Abstract:
There is disclosed herein a tri-metal-layer precircuit  50  which may be selectively etched to provide a multilayer electronic circuit  60  having air bridge crossovers  49 . The enlarged ends  44  of the upper air bridge elements  42 , and/or the top pads  41  of the tower elements  43 , are specially designed such that undercutting of the ends  44  and/or top pads  41  is minimized, thereby minimizing the risk of air bridge/top pad delamination.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to multilayer circuit assemblies, and more particularly to pad shapes for improved etching of tri-metal-layered multilayer circuit assemblies. 
     2. Disclosure Information 
     U.S. Pat. No. 3,801,388 to Akiyama et al. (hereinafter “Akiyama”), U.S. Pat. No. 4,404,059 to Livshits et al. (hereinafter “Livshits”), and U.S. Pat. No. 5,738,797 to Belke, Jr. et al. (hereinafter “Belke”), all of which are incorporated herein by reference, disclose various methods for making electronic circuits which feature circuit crossovers or “air bridges” using various combinations of plating, masking, and etching steps. The methods disclosed in these references are useful for making multilayer circuits by selectively etching a tri-metal-layered precircuit such that the desired air bridge and circuit layout structures are created. 
     For example, the tri-metal precircuit may comprise a structure similar to that illustrated in FIGS. 1-3. Here, the middle layer  10  is a continuous 6-mil-thick aluminum sheet or foil having a top surface  14  and a bottom surface  12 , with a first (lower) conductor pattern  20  made of 2-mil-thick copper patterned onto the bottom surface  12 , and a second (upper) conductor pattern  40  made of 2-mil-thick copper patterned onto the top surface  14 . The lower conductor pattern  20  includes a plurality of base pads  26 , pedestal pads  22 , and circuit traces  24 , while the upper conductor pattern  40  includes a plurality of top pads  41  and bridging elements  42 . Each top pad  41  is arranged on the middle layer top surface  14  opposite a respective one of the base pads  26 . Each bridging element  42  has first and second enlarged ends  44  and a constricted portion  46  between and contiguous with the enlarged ends. The bridging elements  42  are arranged on the top surface  14  such that each enlarged end  44  is disposed opposite a respective one of the pedestal pads  22  and each constricted portion  46  is disposed opposite and transverse to a respective one of the circuit traces  24 . 
     This precircuit may then be affixed to a substrate  30  having an electrically insulative surface  32  by attaching the first conductor pattern  20  to this surface  32 . Then, the precircuit may be exposed for a predetermined amount of time to an etchant (e.g., sodium nitrate) which etches substantially only the aluminum, resulting in the final circuit structure illustrated in FIGS. 4-6. Here, those portions of the aluminum foil  10  which are sandwiched between an enlarged end  44  and a pedestal pad  22 , or between a top pad  41  and a base pad  26 , are more protected from attack by the etchant than are other portions of the foil  10 . After the predetermined amount of time has elapsed and the etching has ceased, most of the foil  10  has been etched away, except for a pedestal  48  of aluminum remaining sandwiched between (1) each matched pair of upper enlarged ends  44  and lower pedestal pads  22 , and between (2) each matched pair of upper top pads  41  and lower base pads  26 . This provides a plurality of air bridges  42  and a plurality of “towers”  43 , as shown in FIGS. 4-6. 
     The air bridges  42  created by this process serve as three-dimensional crossovers. For example, signal or current may flow from point A to point B along the lower conductor pattern, then rise to point C through an air bridge pedestal, then flow across the air bridge to point D, then down the other pedestal to point E, and then on across the lower conductor pattern to point F, thus allowing the circuit trace path ABCDEF to “cross over” the circuit trace path between points G and H. 
     Each tower  43  comprises a top pad  41  atop a pedestal  48  atop a base pad  26 , as shown in FIG.  6 . These towers  43  may be sized and arranged to serve in a variety of interconnect configurations. For example, a given set of towers  43  may serve as wirebond pads, solder joint pads (e.g., for reflowed chip components), direct chip attachment, and the like. A circuit trace  24  is typically attached to the base pad  26  of each tower element  43 . 
     The conductor patterns  20 / 40  may be formed on the aluminum sheet  10  by various methods disclosed in Belke, Livshits, and Akiyama. These references teach that the enlarged ends  44  and/or top pads  41  should be made a certain minimum size (e.g., 40 mils in diameter or smallest width, for the 2/6/2-mil example above) and the constricted portion  46  made a certain maximum size (e.g., no more than 5 mils wide), so that the foil  10  sandwiched between each pair of enlarged ends  44  and pedestal pads  22  and each pair of top pads  41  and base pads  26  is only partly etched through in the X direction leaving the desired pedestals  48 , while all other portions of the foil  10  (including those portions underneath the constricted portions  46 ) are completely etched away. 
     Whenever there is ample circuit space available around a given air bridge or tower, one may design the enlarged ends  44  and/or top pads  41  well above the recommended minimum size, thus assuring the formation of sturdy pedestals  48  and a robust metallurgical connection between each pedestal and its upper enlarged end/top pad  44 / 41 . However, in fine-pitch applications, or where circuit layout is particularly crowded, it may not be possible to design the ends/pads  44 / 41  oversized; in fact, when the size of the ends/pads  44 / 41  is kept close to the recommended minimum size, or even more so when it is desired to make these features even smaller than recommended, there is often a danger of the ends/pads  44 / 41  becoming delaminated from their respective pedestals  48  during etching, as illustrated in FIG.  7 . 
     It would be desirable, therefore, to provide a way of keeping the size of the enlarged air bridge ends  44  and top pads  41  small, while minimizing the aforementioned risk of delamination. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of the prior art approaches by providing specially designed multilayer precircuit and final circuit structures which enable the use of smaller air bridge and/or tower structures. 
     It is an object and advantage that the present invention provides precircuit and final circuit structures which have specially designed enlarged air bridge ends and/or top pads which minimize undercutting of these ends/pads, thereby decreasing their susceptibility to delamination. 
     Another advantage is that the present invention allows the use of smaller air bridge/tower structures with the same or better integrity and reliability as prior art approaches. 
     These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-3 are top, side sectional, and bottom views, respectively, of a precircuit for use in making a multilayer air bridge circuit according to the prior art. 
     FIGS. 4-6 are top and side sectional views of a final multilayer air bridge circuit after etching of the recircuit shown in FIGS. 1-3, according to the prior art. 
     FIG. 7 is an enlarged side view of a multilayer air bridge circuit being etched according the prior art, showing severe undercutting and delamination of an air bridge and a top pad. 
     FIG. 8 is a top view of a multilayer air bridge circuit according to a first embodiment of the present invention, wherein each top pad and each enlarged bridge element end is generally shaped as an n-sided polygon, where n≧5. 
     FIGS. 9-11 are top views of multilayer air bridge circuits according to a second embodiment of the present invention, wherein each top pad and each enlarged bridge element end is generally shaped as an n-sided polygon, where n≧6 and wherein the polygonal shape is generally cross-shaped, asterisk-shaped, star-shaped, or truncated-star-shaped. 
     FIG. 12 is a top view of a multilayer air bridge circuit according to a third embodiment of the present invention, wherein each top pad and each enlarged bridge element end has a generally round or polygonal outer periphery with a plurality of notches extending inward from each outer periphery. 
     FIG. 13 is a top view of a preferred embodiment of the present invention, wherein a top pad or enlarged bridge element end has an eight-armed truncated-star shape. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIGS. 8-13 show three different embodiments of a tri-metal-layer precircuit  50  for use in making a multilayer electronic circuit:  60 . A first embodiment of the precircuit  50  comprises: (1) a metallic foil  10  made of a first metal and having a top surface  14  and a bottom surface  12 ; (2) a first conductor pattern  20  attached to the bottom surface  12  of the metallic foil and made of a second metal, wherein the first conductor pattern comprises a plurality of base pads  26 , pedestal pads  22 , and circuit traces  24 ; and (3) a second conductor pattern  40  attached to the top surface  14  of the metallic foil made of a third metal. The second conductor pattern  40  comprises (a) a plurality of top pads  41  each arranged opposite a respective one of the base pads  26 , and (b) a plurality of bridging elements  42  having first and second enlarged ends  44  and a constricted portion  46  between and contiguous with the enlarged ends. The bridging elements  42  are arranged on the top surface  14  such that each enlarged end  44  is disposed opposite a respective one of the pedestal pads  22  and each constricted portion  46  is disposed opposite and transverse to a respective one of the circuit traces  24 . In the present embodiment, each of the enlarged ends  44  and/or each of the top pads  41  is generally shaped as an n-sided polygon, wherein n≧5. 
     To assist the reader in understanding the present invention, all reference numbers used herein are summarized in the table below, along with the elements they represent: 
       10 =Metallic foil [first metal] 
       12 =Bottom surface of metallic foil 
       14 =Top surface of metallic foil 
       20 =First (lower) conductor pattern [second metal] 
       22 =Pedestal pad 
       24 =Circuit trace 
       26 =Base pad 
       30 =Substrate 
       32 =Electrically insulative surface of substrate 
       40 =Second (upper) conductor pattern [third metal] 
       41 =Top pad 
       42 =Bridge element 
       43 =Tower element 
       44 =Enlarged end portion of bridge element 
       46 =Constricted portion between end portions 
       48 =Pedestals of metallic foil [first metal] 
       49 =Air bridge crossover 
       50 =Precircuit 
       60 =Final multilayer air bridge circuit, after etching 
       70 =Center portion of each cross/star/asterisk 
       71 =Arm of cross/star/asterisk configuration 
       73 =Generally radial edges of each arm 
       75 =Point of arm in star configuration 
       77 =Distal/truncated end of each arm 
       79 =Truncated end of each arm in star configuration 
     a=Subscript denoting asterisk-shaped pad/end portion 
     c=Subscript denoting cross-shaped pad/end portion 
     s=Subscript denoting star-shaped pad/end portion 
     t=Subscript denoting truncated-star pad/end portion 
     N=Notches formed in pad/end portion 
     R=Distance from pad/end outer periphery to center 
     The inventors of the present invention have discovered that the rate of undercutting of the metallic foil  10  beneath each enlarged end  44  and/or top pad  41  may be reduced significantly by providing the ends  44  and/or pads  41  with a multi-sided (n≧5) polygonal shape. The simplest of such shapes would be a pentagon (n=5), then a hexagon (n=6), septagon (n=7), octagon (n=8), and so forth, as illustrated in FIG.  8 . However, the inventors have noted even better results in two other configurations: (1) when the polygonal shape is generally cross-shaped  41   c / 44   c  (n≧12), asterisk-shaped  41   a / 44   a  (n≧15), star-shaped  41   s / 44   s  (n≧6), or truncated-star-shaped  41   t / 44   t  (n≧9) as illustrated in FIGS. 9-11 according to a second embodiment of the present invention; and (2) when the shape of each end  44  and/or pad  41  is either generally round or polygonal with notches N formed therein as illustrated in FIG. 13 according to a third embodiment of the present invention. 
     It is believed that the corners, notches, and arms about the outer periphery of these pad/end configurations provide geometries which disrupt the flow of etchant around these pads/ends  41 / 44  during etching of the middle layer  10 . These flow-disrupting features appear to induce flow separation about the pads/ends; as the etchant “sloshes” about the pads/ends, the separated flow reduces the rate of etching and undercutting on the pedestals  48  beneath these flow-disrupting pads/ends  41 / 44 . 
     The cross-shaped configuration of the second embodiment (see FIG. 9) has four “arms”  71  extending outward from a generally square or rectangular center portion  70 . Each arm  71  has three sides or edges—two generally radial edges  73  and one distal edge  77  generally orthogonal to the radial edges  73 —thus providing twelve sides (n=12) to the overall cross-shaped polygon. 
     The asterisk-shaped configuration  41   a / 44   a  is illustrated in FIG.  10 . This configuration is similar to the cross-shaped configuration, except that more than four arms  71  are provided. 
     The star-shaped configuration  41   s / 44   s  and the truncated-star-shaped configuration  41   t / 44   t  are shown in FIG.  11 . These configurations preferably have at least three arms  71  (n=6), with five to eight arms being most preferred. The star-shaped configuration  41   s / 44   s  has arms  71  which terminate generally in a point or vertex  75 ; alternatively, the end  75  of each arm  71  may instead terminate in a rounded fillet or other non-pointed geometry. The truncated-star configuration  41   t / 44   t  is similar to the star configuration  41   s / 44   s , except that each arm  71  is generally truncated  77 . For example, in FIG. 11 it may be noted that a truncated-star shape has been created by forming a generally star-shaped pattern wherein a distal portion  79  of each arm  71  has been “truncated”. 
     The difference between an asterisk-shaped configuration and the star/truncated configurations is illustrated in FIGS. 10-11, where each configuration has five arms  71 , thus providing a generally pentagon-shaped center portion  70 . Note that each arm  71  is generally rectangular in the asterisk-shaped configuration, while the arms  71  in the star and truncated configurations are generally triangular and trapezoidal, respectively. 
     In the third configuration (see FIG.  12 ), each top pad  41  and/or air bridge end  44  has a generally round or polygonal outer periphery, with a plurality of notches N extending inward from the outer periphery. Each notch N should extend inward somewhere between 5% and 80% of the distance R from the outer periphery to the center of the pad/end; for a circular pad/end R would be the radius of the circle, whereas for a square pad/end R would be one-half the distance between two opposing edges. A preferred range for the notch depth is between 10% and 50% of R, with a most preferred range being 20% to 40% of R. 
     This same set of ranges for the notch depth may be applied to the second configurations, where the “notch” here would be the gap N between adjacent arms  71 . This is illustrated in FIG. 13, where a preferred, eight-armed truncated-star-shaped configuration is illustrated. Here, the overall pad size is 34 mils (i.e., R=17 mils), and the length of each arm  71  and the depth of each “notch” N is 7 mils. (Thus, each notch extends to a depth of 7/17 or approximately 41% of R.) 
     It should be noted that in all of the foregoing configurations, the arms/notches are preferably arranged generally evenly-spaced circumferentially about each pad/end  41 / 44 . 
     The first and second conductor patterns  20 / 40  may be made from the same metal, rather than from two different metals. As alluded to in Belke, Akiyama, and Livshits, a wide variety of metals may be used as the first, second, and third metals, such as aluminum, copper, nickel, steel, and so forth. 
     Once the precircuit  50  is provided, it may then be attached to a suitable substrate  30 . The substrate may be made of metal, polymer, ceramic, and the like, but must have a electrically insulative surface  32  thereon, to which the first conductor pattern  20  is attached. An adhesive may be interposed between the insulative surface  32  and the lower conductor pattern  20 , or an adhesive may comprise the insulative surface  32  itself, such as when the substrate  30  is a metal. 
     After the substrate  30  is attached, the entire precircuit  50  is then subjected to an etchant which etches substantially only the metallic foil/first metal (i.e., not the second/third metals, and thus not the upper/lower conductor patterns  20 / 40 ). The etching may be performed by spraying the etchant onto the precircuit, or dipping the precircuit into an etchant bath, or by other known means. After a predetermined amount of exposure time to the etchant, the metallic foil  10  is mostly etched away, except for those portions which are sandwiched either between a top pad  41  and a base pad  26 , or between an enlarged air bridge end  44  and a pedestal pad  22 . These sandwiched portions of the foil  10  are much less aggressively etched and form the pedestals  48  of each tower element  43  and each air bridge  49 . Those pedestals whose top pads  41  or enlarged ends  44  have one of the configurations disclosed herein will be even less aggressively etched, allowing such tower elements and air bridges to be made significantly smaller and/or more densely grouped. 
     The resulting multilayer electronic circuit  60  will have the same specially configured pads  41  and ends  44  as were present in the precircuit  50  prior to etching of the middle foil  10 . The finished circuit  60  will comprise: (1) a substrate  30  having an electrically insulative surface  32 ; (2) a first conductor pattern  20  attached to the electrically insulative surface  32 , wherein the conductor pattern is made of the second metal and comprising a plurality of base pads  26 , pedestal pads  22 , and circuit traces  24 ; (3) a plurality of pedestals  48  made of the first metal, wherein a respective one of the plurality of pedestals is disposed atop each base pad  26  and  22  pedestal pad; and (4) a second conductor pattern  40  made of the third metal and comprising (i) a plurality of top pads  41  each attached to a respective one of the pedestals  48  atop a respective one of the base pads  26 , and (ii) a plurality of bridging elements  42  having first and second enlarged ends  44  and a constricted portion  46  between and contiguous with the enlarged ends, wherein the bridging elements  42  are arranged such that each enlarged end  44  is disposed atop a respective one of the pedestals  48  atop a respective one of the pedestal pads  22 , and wherein each constricted portion  46  is disposed transverse to and spaced apart from a respective one of the circuit traces  24 . 
     Various other modifications to the present invention may occur to those skilled in the art to which the present invention pertains. Other modifications not explicitly mentioned herein are also possible and within the scope of the present invention. It is the following claims, including all equivalents, which define the scope of the present invention.