Abstract:
A method for affixing a resonator to a printed circuit board is provided. The method uses a thin metal film which may be affixed to a surface of a stand off, or directly to a surface of a resonator. The metalized surface may be affixed to a printed circuit board using a molten agent with a surface tension which withstands the downward force exerted by the puck&#39;s weight. The metalized surface may be affixed to a printed circuit board using a solder paste and the solder is allowed to reflow. The surface tension of the molten solder causes the resonator (or resonator puck) to self-center, ensuring proper placement and eliminating the need for repositioning. Since the resonator is not positioned using traditional glues which are subject to shifting during transportation or curing, then the resonator is not subject to shifting, eliminating the need to reposition the resonator after the resonator becomes fixed.

Description:
FIELD OF INVENTION  
         [0001]    The invention relates to a method for the placement of a dielectric resonator on a printed circuit board.  
         BACKGROUND OF THE INVENTION  
         [0002]    Dielectric resonators are commonly used in filters, oscillators and other electronic devices. Although different forms of dielectric resonators are commercially available, the dielectric resonators that are most often used have the form of a short circular straight-wall cylinder which may have or may not have an axially-extending hole in the center of the cylinder and a length-to-radius ratio which is often close to one. In some instances, the resonators are often surrounded by a shielding casing to prevent radiation losses. The casing defines a microwave filter cavity.  
           [0003]    Although not achievable in practice, the dielectric resonators should ideally hang freely in space. It is therefore necessary to mount the dielectric resonator firmly within a cavity with the aid of some form of mounting device. Conventionally, the mounting device may be a “stand off.” The stand off typically consists of a ceramic disc which is suitably fastened to the resonator. In combination, the dielectric resonator and the stand off may be termed the dielectric resonator puck (“DR puck”).  
           [0004]    In practice, the DR puck couples to a transmission line and causes a sharp resonance in the frequency response of the circuit. The position of the DR puck with respect to the transmission line is critical and must be controlled to ensure proper circuit performance. The coupling of the DR puck to the transmission line must be controlled in the x, y, and z directions. Therefore, it is critical that the DR puck be centered properly and not allowed to move position.  
           [0005]    [0005]FIG. 1 depicts a typical mounted DR puck in accordance with the prior art. FIG. 2 illustrates a prior art method which may be used to mount the DR puck. The method illustrated may be practiced using any conventional circuit board assembly apparatus machines or robots as may be found in the prior art. As shown in FIG. 1, the resonator  120  may be mounted to a stand off  122 . The resonator  120  may be affixed to the stand off  122  prior to the resonator  120  and stand off  122  combination (hereinafter called “DR puck  120 ,  122 ”) being mounted onto the printed circuit board  126 .  
           [0006]    In a typical prior art mounting method shown in FIG. 2, the circuit substrate  126  may be screen printed with the intended circuit design (step  102 ). The screen-printing may indicate where on the printed circuit board  124  a specific component is to be affixed. That is, the circuit design may be imprinted on the substrate  126  to indicate the specific placement of the passive, active, or chip and/or other circuit components. A solder paste may be applied to the printed circuit board for holding the circuit components in place. The solder paste may be applied to the circuit board such that the applied solder paste traces (e.g., follows) the imprinted circuit design. The circuit components may then be placed on the printed circuit boarding the solder paste in accordance with the screen printing (step  104 ).  
           [0007]    Once the circuit components are placed on the printed circuit board, the board may be heat treated to “reflow” the solder. Heat treating may be accomplished by passing the circuit board through a reflow oven at sufficient temperature to soften or melt the solder. Upon removing the printed board from the heat, the solder is permitted to harden, and the circuit components are held in place. In this context, “reflow” may mean the controlled process of melting a solderpaste, which consists of various soft, low melting point metals in ball or powder form mixed with a liquid or paste fluxing agent. The solderpaste may typically be melted in a high temperature environment, such as, for example, a heated plate or an oven that is heated by infrared or thermal heat sources containing hot air or hot gas that is recirculated. In some instances, the solderpaste may be heated in a vapor phase chamber that contains both the liquid and vapor phases of a boiling liquid. The resulting melted solder may then be returned to a lower temperature where it re-solidifies.  
           [0008]    It should be noted that the prior art method for assembling circuit board components typically involves mounting the DR puck  120 ,  122  under a different process, and at a different time than is used with the circuit components. The process for mounting the DR puck  120 ,  122  typically involves identifying the location for mounting the DR puck  120 ,  122  and applying epoxy  128  to that location (step  108 ). The DR puck  120 ,  122  may then be placed on the epoxy  128  to be held into place (step  110 ). The epoxy may then be allowed to dry or may be cured to speed up the drying process (step  112 ). The finished circuit board device  100  may be subjected to various quality control test to ensure that the circuit is operating as designed (step  114 ).  
           [0009]    In some instances, the position of the DR puck  120 ,  122  may shift during assembly of circuit board product, which may severely degrade DR  120  performance. For example, such shifting may be due to shifting of the DR puck  120  during assembly, during transportation of the board, and/or while the epoxy  128  is still uncured, or during the curing process, etc. To ensure proper placement of the components, the finished circuit board product is subject to quality control testing (step  114 ). Where the DR puck  102 ,  122  is found to be improperly positioned due to shifting or improper placement, the DR puck may be removed (step  116 ) and reset. That is, the epoxy may be re-dispensed (step  108 ) at the location where the DR puck  120 ,  126  is to be placed, and DR puck  120 ,  122  may be repositioned (step  110 ) and the epoxy re-cured (step  112 ).  
           [0010]    As can be seen then, a more efficient method of placing the DR puck  120 ,  122  is needed. A preferable method may eliminate or substantially reduce the need to reposition the DR puck  120 ,  122  during circuit board product assembly. The preferred method may include less processing steps which would make the method more efficient than the prior art.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention addresses many of the shortcomings found in the prior art, especially in the area of automated resonator placement. In one aspect, the present invention takes advantage of cohesive properties (e.g., surface tension) of solder to ensure proper positioning of a metalized dielectric resonator puck and support. Once the metalized resonator puck and support are positioned on the printed circuit board, the printed circuit board is heated permitting the solder to “reflow,” and center the resonator puck automatically. That is, the surface tension of the solder causes the puck and support assembly to center itself over the solder application area on the printed circuit board. Since the metalized dielectric resonator puck and support assembly centers itself due to the solder surface tension, the puck is properly positioned. Thus, after being initially placed on the printed circuit board and subjected to solder reflow, the need to remove and reposition the dielectric resonator puck is eliminated. Consequently, in another aspect, the invention provides a method which is more efficient than the prior art, since processing steps are eliminated.  
           [0012]    In one exemplary embodiment of the present invention, a dielectric resonator (DR) puck (e.g., stand off and resonator combination) is provided which includes a metallization film on one surface of the puck stand off to aid the puck in self-centering as the solder reflows. The resonator support surface opposite the puck would be coated with a metal film, such as for example, silver, nickel, tin lead, or the like. In particular, the surface of the puck opposite the affixed resonator is coated with a thick film metal which may be fired into place. The metalized puck may then be mounted onto a printed circuit board along with the other circuit components. In one exemplary embodiment the metalized puck may be placed on the printed circuit board using any standard component application process and or conventional manufacturing machine (e.g., robot).  
           [0013]    In another exemplary embodiment, a dielectric resonator is disclosed which is metalized on one surface. That is, one surface of the resonator is coated with a thin metal film of solderable material. The metalized resonator may then be positioned on a circuit board by placing the metalized surface of the resonator in a solderpaste and subjecting the solder paste to a reflow process, during which the dielectric resonator is repositioned (e.g., self-centered).  
           [0014]    In another exemplary embodiment, the present invention discloses a circuit board device including a metalized puck which has been affixed to a printed circuit board using solder. The circuit board device may include various electrical components which are soldered in place, and may be used in any circuit device requiring a resonator.  
           [0015]    In yet another embodiment of the invention, a method for assembling a circuit board device with a metalized puck is disclosed. The method includes placing a solder paste onto a printed circuit board, aligning the metalized puck, and sending the printed circuit board containing the puck through an oven to reflow the solder.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The accompanying drawings, wherein like numerals depict like elements, illustrate exemplary embodiments of the present invention, and together with the description, serve to explain the principles of the invention. In the drawings:  
         [0017]    [0017]FIG. 1 is an exemplary depiction of a prior art dielectric resonator puck affixed to a printed circuit board;  
         [0018]    [0018]FIG. 2 is a flowchart of an exemplary prior art method for placement of circuit components onto a printed circuit board;  
         [0019]    [0019]FIG. 3 is a depiction of a metalized stand off in accordance with the present invention;  
         [0020]    [0020]FIG. 4 is an exemplary dielectric resonator puck in accordance with the present invention;  
         [0021]    [0021]FIG. 5 is a portion of an exemplary printed circuit board in accordance with the present invention;  
         [0022]    [0022]FIG. 6 is a depiction of a mounted dielectric resonator puck in accordance with the present invention; and  
         [0023]    [0023]FIG. 7 is a flow chart of an exemplary method for mounting a dielectric puck in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]    The present invention provides a circuit board including a metalized dielectric resonator (“DR”) puck affixed to the circuit board using solder. The invention takes advantage of the cohesive property (e.g., surface tension) of the solder which permits the metalized DR puck assembly to self-center during solder reflow. By allowing the DR puck to self-center, the placement of the puck is precisely aligned to the initial placement of the solder. That is, once the solder is accurately placed, the metalized DR puck assembly may be positioned on the solder and the puck may center itself in relation to the solder&#39;s placement.  
         [0025]    As used herein the term puck may refer to any combination of a resonator and a support (e.g., stand off) wherein the resonator is attached to one surface of the stand off. In general, the stand off may be any support permitting the resonator to be positioned above the planar surface of a printed circuit board. In one exemplary embodiment, the stand off may be ceramic, Cordierite (Mg, Al, Silicate), Forsterite (Mg, Silicate), Rexolite, or some other material suitable for supporting a dielectric resonator. The stand off may include a short circular straight-wall cylinder which may have or may not have an axially extending hole in the center of the cylinder and a length-to-radius ratio which is often close to one. The stand off may include a first flat surface parallel to a second flat surface, where the first and second flat surface may be on opposite ends of and perpendicular to the cylinder walls. The resonator may be affixed to the first flat surface using any conventional method for affixing. For example, the resonator may be affixed to the stand off using an epoxy. Such epoxies are known to those skilled in the art.  
         [0026]    A typical resonator for use with the present invention may be coupled to other circuitry on the printed circuit board. In one exemplary embodiment, the resonator may be cylindrical and may be suitably shaped or configured to be affixed to the first flat surface of the stand off. As noted, the resonator may be affixed to the first flat surface using any epoxy suitable for such purposes.  
         [0027]    In accordance with one exemplary embodiment, the second flat surface of the puck may be metalized in order to adhere to the solder during the reflow process. That is, the second flat surface may include a thin film coating of metal. In a preferred embodiment, the metal may be solderable. The metal may be affixed in any conventional method as may be suitable for the composition of the stand off. For example, where the stand off is ceramic, the thin metal layer may be fired into place. The metal film may consist of gold, silver, tin lead, nickel or any other solderable metal suitable for forming a thin metal film.  
         [0028]    The metalized puck may be placed on a printed circuit board using any conventional device for placing circuit components on a circuit board. For example, a conventional manufacturing robot may be used to place the puck onto a printed circuit board surface. In one exemplary embodiment, the conventional device for placing circuit components may place the puck onto the circuit board in a similar manner as is typically done with electrical circuit components. The puck may be placed simultaneously with the other circuit components or may be placed in turn with the circuit components.  
         [0029]    It should be noted that the present invention may be described with respect to a dielectric resonator and stand off. However, the invention is not so limited. For example, the present invention may be practiced including a dielectric resonator wherein one surface of the resonator is metalized with a thin metal film as described above with respect to the puck. That is, one surface of the dielectric resonator may be coated with a thin metal coating of solderable material, such as, for example, nickel, silver gold, lead, or tin. The coated surface may be placed in a solderpaste for soldering the dielectric resonator to, for example, a printed circuit board during a typical reflow process.  
         [0030]    In accordance with another exemplary embodiment of the present invention, a printed circuit board is disclosed which includes metalized pucks affixed to the board using solder. Once placed, metalized pucks are heat treated along with other circuit components so that the solder may be permitted to reflow. The solder may then be permitted to cool, holding the circuit components and the puck into place.  
         [0031]    In accordance with another exemplary embodiment of the invention a method for affixing the DR puck is disclosed. The method is more efficient than the prior art methods in that the method has a reduced number of steps required for affixing the DR puck to a printed circuit board. For example, the method according to the present invention excludes the steps of dispensing epoxy, independently placing the DR puck, and curing the epoxy. The method according to the present invention permits the metalized DR puck assembly (or metalized resonator) to be placed on a printed circuit board simultaneously with the placement of other electrical circuit components. In addition, the metalized DR puck assembly may be affixed to the printed circuit board using conventional solder. The solder may be placed on the circuit board in the location for eventual placement of the DR puck (herein called, “DR puck solder location”). The DR puck may then be placed on the DR puck solder location using any conventional machine or robot for placing circuit components on a circuit board. The exact placement of the DR puck on the printed circuit board is not critical in the present invention due to the self-centering of the DR puck on the metalized circuit board during the reflow process. This allows for standard pick and place machines to be used in the assembly, eliminating the requirement for highly accurate and costly placement procedures.  
         [0032]    Once the DR puck is placed, the solder is subjected to heat to reflow the solder. The solder then is changed into a substantially liquid form wherein the surface tension of the solder floats the puck in the solder liquid form, and repositions the puck until it is centered in the DR puck solder location. Accordingly, the method according to the present invention eliminates the need to reposition the DR puck due to shifting of the puck caused by moving or jostling the board prior to the solder being permitted to harden. Thus, the method according to the present invention is more efficient and less expensive than conventional prior art DR puck application methods.  
         [0033]    [0033]FIG. 3 illustrates an exemplary support  320  (e.g., stand off  320 ) in accordance with an exemplary embodiment of the invention. The stand off  320  may cylindrical and low profile in shape. Thus, the stand off  320  may include a straight circular side wall  322  and a first flat surface  324  situated perpendicular to the straight side wall  322 . Stand off  320  may further include a second flat surface  326  parallel to the first flat surface  324  and perpendicular to the side wall  322 . Positioned along the central axis of the stand off  320  from the first flat surface to the second flat surface may be a central bore  328 , although the central bore is not mandatory.  
         [0034]    In one exemplary embodiment, the second flat surface  326  of the stand off  320  may be coated with a thin metal film  330 . In this context the coating of the stand off with the metal film may be called metalizing the stand off. The thin metal film may be composed of any metal suitably capable of being securely affixed to the second flat surface  326  and which may be suitable for bonding with solder. Typical examples of the metal which may be used includes gold, silver, tin lead, and nickel, although others are contemplated to be within the scope of the invention.  
         [0035]    [0035]FIG. 4 illustrates an exemplary metalized DR puck  400  in accordance with the present invention. DR puck  400  includes the stand off  320  of FIG. 3. As can be seen, a dielectric resonator (DR)  402  is affixed on the first flat surface  324  of stand off  320 . The DR  402  may be any DR puck that couples to microwave circuitry to provide a resonance in the response of microwave signals. For example, typical DRs which may be used cylinders of dielectric material for the purpose of creating a resonance at a desired frequency. Other shapes of dielectric material may also be used as resonators. The DR may be affixed to the first flat surface of using an adhesive, such as for example, an epoxy. As previously noted, stand off  320  includes a thin layer of metal on second flat surface  326 . Thus, in combination, the resonator  402  and the stand off  320  comprise the metalized DR puck  400 . Once the stand off  320  is metalized and the resonator  402  is affixed to the stand off  320  the DR puck  400  may then be prepared for use on a circuit board.  
         [0036]    FIGS.  5 - 6  illustrates an exemplary circuit board including a DR puck  400  in accordance with the present invention. Particularly, FIG. 5 illustrates an exemplary circuit board  500  including a circuit board substrate  520 . Circuit board substrate  520  may be screen-printed with a metalized circuit design  502  for identifying the locations at which the circuit components are to be affixed. For example, locations  510  may identify positions for affixing various circuit components, such as for example, lumped circuit elements, or microchips, etc. As will be discussed in detail below, situs  504  designates the DR puck solder location  504  where the DR puck  400  may be to be placed.  
         [0037]    As can be seen the screen printing typically forms a particular circuit design  502  on the circuit board substrate  520  which comprises lines  512  connecting the locations  510  for affixing the circuit components. Typically, the circuit component locations  510  (and DR puck solder location  504 ) are overlaid with a solder paste (not shown). The solder paste may be any solder paste for use in affixing circuit components to a circuit board  520 . The solder paste may be placed on the board  520  such that the solder paste follows (e.g., traces) the screen printed circuit design. Once the solder paste is applied, the circuit elements may be placed at their intended locations, in accordance with the circuit requirements.  
         [0038]    As previously noted with respect to FIG. 1, in the prior art a typical DR puck is attached to a printed circuit board using an epoxy. The DR puck may shift during transportation and/or during curing of the epoxy adhesive. Thus, prior art methods for affixing DR pucks to a printed circuit board are subject to imprecise placement of the DR puck which would later have to be removed and the placement process repeated or the assembly would require additional costly tuning methods. Thus, the prior art placement of the DR puck is inefficient in that it may lead to repetition in the puck placement or more costly tuning of the assembly. A more efficient method is illustrated in FIGS. 6 and 7, which show a DR puck placement method having fewer steps, and ensuring more efficient DR puck placement over the prior art.  
         [0039]    [0039]FIG. 6 illustrates an exemplary printed circuit board  600  including metalized DR puck  400  affixed to its surface, in accordance with exemplary embodiments of the present invention. With brief reference to FIG. 5, DR puck solder location  504  identifies the location at which a DR puck  400  may be placed. The DR puck solder location  504  may be prepared by injecting or placing a solder paste centered at the DR puck solder location  504 . Returning now to FIG. 6, DR puck  400  is shown placed at DR puck solder location  504 . DR puck  400  is held in place by solder  602  centered at the DR puck solder location  504 . As previously noted, the solder is placed as a solder paste (not shown) upon which the DR puck  400  is positioned using any conventional machine or method of placing circuit components on a printed circuit board (e.g., manufacturing robot).  
         [0040]    An exemplary method in accordance with the present invention by which the DR puck  400  is secured to a printed circuit board at puck solder location  504  is illustrated in FIG. 7. As shown, the exemplary method may begin with the circuit board substrate  520  being screen printed (step  702 ) with the layout of the desired circuit. A solder paste may be placed on the circuit board substrate  520  in such a way that it follows (e.g., traces) the screen printed circuit. The circuit components along with the DR puck may then be placed in the solder at predetermined locations (step  704 ). The circuit components and the DR puck may be placed using any conventional machinery as is found in the prior art. As such, the machinery will not be discussed herein, for brevity.  
         [0041]    As previously noted, the DR puck  400  is metalized on the surface of the puck which is positioned against the solder paste. Once the DR puck  400  is so positioned, the printed circuit substrate  520 , including the components positioned in the solder paste, may be heat-treated to cause the solder paste to reflow (step  706 ). In one exemplary method, the printed circuit substrate  520  and components are placed in a reflow oven at a temperature sufficient for causing the solder paste to melt into a solder liquid which will later harden and fix the components in place. The temperature of the oven may be determined by the type and composition of the solder. Reflow temperatures for a particular solder are well known in the art and as such will not be discussed herein. In another exemplary embodiment, the solder may be subjected to infrared or vapor phase reflow, or the like, for ensuring converting the solder into a molten form which hardens and fixes the circuit components into place.  
         [0042]    It should be noted that once the solder is converted to a liquid or molten form, the liquid solder may lift and center the DR puck  400  due to the cohesive nature of the solder molecules at the surface of the solder liquid. As is well known, the cohesive nature of the molten solder molecules at the surface causes the surface molecules to cohere to each other. Unless the cohesive forces are broken, the solder molecules will stick together and support (e.g., float, lift, etc.) any object which does not provide sufficient force for interrupting the molecular cohesiveness. Thus, the intermolecular surface forces (e.g., surface tension) which cause the surface molecules to cohere will also cause objects which are not weighted enough to break the intermolecular forces to float on the solder surface. Consequently, since the weight of the DR puck  400  is not enough to interrupt or break the cohesive forces of the at the surface of the molten solder, the DR puck  400  will float on the solder&#39;s surface and the DR puck  400  will self-center at the solder location  504 . In this way, the puck  400  may be positioned as desired, and will not require replacement once the solder is permitted to harden. Once the solder hardens, the circuit components will be fixed into place. The printed circuit board with the fixed components may then be subjected to quality control testing to ensure that the circuit operates as desired (step  708 ).  
         [0043]    The present invention has been described above with reference to various exemplary embodiments. However, those skilled in the art will recognize that changes and modifications may be made to the exemplary embodiments without departing from the scope of the present invention. For example, the various operational steps, as well as the components for carrying out the operational steps, may be implemented in alternate ways depending upon the particular application or in consideration of any number of cost functions associated with the operation of the system, e.g., various of the steps may be deleted, modified, or combined with other steps. Alternatively, additional steps (e.g. solder paste placement step may be added to illustrate alternate embodiments of the invention. In addition, the various circuit component placement systems disclosed herein may be modified or changed to accommodate additional pucks or circuit components as may be desired. The changes and/or modifications described above are intended to be included within the scope of the present disclosure, as set forth in the following claims.