Abstract:
A surface mount technology (“SMT”) apparatus for use in routing radio frequencies (“RF”) between cavities that require a high level of isolation on a single printed circuit board (“PCB”). The SMT part is attached to the PCB over a stripline-ready trace which transitions to microstrip before and after the SMT stripline part to maintain consistent characteristic impedance. When presented with a high isolation need between two cavities using microstrip transmission lines, the proposed stripline SMT apparatus under the isolation wall will tend to provide the necessary isolation. The present invention provides a repeatable and reliable interconnect while improving the electrical match between the two cavities. Furthermore, the invention removes the costs associated with manually forming and soldering cables between PCBs.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to radio frequency (“RF”) and electromagnetic interference (“EMI”) protection for electronic circuitry, and in particular, to a surface mount technology (“SMT”) stripline structure used in combination with an isolation wall for routing radio frequencies between two cavities that require a high level of isolation. 
     Printed circuit boards (“PCB”) comprise a dielectric substrate which supports the printed wiring, including both circuit and ground traces. The detailed method of construction of printed circuit board and the materials used are well known in the art. Many PCBs require a means of signal isolation to reduce or prevent RFI and EMI between various groups or blocks of circuitry. The circuit blocks comprise both active and passive components. In accordance with conventional SMT practice, these electrical components are mounted on the top surface of the printed circuit board during the automated (“component population”) phase of the manufacturing process. As used here, the terms EMI and RFI denote RF signals unintentionally coupled, radiated, or otherwise transmitted between circuit blocks that are intended to be mutually isolated. Ideally, circuit blocks that are likely to create, or to be susceptible to, EMI in or from other nearby circuitry would be contained within a single shielded enclosure or cavity. 
     When a shielded transmission line is used to connect or route RF signals between physically separated shielded cavities, the shielded transmission line minimizes EMI by providing a continuous extension of the shielding surfaces of the separated cavities. The internal structure of the transmission line is selected so that its characteristic impedance within the frequency bands of interest is both well defined and controlled in order to minimize RF signal distortion and maximize the transfer of desired RF signal power. Conventional examples of shielded transmission line include stripline and coaxial cable. 
     The term stripline commonly denotes a structure comprising a signal conducting strip and two ground planes which extend considerably in transverse directions. The space between the ground planes is filled with a dielectric medium and the central strip is embedded in this dielectric. The ground planes are at zero RF potential relative to each other. Coaxial cable utilized as an isolated interconnecting transmission line is not compatible with the automated SMT assembly process flow and generally must be hand formed and soldered at a much greater expense. 
     An example of an unshielded transmission line is microstrip, comprising a single dielectric substrate with ground plane on one side and a signal conducting strip on the other face. Unlike stripline, SMT components can be attached directly to the signal conducting top layer of microstrip. Microstrip is also subject to EMI from nearby conductors because of its unshielded structure. 
     When coaxial cable is not used, the conventional structure of SMT assemblies requiring a shielded transmission line interconnection is a multilayer PCB incorporating at least one stripline structure as described above (ground layer, dielectric, signal conducting strip, dielectric, ground layer) and optional layers for routing other signals. The performance constraints imposed by this solution include: 
     1) The multilayer PCB structure exhibits increased thermal resistance from the SMT component side to the reverse side which is commonly attached to a heatsink. 
     2) In practice, the two ground planes are conventionally connected together along two paths parallel to the entire length of the central conducting strip, in order to minimize the difference of potential between the ground planes, and thereby minimize coupling of RF signals between the stripline structure and adjacent circuitry. The ground plane interconnection is normally accomplished with conductive through-holes, thereby adding significant cost to the final PCB assembly. 
     3) PCB material handling and processing costs are much lower when the unpopulated PCB consists of only a back conductive layer, one dielectric layer, and a top SMT compatible layer. Converting the entire PCB to a multilayer PCB structure when only a small number of shielded interconnections are required adds unnecessarily to the final assembly weight, size and cost. 
     4) Specialized circuit functions including RF power amplifiers are conventionally fabricated on PCB materials, including hard or brittle ceramic substrates, that are not compatible with multilayer PCB fabrication techniques, thereby precluding the inclusion of stripline as an inherent part of the unpopulated PCB. 
     Those having skill in the art would understand the desirability of having a radio frequency interconnection that has high isolation, without and does not need extensive hand assembly to produce. This type of radio frequency interconnection would necessarily provide sufficient isolation, and allow surface mount technology to be utilized, thus allowing the cost efficient manufacturing of high frequency circuit assemblies to be achieved. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, there is provided a SMT stripline structure for routing RF between cavities that require a high level of isolation. The structure comprises a PCB with the printed ground and circuit trace and an SMT stripline part which is attached to the base PCB during the normal component population. The SMT stripline part comprises an upper and lower ground plane pattern, with a layer of dielectric in between. The SMT stripline part is installed on the PCB stripline-ready trace using conventional SMT component attachment means. The PCB stripline-ready trace transitions to microstrip before and after the SMT stripline structure to maintain consistent characteristic impedance. 
     The proposed invention allows for a more repeatable and reliable interconnect while improving the electrical match between the two cavities. Additionally, the invention removes the costs associated with manually forming and soldering cables between PCBs. 
    
    
     
       DETAILED DESCRIPTION 
       These and other features and advantages of the present invention will be better understood from the following detailed description read in light of the accompanying drawings, wherein: 
         FIG. 1  is a view a printed wiring assembly having one or more highly isolated RF interconnections; 
         FIG. 2  is a magnified view of the printed wiring assembly including a SMT component and a printed wiring conductor pattern to form a highly isolated RF interconnection; 
         FIG. 3  is a view of the SMT component; 
         FIG. 4  is a view of a conductive area disposed upon the printed wiring assembly to which the SMT component is coupled; and 
         FIG. 5  is a side cross-section view of the attachment of the SMT component to the printed wiring assembly and to the shield assembly. 
     
    
    
     Like reference numerals are used to designate like parts in the accompanying drawings. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present invention relates generally to RF and electromagnetic interference protection for electronic circuitry, and in particular, to a SMT stripline structure for routing of RF between two areas that need a high level of isolation. 
       FIG. 1  is a view a printed wiring assembly  114  having one or more highly isolated REF interconnections. In the embodiment shown the printed wiring assembly (“PWB assembly”)  101  includes one or more specially formed conductive areas  113  disposed underneath and, used in cooperation with a surface mount component (“SMT component”)  108 . A cover  103  is also coupled to the SMT component  108 . The printed wiring assembly  101  includes one or more conventionally constructed microstrip transmission lines  102 . 
     The microstrip transmission line  102  is typically disposed on a first side of a dielectric slab  112 . The microstrip transmission line  102  is suspended above a ground plane  100  that is disposed on a second side of the dielectric sheet. 
     In alternative embodiments the ground plane may be one layer of a multi-layer printed wiring assembly made up of multiple dielectric sheets having conductors disposed on one or more sides. Each of the multiple dielectric sheets are bonded together, typically with a pre-preg bonding material, as is known to those skilled in the art. Alternatively, dielectric sheets may be coupled, or bonded together with solder. Microstrip transmission lines are typically constructed to have a strip width that will yield a desired characteristic impedance when separated from a ground plane by a dielectric sheet of a given thickness. 
     A conventionally constructed shield assembly  103  is disposed on the printed wiring assembly  101  to shield a first area of circuitry  104  from electromagnetic radiation that is typically impinging on the printed wiring assembly, or is generated by circuitry disposed on a second area  105  of the printed wiring assembly  101 . The shield is typically a piece of sheet metal formed to enclose the first area of circuitry  104 . The shield typically includes a lid  106  to cover the assembly. The lid is typically electrically and mechanically coupled to a wall assembly  107 . The shield is typically electrically coupled to the printed wiring assembly  101  such that it is at a ground potential. In a microstrip circuit assembly the shield is typically coupled electrically and mechanically to the ground plane by methods known to those skilled in the art, such as soldering to a grounded conductor area  109 , disposed on the top of the printed wiring assembly. Grounded conductor area  109  is typically coupled to the ground plane via edge plating  110 , plated through via holes  111  and the like. It is often desirable to bring a controlled impedance transmission line such as a microstrip transmission line, through a shield assembly with minimal disruption in its impedance. 
     A surface mount component (“SMT component”)  108  allows a controlled impedance transmission line, such as a microstrip transmission line, to pass through a shield assembly. The SMT component tends to allow simplified and cost effective production of printed wiring assemblies having a shield under the SMT component  108  the microstrip transmission line  102  is transformed by changing its width. The strip transmission line includes a line width reduction and shielding that occurs in the conductive area. 
     The transformation to a strip transmission line is possible because the ground plane  100  is coupled to the surface mount component  108  by means of grounded attachment footprint  113  to form a second ground plane as a part of the SMT component. The shield advantageously couples mechanically and electrically to the second ground plane formed on the SMT component. Those skilled in the art will realize that a strip transmission line is a conductor disposed between two ground planes, while a microstrip line is a conductor disposed above a single ground plane as previously discussed. Thus, by utilizing the SMT component  108  the microstrip conductor  102  passes under the shield  103 , while interference to the shielded circuit  104  tends to be minimized. When the microstrip conductor passes under the SMT component its width is changed to that of a stripline conductor. 
       FIG. 2  is a magnified view of the printed wiring assembly including a SMT component and a printed wiring conductor pattern to form a highly isolated RF interconnection. A base printed circuit board (“PCB”), including a first ground layer  100 , a first dielectric layer  112 , printed circuit signal trace  102 , printed circuit ground layer  109 , a SMT component  108 , one or more isolation walls, including a first isolation cavity wall  220 , and a second isolation cavity wall, and a plurality of conductive through-holes  260  between layers  109  and  100 . 
     A printed wiring assembly (“PWB Assembly”)  114  includes a SMT component  108  disposed upon a base PCB  115 . Also included in the PWB assembly are a first shield assembly, having a wall shown in partial cross section  220 , and a second shield assembly, having a wall shown in partial cross section  221  that are mechanically and electrically coupled to the base PWB  115  and SMT component  108 . 
     PCB  115  includes a ground plane  100  disposed upon a first side of a substrate, or dielectric slab  112  and a ground area  109  disposed upon a second side of the substrate. Also disposed upon the second side of the substrate is a conductor, or PCB signal trace,  102 . The second side of the substrate also has a conductive pattern  113  disposed on it. Dielectric slabs typically include materials such as glass teflon, glass epoxy, ceramic and the like. Alternative embodiments include dielectrics that are suitable for the construction of flexible PCBs. Further alternative embodiments include multi layer printed wiring assemblies including one or more layer structure as described above in addition to additional layers of conductors and dielectrics, fabricated as known to those skilled in the art. 
     The grounds  100 ,  109  disposed upon the PWB assembly  114  typically include copper, or copper having a solder coating, or equivalent materials that has been disposed upon the dielectric and etched away to form a conductive pattern by methods known to those skilled in the art. The grounds may be formed from copper or any other conductive material that may be conveniently disposed upon the chosen dielectric material  112 . As shown ground areas  109  are typically disposed adjacent to conductors  102  to provide shielding and a means of coupling external components that may be present to a desired ground. In alternative embodiments the ground areas  109  need not be disposed adjacent to the microstrip line. 
     A plurality of feed through holes  260  are used to tie the ground plane  100  to the conductive pattern. As will be appreciated by those skilled in the art, radio frequency (“RF”) circuits typically utilize a plurality of feed through holes to minimize a path length from the ground areas  109  to the ground plane  100 . Feed through holes  260  may be constructed as plated through holes, “z” wires, grommets and the like. In alternative embodiments the ground plane  100  may be tied to the conductive areas by edge plating. Equivalently a combination of edge plating and feed through holes may be used. A typical use of ground areas  109  on the second side of a printed wiring assembly  114  is to couple a shield assembly  220  to the ground plane  100 . 
     A shield assembly such as first shield assembly  220  or second shield assembly  221  are typically coupled electrically and mechanically to a PCB by a solder connection to a ground layer  109  disposed upon the surface of the PCB. The solder connection may be made by soldering the walls of the shield assembly  220  directly to the surface ground layer  109 . Equivalently the shield may have pins that extend into feed through holes, with the shield being soldered into place. In further equivalent embodiments the shield may be coupled to the ground layer by mechanical means such as screws, clips and the like. 
     The material of the shield may include ferromagnetic and non-ferromagnetic materials such as copper, solder coated copper, iron and the like. As will be appreciated by those skilled in the art the shield may be solid, perforated or constructed of wire mesh, with the allowable mesh opening depending upon the frequency of the interference that has been deemed problematic. 
     In addition to being coupled to the ground area  109  the shield is also coupled electrically and mechanically to a ground conductor disposed upon a first surface of the SMT component  108 . 
     A SMT Component  108  has a first surface that is made of a conductive material that is coupled to the shield. Coupling is typically achieved with solder, conductive epoxy or the like. In an alternative embodiment a gap is present between the shield and the first surface of the SMT component. In a further alternative embodiment the shield is coupled to the first surface of the SMT component by a dielectric material such as epoxy, or the like that has a relative dielectric constant typically greater than one. The first surface of the SMT component forms a strip line ground plane. Those skilled in the art will realize that the ground plane formed with the first surface of the SMT component operates in cooperation with the microstrip ground plane  100  to form the pair of ground planes utilized in conjunction with a center conductor  102  to form a strip line circuit. A conductive area  113  that is connected to ground plane  100  by means of conductive through holes  260  serves to connect the first surface ground plane of the SMT to the ground plane  100  and to attach the SMT component to the PCB surface. 
     In a first embodiment of the invention a fully shielded interconnection between shielded cavities  220 ,  221  is provided. The SMT component  108  when applied to a printed wiring assembly having a conductive pattern  113  appropriate to form part of a stripline structure may be utilized to form a completely shielded interconnection between a first  220  and a second  221  shielded cavity. The completely shielded interconnection is formed from the printed wiring board by virtue of the conductive pattern, functioning in conjunction with the SMT part disposed on the printed wiring board. 
     The SMT component  108  is attached to the base PCB  115  so that conductive layers  42  and  113  are substantially aligned and in electrical contact. One or more grounded isolating walls  220 ,  221  surround and are in electrical contact with the top layer  46  of the SMT component  108 , thereby minimizing EMI between the SMT structure and surrounding circuit blocks or structures shielded, and further minimizing EMI between separate circuit cavities, of which walls  220 ,  221  are contiguous parts. 
     In a second embodiment of the invention, the construction takes into account that depending on the highest frequency component of the EMI vs. the aperture size, it may not be needed for the shield to actually contact the upper ground layer of the SMT component in order to realize adequate shielding. Thus a gap remains between the upper ground layer and the upper ground layer of the SMT. The gap may be left unfilled, or alternatively filled with a dielectric material. The dielectric material typically has a relative dielectric constant of greater than 1, where 1 is approximately the relative dielectric constant of air. The SMT component  108  is again attached to the base PCB in the manner described in the first embodiment. As would be understood by those skilled in the art, electromagnetic radiation from a gap or slot decreases rapidly when the wavelength of such radiation substantially exceeds the greater of the length or width of a slot or aperture in a shielding structure. Therefore, depending on both the particular requirements for EMI performance, and the highest frequency utilized within or in proximity to the PCB, it may not be necessary for the shields  220  to directly contact or connect to the top layer  46  of the SMT part in order to provide sufficient EMI protection. It will also be appreciated by those skilled in the art that in many types of PCB circuits, erratic operation or noise may be created when an electrical connection is maintained by a mechanical or pressure contact between metal surfaces, without benefit of solder, conductive adhesive, or the like. The second embodiment therefore can have a fabrication cost advantage over the first embodiment when either a pressure or solder contact can be omitted. 
     When overcrowding of components or signal traces on a printed wiring assembly occurs it often leads to a localized EMI condition (“crosstalk”) on a single-layer (i.e. ceramic) substrate, the SMT part in conjunction with the conductive pattern by itself, without any additional shielding, can provide a benefit. In other words, the first, second and third embodiments each provide a different level of EMI attenuation, attendant with a varying degree of mechanical complexity and/or assembly cost. For example, the fully shielded (#3) version might not be compatible with an SMT-only assembly flow. 
     In a third embodiment of the invention the SMT component  108  is again attached to the base PCB in the manner described for the first embodiment. As will be appreciated by those skilled in the art, it is possible that, for a particular PCB, the incorporation of one or more shielded enclosures or cavities provides more EMI suppression than is actually required. In situations wherein undesired electromagnetic coupling exists primarily between adjacent PCB traces, or primarily between a PCB trace and an adjacent component, the SMT component itself, without the inclusion of additional shield walls, comprises a shield over and around the EMI susceptible trace, thereby providing a reduction in EMI. The third embodiment therefore can have a fabrication cost advantage over the first and second embodiments when shielded enclosures or cavities can be omitted. 
       FIG. 3  is a view of the SMT component. The SMT component  108  comprises a lower conductive layer  42 , a second dielectric layer  44 , an upper conductive layer  46 , a plurality of conductive through-holes  48  between layers  42  and  46 , and in an alternative embodiment, edge plating  47  between layers  42  and  46 . 
     In a first embodiment of the SMT component, layers  42  and  46  are coupled with conductive through-holes  48 , comprising plated through holes, silver epoxy filled holes, or the equivalent. In a second embodiment of the SMT component, layers  42  and  46  are coupled with conductive, side edge plating (or “wrap metalization”)  47 , typically including deposited copper or the like. In a third embodiment of the SMT component, layers  42  and  46  are coupled with both conductive through-holes  48 , comprising plated through holes, silver epoxy filled holes, or the equivalent, and conductive, side edge plating (or “wrap metalization”)  47 , typically including deposited copper or the like. In this embodiment the wrap metalization provides full shielding past the via, or plated through, holes. In a fourth embodiment of the SMT component, layers  42  and  46  are coupled through one or more external shield walls  220 , by the coupling common to the walls, the conductive pattern  113 , ground area  109 , and layers  42  and  46 . 
       FIG. 4  is a view of the conductive area, or pattern,  113  disposed upon the base PCB ( 115  of  FIG. 2 ) to which the SMT component is coupled. The circuit trace ( 102  of  FIG. 2 ) that passes underneath the SMT part is comprised of a microstrip transmission line  102  which transitions into stripline  116  transmission line, under the SMT stripline part  108 , and then back to a microstrip transmission line. The microstrip transmission line utilizes a single ground plane ( 100  of FIG.  2 ), while a stripline utilizes two ground planes ( 100  and  46  of FIG.  2 ). The conductive pattern of the signal trace is changed in width to maintain a constant characteristic impedance. Although 50-ohm microstrip and stripline is discussed, it is noted that characteristic line impedances greater or lesser than 50 ohms may be used if desired. Those knowledgeable in the art will recognize that for a fixed thickness and dielectric constant of dielectric layers  44  and  112 , microstrip trace  102  will be wider than stripline trace  116  when both the microstrip and stripline are correctly proportioned to exhibit the same characteristic impedance 
     In a first embodiment of the conductive area  113 , the locations of the multiple conductive plated through holes  260  may be made relatively congruent with the through holes  48  disposed within the SMT component, for the purpose of minimizing the inductance between the pair of ground planes comprising the stripline region. top and bottom. However this configuration may be difficult to produce and inspect, and the holes  260  may wick the solder away from the SMT component mounting interface  113 . 
     In a second embodiment of the conductive area  113 , the locations of the multiple conductive plated through holes  260  fall outside of the outline of the SMT part, but are close enough to it to still provide sufficient grounding and a good solder joint. Solder wicking as described above can be prevented by selectively applying an optional pattern or layer of solder resisting film (not shown) over holes  260 , using methods known to those skilled in the art. 
       FIG. 5  is a side cross-section view of the attachment of the SMT component to the printed wiring assembly and to the shield assembly. As shown shields  220 , 221  are coupled to The SMT component, which is in turn coupled to the base PCB  115  that includes the conductive pattern. 
     This invention has been described in detail in connection with the preferred embodiments. These embodiments are examples only and the invention is not restricted thereto. It will be easily understood by those skilled in the art that variations and modifications can be made to the invention within the scope of the appended claims.