Abstract:
A method for interconnecting a first flex printed circuit board (PCB) with a second flex PCB. The method includes: providing the first flex PCB with holes at contact locations to be electrically coupled to the second flex PCB; providing the second flex PCB with electrical pads corresponding to the holes at the contact locations; applying a non-conductive material between the first PCB and the and second PCB with clearances for each of the electrical pads; aligning the first PCB with the second PCB so that the holes in the first PCB are in line with the corresponding electrical pads on the second PCB; bonding a portion of flat areas on the first and second PCBs together; dispensing a conductive adhesive into the holes to fill the space created by the holes, corresponding clearances of the non-conductive material, and the corresponding electrical pads; and curing the conductive adhesive.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention disclosure is related to Government contact number FA8750-06-C-0048 awarded by the U.S. Air Force. The U.S. Government has certain rights to this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to flexible printed circuit board (PCB) assemblies and more specifically to an adhesive reinforced open hole interconnect of flexible PCBs. 
     BACKGROUND 
     Next generation large area multifunction mixed signal printed circuit board (PCB) assembly for space and airship applications, such as, radar and communication systems need to be lighter weight and more conformal than what can be achieved with current multilayer rigid and flexible (flex) PCB assembly technology. Methods have been created and demonstrated that create antenna structures out of individual layers of flex PCBs that require electrical interconnect between one another. However, many interconnects need to align and bond to form the working circuits. Furthermore, traditional methods of electrically bonding individual layers of flex PCBs have problems with alignment of the electrical pads that require connection. 
     In more recent attempts, these connections were created with a blind bond attached between pads on the two flex PCBs with dispensed conductive and non-conductive pastes. Nevertheless, in many instances, pads do not align or conductive adhesive short between adjacent pads. Moreover, issues with the quality of the bonds are not known until after the parts are cured and tested. Accordingly, extensive and expensive rework is required if the alignment is off. 
     Therefore, there is a need for a more accurate and higher quality interconnect of flexible PCBs, specially in Radio Frequency (RF) range. 
     SUMMARY 
     In some embodiments, the present invention is a method for interconnecting a first flex PCB with a second flex PCB. The method includes: providing the first flex PCB with holes at contact locations to be electrically coupled to the second flex PCB; providing the second flex PCB with electrical pads corresponding to the holes at the contact locations; applying a non-conductive material between the first PCB and the and second PCB with clearances for each of the electrical pads; aligning the first PCB with the second PCB so that the holes in the first PCB are in line with the corresponding electrical pads on the second PCB; bonding a portion of flat areas on the first and second PCBs together; dispensing a conductive adhesive into the holes to fill the space created by the holes, corresponding clearances of the non-conductive material, and the corresponding electrical pads; and curing the conductive adhesive. 
     The non-conductive material may be a non-conductive adhesive film and the clearances of the non-conductive material may be cut holes in the adhesive film at the contact locations. The non-conductive material may be a non-conductive paste and the clearances of the non-conductive material may exclude the non-conductive paste at the contact locations. 
     In some embodiments, the present invention is a method for interconnecting a first flex PCB with a second flex PCB. The method includes: providing the first flex PCB with holes at contact locations to be electrically coupled to the second flex PCB; providing the second flex PCB with electrical pads corresponding to the holes at the contact locations; aligning the first PCB with the second PCB so that the holes in the first PCB are in line with the corresponding electrical pads on the second PCB; mechanically attaching the first PCB to the second PCB by fusion bonding a portion of areas of bare dielectric material on each of the first and second PCB; dispensing a conductive adhesive into the holes to fill the area created by the holes and the corresponding electrical pads; and curing the conductive adhesive. 
     In some embodiments, the present invention is a flex PCB assembly which includes: a first flex PCB including holes at electrical contact locations; a second flex PCB including electrical pads corresponding to the holes at the electrical contact locations; a non-conductive material between the first PCB and the and second PCB with clearances for each of the electrical pads; and a conductive adhesive dispensed in the holes to fill the space created by the holes, corresponding clearances of the non-conductive material, and the corresponding electrical pads to provide electrical contacts between the hole of the first PCB and the pads of the second PCB, wherein the first PCB is aligned with the second PCB so that the holes in the first PCB are in line with the corresponding electrical pads on the second PCB. 
     The first flex PCB may also include conductive material at the holes with an annular ring shape. Also, the size of the pads may be larger than the size of the holes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  show a stack up of two open hole PCBs, according to some embodiments of the present invention. 
         FIG. 2  shows an exemplary process flow for interconnection of two or more PCBs with non-conductive film, according to some embodiments of the present invention. 
         FIG. 3  shows an exemplary process flow for interconnection of two or more PCBs with non-conductive paste, according to some embodiments of the present invention. 
         FIG. 4  shows an exemplary process flow for interconnection of two or more PCBs with fusion bond, according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In some embodiments, the present invention is a method for interconnect of flexible PCBs using adhesive reinforced open holes and/or a PCB assembly using the same method. The method of the present invention facilitates bonding of two or more separate flex PCBs that only need to attach in discrete areas to make electrical connection. If these were conventional flat circuits, standard drill and plate techniques could be used. However, since these PCBs (circuits) are only bonded in discrete areas, new processes are incorporated to allow for alignment inspection and post bonding electrical connection. 
       FIGS. 1A to 1C  show a stack up of two open hole PCBs, according to some embodiments of the present invention. Although, two PCBs are used for simplicity in the examples through out this disclosure, the present invention is not limited to two PCBs and is also applicable to more than two flexible PCBs to be interconnected. As shown in  FIG. 1A , a top flex PCB  12  includes at least one open hole (through hole)  14  with an annular ring  11  on both sides of the PCB anywhere an electrical contact is required. A bottom 3-D flex PCB  16  includes at least one electrical pad  17  that corresponds to the at least one through hole  14 , at each location. The electrical pad(s)  17  has the same or slightly larger diameter as the through holes  14  including the annular ring  11 . The through hole  14  in the top PCB  12  is used for visual alignment of the top PCB with the bottom PCB  16 , and for electrical connection of the two PCBs  12  and  16 . 
     The two PCBs  12  and  16  are bonded together using, for example, a non-conductive adhesive  13 , which includes a cutout pad  15 , to create mechanical support with clearances around the electrical connection pads  17 . In some embodiments, the non-conductive adhesive may be cured to strengthen the boding. After the non-conductive adhesive  13  is applied (or prior to a fusion bond), the top PCB  12  is aligned with the bottom PCB  16  with appropriate tooling. Once in the alignment tooling, alignment is assured by inspecting to see if the holes  14  in the top PCB  12  align with the pads  17  on the bottom PCB  16 . Once verified, the non-conductive bond may be cured, for example, by heat, pressure and time. The bond provides a seal around the pads preventing conductive adhesive from squeezing in between layers, which could cause electrical shorts. 
       FIG. 1B  shows the two PCBs  12  and  16  assembled together. Once the non-conductive adhesive  13  is cured (or a fusion bond is completed), a conductive bond, for example, a conductive paste, is dispensed into the hole  14  of the top PCB  12  to the electrical pad  17  below on the bottom PCB  16 , and onto the annular ring  11  on the top PCB  12  to make the electrical connection, as shown in  FIG. 1C . In some embodiments, the conductive paste is dispensed to fill the cavity created by the hole  14 , non-conductive adhesive  13  and pad  17  ( FIG. 1B ) by syringe or dispense jetting techniques that can get in between the 3-dimensional cavities between the two PCBs. 
     In some embodiments, the top PCB  12  and the bottom PCB  16  are mechanically bonded together by any of the following methods. A non-conductive film may be placed onto the bottom PCB  16  with clearances around the electrical bonding pads  17  ( FIG. 2 ); a non-conductive paste adhesive may be dispensed onto the bottom PCB  16  with clearances around the electrical bonding pads  17  ( FIG. 3 ); or base dielectric layers (e.g., flex circuit plastic) are fusion bonded together around the pads  17  ( FIG. 4 ). At this point, conductive adhesive is dispensed into the holes of the top circuit to fill the cavity created by the hole, non-conductive adhesive and pad on bottom ( FIG. 1   c ) by syringe or dispense jetting techniques that can get in between the 3-dimensional features of the circuit. Enough conductive adhesive is dispensed to fill the cavity and squeeze up onto the annular ring. The conductive adhesive is then cured to complete the electrical connections. 
     In some embodiments, standard techniques (circuit etching, drilling and plating) are used to fabricate the circuits (PCBs) separately, however, the open hole of the top PCB may be larger than a standard via, which allows for visual inspection of the alignment with the bottom PCB and subsequent dispensing of conductive adhesive to make the electrical connections. Here, the non-conductive bond used for the mechanical attachment of the two PCBs also provides a seal around the pads for electrical connection that prevents the conductive material from squeezing between circuits, which could cause electrical short circuits. 
     In some embodiments, the design of the individual PCBs is made such that the top PCB  12  contains large diameter plated through holes  14  with an annular ring of at least 0.002″ on top (e.g., in  FIG. 1A ). The hole  14  is large enough to dispense conductive adhesive through. The catch electrical pad  17  on the bottom PCB  16  is made to the same or larger diameter of the outer diameter of the annular ring  11 . In RF applications, the 2 diameters will most likely be substantially the same, depending on what is required for impedance matching of the system. For non-RF pads, the bottom pad could be larger to improve registration. 
       FIG. 2  shows an exemplary process flow for interconnection of two or more PCBs with non-conductive film, according to some embodiments of the present invention. As shown in block  202 , a top PCB is provided (e.g., designed and fabricated) with holes and (annular) rings at the attachment locations. Similarly, in block  204 , a bottom PCB is provided (e.g., designed and fabricated) with pads matching the location of the pads on the top PCB at the attachment locations. Clearance for the pads in the non-conductive adhesive film is provided by pre-cutting matching holes in the non-conductive adhesive film where the electrical connections (of the two PCBs) will occur, as shown in block  206 . In block  208 , the top PCB is aligned with the bottom PCB so that the holes in the top PCB are in line (aligned) with the corresponding pads on the bottom PCB. The non-conductive adhesive film is aligned on the bottom PCB and tacked in place, as shown in block  210 . A portion of the flat areas of the two PCBs are then bond together with, for example, heat and pressure using the non-conductive film for mechanical interconnect, as shown in block  212 . 
     In block  214 , a conductive adhesive is dispensed into the holes to fill the area created by the holes, the pads and the non-conductive film, and on top of the pads. The conductive adhesive is then cured by, for example, heat in block  216 . 
       FIG. 3  shows an exemplary process flow for interconnection of two or more PCBs with non-conductive paste, according to some embodiments of the present invention. As depicted in block  302 , a top PCB is provided (e.g., designed and fabricated) with holes and (annular) rings at the attachment locations. Similarly, in block  304 , a bottom PCB is provided (e.g., designed and fabricated) with electrical pads matching the location of the pads on the top PCB at the attachment locations. Here, the non-conductive paste is dispensed on the bottom PCB with clearance for the pads on the bottom PCB, as shown in block  306 . In block  308 , the top PCB is aligned over the bottom PCB so that the hole in the top PCB are in line with the pads on the bottom PCB. A portion of the flat areas of the two PCBs are then bond together with, for example, heat and pressure to cure the non-conductive paste for mechanical interconnect, as shown in block  310 . In block  312 , a conductive adhesive is dispensed into the holes to fill the area created by the holes, the pads and the non-conductive paste, on top of the pads. The conductive adhesive is then cured by, for example, heat in block  314 . 
       FIG. 4  shows an exemplary process flow for interconnection of two or more PCBs with fusion bond, according to some embodiments of the present invention. As shown in block  402 , a top PCB is provided (e.g., designed and fabricated) with holes and (annular) rings at the attachment locations. Likewise, in block  404 , a bottom PCB is provided (e.g., designed and fabricated) with electrical pads matching the location of the pads on the top PCB at the attachment locations. In block  406 , the top PCB is aligned over the bottom PCB so that the hole in top PCB are in line with the pads on the bottom PCB. The two PCBs are then mechanically attached together by fusion bonding a portion of the areas of bare dielectric material on the two PCBs in block  408 . In block  410 , a conductive adhesive is dispensed into the holes to fill the area created by the holes, the pads and the non-conductive paste, on top of the pads. The conductive adhesive is then cured by, for example, heat in block  412 . 
     It will be recognized by those skilled in the art that various modifications may be made to the illustrated and other embodiments of the invention described above, without departing from the broad inventive scope thereof. It will be understood therefore that the invention is not limited to the particular embodiments or arrangements disclosed, but is rather intended to cover any changes, adaptations or modifications which are within the scope and spirit of the invention as defined by the appended claims.