Patent Publication Number: US-7581994-B2

Title: Method and assembly for establishing an electrical interface between parts

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to electrical assemblies configured according to assembly methods, and especially to electrical assemblies and methods effecting inter-part electrical coupling in an assembled state. 
     Some electrical assemblies are designed to effect electrical contact between or among respective parts by pressure contact between or among parts during an assembly process that results in an assembled configuration of parts. By way of example and not by way of limitation, RF (radio frequency) microwave integrated circuit (MIC) modules are bolted into housings using mounting screws. A MIC module typically includes a two-conductor microstrip transmission line for conveying microwave signals. Establishing a required RF ground under the microstrip transmission line relies upon the pressure contact established between the MIC module and the housing as the MIC module is bolted into the housing. Because of surface irregularities and flatness issues regarding both of the parts—the housing and the MIC module—intermittent contact may be established at the interface between the housing and the MIC module. An intermittent contact interface may cause output power perturbations during operation of the assembly. 
     The primary conductor in the microstrip transmission line is typically a gold trace printed on dielectric material supporting the microstrip transmission line. Representative materials employed for such dielectric material includes, by way of example and not by way of limitation: alumina, Rogers 4003, TMM-10, duroid and other dielectric materials known in the art of MIC modules. By way of further example and not by way of limitation, the dielectric material may be attached to a gold or silver plated conductive header such as kover, silvar, stainless steel or another header material known in the art of MIC modules. The header material may be employed to act as the second conductor in the two-conductor microstrip transmission line. 
     Different microstrip modules may be electrically connected together via a gold ribbon that is welded or soldered between the respective microstrip transmission lines that are printed or otherwise affixed to the dielectric material. The header may typically be employed as a ground conductor and may be bolted to the housing. Bolting the parts together is preferred some assemblies for both mechanical and electrical reasons. However, bolting the header to the housing is known to experience the problem of intermittent contact between the housing and the header. Intermittent contact is caused by irregularities in the surface finish of the bottom of the header and the housing surface that contributes to less than ideal flatness of the interface between the header and the housing surface. Small high points, often of microscopic scale, make contact at the two surfaces presented at the interface between the housing and the header. These high points may move as the assembly is thermally cycled or vibrated. This movement may cause the inter-part contact points to move or shift. This moving or shifting action causes intermittent contact between parts at the header-to-housing interface that is manifested in perturbations shown in output power plots over a range of temperature. 
     This intermittent contact, sometimes referred to as “ground jumps”, is often found in temperature cycling and may require extensive troubleshooting and rework to eliminate. Usually, these power jumps or perturbations are discovered during RF performance testing that typically involves temperature cycling. Troubleshooting and fixing the power jumps is very time consuming and costly as they require disassembly and reassembly using additional gold conducting ribbons between the header-to-housing interface. 
     One solution employs gold ribbon that is place between the header-to-housing interface underneath the RF transmission line to create a “gasket” affect so that the carrier (i.e., the microstrip transmission line) establishes a pressure contact in the region of the gold ribbon. Size and placement of ribbon may be variable geometry of the header-to-housing interface. 
     Incorrectly placed or incorrectly sized ribbons may lead to physical damage to solder or epoxy bond lines between the substrate and carrier or may lead to physical cracking of the substrate. Sufficiently reducing this intermittent contact problem to assure reliable performance by the header-to-housing interface usually involves a plurality of iterations of RF performance testing or temperature cycling. 
     There is a need for an assembly and a method for effecting the assembly that establishes a substantially continuous electrical interface between parts of the assembly in response to an urging together of the parts of the assembly. 
     SUMMARY OF THE INVENTION 
     A method for establishing a substantially continuous electrical interface between a first expanse of a first electrical part and a second expanse of a second electrical part includes the steps of: (a) in no particular order: (1) Adhering a first layer of substantially pure gold material to at least a portion of the first expanse; and (2) adhering a second layer of substantially pure gold material to at least a portion of the second expanse. (b) Urging the first expanse and the second expanse together. 
     An assembly configured according to the present invention includes: (a) A first part having a first expanse plated with substantially pure gold material. (b) A second part having a second expanse plated with substantially pure gold material. (c) An urging structure pressing the first expanse and the second expanse together to establish an electrical interface between the first part and the second part. 
     It is, therefore, a feature of the present invention to provide an assembly and a method for effecting the assembly that establishes a substantially continuous electrical interface between parts of the assembly in response to an urging together of the parts of the assembly. 
     Further features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic elevation view of a first embodiment of an assembly configured according to the prior art. 
         FIG. 2  is a schematic elevation view of a second embodiment of an assembly configured according to the prior art. 
         FIG. 3  is a schematic elevation view of a first step in effecting a diffusion bonding of two gold interfaces. 
         FIG. 4  is a schematic elevation view of a second step in effecting a diffusion bonding of two gold interfaces. 
         FIG. 5  is a schematic elevation view of a third step in effecting a diffusion bonding of two gold interfaces. 
         FIG. 6  is a schematic elevation view of a fourth step in effecting a diffusion bonding of two gold interfaces. 
         FIG. 7  is a schematic section view of an assembly configured according to the present invention. 
         FIG. 8  is a flow chart illustrating the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention advantageously employs the phenomenon of gold diffusion to create an electrical and mechanical bond at the interface between parts of an assembly, such as by way of example and not by way of limitation a header-to-housing interface of an RF (radio frequency) microwave integrated circuit (MIC) module. An advantageous result is that intermittent electrical contact between parts at the header-to-housing interface is substantially reduced or eliminated, even in environments presenting temperature cycling or vibration. 
       FIG. 1  is a schematic elevation view of a first embodiment of an assembly configured according to the prior art. In  FIG. 1 , an assembly  10  includes a first part  12  and a second part  14 . First part  12  has a substantially planar upper face  20 . Second part  14  has a generally concave lower face  22  in facing relation with respect to face  20  when assembly  10  is in an assembled orientation, as illustrated in  FIG. 1 . In the assembled orientation illustrated in  FIG. 1 , faces  20 ,  22  cooperate to establish a gap Δ 1  at widest separation between faces  20 ,  22 . A ribbon structure  24  is installed between faces  20 ,  22  to at least partially fill gap Δ 1 . Preferably, ribbon structure  24  is gold material, and when faces  20 ,  22  are urged together an improved electrical interface is provided to an interface between faces  20 ,  22  by ribbon structure  24  compared with an interface between faces  20 ,  22  without ribbon structure  24  situated therebetween. 
       FIG. 2  is a schematic elevation view of a second embodiment of an assembly configured according to the prior art. In  FIG. 2 , an assembly  30  includes a first part  32  and a second part  34 . First part  32  has a substantially planar upper face  40 . Second part  34  has a generally convex lower face  42  in facing relation with respect to face  40  when assembly  30  is in an assembled orientation, as illustrated in  FIG. 2 . In the assembled orientation illustrated in  FIG. 2 , faces  40 ,  42  cooperate to establish a gap Δ 2  at widest separations between faces  40 ,  42  substantially at ends  36 ,  38  of assembly  30 . Ribbon structures  44 ,  46  are installed between faces  40 ,  42  to at least partially fill gap Δ 2  at ends  36 ,  38 . Preferably, ribbon structures  44 ,  46  are gold material, and when faces  40 ,  42  are urged together an improved electrical interface is provided to an interface between faces  40 ,  42  by ribbon structures  44 ,  46  compared with an interface between faces  40 ,  42  without ribbon structures  44 ,  46  situated therebetween. 
     Prior art practice illustrated in  FIGS. 1-2  provides for plating faces  20 ,  22  or  40 ,  42  using Type 2 (99.0% purity) gold plating on all mating portions of faces  20 ,  22  or  40 ,  42 . Prior art practice also provides for ribbon structure to use Type 2 (99.0% purity) gold material. Prior art assemblies  10 ,  30  purposely employed lesser purity gold material (e.g., Type 2 (99.0% purity) gold material) for establishing electrical contact at an interface such as an interface between faces  20 ,  22  or  40 ,  42  in order to avoid occurrence of diffusion bonding by gold diffusion. 
     When mating gold surfaces are placed under pressure at an elevated temperature, the process of diffusion bonding starts to occur. Gold material is particularly susceptible to diffusion bonding, especially in structures involving substantially pure gold material. The process of diffusion bonding may be alternately referred to as “gold diffusion” elsewhere in this disclosure. This process of gold diffusion, also referred to as cold welding, can result in a strong bond between parts  12 ,  14  or  32 ,  34 . Assembly designers have sought to avoid the occurrence of gold diffusion or cold welding in the past because the strong bond established by that process makes parts difficult to position and difficult to move once installed, such as when removal may be required for replacement or repair of an assembly. 
     When mating gold surfaces are placed in facing relationship under pressure at an elevated temperature, the process of gold diffusion starts to occur. The higher the elevated temperature (and pressure) the faster will be the diffusion rate. After a period of time, the diffusion is complete at the interface establishing a robust electrical contact and structural joint. Gold has the property of being able to diffuse and join to itself under a relatively low temperature and pressure. 
       FIG. 3  is a schematic elevation view of a first step in effecting a diffusion bonding of two gold interfaces.  FIG. 4  is a schematic elevation view of a second step in effecting a diffusion bonding of two gold interfaces.  FIG. 5  is a schematic elevation view of a third step in effecting a diffusion bonding of two gold interfaces.  FIG. 6  is a schematic elevation view of a fourth step in effecting a diffusion bonding of two gold interfaces. Regarding  FIGS. 3-6  together, an assembly  50  includes a first part  52  and a second part  54 . First part  52  has an upper face  60 . Second part  54  has a lower face  62  in facing relation with respect to face  60  when assembly  50  is in an assembled orientation, as illustrated in  FIGS. 3-6 . Faces  60 ,  62  present gold material toward each other (not shown in detail in  FIGS. 3-6 ). The gold material may be plated on or otherwise adhered with parts  52 ,  54 . 
     In the first assembly step illustrated in  FIG. 3 , faces  60 ,  62  cooperate to establish a first gap (represented in  FIG. 3  by gap Δ 3 ) between faces  60 ,  62 . Applying a force F (indicated by arrows  64 ,  66  in  FIGS. 3-6 ) and elevating temperature of assembly  50  urges parts  52 ,  54  together and promotes gold diffusion or cold welding. 
     In the second assembly step illustrated in  FIG. 4 , application of force F and elevation of temperature result in faces  60 ,  62  establishing a second gap (represented in  FIG. 4  by gap Δ 4 ) between faces  60 ,  62  that is a lesser gap than first gap Δ 3  ( FIG. 3 ). 
     In the third assembly step illustrated in  FIG. 5 , application of force F and elevation of temperature result in faces  60 ,  62  establishing a third gap (represented in  FIG. 5  by gap Δ 5 ) between faces  60 ,  62  that is a lesser gap than second gap Δ 4  ( FIG. 4 ). The gap between faces  60 ,  62  is substantially zero in some localities, such as locality  68 , as gold diffusion has substantially occurred in those localities. 
     In the fourth assembly step illustrated in  FIG. 6 , application of force F and elevation of temperature result in faces  60 ,  62  establishing a fourth gap (represented in  FIG. 6  by gap Δ 6 ) between faces  60 ,  62  that is a lesser gap than second gap Δ 5  ( FIG. 5 ). Fourth gap Δ 6  is substantially zero wherever faces  60 ,  62  abut as gold diffusion has substantially occurred across the full interface of faces  60 ,  62 , thereby establishing a cold welded junction having substantially continuous electrical properties and strong physical properties resistant to breaking the bond between parts  52 ,  54 . 
     The present invention advantageously employs the phenomenon of gold diffusion bonding or cold welding between two gold surfaces to establish electrical and structural bond of sufficient quality and continuity to substantially eliminate intermittent contact between parts of an assembly. By way of example and not by way of limitation, the present invention advantageously employs the phenomenon of gold diffusion bonding or cold welding between two gold plated surfaces to establish an electrical and structural contact that substantially ensures a continuous electrical interface under a microstrip transmission line. 
     Cold welding of gold surfaces is a phenomenon that has been avoided by assembly designers for many years. Cold welding of gold surfaces has long been regarded as a problem to be avoided because it made parts difficult to move or position with respect to each other and for other assembly-related reasons. However, the present invention takes advantage of the phenomenon of cold welding of gold surfaces to solve the “ground jump” anomaly described above. 
       FIG. 7  is a schematic section view of an assembly configured according to the present invention. In  FIG. 7 , an assembly  70  includes a first part  72  and a second part  74 . First part  72  is preferably plated with a layer  80  of gold material and has an upper face  82 . Second part  74  is preferably plated with a layer  84  of gold material and has a lower face  86  in facing relation with respect to face  82  when assembly  70  is in an assembled orientation, as illustrated in  FIG. 7 . Faces  82 ,  86  are in facing relation with each other. The gold material may be plated on or otherwise adhered with parts  72 ,  74 . 
     A microwave transmission line  88  is carried upon a substrate  89  on an upper surface  83  of part  74 . Screw fasteners  90 ,  92  cooperate with parts  72 ,  74  to urge parts  72 ,  74  with a pressure force P indicated by an arrow  94 . Elevating temperature of assembly  70  while screwing screw fasteners  90 ,  92  to apply pressure force P between parts  72 ,  74  applies a compression force between faces  82 ,  86  sufficient to affect gold diffusion or cold welding as described earlier herein in connection with  FIGS. 3-6  where faces  82 ,  86  abut. An alternate embodiment of assembly  70  may provide for a gold ribbon member  98  situated between faces  82 ,  86 . 
     Assembly  70  advantageously employs the phenomenon of gold diffusion or cold welding for establishing a substantially continuous electrical interface between faces  82 ,  86  with strong physical properties resisting separation of parts  72 ,  74 . An alternate embodiment of assembly  70  (using ribbon member  98 ) employs the previously avoided prior art practice of placing gold ribbons between parts, such as between a carrier-to-housing interface ( FIGS. 1-2 ) to advantageously enhance electrical and physical properties of a bond between the parts. 
     Assembly  70  employs a high purity gold plating (e.g., AMS B 488 Type 3; purity=99.90%) for gold layers  80 ,  84 . Prior art practice preferred using gold plating having a lesser purity (e.g., AMS B 488 Type 2; purity=99.0%) in order to avoid the occurrence of gold diffusion or cold welding. Type 3 gold is a high purity gold plating having substantially no brighteners or additives. When employed, ribbon member  98  is preferably a high purity gold ribbon (e.g., AMS B 488 Type 3; purity=99.90%). A preferred thickness for ribbon member  98  is generally in the range of 0.001 to 0.0005 inches. This preferred combination of materials for configuring assembly  70  encourages occurrence of gold diffusion or cold welding when the parts  82 ,  84  and (if employed) ribbon member  94  are joined in an interface structure during a bolting down process, such as by tightening screw fasteners  90 ,  92 . Performing temperature cycling or providing elevated temperature while effecting a bolting down process will assist or accelerate the gold diffusion or cold welding process. Once gold diffusion or cold welding has occurred, the interface between parts  72 ,  74  will not have significant intermittent contact as temperature changes or vibration occur, thereby establishing a robust microwave ground interface connection. 
     In an alternate embodiment of assembly  70 , lesser purity gold plating (e.g., Type 2) may be plated on one or more of parts  82 ,  86  so long as ribbon member  98  is configured using high purity gold ribbon (e.g., Type 3) is gap welded to the surface of the part or parts  82 ,  86  plated with lesser purity gold. Ribbon member  98  used in assembly  70  may be in a fully annealed condition (preferred), an intermediately annealed condition or a full hard condition. 
     Surface preparation of faces  82 ,  86  (and when used, ribbon member  98 ) prior to the diffusion bonding process is preferred in order to clean off contaminating material that may be present at the interface between parts  72 ,  74  that is to be diffusion bonded. Contaminants may act as diffusion barriers and reduce the effectiveness or completeness of the diffusion bonding or cold welding process. 
     Once assembly  70  is bolted down to apply pressure P to the interface between parts  72 ,  74 , assembly  70  is preferably allowed to thermally age at an elevated temperature to allow gold diffusion or cold welding to take place. 
       FIG. 8  is a flow chart illustrating the method of the present invention. In  FIG. 8 , a method  100  for establishing a substantially continuous electrical interface between a first expanse of a first electrical part and a second expanse of a second electrical part begins at a START locus  102 . Method  100  continues with the step of, in no particular order: (1) adhering a first layer of substantially pure gold material to at least a portion of the first expanse, as indicated by a block  104 ; and (2) adhering a second layer of substantially pure gold material to at least a portion of the second expanse, as indicated by a block  106 . Method  100  continues by urging the first expanse and the second expanse together, as indicated by a block  108 . Method  100  terminates at an END locus  110 . 
     It is to be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for the purpose of illustration only, that the apparatus and method of the invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims: