The transitional term “comprising” as used herein is synonymous with “including”, and is inclusive or open-ended and does not exclude additional element or method steps not explicitly recited. The articles "a", "an" is used herein, such as "a component” or "an electrode" have the meaning of "one or more" that is "one or more components", "one or more electrodes".

[0052] All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

[0053] Although selected features of the present invention have been shown and described, it is to be understood the present invention is not limited to the described features.

Claims

CLAIMS1. A component of an electrostatic acoustic device, the component comprising: a cladded printed circuit board (PCB) material including a flat dielectric substrate with an electrically conductive cladding configured to include a plurality of first regions and a plurality of second regions of the cladding, wherein the second regions fully enclose the respective first regions; wherein a previously determined height difference is previously specified between the first and second regions, wherein a height of the second regions is greater than a height of the first regions, wherein the height difference is measurable relative to a reference plane parallel to the cladded surface; wherein the cladding of the PCB material is processed until a height difference between the first regions and the second regions is within a previously specified height difference tolerance from the previously determined height difference; wherein a portion of the cladding is removed to electrically isolate the first regions from the respective second regions; and through-holes in the first regions through the cladding and fully through the dielectric substrate.

2. The component of claim 1, further comprising: electrical connections through the dielectric substrate to the first regions of the first and second components and to at least one of the second regions of the first and second components.

3. The component of claim 1, wherein the cladded printed circuit board (PCB) material including at least one of the first regions and at least one of the respective second regions is entirely separable from a remaining cladded printed circuit board (PCB) material at a border outside the second regions.

4. The component of claim 1, wherein the previously determined height difference between between the first and second regions, is between 17 and 150 microns; wherein the previously specified height difference tolerance is less than absolute value of ± five microns.

5. The component of claim 1, wherein the second regions are circular of outer diameter between three and twenty millimetres or the second regions are elliptical with an outer major axis of length between three and twenty millimetres.

6. The component of claim 1, wherein a thickness of the dielectric substrate is between 0.2 and 1.6 millimetres.

7. The component of claim 1, wherein the through-holes are of diameter between 0.3 and 1.5 millimetre wherein total area of the through-holes is between thirty and fifty per cent of an area of the first regions.

8. An electrostatic acoustic device comprising: a first component and a second component each including: a cladded printed circuit board (PCB) material including a flat dielectric substrate with an electrically conductive cladding configured to include a first region and a second region, wherein the second region fully encloses the first region; wherein a previously determined height difference is previously specified between the first and second region, wherein a height of the second region is greater than a height of the first regions, wherein the height difference is measurable relative to a reference plane parallel to the cladding; wherein the cladding of the PCB material is processed until the height difference between the first region and the second region is within a previously specified height difference tolerance from the previously determined height difference; wherein a portion of the cladding is removed to electrically isolate the first region from the respective second regions; through-holes in the first region through the cladding and fully through the dielectric substrate; and a membrane tensioned across and attached to the second region of the first component without contacting the first region of the first component at a centre of the membrane to produce a membrane sub-assembly, wherein the second component is attachable to the membrane subassembly by contacting the membrane of the sub-assembly to the second region of the second component opposite the second region of the first component, without contacting the first region of the second component to a centre of the membrane.

9. The electrostatic acoustic device of claim 8, further comprising: electrical connections through the dielectric substrate to the first regions of the first and second component and to at least one of the second regions of the first and second components.

10. The electrostatic acoustic device of claim 8, wherein respective through-holes of the first and second components are not fully opposing.

11. The electrostatic acoustic device of claim 8, wherein the previously determined height difference between the first and second regions, is between 17 and 150 microns.

12. The electrostatic acoustic device of claim 8, wherein the second regions are circular of outer diameter between three and twenty millimetres or elliptical with an outer major axis of length between three and twenty millimetres.

13. The electrostatic acoustic device of claim 8, wherein a thickness of the dielectric substrate is between 0.2 and 1.6 millimetres.

14. The electrostatic acoustic device of claim 8, wherein the through-holes are of diameter between 0.3 and 1.5 millimetre wherein total area of the through-holes is between thirty and fifty per cent of an area of the first regions.

15. A method of manufacture of a component of an electrostatic acoustic device, the method comprising: providing a cladded printed circuit board (PCB) including a flat dielectric substrate of uniform thickness with a cladding of a conductive material of previously defined cladding thickness on a surface of the dielectric material; specifying a plurality of first regions and a plurality of second regions of the cladded surface of the PCB, wherein the second regions border and fully enclose the respective first regions; specifying a previously determined height difference between the first and second regions, wherein a height of the second regions is greater than a height of the first regions, wherein the height difference is measurable relative to a reference plane parallel to the cladding; processing the cladding of the PCB until the height difference between the first regions and the second regions is within a previously specified height difference tolerance; removing a portion of the cladding thereby electrically isolating the first regions from the respective second regions; drilling through-holes in the first regions through the cladding and fully through thedielectric substrate.

16. The method of claim 15, further comprising: providing an electrical connection through the dielectric substrate to the first regions of the first and second components and to at least one of the second regions of the first and second components.

17. The method of claim 15, further comprising: producing a plurality of the components including a first component and a second component; tensioning a membrane and attaching parallel to the reference plane to the second regions of the first component without contacting the membrane to the first regions of the first component at a centre of the membrane, to produce a membrane sub-assembly; and attaching the membrane sub-assembly by contacting the membrane of the membrane sub-assembly to the second regions of the second component opposite the second regions of the first component, without contacting the first regions of the second component to a centre of the membrane, producing thereby an assembly including at least one of the electrostatic acoustic devices.

18. The method of claim 17, further comprising: separating at least one electrostatic acoustic device from remaining electrostatic acoustic devices of the assembly by separating the assembly at an outer border outside the second regions.

19. The method of claim 17, wherein in the assembly, respective through-holes of the first and second components are not fully opposing.

20. The method of claim 15, wherein the previously determined height difference between between the first and second regions, is between 17 and 150 microns.

21. The method of claim 15, wherein the second regions are circular of outer diameter between three and twenty millimetres or elliptical with an outer major axis of length between three and twenty millimetres.

22. The method of claim 15, wherein a thickness of the dielectric substrate is between 0.2 and 1.6 millimetres.

23. The method of claim 15, wherein the through-holes are of diameter between 0.3 and 1.5 millimetre wherein total area of the through-holes is between thirty and fifty per cent of an area of the first regions.

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