Abstract:
Systems and methods of manufacturing printed circuit boards using blind and internal micro vias to couple subassemblies. An embodiment of the invention provides a method of manufacturing a printed circuit including attaching a plurality of metal layer carriers to form a first subassembly including at least one copper foil pad on a first surface, applying an encapsulation material onto the first surface of the first subassembly, curing the encapsulation material and the first subassembly; applying a lamination adhesive to a surface of the cured encapsulation material, forming at least one via in the lamination adhesive and the cured encapsulation material to expose the at least one copper foil pad, attaching a plurality of metal layer carriers to form a second subassembly, and attaching the first subassembly and the second subassembly.

Description:
The present application claims priority to and the benefit of Provisional Application No. 61/351,253, filed Jun. 3, 2010, entitled “METHODS OF MANUFACTURING PRINTED CIRCUIT BOARDS USING INTERNAL STACKED MICRO VIAS TO COUPLE SUBASSEMBLIES”, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to printed circuit boards and methods of manufacturing the same, and more particularly, to printed circuit boards having circuit layers laminated with blind and internal micro via(s) and methods of manufacturing the same. 
     BACKGROUND 
     Most electronic systems include printed circuit boards with high density electronic interconnections. A printed circuit board (PCB) may include one or more circuit cores, substrates, or carriers. In one fabrication scheme for the printed circuit board having the one or more circuit carriers, electronic circuitries (e.g., pads, electronic interconnects, etc.) are fabricated onto opposite sides of an individual circuit carrier to form a pair of circuit layers. These circuit layer pairs of the circuit board may then be physically and electronically joined to form the printed circuit board by fabricating an adhesive (or a prepreg or a bond ply), stacking the circuit layer pairs and the adhesives in a press, curing the resulting circuit board structure, drilling through-holes, and then plating the through-holes with a copper material to interconnect the circuit layer pairs. 
     The curing process is used to cure the adhesives to provide for permanent physical bonding of the circuit board structure. However, the adhesives generally shrink significantly during the curing process. The shrinkage combined with the later through-hole drilling and plating processes can cause considerable stress into the overall structure, leading to damage or unreliable interconnection or bonding between the circuit layers. Thus, there is a need for material and associated processes which can compensate for this shrinkage and can provide for a more stress-free and reliable electronic interconnection between the circuit layer pairs. 
     In addition, the plating of the through-holes (or vias) with the copper material requires an additional, expensive, and time consuming process sequence that is difficult to implement with a quick turnaround.  FIG. 1  is a flowchart of a sequential lamination process for manufacturing a printed circuit board having stacked vias including expensive and time consuming sequential lamination and plating steps. Thus, there is a need to provide for a printed circuit board and a method of manufacturing the same that can be quickly and easily fabricated and/or ensure alignment of the interconnections (or through-holes or micro vias) on the printed circuit board by reducing iterations of key processes to thereby reduce manufacturing time and cost. 
     SUMMARY 
     Aspects of embodiments of the invention relate and are directed to systems and methods of manufacturing printed circuit boards using blind and internal micro vias to couple subassemblies. An embodiment of the invention provides a method of manufacturing a printed circuit including attaching a plurality of metal layer carriers to form a first subassembly including at least one copper foil pad on a first surface, applying an encapsulation material onto the first surface of the first subassembly, curing the encapsulation material and the first subassembly; applying a lamination adhesive to a surface of the cured encapsulation material, forming at least one via in the lamination adhesive and the cured encapsulation material to expose the at least one copper foil pad, attaching a plurality of metal layer carriers to form a second subassembly, and attaching the first subassembly and the second subassembly. 
     Another embodiment of the invention provides a method of manufacturing a multi-layer printed circuit board including forming a first subassembly including (a) attaching at least one metal layer carrier to form a first subassembly including at least one copper foil pad on a first surface, (b) applying an encapsulation material onto the first surface of the first subassembly, (c) curing the encapsulation material and the first subassembly, (d) forming at least one first via in the cured encapsulation material to expose the at least one copper foil pad, (e) forming a conductive pattern on a surface of the cured encapsulation material, the conductive pattern including a conductive pad coupled to the at least one first via, (f) applying a lamination adhesive to the surface of the cured encapsulation material, (g) forming at least one hole in the lamination adhesive proximate the at least one first via, (h) filling the at least one hole with a conductive material to form at least one second via, repeating (a) through (e) to form a second subassembly, attaching the first subassembly and the second subassembly such that the at least one second via of the first subassembly is about aligned with the conductive pad of the second subassembly. 
     Yet another embodiment of the invention provides an attachment structure for coupling subassemblies of a multi-layer printed circuit board, the structure including a first assembly including a first metal layer carrier including a first blind via including a first capture pad positioned in a top surface of the first metal layer carrier, a first laminate adhesive layer positioned along the top surface and the first capture pad, and a first via about filled with a conductive material positioned in the first laminate adhesive, the first via in contact with the first capture pad, and a second assembly including a second metal layer carrier including a second blind via including a second capture pad positioned in a top surface of the second metal layer carrier, where the first assembly is attached to the second assembly using the first laminate adhesive layer such that the first via in the first adhesive is about aligned with the second capture pad of the second blind via. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart of a sequential lamination process for manufacturing a printed circuit board having stacked vias including sequential lamination and plating steps. 
         FIGS. 2   a - 2   f  illustrate a process for attaching subassemblies to form a multi-layer printed circuit board using internal micro vias positioned in encapsulation and adhesive layers in accordance with one embodiment of the present invention. 
         FIG. 2   g  is a cross sectional view of the finalized multi-layer printed circuit board of  FIGS. 2   a - 2   f  in accordance with one embodiment of the present invention. 
         FIG. 3  is a cross sectional view of a multi-layer printed circuit board having three subassemblies attached using the process of  FIGS. 2   a - 2   f  in accordance with one embodiment of the present invention. 
         FIGS. 4   a - 4   j  illustrate an alternative process for attaching subassemblies to form a multi-layer printed circuit board using internal micro vias positioned in an adhesive layer in accordance with one embodiment of the present invention. 
         FIG. 5  is a cross sectional expanded view of a subassembly to subassembly attachment including two blind vias coupled by adhesive and conductive paste to form a thin via in accordance with the process of  FIGS. 4   a - 4   j.    
         FIG. 6  is a cross sectional expanded view of another subassembly to subassembly attachment including stacked vias on each subassembly coupled by adhesive and conductive paste to form a via in accordance with one embodiment of the present invention. 
         FIG. 7  is a cross sectional expanded view of another subassembly to subassembly attachment using a conductive paste micro via located between two mechanically drilled vias having enlarged surface areas in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the described exemplary embodiments may be modified in various ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, rather than restrictive. There may be parts shown in the drawings, or parts not shown in the drawings, that are not discussed in the specification as they are not essential to a complete understanding of the invention. Like reference numerals designate like elements. 
       FIG. 1  is a flowchart of a sequential lamination process for manufacturing a printed circuit board having stacked vias including sequential lamination and plating steps. 
       FIGS. 2   a - 2   f  show a process for manufacturing a printed circuit board including attaching laminated subassemblies using internal micro vias positioned in encapsulation and adhesive layers in accordance with one embodiment of the present invention. 
     In  FIG. 2   a , the process begins when a laminated subassembly  100  having four layers and copper pads (e.g., foil)  102  on both sides is provided. The laminated subassembly  100  further includes two plated or filled through hole vias  104 . The layers of the subassembly can be made of metal, ceramic, or insulating material (e.g., FR4, LCP, Thermount, BT, GPY, such as Teflon, thermally conducting carbon (stablecor), halogen free, etc., where GPY is a laminate that does not fit in the FR4 category, such as polyimide, aziridine cured epoxy, bismalimide, and other electrical grades of laminate). The present invention, however, is not thereby limited. In other embodiments, other suitable substrate and conductive layer materials can be used. In the embodiment shown in  FIG. 2   a , the subassembly layers have a thickness ranging from about 3 to 4 mils. However, in other embodiments, the subassembly layers and other components can have other suitable dimensions. 
     In several embodiments, the laminated subassembly  100  can be manufactured using the process described in  FIG. 1 . In other embodiments, the subassembly can be a single lamination subassembly having multiple single metal layer carriers and stacked micro vias. Aspects of single lamination processes for manufacturing circuit boards are further described in U.S. Pat. No. 7,523,545, U.S. Prov. Pat. Appl. No. 61/189,171, and U.S. patent application Ser. No. 12/772,086 the entire content of each of which is incorporated herein by reference. 
     In the embodiment illustrated in  FIG. 2   a , the laminated subassembly  100  includes four metal layers. In other embodiments, the laminated subassembly can include more than or less than three metal layer carriers. In the embodiment illustrated in  FIG. 2   a , the laminated subassembly includes two through hole vias. In other embodiments, the laminated subassembly can have more then or less than two vias. In other embodiments, the through hole vias can be replaced with stacked micro vias, buried vias, and/or blind vias. 
     In  FIG. 2   b , the process applies an encapsulation material  106  to a top surface of the laminated subassembly  100  and cures it. In several embodiments, the encapsulation material is a dielectric material. In several embodiments, the curing is achieved by heating the subassembly and encapsulation material thereon at a pre-selected temperature for a pre-selected duration. 
     The encapsulation material can be any suitable non-cured insulating material, including, without limitation, FR4, LCP, Thermount, BT, GPY, such as Teflon, thermally conducting carbon (stablecor), halogen free, etc., where GPY is a laminate that does not fit in the FR4 category, such as polyimide, aziridine cured epoxy, bismalimide, and other electrical grades of laminate. 
     In  FIG. 2   c , the process applies a laminate adhesive  108  to a top surface of the cured encapsulation material  106 . 
     In  FIG. 2   d , the process forms holes  110  for micro vias by drilling through the laminate adhesive  108  and encapsulation material  106  up to a top surface of the copper pads  102 . Each of the micro vias can be formed by laser drilling (and/or mechanical drilling) holes with a diameter ranging from about 4 to 10 mils. In other embodiments, other suitable techniques for forming via holes can be used. In addition, other via sizes can be used. 
     In  FIG. 2   e , the holes  110  are filled with conductive paste thereby forming micro vias  112 . In some embodiments, the micro vias are filled with copper instead of conductive paste. In one embodiment, conductive paste is used when the via holes are laser drilled and copper is used when the holes are mechanically drilled. 
     In  FIG. 2   f , a second laminated subassembly  200  having copper pads  202  on both sides is provided and brought in proximity to the first laminated subassembly  100 . 
       FIG. 2   g  is a cross sectional view of the finalized multi-layer printed circuit board of  FIGS. 2   a - 2   f  in accordance with one embodiment of the present invention. In  FIG. 2   g , the first and second subassemblies ( 100 ,  200 ) are brought together and attached. In some applications it can be difficult to connect and manufacture boards having high aspect ratio vias. By attaching the laminated subassemblies using the process described above, the method of attachment and manufacturing is made much easier. In the embodiment illustrated in  FIG. 2   g , the process of  FIGS. 2   b - 2   e  is performed on the top surface of the first laminated subassembly  100 . In other embodiments, the process of  FIGS. 2   b - 2   e  is performed on both the top and bottom surfaces of the laminated subassembly  100  to allow for attachment of more than one second subassembly  200  to the first subassembly  100 . 
       FIG. 3  is a cross sectional view of a multi-layer printed circuit board  300  including three subassemblies attached using the process of  FIGS. 2   a - 2   f  in accordance with one embodiment of the present invention. In other embodiments, more than three subassemblies can be attached using the processes of  FIGS. 2   a - 2   f . The PCB  300  includes three subassemblies having multiple copper pads  302  and through hole vias  304 . The subassemblies are attached by internal micro vias  312  embedded in the encapsulation layers ( 306 - 1 ,  306 - 2 ) and adhesive layers ( 308 - 1 ,  308 - 2 ). In the embodiment illustrated in  FIG. 3 , the subassembly to subassembly attachment is implemented using a micro via filled with a conductive paste. In other embodiments, the subassembly to subassembly attachment can be implemented using a solid copper plated micro via or solid copper through hole via. 
       FIGS. 4   a - 4   j  illustrate an alternative process for attaching subassemblies to form a multi-layer printed circuit board using internal micro vias in accordance with one embodiment of the present invention. 
     In  FIG. 4   a , the process begins when a laminated subassembly  400  having four layers and copper pads (e.g., foil)  402  on both sides is provided. The laminated subassembly  400  further includes two plated or filled blind vias  404  coupled to another two plated or filled blind vias  405 . The layers of the subassembly can be made of metal, ceramic, or insulating material (e.g., FR4, LCP, Thermount, BT, GPY, such as Teflon, thermally conducting carbon (stablecor), halogen free, etc., where GPY is a laminate that does not fit in the FR4 category, such as polyimide, aziridine cured epoxy, bismalimide, and other electrical grades of laminate). The present invention, however, is not thereby limited. In other embodiments, other suitable substrate and conductive layer materials can be used. In the embodiment shown in  FIG. 4   a , the subassembly layers have a thickness ranging from about 3 to 4 mils. However, in other embodiments, the subassembly layers and other components can have other suitable dimensions. 
     In several embodiments, the laminated subassembly  400  can be manufactured using the process described in  FIG. 1 . In other embodiments, the subassembly can be a single lamination subassembly having multiple single metal layer carriers and stacked micro vias. Aspects of single lamination processes for manufacturing circuit boards are further described in the above referenced patents and patent applications. 
     In the embodiment illustrated in  FIG. 4   a , the laminated subassembly  400  includes four metal layers. In other embodiments, the laminated subassembly can include more than or less than three metal layer carriers. In the embodiment illustrated in  FIG. 4   a , the laminated subassembly includes four blind vias. In other embodiments, the laminated subassembly can have more then or less than four vias. In other embodiments, the blind vias can be replaced with through hole, buried vias, and/or stacked vias. 
     In  FIG. 4   b , the process applies an encapsulation material  406  to a top surface of the laminated subassembly  400  and cures it. In several embodiments, the encapsulation material is a dielectric material. In several embodiments, the curing is achieved by heating the subassembly and encapsulation material thereon at a pre-selected temperature for a pre-selected duration. 
     The encapsulation material can be any suitable non-cured insulating material, including, without limitation, FR4, LCP, Thermount, BT, GPY, such as Teflon, thermally conducting carbon (stablecor), halogen free, etc., where GPY is a laminate that does not fit in the FR4 category, such as polyimide, aziridine cured epoxy, bismalimide, and other electrical grades of laminate. 
     In  FIG. 4   c , the process forms holes  410  for micro vias (or vias) by drilling through the encapsulation material  406  up to a top surface of the copper pads  402 . Each of the micro vias can be formed by laser drilling (and/or mechanical drilling) holes with a diameter ranging from about 4 to 10 mils. In other embodiments, other suitable techniques for forming via holes can be used. In addition, other via sizes can be used. 
     In  FIG. 4   d , the holes  410  are filled with copper thereby forming solid copper micro vias  412 . In some embodiments, the micro vias  412  are filled with conductive paste instead of copper. In one embodiment, conductive paste is used when the via holes are laser drilled and copper is used when the holes are mechanically drilled. 
     In  FIG. 4   e , the process images, develops, plates copper, adds resist and strips the resist to form a conductive pattern on the encapsulation layer  406  and on vias  412 . The conductive pattern includes capture pads  414  positioned on top of vias  412 . 
     In  FIG. 4   f , the process applies a laminate adhesive  416  to a top surface of the cured encapsulation material  406  and the capture pads  414 . 
     In  FIG. 4   g , the process forms holes  418  for thin micro vias by drilling through the laminate adhesive  416  up to a top surface of the capture pads  414 . Each of the thin micro vias can be formed by laser drilling (and/or mechanical drilling) holes with a diameter ranging from about 1 to 3 mils. In other embodiments, other suitable techniques for forming via holes can be used. In addition, other via sizes can be used. 
     In  FIG. 4   h , the holes  418  are filled with conductive paste thereby forming micro vias  420 . 
     In  FIG. 4   i , a second laminated subassembly  400 - 2  having substantially similar features on one surface thereof to the first subassembly  400  of  FIG. 4   e , including two blind solid copper micro vias with conductive pads positioned thereon, is formed and aligned such that the thin conductive paste filled micro vias of the first laminated assembly  400  and corresponding conductive pads of the second laminated assembly  400 - 2  will be physically and electrically coupled when they are brought together for attachment, and secured by the laminate adhesive  416 . 
       FIG. 4   j  is a cross sectional view of the finalized multi-layer printed circuit board of  FIGS. 4   a - 4   i  in accordance with one embodiment of the present invention. In  FIG. 4   j , the first and second subassemblies ( 400 ,  400 - 2 ) are brought together and attached. In some applications it can be difficult to connect and manufacture boards having high aspect ratio vias. In some applications, complex via structures can be too difficult to manufacture using traditional manufacturing methods. By attaching the laminated subassemblies using the process described above, the method of attachment and manufacturing is made much easier. In addition, the conductive paste or conductive ink micro via between the laminated subassemblies is very thin (e.g., 3 to 5 mils). While not bound by any particular theory, the thin micro via or joint can provide good high frequency conductivity. In several embodiments, the electrical conductivity of the joint is not as good as a highly conductive metal such as copper. However, because the joint is thin, it can provide the good conductivity for signals having high frequency characteristics (e.g., radio frequency type signals and the like). In addition, the thin copper paste joint can provide minimal disruption to the electrical current flowing therethrough. 
     In embodiments illustrated in  FIGS. 4   a - 4   j , the process is performed on the top surface of the first laminated subassembly  400 . In other embodiments, the process of  FIGS. 4   a - 4   j  is performed on both the top and bottom surfaces of the laminated subassembly  400  to allow for attachment of more than one second subassembly  400 - 2  to the first subassembly  400 . 
     In several embodiments, the conductive paste or conductive ink can include a mixture of copper and tin. In other embodiments, other suitable conductive materials can be used for the conductive paste. 
       FIG. 5  is a cross sectional expanded view of a subassembly to subassembly attachment  500  including two blind vias ( 512 - 1 ,  512 - 2 ) coupled by adhesive (not shown) and conductive paste  520  to form a thin via in accordance with the process of  FIGS. 4   a - 4   j . Each of the blind vias ( 512 - 1 ,  512 - 2 ) includes conductive pads ( 502 - 1 ,  502 - 2 ) on outer surfaces thereof and conductive pads ( 514 - 1 ,  514 - 2 ) on inner surfaces thereof. The conductive paste structure  520  forms a thin micro via within the adhesive (see  FIG. 4   j ), which can have the desirable properties discussed above. 
       FIG. 6  is a cross sectional expanded view of another subassembly to subassembly attachment  600  including stacked vias ( 602 ,  604 ) on each subassembly coupled by adhesive (not shown) and a conductive paste via  606  in accordance with one embodiment of the present invention. As compared to the subassembly attachment of  FIG. 5 , the conductive paste via  606  is substantially taller (e.g., z-axis length). This taller form of the conductive paste via can be easier to manufacture and provides good control of the impedance between board layers. 
       FIG. 7  is a cross sectional expanded view of another subassembly to subassembly attachment  700  using a conductive paste micro via  702  located between two mechanically drilled vias ( 704 ,  706 ) having enlarged surface areas ( 708 ,  710 ) in accordance with one embodiment of the present invention. 
     While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as examples of specific embodiments thereof. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents. For example, while certain components have been indicated to be formed of copper, other suitable conductive materials may be used instead of copper.