Patent Application
20090027143
1. Field of the Invention
The present invention relates generally to radio frequency (RF) devices, and particularly to stripline transmission device packages.
2. Technical Background
Stripline devices are used in RF and microwave circuit applications. A stripline transmission line is implemented by sandwiching a planar conductor between a pair of ground planes. A dielectric material is interposed between the planar conductor and each of the ground planes. Of course, transmission lines are used to interconnect circuit elements and implement impedance transformation networks. Stripline structures may also be used to realize directional couplers, baluns, power dividers and other such devices commonly used in RF and microwave circuits.
A coupler is a four-port device that includes a primary transmission line disposed in close proximity with a secondary transmission line. Power is directed into the device by way of an input port connected to the primary transmission line. The power propagates along the primary transmission line and the electromagnetic waves are coupled onto the secondary line. The coupler structure, therefore, would be implemented in a stripline configuration by disposing the primary conductor and the secondary conductor on opposite sides of a very thin dielectric material. The coupled structure is subsequently disposed between the two ground planes in the manner previously described. Of course, baluns, power combiners, power splitters and the like may be implemented using coupler structures.
The stripline devices discussed above may be implemented in surface mounted packages. Surface mounted devices are advantageous because they are dimensionally small (i.e., less than 1.0 inch) and typically have a low profile (less than 0.15 inches). Of course, the dimensions will vary as a function of power and frequency. Accordingly, these devices are ideally suited for relatively small printed circuit boards used in wireless infrastructure applications, such as power chips, etc.
In one approach, the manufacturing process produces a rectangular panel having a two-dimensional array of devices disposed thereon. The panel is implemented by sandwiching several layers of softboard dielectric materials. The stripline structure itself is formed using a very thin softboard layer (less than 10 mils) that includes copper foil disposed on either side. The array of stripline structures are typically formed by using standard photolithography techniques, i.e., the array of stripline devices are imaged onto the copper surfaces and subsequently etched, removing any excess copper material. This process is quite accurate and allows the placement of coupled transmission lines on either side of the thin panel within very high tolerances. A relatively thicker softboard dielectric panel (e.g., less than 60 mils) is disposed on either side of the stripline structure. The thicker panels have a dielectric surface positioned next to the stripline structure and an exterior copper foil surface that functions as the ground plane. The softboard sandwich is laminated by applying heat and pressure. The laminated panel is easily cut and divided into its constituent individual components.
One of the benefits of the approach briefly described above is its extreme accuracy. The process is well understood and produces very smooth and well defined lines. Device performance parameters such as amplitude balance, phase balance, insertion loss, etc. are quite predictable. The process is very efficient, very large panels may be produced using very high levels of automation. Thus, the method is very conducive to low cost, high volume manufacturing. However, one drawback associated with this technique relates to the poor thermal conductivity of softboard materials. As those of ordinary skill in the art will appreciate, thermal conductivity is proportional to the amount of power that the individual component is able to dissipate.
What is needed, therefore, is a method for making stripline devices that is characterized by all of the above advantages associated with softboard devices, while at the same time, being characterized by high thermal characteristics and high power handling capabilities.
The present invention addresses the needs described above by providing a method for making stripline devices that is characterized by all of the advantages associated with softboard devices, while at the same time, being characterized by high thermal characteristics and high power handling capabilities.
One aspect of the present invention is directed to a method for making a hybrid material stripline device. The method includes providing an inner layer of material, the inner layer including a dielectric material and at least one conductive sheet. At least one stripline device is formed in the inner layer by processing the at least one conductive sheet. The at least one stripline device is characterized by a surface area footprint. A first outer layer and a second outer layer are provided. At least one of the first outer layer and/or the second outer layer includes at least one ceramic portion. The at least one ceramic portion has a ceramic surface area greater than or substantially equal to the surface area footprint of the at least one stripline device. At least one of the first outer layer and/or the second outer layer further includes a softboard dielectric material. The inner layer of material is sandwiched between the first outer layer and the second outer layer. The first outer layer, the inner layer and the second outer layer are laminated to form a laminate panel structure, a surface of the first outer layer forming a first major surface of the laminate panel structure and a surface of the second outer layer forming a second major surface of the laminate panel structure. A first conductive sheet is disposed over the first major surface and a second conductive sheet is disposed over the second major surface, the first conductive sheet and the second conductive sheet being configured as parallel ground planes for the at least one stripline device.
In another aspect, the present invention is directed to a stripline structure that includes a first outer layer and a second outer layer disposed in substantially parallel planes one to the other. At least one of the first outer layer and/or the second outer layer includes at least one ceramic portion having a ceramic surface area. At least one of the first outer layer and/or the second outer layer comprises a softboard dielectric material. An inner layer is sandwiched between the first outer layer and the second outer layer, the inner layer having at least one stripline device formed therein. The at least one stripline device is characterized by a surface area footprint. The ceramic surface area is greater than or substantially equal to the surface area footprint. The first outer layer, the inner layer, and the second outer layer are laminated to form a panel, the panel having a first outer major surface and a second outer major surface. A first conductive sheet is disposed over the first outer major surface and a second conductive sheet is disposed over the second outer major surface. The first conductive sheet and the second conductive sheet are configured as parallel ground planes for the at least one stripline device. A plurality of conductive vias are formed in the first outer layer and/or the second outer layer. The plurality of conductive vias are in electrical communication with the at least one stripline device.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.
Reference will now be made in detail to the present exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. An exemplary embodiment of the stripline component of the present invention is shown in
As embodied herein and depicted in
The inner circuit layer 14 is formed from a relatively thin softboard dielectric material 146 that includes a top conductive layer and a bottom conductive layer disposed on either side of dielectric 146. Each stripline device 140 is formed using standard photolithographic techniques. As shown, the stripline transmission line structures (142, 144) that form each device 140 are imaged onto the top and bottom layers disposed on dielectric layer 146 and excess material is etched away. The transmission lines (142, 144) formed on the top and bottom surfaces may be disposed aligned thereon with great accuracy. Layer 14 includes an N×M array of stripline devices 140 formed thereon. In the example shown in
In the example embodiment provided in
Of course, conductive vias (200, 120, 146, 1600) are formed in the various layers of the device. As those of ordinary skill in the art will appreciate, the conductive vias are in electrical communication with the various ports of each stripline device 140. The ports are subsequently connected to pads disposed on the exterior of the component package.
The metal/dielectric laminates used to fabricate upper layer 12 (20), inner layer 14, and lower layer 162 (22) are often called “soft-board.” The laminate materials may be selected to optimize physical properties such as mechanical stability, electrical performance, planarity and rigidity. Softboard laminate materials may be employed to produce extremely accurate circuit traces having very small line widths. For example, the softboard dielectric may be formed using materials such as FR-4, Rogers RO 3003, Pyralux, etc. FR-4, of course, is produced by applying high pressure to a woven glass woven fabric impregnated with an epoxy resin binder. FR-4 is commonly used dielectric material because it is characterized by excellent mechanical properties, electric properties, and is dimensionally stable (i.e., very low shrinkage). Pyralux is a composite material and RO 3003 is a ceramic-filled PTFE composite material. Both materials used for circuit boards in microwave applications.
The ceramic pucks may be formed using any suitable ceramic material such as LTCC, alumina, AIN, and BeO ceramics. Low temperature co-fired ceramics (LTCC) are formed from “green tape.” A green tape is produced using selected glass compositions and/or ceramic powders. The powder is mixed with a binder and a solvent. The mixture is subsequently cast and cut to form the green tapes. In accordance with the present invention, interconnecting vias may be formed in the green tape before firing. As those of ordinary skill in the art will understand, the composition of the glass and ceramic powders, of course, determines the coefficient of thermal expansion, the dielectric constant and the compatibility of the ceramic material to the other various materials employed herein.
As embodied herein and depicted in
The physical dimensions of the coupling elements are typically computed using suitable computational tools. Of course, coupling requirements dictate line widths, line spacing and ground plane spacing. In a hybrid coupler, three (3) dB coupling between the conductors with a transmission line impedance of 50 ohms is often desirable. The width of the conductors required to obtain this impedance for a given dielectric (e.g., FR-4) may be readily computed. The length of the conductors is selected to be a quarter wavelength at the frequency of interest.
As shown in the “one-device” detail view of
Referring to
The outer dielectric layers 12 and 16 are typically less than 60 mils thick. Each bonding layer 18 is several mils thick and may be implemented using any suitable epoxy, such as Rogers Bond ply, DuPont PFA, DuPont FEP, etc. As noted above, the metallic layers (20, 22) disposed on the outer dielectric boards (12, 16) serves as ground planes.
As embodied herein and depicted in
As embodied herein and depicted in
As shown in
In one example embodiment, the length (“x”) is approximately 0.250 inches or less. The width (“y”) is approximately 0.200 inches or less. As noted above, component 100 is a low profile device, and accordingly, the height (dimension z) is approximately 0.05 inches or less. Referring to
Coupler 100 may be employed in a wide variety of RF and microwave applications such as balanced amplifiers, variable phase shifter, attenuators, LNAs, etc. The surface mount component 100 is ideally suited for use on relatively small printed circuit boards, such as those employed in wireless systems.
The ceramic pucks (
As embodied herein and depicted in
As before, the inner circuit layer 14 is formed from a relatively thin softboard dielectric material 146 that includes a top conductive layer and a bottom conductive layer disposed on either side of dielectric 146. Each stripline device 140 is formed using standard photolithographic techniques wherein stripline transmission line structures (142, 144) are imaged on the top and bottom layers of dielectric layer 146. The excess material is etched away to form an array of devices 140. This technique is used to dispose and align the transmission lines (142, 144) on the top and bottom surfaces with great accuracy. Layer 14 is shown to include an N×M array of stripline devices 140. While the example provided in
Top dielectric layer 12 is formed from one of the softboard dielectric materials disclosed herein. A conductive sheet 20 is disposed over the top surface of layer 12 and functions as a ground plane. In this embodiment, the bottom dielectric layer 16 is formed from a single sheet of ceramic material. A second conductive sheet 20 is disposed on the bottom surface of the ceramic dielectric material 16 and functions as a second ground plane.
Once again, conductive vias (200, 120, 148, 160) are formed in the various layers of the device to provide input/output ports for each of the stripline devices 140. The ports are subsequently connected to pads disposed on the exterior of the component package in the manner shown in
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The term “connected” is to be construed as partly or wholly contained within, attached to, or joined together, even if there is something intervening.
The recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein.
All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate embodiments of the invention and does not impose a limitation on the scope of the invention unless otherwise claimed.
No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. There is no intention to limit the invention to the specific form or forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention, as defined in the appended claims. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
