Abstract:
A printed circuit architecture includes a relatively thick, stiffening base of thermally and electrically conductive material, and a laminate of conductive layers including a printed circuit structure, interleaved with dielectric layers, disposed atop the base. The patterned conductive layers contain an integrated circuit structure that is configured to provide RF signaling, microstrip shielding, and digital and analog control signal leads, and DC power. Low inductance electrical connectivity among the conductive layers and also between conductive layers and the base is provided by a plurality of conductive bores. Selected bores are counter-drilled at the RF signaling layer and filled with insulating plugs, which prevent shorting of the RF signal trace layer to ground, during solder reflow connection of leads of circuit components to the RF signaling layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a divisional of application Ser. No. 09/489,505 filed Jan. 21, 2000 now U.S. Pat. No. 6,466,113, which claims the benefit of previously filed co-pending Provisional Patent Application, Ser. No. 60/116,653, filed Jan. 22, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to communication circuits and components and support structures therefor, and is particularly directed to a new and improved multi-layer printed circuit architecture for high power RF devices that provides low-inductance and low thermal resistance interconnections to a relatively thick, thermal dissipation, ground plane support substrate. 
     BACKGROUND OF THE INVENTION 
     Associated with continuing improvements in component micro-miniaturization, integration density and operational frequencies of signal processing and communication circuits, especially those employed in high frequency and high power RF applications, are packaging design and fabrication techniques that will facilitate the practical implementation of an integrated circuit architecture. As diagrammatically illustrated in FIG. 1, a typical printed circuit board structure, such as that employed for RF applications, is configured as a multi-layered laminate of dielectric layers (D 1 , D 2 , D 3 ) interleaved with patterned conductive layers (L 1 , L 2 , L 3 ), which respectively provide RF signaling and shielding, digital and analog control, and DC power functions. 
     This multi-layer laminate is supported atop a conductive (e.g., copper) ground plane substrate L 4 , that may serve as or be attached to a thermal dissipation medium, serving as a ground plane and mechanically stable support. Integrated circuit components and devices  10  may be surface-mounted to signal traces of the topside patterned conductor layer L 1  formed on the relatively thick dielectric layer D 1 . The multi-layer laminate structure contains a distribution of conductively plated through-holes or vias (one of which is shown at  20 ), which provide ‘vertical’ or ‘through-the-stack’ interconnections among the various conductive layers of the laminated structure. 
     As shown in the interconnect schematic diagram of FIG. 2, such plated through-holes typically include the following: 1) through-hole interconnects  21  between the (RF signaling) conductive layer L 1  and the (analog and digital signaling) layer L 3 ; 2) through-hole interconnects  22  between (microstrip ground/RF shielding) layer L 2  and the underlying ground plane and thermal dissipation support plate L 4 ; and 3) through-hole interconnects  23  among the RF signaling layer L 1 , the microstrip ground layer L 2  and the ground plane and thermal dissipation support plate L 4 . 
     In order to ensure proper operation of the composite circuit architecture, it is essential to minimize the reactance (parasitic capacitance, and inductance in particular) of interconnects. This mandates the use of shorter sections of conductive material, particularly at higher RF frequencies. Since the effective length of a section of interconnect includes both the vertical plated through-hole dimension and the horizontal dimension of a patterned conductive layer Li to which it is joined, a very efficacious technique to minimize grounding lead inductance is to fabricate such leads as a large number of closely spaced plated ground interconnect vias  22 , that extend between the RF ground/shielding layer L 2  and the bottom ground plate L 4 . 
     Unfortunately, this gives rise to a significant fabrication issue—ensuring that the plated ground vias  22  between the bottom layer L 4  and RF ground/shield layer L 2  do not extend all the way through the topside dielectric layer D 1 . If they did, the vias  22  would intersect the RF signal trace layer L 1 , and thereby short the RF signaling layer L 1  to ground during a solder reflow step customarily used in the fabrication process. The basic problem is the substantial thickness of the copper substrate L 4  upon which the interleaved dielectric and conductive layer laminate is mounted. In particular, providing the ground interconnects  22  requires the formation of conductive through holes through the stack between the RF shielding layer L 2  and the ground plane layer L 4 . 
     One way to form the RF shield to ground vias  22  would be to drill holes from the bottom surface of the layer L 4  up into the laminate, so as to intersect the RF shield layer L 2 . However, this approach demands a very exact (and therefore prohibitively expensive) vertical drilling depth through the dielectric—patterned conductor stack. This is especially true, if ground plane layer L 4  has substantial thickness. The hole depths would have to be sufficient to intersect the target RF shield layer L 2 , but not puncture the topside dielectric layer D 1 . It may be noted that the problem cannot be avoided by simply increasing the thickness of the dielectric layer D 1  (in order to increase the tolerance of the drill depth), since the characteristics of the dielectric layers (particularly those of the topside dielectric layer D 1 ), including thickness and dielectric properties, must be tailored for proper circuit operation. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the above-described problems are effectively obviated by a new and improved multi-layer printed circuit architecture and fabrication process therefor, that facilitates forming a large number of closely spaced plated vias between a robust underlying ground plane support pallet and the RF shielding layer, in a manner that minimizes interconnect inductance, while at the same time preventing unwanted shorting of the RF signal trace layer to ground, during solder reflow for connection to ‘wide lead’ power devices. 
     By ‘wide lead’ is meant an interconnect medium having a dimension equal to or greater than one-twentieth of a wavelength of propagation within the dielectric material of the RF transmission line. The invention successfully addresses the issue of inductance in the ground return path of the high power device to be mounted in a device well. The sensitivity of the path between the RF shielding layer and the base of the device (which is attached to the underlying ground plane pallet) varies according to the input and output impedances of the device. For large power transistors, these impedances are very low, and the circuit is very sensitive to stray inductance. 
     As will be described, the multilayer printed circuit structure of the invention includes an interleaved laminate of patterned dielectric layers and patterned conductive layers. The conductive layers are used for RF signaling, RF microstrip shielding/ground, digital and analog control signal leads, and DC power. A vertical interconnect between the RF signaling layer and the control/DC conductive layer is provided by way of a plated bore that intersects material of each of these conductive layers. The RF shielding layer is patterned adjacent to the bore, so as to be laterally offset from bore and thereby prevent conductive material plated in the bore from electrically bridging the RF shielding layer. 
     A vertical interconnect that joins the RF signaling layer, the RF microstrip shielding layer and the underlying ground plane support pallet is realized by forming a plated bore completely through the laminate structure from the RF signaling layer down through the bottom dielectric layer and into or through the conductive pallet. The support pallet preferably comprises a relatively thick metallic substrate, that is patterned to provide recesses of appropriate depth that conform with each of device capture slots and bores in the laminate structure. Although this bore intersects each of the RF signaling layer and the RF microstrip layer, the DC/control layer is patterned so that the plated bore is laterally offset from it, to prevent the plated bore from contacting the DC/control layer. 
     In the course of forming a vertical interconnect that electrically joins the RF microstrip shielding layer with the underlying ground plane support pallet, a further bore is drilled completely through the laminate structure from the RF signaling layer down to and at least partially through the ground plane pallet. The further bore intersects each of the RF signaling layer and the microstrip shielding layer; however, the DC/control layer is patterned so as to be laterally offset from the further bore, to prevent conductive material plated in the bore from contacting the DC/control layer. 
     The bores used for ground plane interconnections are preferably formed through the overall laminate structure (including the pallet) after the conductor—dielectric laminate structure has been bonded to the pallet. The bores are then plated to interconnect the RF signaling layer to the bottom of each bore. Although this operation provides the intended interconnects for the RF signaling layer, it results in an unwanted shorting of the RF signaling layer to the vertical interconnect between the RF shield layer and the support pallet. 
     Pursuant to the invention, this problem is obviated by counter-drilling the plated bore used for the vertical interconnect between the RF shield layer and the ground plane pallet with an oversized drill, to form an oversized counterbore that extends to a prescribed depth from the RF signaling layer into the topside dielectric layer. Because the counter-drilling of this bore is from the top surface of the laminate and directly into the relatively thicker topside dielectric layer, precise control of the depth of the counterbore is readily achieved. In addition, the radius of the counter-drill is sufficiently larger than the radius of the bore per se, so that the circular perimeter of the counterbore overlaps and removes a prescribed portion of the conductive material plated in the bore. 
     The upper portions of the bores are then filled with electrically insulating (e.g., epoxy, glass, or other suitable insulating material) plugs. Because the counter-drilled bore no longer has plated material intersecting the RF signaling layer, its non-conductive plug provides an insulating barrier between the RF signaling layer and the plated conductor remaining in the bore. The conductive material remaining in the bore beneath the plug still provides the intended interconnect between the RF shielding layer and the ground plane pallet. 
     This insulator-filled counterbore structure allows a large number of such bores to be placed immediately adjacent to well regions where high power devices, such as RF power transistors and the like, are installed, so as to provide low inductance electrical and thermal interconnects between the RF ground, common terminals of such devices, and the ground plane/heat sink pallet, without the danger of being shorted to the topside RF signaling layer, during solder reflow of interconnect leads for the topside RF signaling layer and the well-installed device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 diagrammatically illustrates a printed circuit board structure configured as a multi-layered laminate of dielectric layers and patterned conductive layers atop a relatively thick, thermal dissipation, ground plane support; 
     FIG. 2 is an interconnect schematic diagram associated with the printed circuit board laminate structure of FIG. 1; 
     FIGS. 3A and 3B contain a flow chart containing respective steps of a processing flow sequence for fabricating a multi-layer printed circuit architecture in accordance with the present invention; and 
     FIGS. 4-18 are reduced complexity cross-sectional illustrations of the multi-layer printed circuit architecture associated with the respective steps of the processing flow sequence of FIGS.  3 A and  3 B. 
    
    
     DETAILED DESCRIPTION 
     Attention is now directed to FIGS. 3A and 3B, which show respective steps of a processing flow sequence for fabricating a multi-layer printed circuit architecture in accordance with the present invention, and FIGS. 4-18, which are reduced complexity cross-sectional illustrations of the multi-layer printed circuit architecture associated with the respective steps of the processing flow sequence of FIG.  3 . For purposes of providing a non-limiting example, the present invention will be described for the case of implementing a multi-layer printed circuit architecture that contains a four conductor (L 1 -L 4 ), three dielectric (D 1 -D 3 ) laminate employing the three types of vertical interconnects shown schematically at  21 ,  22  and  23  in the interconnect diagram of FIG.  2 . 
     The process begins by preparing and laminating together a plurality (three in the present example) of patterned metal-coated dielectric layers into a composite assembly. This assembly, in turn, is then laminated or bonded onto an underlying metal pallet. The resulting structure is then subjected to further processing, including the counter-drilling and insulator fill operation of the present invention, described briefly above. 
     More particularly, at a step  301 , a first, dual metalized layer of dielectric material D 1  for the RF or microwave signaling portion of the structure is provided, as shown in FIG.  4 . As a non-limiting example dielectric layer D 1  may comprise a layer of RO4350 dielectric material supplied by Rogers Corp., having a thickness on the order of 20 mils, and its upper and lower surfaces coated with conductive material, such as respective one ounce copper, to form the first and second conductive layers L 1  and L 2 . 
     The thickness of the dielectric layer D 1  is defined, so as to provide a prescribed transmission line impedance at the intended operational frequency of the RF circuit. As in the structure of FIG. 1, the upper or topside metal layer L 1  is employed for RF signaling, while the lower metal layer L 2  serves as the RF ground/shield of the transmission line. 
     In a parallel step  311 , a dual metalized layer of dielectric material D 3 , shown in FIG. 5, upon which the DC/control signaling portion of the structure is supported, is provided. As a non-limiting example, the dual metalized dielectric layer D 3  may comprise a commercially available metalized dielectric FR-4 laminate having a thickness on the order of four mils, and both its upper and lower surfaces coated with one ounce copper layer. 
     In step  302 , the RF ground plane metal L 2  of the dielectric layer D 1  is selectively patterned to realize the structure shown in FIG. 6, while in companion step  312 , the DC/control metal layer L 3  on dielectric layer D 3  is selectively patterned and the lower metal layer on bottom surface of dielectric layer D 3  is completely stripped off, thereby realizing the structure shown in FIG.  7 A. In step  313 , the metallic layer L 2  undergoes surface oxidation (of the copper) to prepare it for bonding with an FR 4  prepreg layer which, after curing, constitutes the dielectric layer D 2 , shown in FIG.  7 B. As shown in FIG. 8, dielectric layer D 2  serves as the bonding vehicle between patterned conductor L 2  of dielectric layer D 1  (FIG. 6) and metal layer L 3  patterned atop dielectric layer D 3  (FIG.  7 A). 
     Next, in step  304 , using a standard adhesive (prepreg), the metalized dielectric layers D 1 , D 2  and the upper portion D 3 (A) of layer D 3  of FIGS. 6 and 8 are bonded together to form the multilayer metal and dielectric laminate structure of FIG.  9 . In step  305 , a set of blind vias or through holes for providing the vertical ground interconnect  21  that electrically joins the RF signaling layer L 1  with the underlying ground DC/control layer L 3 , is realized by forming respective bores, one of which is shown at  71  in FIG. 10, completely through the laminate structure from the RF signaling layer L 1  down through the upper portion D 3 (A) of dielectric layer D 3 . 
     Although the bore  71  intersects material of each of the RF signaling layer L 1  and the conductive layer L 3 , it may be noted that the RF shielding layer L 2  has been patterned adjacent to the bore  71  in step  302 , so that the bore  71  does not intersect, but is laterally offset from, the RF shielding layer L 2 . This patterned offset prevents conductive material formed (e.g., plated) in the bore  71  from contacting RF shielding layer L 2 . 
     Next, in step  306 , the bores  71  and the metal layer L 1  are exposed to a plasma etch to prepare their surfaces for an electrolytic metallic plate. Then, in step  307 , a suitable conductive metal, such as copper, as a non-limiting example, is electroplated onto the metal layer L 1  and into the bores  71 , to produce the electroplated plated structure shown in FIG. 11, which is to be bonded to a relatively thick ground plane pallet L 4 . 
     For this purpose, at step  308  a relatively thick (e.g., on the order of 60+ mils) copper plate to serve as the underlying ground plane layer L 4  is provided. Next, in step  309 , the pallet L 4  undergoes surface layer oxidation, to prepare it for bonding with an FR 4  prepreg layer (the lower portion D 3 (B) of dielectric layer D 3 ), and then is integrated with the previously formed laminate of L 1 -D 1 -L 2 -D 2 -L 3 -(upper portion D 3 (A) of layer D 3 ), to form the structure of FIG.  12 . 
     In step  310 , suitable tooling holes (not shown) are drilled into the pallet layer L 4  for holding the structure during subsequent processing. Then, in step  311 , using a suitable prepreg material, the laminate structure L 1 -D 1 -L 2 -D 2 -L 3 -D 3 (A) of FIG. 11 is bonded to the oxide-coated pallet D 3 (B)-L 4  of FIG. 12, to obtain the composite laminate structure L 1 -D 1 -L 2 -D 2 -L 3 -D 3 -L 4  of FIG.  13 . 
     Once the overall laminate structure has been assembled in step  311 , through holes or bores  72  that provide vertical interconnect  22  to electrically join RF shielding layer L 2  with underlying ground plate L 4 , and bores  73  that provide vertical interconnect  23  electrically joining the RF signaling layer L 1  and the RF shielding layer L 2  with the underlying ground plane pallet L 4 , are formed in step  312 . Each of bores  72  and  73  may be formed by drilling a plurality of holes completely through the laminate structure from RF signaling layer L 1  down through ground plane pallet L 4 , as shown in FIG.  14 . 
     Bore  72  intersects each of the RF signaling layer L 1  and the RF shielding layer L 2 . However, as the DC/control layer L 3  has been patterned in step  312  adjacent to where the bore  72  is drilled, the bore  72  is laterally offset from the DC/control layer L 3 , to prevent conductive material to be plated into the bore  72  from contacting the DC/control layer L 3 . Similarly, bore  73  intersects each of RF signaling layer L 1  and RF shielding layer L 3 . However, the RF shielding layer L 2  has been patterned in step  302  adjacent to the bore  73 , so that the bore  73  is laterally offset therefrom, to prevent conductive material to be plated in bore  73  from contacting RF ground layer L 3 . 
     As described briefly above, the bores  72 , in which vertical interconnects  22  between the RF shielding layer L 2  with the underlying ground plate L 4  are formed, are spatially located so as to be immediately adjacent (i.e., as is close as practically possible to fabricate) to locations where wells for devices such as ‘wide lead’ power transistors are to be formed (in step  321 , to be described). 
     As pointed out above, by ‘wide lead’ is meant an interconnect medium having a dimension equal to or greater than one-twentieth of a wavelength of propagation within the dielectric material of the RF transmission line. As a non-limiting example, at a frequency on the order of 2.5 GHz, the lead dimension may be on the order of 0.13 inches. Typically, wide lead power devices may have lead widths on the order of 0.20 to 0.50 inches. 
     In step  313 , the bores and the topside metal layer L 1  are exposed to a plasma etch to prepare their surfaces for a further metallic plate in subsequent step  315 . Prior to this further metal plating operation, the RF signaling layer L 1  is patterned in step  314 , to remove L 1  material at regions  84  therethrough. The patterned RF signaling L 1  and the bores are then plated in step  315  with a suitable conductor (e.g., Cu), followed by a pattern mask strip operation in step  316 , leaving the plated structure of FIG.  15 . 
     Next, in query step  317 , those ones of plated holes  71 ,  72  and  73 , which are to be counter-drilled (i.e., holes  72 ), are identified. For those holes (i.e. holes  71  or  73 ) that are not to be counter-drilled (the answer to query step  317  is NO), the processing routine transitions to step  319 . However, if a respective hole (hole  72 ) is to be counter-drilled (the answer to query step  317  is YES), the processing routine transitions to counter-drill step  318 . 
     As described above, pursuant to the invention, the counter-drilling of bores  72  (step  318 ) prevents unwanted shorting of the vertical interconnect  22 , that extends to and intersects the ground plane pallet L 4 , to the RF signaling layer L 1 . In particular, as shown in FIG. 16, each of the holes  72  is subjected to a counter-drilling operation using an oversized drill to bore a larger diameter hole or oversized counterbore  92  through the RF signaling layer L 1  to a prescribed depth  93  into the dielectric layer D 1 . 
     Because the counter-drilling of bores  72  is from the top surface of the laminate structure and directly into the relatively thicker dielectric layer D 1 , precise control of the depth  93  of the counterbore  92  is readily achieved. The radius of the counterbore  92  is preferably sufficiently larger than the radius of the bore  72 , so that the circular perimeter of the counterbore  92  overlaps that of bore  72 , thereby removing not only additional material of the RF signaling layer L 1  and dielectric layer D 1 , but also a depth of the plated conductive material  82  in the bore  72  that had been joined to topside RF signaling layer L 1  in plating step  315 . 
     Once counterboring of all holes  72  has been completed, a routing step  319  is performed, to form one or more slots or wells  61  for receiving circuit devices, such as power transistors, as shown in FIG.  17 . Because the ground plane pallet L 4  is relatively thick, the depths of the wells  61  may be variably dimensioned, to facilitate mounting different sized circuit devices therein, so that the devices may have their terminal contacts positioned at the proper height above the top surface of the laminate for effectively ‘common-plane’ interconnections with the adjacent RF signaling layer L 1 . 
     Next, in step  320 , the SnPb mask is stripped so that all exposed metal is copper. Then, in step  321 , metal in the bores and layer L 1  are chemically plated with a suitable protective alloy, such as Ni/Au. This plate does not deposit on dielectric material. In step  322 , a suitable insulating material, such as epoxy, is introduced to a prescribed depth in each of the holes  71 ,  72  and  73 , forming a set of dielectrically insulating plugs  101 ,  102  and  103 , respectively, shown in FIG.  18 . 
     Because the walls of the counterbore  92  contain no metal that would otherwise conductively join the RF signaling layer L 1  with any of the metal layers L 2 , L 3  or L 4 , (epoxy) plug  102  forms a substantial insulating barrier between the RF signaling layer L 1  and plated conductive material  82  remaining in the bore  72 . The conductive material  82  remaining in the bore  72  provides the intended interconnection  22  between microstrip shielding layer L 2  and the ground plate L 4 , without unwanted shorting of the vertical interconnects  22  to the topside RF signaling layer L 1  during solder reflow for the RF signaling layer L 1 . 
     This dielectric-filled counterbore structure thereby allows placement of closely spaced interconnect bores  72  at locations of the printed circuit board, in particular as immediately adjacent to regions (wells)  61  where high power devices, such as transistors and the like, are installed, and provides low inductance electrical and thermal interconnects between the microstrip ground, common terminals of such devices, and the backing support ground plane/heat sink layer L 4 . Next in step  323 , a solder mask for subsequent lead connections between circuit devices and the RF layer L 1  is formed. Then, in step  324 , the laminate is separated into respective printed circuit boards for subsequent component population during final fabrication. 
     As will be appreciated from the foregoing description, by means of a dielectric filled counterbore, the multi-layer printed circuit architecture and fabrication process of the present invention facilitates forming a large number of closely spaced, low inductance plated vias between the underlying ground plane, heat sink pallet and the microstrip shielding layer, minimizing interconnect inductance, while at the same time preventing unwanted shorting of the RF signal trace layer to ground, during solder reflow connection of circuit components to the RF signaling layer. Because the counter-drilling of the ground connect bore is from the top surface of the laminate and directly into a relatively thicker topside dielectric layer, precise control of the depth of the counterbore is readily achieved. 
     While we have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.