Abstract:
In an exemplary method, three dimensional printing forms a micro lattice truss structure with a first end formed in contact with a conductive area on a PCB so that the truss structure is adhered to the conductive area due to the three dimensional printing. The truss structure extends outward from the PCB and has a distal end. The truss structure is formed with resiliency so that the truss structure maintains structural integrity during end-to-end compression. The resiliency of the micro lattice truss structure enables the truss structure to return to substantially its uncompressed length when the compression is removed. The truss structure is conductive so that a resilient electrical connection can be formed between the conductive area of the PCB and another spaced apart surface parallel with the PCB when the distal end of the truss structure is in contact with and compressed by the other surface.

Description:
BACKGROUND 
       [0001]    This invention generally relates to electrical connections and more specifically relates to establishing a resilient conductive connection between two parallel spaced-apart surfaces, e.g. printed circuit boards. 
         [0002]    Modern electronics often contain circuitry formed on a plurality of stacked printed circuit boards (PCB). In many situations, there exists a need to couple an electrical signal from circuitry on one PCB to another PCB. One straightforward approach is to connect the ends of a wire or coaxial cable between the respective areas on the two PCBs. In another approach, a plurality of rigid perpendicular conductive pins extend from one PCB and are in alignment with corresponding receptacles or holes in the other PCB to establish electrical connections that may or may not be soldered after the connections are engaged. 
         [0003]    Conventional metal springs disposed between PCBs have also been utilized to establish connections with respectively aligned contacts on adjacent parallel PCBs. However, such interconnections are normally time-consuming to install and may be tedious to assemble. Additionally, disassembly of such connected PCBs for maintenance or repair of the circuitry may result in even greater difficulties where such interconnections and careful alignment are required to be manually reestablished during reassembly of the respective PCBs. Thus, there exists a need for an improved electrical interconnection that facilitates ease of initial assembly and also provides for easy reassembly following a disassembly of the respective PCBs for maintenance or repair. 
       SUMMARY 
       [0004]    It is an object of the present invention to satisfy this need. 
         [0005]    An exemplary apparatus includes a base board with a conductive area on its external surface and can make a resilient electrical connection with another surface. A resilient, conductive, three dimensional, micro lattice truss has a first end permanently attached to the conductive area. The truss extends substantially perpendicular from the base board and has a second distal end. An unengaged state of the apparatus occurs when the second distal end of the truss is not in engagement with the other surface and the truss is not compressed in a direction towards the base board. An engaged state of the apparatus occurs when the second distal end of the truss engages the other surface and the truss is compressed a direction towards the base board. In the engaged state, the truss establishes an electrical connection between the conductive area on the base board and the other surface. 
         [0006]    An exemplary method for making an electrical interconnector disposed on a first PCB includes using three dimensional printing to form a micro lattice truss structure from a bath of material. An external planar surface of the first PCB has a conductive area disposed within the bath of material. A first end of the truss structure is formed in contact with the conductive area so that the truss structure is adhered to the conductive area due to the three dimensional printing. The micro lattice truss structure extends outward away from the first PCB and has a second distal end opposing the first end. The micro lattice truss structure is formed with resiliency so that the truss structure maintains structural integrity during moderate compression of its second end towards the first end. The resiliency of the micro lattice truss structure enables the truss structure to return to substantially its uncompressed length when the compression is removed. The micro lattice truss structure is conductive so that a resilient electrical connection can be formed between the conductive area of the first PCB and another spaced apart surface parallel with the first PCB when the second distal end of the truss structure is in contact with and compressed by the other surface. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]    Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which: 
           [0008]      FIG. 1  illustrates an exemplary micro lattice connector assembly installed to establish electrical connection between circuitry on two spaced apart surfaces. 
           [0009]      FIG. 2  illustrates an exemplary micro lattice connector in an assembled position. 
           [0010]      FIG. 3  illustrates an exemplary micro lattice connector in an unassembled position. 
           [0011]      FIG. 4  illustrates another embodiment of a micro lattice connector assembly. 
           [0012]      FIG. 5  illustrates an initial step in the fabrication of an exemplary micro lattice connector in accordance with the present invention. 
           [0013]      FIG. 6  is a representation of atomic layer deposition associated with an exemplary micro lattice connector. 
           [0014]      FIG. 7  illustrates an exemplary micro lattice connector that has been milled to a desired physical dimension. 
           [0015]      FIG. 8  illustrates a final structure of one embodiment of an exemplary micro lattice connector. 
           [0016]      FIG. 9  shows a side elevational view of another embodiment of an exemplary micro lattice connector. 
           [0017]      FIG. 10  shows a top view of the embodiment as shown in  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    One aspect of the present invention resides in the recognition of the difficulties associated with the installation, assembly and reassembly of other interconnect mechanisms for coupling signals between adjacent surfaces/PCBs disposed parallel to each other in different planes. It was recognized that by manufacturing a resilient conductive micro lattice connector as part of one of the surfaces to be connected, the tediousness in the assembly with other surfaces/PCBs would be substantially eliminated and the time required for assembly would be significantly reduced. Additionally, difficulties associated with the manual reassembly of the respective PCBs following separation of the PCBs for maintenance or repair would be much reduced. It was further recognized that using 3D printing to initially form a non-conductive micro lattice connector on the surface and later plating it with a conductive material would provide a flexible and economical approach to making a conductive micro lattice connector assembly. 
         [0019]      FIG. 1  shows an exemplary micro-lattice connection assembly  100  which includes a PCB  105  having a plurality of conductive plated through holes  110  and a conductive micro-lattice truss  115 . As used herein, a micro-lattice truss is a three dimensional structure of interconnected strands. Layers of such trusses may be used to form various lengths. However, a conventional helical wound spring is not a micro-lattice truss. Another PCB  125 , being in a plane substantially parallel to that of PCB  105 , has a plurality of conductive plated through holes  120  that are aligned with plated through holes  110 . A connection between the plated through holes on PCB  105  and  125  can be made in a variety of conventional ways, e.g. soldering. The micro-lattice truss  115  has one end  130  that is permanently attached to PCB  115  and another distal end  135 . In this illustrative example, another parallel PCB  145  has a plurality of plated through holes  140  aligned to be engaged by the distal end  135  of the truss  115  in the assembled state. The PCB  145  may contain a variety of circuitry connected to the plated through holes  140 , e.g. in this example a strip line  150  is connected to the respective plated through holes  140 . In the assembled state as shown in  FIG. 1 , the micro-lattice connection assembly  100  is used to provide a connection between circuitry on PCB  125  associated with plated through holes  120  and circuitry on PCB  145  associated with plated through holes  140 . In this example the resilient conductive micro-lattice trusses  115  have sides parallel to the longitudinal axis of the trusses that are bowed outwardly indicating that the trusses are engaged and in compression between PCB  105  and  145  so that the respective ends  135  of each truss firmly engage the respective plated through holes  14  in order to establish a good electrical connection with very little ohmic resistance. Although PCBs are referred to in the described embodiment, the subject invention is suitable for providing interconnections between a variety of parallel surfaces. 
         [0020]      FIG. 2  shows an enlarged view of the exemplary micro-lattice connection assembly showing only a single truss  115  in which the distal end  135  is in engagement with plated through hole  140  of PCB  145 . A longitudinal axis  200  is shown with respect to truss  115 . The width of truss  115  transverse to axis  200  and equidistant between PCB  105  and  145  is significantly greater than the width of the truss at either the attached end  130  or the distal end  135 . Because the truss is resilient, this indicates that the truss is under compression along axis  200 . 
         [0021]      FIG. 3  shows an enlarged view of the exemplary micro-lattice connection assembly showing only a single truss  115  in an unassembled state. That is, the distance between PCB  105  and PCB  145  is greater than the fully extended length of truss  115  in an uncompressed state. In this illustrative example, the side view of the truss  115  presents a substantially rectangular profile. The distal end  135  is generally planar and disposed in a plane substantially parallel to that of PCB  145 . This maximizes the surface of contact between the truss  115  and the plated area  140  of PCB  145  in an assembled state. Similarly, the end  130  of truss  115  is substantially parallel to PCB  105  so as to enhance the surface area of contact between the end  130  in the plated through hole  110 . Although the exemplary embodiment shows plated through holes in PCBs as areas of engagement for the truss  115 , it will be apparent that the exemplary truss can provide a conductive connection between any types of surfaces. 
         [0022]      FIG. 4  shows an alternative embodiment of a micro-lattice connection assembly comprising a PCB  400  with plated through holes  405  and opposing micro-lattice trusses  410  and  415  extending from the opposing surfaces of plated through holes  405  on PCB  400 . This embodiment provides a suitable mechanism for interconnecting parallel areas on opposing surfaces in which the illustrated embodiment is sandwiched between the opposing surfaces and the surfaces moved close enough to each other so as to compress and engage the respective trusses  410  and  415 . 
         [0023]      FIGS. 5-8  provide assistance in understanding one exemplary method of manufacturing a micro lattice interconnect assembly. This exemplary method utilizes three-dimensional (3D) printing of a micro lattice truss structure  510  on a surface  505  of a support  500 , e.g. PCB. More specifically, a two-photon polymerization (TPP) process can be utilized to deposit the truss structure on surface  505 . This process utilizes rapid, ultrashort duration laser beams  520 ,  525  that focus on a point  515  causing the material at the point of focus to polymerize/solidify. For example, the TPP process may utilize femtosecond lasers that can reach powers up to  200  GW. Photoresist such as SU- 8 , PMMA, or PMGI could be utilized as the base material, i.e. the bed of material that is selectively solidified to form the lattice truss elements. The support  500  is placed in the bed of material so that an end portion of the truss structure is solidified/formed in contact with the surface  505 . 
         [0024]    The support  500  with surface  505  is coated or submerged in a bath of polymer where that material comes in direct contact with the support and surface  505 . When that material is exposed to sufficient energy (such as two coherent laser beams) the material solidifies and is adhered in place on the surface  505  of the board/support  500 . The conductive plating on this polymer, which is also adhered to surface  505 , makes the electrical contact to the metallic surface  505  on the board. 
         [0025]      FIG. 6  represents that once the entire micro lattice truss structure has been formed by 3D printing, the structure may then be subjected to an atomic layer deposition process. In this process the surface of the truss structure is subjected to an alternating gaseous species to deposit additional material. For example, the gases can be used to produce a monolayer formed from components of the gas on the surface of the structure. There could be two gases involved in the process with gas A depositing one layer, gas B depositing another layer, and then gas A would be used again, etc. A variety of gases could be used such as metal oxides (Aluminum Oxide, Titanium Oxide, etc), polymers (polyethylene, PMMA, etc), or pure metals (Au, Ag, Ta, Ti, etc). Depositing the material before plating ensures a consistent, clean surface with the desired dimensions. 
         [0026]      FIG. 7  illustrates that the structure  700  is then side exposed and plasma etched such as with oxygen. Side exposure is the process of using a plasma to cut away at the sides of the lattice structure to a desired dimension and to expose the interior 3D printed strands adjacent the cut. This allows the plasma to etch away at the inside of the structure, i.e. the 3D printed material, without compromising the outside, plated material. 
         [0027]    The cut could be along the distal end of the lattice structure where the ends of the lattice strands will be in contact with the surface for which a connection is to be made. Plasma is one, but not the only, process that can be used to remove the 3D printed material. Preferably, a significant portion of the original 3D printed strands of the lattice are removed leaving only the conductive outer layer which will then resemble interconnected hollow tubes. Oxygen is a gas suitable for such plasma etching because it is low in cost and easy to obtain. The O2 plasma cleans well and is relatively simple to use. However, other elements could be used for etching, such as Argon or Hydrogen. 
         [0028]      FIG. 8  represents a finished, base structure of a micro lattice truss  800 . This structure is then made conductive by plating it with a conductive metal such as gold, copper, silver, nickel, Rhodium, etc. Also conductive non-metals such as metallic glass or carbon nanotubes could be utilized to coat the base structure with a conductive skin. 
         [0029]      FIGS. 9-10  illustrate another embodiment of a micro lattice truss structure  900  having an outer concentric tube  905  and an inner concentric tube  910 . Each of the inner and outer tubes are formed with longitudinal truss members extending along the longitudinal axis  915 . The axial displacement of the micro lattice structure as described herein can accept large displacements (changes in physical dimension) along its axis with little to no distortion to its physical dimensions in the transverse direction. This is helpful in creating and maintaining a desired impedance for RF signal transmission, e.g. inner and outer concentric micro-lattice structures can be dimensioned for a desired impedance, based on materials and spacings, which is maintained both in and out of compression. 
         [0030]    It will be understood that the micro lattice truss structure could have a variety of dimensions and properties while still providing the advantages discussed above. By way of example, and without limitation, each truss element/fiber could have a wall thickness of 0.0001 inch; each micro lattice truss structure could have 40% compliance relative to height; the weight of one entire micro lattice truss structure could be in the order of 10-20 grams; each micro lattice truss structure could have a conductivity in the order of 40-70×10 6  Siemens/m; each micro lattice truss structure could have a longitudinal length of 5-20 mm. 
         [0031]    Based on the teachings herein, it will be observed that there is a balance between contact pressure and compliancy. In an exemplary application, a compression of between 0.002″ and 0.006″ of compression (of a 0.010″ tall contact) may be desired. It may be possible to achieve &gt;75% compression without lasting damage to the micro-lattice structure. Compression, force, electrical resistance and resiliency need to be balanced. The truss should have enough repeating layers so that it is long enough to make contact with both surfaces. For micro-lattice structure of 0.010″ in length, there could be 5 layers or more. 
         [0032]    Even a lesser compression, e.g. 50%-60%, without damage and with substantially retained resiliency, could be utilized. Wall thickness is an important consideration with regard to the amount of deformation of the structure at different loads. For a structure with a 10 nm thick micro truss wall, it could experience a stress of up to 1.2 MPa with very high recoverability. On the other hand, a 50 nm thick micro truss wall would have more overall strength but would not be as recoverable. Hence, the wall thickness and length of the micro-lattice structure should be considered depending on the specific application. In one example with 0.002″ minimum compression target, almost no permanent set is achieved, i.e. almost complete resiliency. It may be a design goal to stay in the compliant region where there will be no permanent set. Although exemplary implementations of the invention have been depicted and described in detail herein, it will be apparent to those skilled in the art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention. For example, a micro-lattice structure could be originally 3D printed using a conductive material that would result in solid metallic trusses/strands as opposed to hollow tubes. However, these solid ‘conductive’ materials may result in less desirable mechanical and electrical properties in comparison to substantially hollow conductive tubes. 
         [0033]    The scope of the invention is defined in the following claims.