Abstract:
A method of forming a circuit includes forming a metallic circuit pattern on a base substrate. The circuit pattern has traces which are connected together by temporary bussing. A resist pattern for defining at least one terminal pad is formed over the circuit pattern. A layer of metal is formed on at least one area of the circuit pattern exposed by the resist pattern to a thickness suitable for serving as the at least one terminal pad for the circuit. A portion of the base substrate at the location of the temporary bussing is removed thereby causing the removal of the temporary bussing.

Description:
BACKGROUND OF THE INVENTION 
     Electronic circuits included on circuit boards often have thickened metallized areas serving as terminal pads which allow electrical devices to be wire bonded thereto. Conventional methods for forming such circuits usually involve forming a photoresist pattern on a copper clad circuit board substrate and electro-plating a thick patterned layer of copper over the copper cladding. Areas of copper are etched away to produce a desired circuit pattern on the circuit board substrate. The thickened areas of the circuit are suitable for wire bonding to electronic devices. 
     A drawback with such a method is that etching the unneeded areas of copper from the circuit board substrate usually requires a relatively long etching process due to the thickened layers of metal. As a result, the side edges of the circuit pattern often become undercut and/or ragged which can affect the performance of the circuit. In addition, temporary bussing pathways may be formed to provide electrical continuity between different portions of the circuit board substrate or between opposite sides thereof. The electrical continuity is required for providing electrical current to areas where the deposition of metallic material by electro-plating or electrolytic deposition is desired. The temporary bussing pathways are later etched away in another etching process. The added etching process may affect the quality of the side edges of the remaining portions of the circuit pattern. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of forming a circuit on a circuit board including thickened areas suitable for wire bonding to electrical devices where the traces of the circuit have limited undercutting and can be manufactured with higher tolerances than possible with previous methods. The method includes forming a metallic circuit pattern on a base substrate. The circuit pattern has traces which are connected together by temporary bussing. A resist pattern is formed over the circuit pattern for defining at least one terminal pad. A layer of metal is formed on at least one area of the circuit pattern exposed by the resist pattern to a thickness suitable for serving as the at least one terminal pad for the circuit. A portion of the base substrate at the location of the temporary bussing is removed, thereby causing the removal of the temporary bussing. 
     In preferred embodiments, the metallic circuit pattern is formed by forming a first resist pattern for defining the circuit pattern over a metallic layer on the base substrate. Areas of the metallic layer on the base substrate exposed by the first resist pattern are etched away thereby forming the metallic circuit pattern under the first resist pattern. The first resist pattern is then stripped from the base substrate to uncover the circuit pattern. The circuit pattern and its side edges are covered with a protective metallic layer. The protective metallic layer is formed by forming a metallic inner barrier layer over the circuit pattern and side edges thereof by electroless deposition and then forming a metallic outer layer over the barrier layer also by electroless deposition. 
     The base substrate preferably has opposing sides each with a metallic layer thereon. In such a case, before forming the metallic circuit pattern, at least one via hole is formed through the base substrate. A conductive pathway is formed through the at least one via hole to provide electrical continuity between the metallic layers on the opposing sides of the base substrate. The conductive pathway later becomes part of the temporary bussing when the circuit pattern is formed. The conductive pathway may be formed by first forming a thin metallic layer within the at least one via hole and over the metallic layers of the base substrate by electroless deposition, and then forming a thick metallic layer over the thin layer as well as within the at least one via hole by electrolytic deposition. The metallic layer which forms the at least one terminal pad is deposited by electrolytic deposition. 
     In one embodiment, the metallic layers of the base substrate which are on opposing sides of the base substrate are made of copper. The conductive pathway is formed by first forming a thin copper layer within the at least one via hole and over the copper layers of the base substrate by electroless copper deposition, and then forming a thick copper layer over the thin layer by electrolytic copper deposition. Consequently, after etching, the resulting metallic circuit pattern is made of copper. The protective metallic layer is formed by forming an inner barrier layer of nickel over the circuit pattern and side edges thereof by electroless nickel deposition and then forming an outer layer of gold over the inner barrier layer of nickel by electroless gold deposition. The terminal pads are formed by electrolytic gold deposition. Finally, the temporary bussing is routed out with a router. 
     In another embodiment, the metallic circuit pattern is formed by providing the base substrate with a metallic layer thereon. A first resist pattern is formed over the metallic layer on the base substrate for defining the circuit pattern. Next, the thickness of the metallic layer is increased in areas of the base substrate exposed by the first resist pattern. The thickened metallic layer in the areas exposed by the first resist pattern is later covered with a protective metallic layer. The first resist pattern is then stripped from the base substrate. Finally, areas of the base substrate not protected by the protective metallic layer are etched from the base substrate, thereby forming the metallic circuit pattern. 
     In the present invention, since the circuit pattern is etched before the thick layer of metal forming the terminal pads is deposited, the etching is performed on a relatively thin layer of metal for a relatively short period of time. As a result, the side edges of the traces of the circuit pattern once formed, are not subjected to a lengthy attack by the etchant and experience very little etching and/or undercutting. In addition, by removing the temporary bussing by routing, the circuit pattern is not subjected to any further etching steps, thereby preserving the quality of the side edges of the traces. Consequently, the present invention is suitable for forming very fine and delicate traces with high yield as well as with high performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a flow chart depicting the steps of one method for fabricating electronic circuits in accordance with the present invention. 
     FIG. 2 is a side sectional view of a portion of a circuit board substrate having a via hole depicting deposited metallized layers. 
     FIG. 3 is a side sectional view of a portion of the circuit board substrate depicting a first pattern of photoresist formed over the metallized layers. 
     FIG. 4 is a side sectional view of the portion of the circuit board substrate of FIG. 3 depicting metallized areas surrounding the first photoresist pattern removed by etching to form a metallized circuit pattern. 
     FIG. 5 is a plan view of a portion of the circuit board substrate having a metallized circuit pattern defined thereon, including temporary bussing pathways. 
     FIG. 6 is a side sectional view of the portion of the circuit board substrate of FIG. 3 depicting protective metallic layers covering the circuit pattern. 
     FIG. 7 is a plan view of the portion of the circuit board substrate of FIG. 5 depicting a second pattern of photoresist and terminal pads formed thereon. 
     FIG. 8 is a plan view of the portion of the portion of the circuit board substrate of FIG. 7 with the second pattern of photoresist removed to show the circuit pattern with the terminal pads. 
     FIG. 9 is a side sectional a view of the portion of the circuit board substrate of FIG. 6 depicting a terminal pad formed thereon. 
     FIG. 10 is a plan view of the portion of the circuit board substrate of FIG. 8 with the temporary bussing pathways routed out. 
     FIG. 11 is a flow chart depicting the steps of another method for fabricating electronic circuits. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts the steps of one method for forming a circuit in accordance with the present invention. In step  10 , a conventional circuit board substrate  36  (FIG. 2) having thin metallic base layers  36   a  covering opposing sides is provided. Typically, the metallic layers  36   a  are formed of copper. The metallic layers  36   a  are cleaned in a cleaning process in preparation for subsequent processes. In step  12 , a series of via holes  38  are formed through substrate  36  and metallic layers  36   a  (FIGS. 2 and 5) at predetermined locations. The via holes  38  are preferably drilled, but alternatively, may be punched, stamped or formed by a laser. Typically, the via holes  38  are positioned on substrate  36  adjacent to a location where an opening in substrate  36  will be later located. The position of the via holes  38  is also at the future location of temporary bussing  42  (FIG.  5 ). The temporary bussing  42  is later removed and an electrical device positioned at the same location. Typically two via holes  38  are formed at the future location of temporary bussing  42 , but alternatively, the number of via holes  38  may vary depending upon the situation at hand. 
     In step  14  (FIG.  1 ), a thin metallic layer of copper  37  is deposited over the metallic layers  36   a  of substrate  36  as well as over the inner surfaces of via holes  38  (FIG.  2 ). The thin layer of copper  37  is typically formed in a bath by electroless copper deposition. The portion of layer  37  extending through the via holes  38  provides electrical continuity between the two separated layers of copper  36   a . Although layer  37  is preferably formed by electroless deposition, alternatively, layer  37  may be formed by other suitable methods such as vapor deposition. 
     Typically, layer  37  is too thin to survive subsequent processing steps. As a result, in step  16  (FIG.  1 ), a thicker metallic layer of copper  39  (FIG. 2) is deposited over the thin layer  37  by electrolytic copper deposition in a electrolyte bath where the copper surfaces are electrically connected to a power source and current passed therethrough. Electrical continuity to the copper surfaces on both sides of substrate  36  is provided by the thin layer of copper  37  within the via holes  38  which permits the electrolytic deposition of copper on both sides of substrate  36  as well as within the via holes  38 . Electrolytic deposition is able to deposit a thicker layer of copper  39  than the layer  37  formed by electroless deposition. The copper layer  39  within the via holes  38  has a thickness that is sufficient to survive subsequent processes and thus maintain electrical continuity to both sides of substrate  36 . 
     In Step  18  (FIG.  1 ), a first photoresist layer  61  (FIG. 3) is deposited over one or both of the copper layers  39  as desired. Patterns of desired circuits are formed from the photoresist by conventional exposure and development processes. The patterns provide masks for forming the desired circuits. Such circuit patterns may be formed on one or both sides of substrate  36  depending upon the application at hand. FIG. 3 depicts a portion of a pattern  41  of photoresist formed on layer  39 . 
     In step  20  (FIG.  1 ), the copper material which is not covered by the photoresist pattern  41  is etched away in an etching bath to form a copper circuit pattern  40  consisting of layers  36   a ,  37  and  39  (FIG.  4 ). The side edges  47  of circuit pattern  40  have the added thicknesses of layers  36   a ,  37  and  39 . The photoresist pattern  41  is then stripped away in step  22  with an appropriate solution in a stripping bath. In the example depicted in FIG. 5, the circuit pattern  40  includes a first radio frequency trace  44 , a second radio frequency trace  46 , a first DC trace  48 , a second DC trace  50  and temporary bussing  42  connected therebetween. The traces  44  and  46  are on opposite sides of temporary bussing  42 , while traces  48  and  50  are side by side between traces  44 / 46 . In the example shown, traces  48  and  50  are relatively narrow in comparison to traces  44  and  46 . As a result, traces  48  and  50  include widened regions  49  over which terminal pads will later be formed for bonding to an electrical device. FIG. 4 depicts a cross sectional view of the portion of circuit pattern  40  forming trace  44 . The temporary bussing  42  extends around and includes the two metallized via holes  38 , a central rectangular region  42   a  and a series of narrow traces  42   b  extending from rectangular region  42   a  to traces  44 ,  46 ,  48  and  50 . The temporary bussing  42  provides electrical continuity between the traces  44 ,  46 ,  48  and  50  of circuit pattern  40 . In addition, temporary bussing  42  provides electrical continuity between circuit pattern  40  and any circuit patterns or metallic areas located on the opposite side of substrate  36 . Although not shown in FIG. 5, temporary bussing  42  may be employed to provide electrical continuity to circuit patterns adjacent to circuit pattern  40  on the same side of substrate  36 . In such a case, another trace would extend therebetween. It is understood that circuit pattern  40  may be of other suitable configurations depending upon the application at hand. It is also understood that other circuit patterns may be formed on the same and/or opposite side of substrate  36 . 
     In step  24  (FIG.  1 ), a thin layer of nickel  43  (FIG. 6) is deposited over the circuit pattern  40  in a bath by electroless nickel deposition. The layer of nickel  43  covers the top surfaces as well as the side edges  47  of circuit pattern  40 . Next, in step  26 , a thin layer of gold  45  is deposited over the layer of nickel  43  by electroless gold deposition and also covers the top surfaces and side edges  47 . The layers of nickel  43  and gold  45  are deposited only over circuit pattern  40  and not over non-metallic areas of substrate  36 . The combined layers of nickel  43  and gold  45  serve as a protective metallic layer or jacket for protecting the top surfaces and side edges  47  of circuit pattern  40  which prevents or reduces etching as well as undercutting during subsequent processing steps. The layer of nickel  43  acts as a barrier layer between the copper and the gold layers to prevent migration between the copper and the gold layers. 
     In step  28  (FIG.  1 ), a second layer of photoresist  56  (FIG. 7) is deposited upon substrate  36  and over circuit pattern  40 . The second layer of photoresist  56  is exposed and developed by conventional methods for forming a pattern  58  of open areas  59 . The areas  59  correspond to desired locations for forming terminal pads for circuit pattern  40 . In step  30 , a layer of gold  54  is deposited by electrolytic gold deposition in an electrolytic bath over portions of circuit pattern  40  exposed by the open areas  59  of the photoresist pattern  58 . The electrolytic layer of gold  54  is positioned on the appropriate areas of circuit pattern  40  to form terminal pads  44   a ,  46   a ,  48   a  and  50   a  for respective traces  44 ,  46 ,  48  and  50 . Layer  54  is formed to a thickness suitable for bonding to electrical devices. The temporary bussing  42  including via holes  38 , provide the necessary electrical continuity within circuit pattern  40  and to other circuit patterns or metallized areas if any, including those on the opposite side of circuit board substrate  36  for the electrolytic gold deposition. 
     In step  32  (FIG.  1 ), the second layer of photoresist  56  is then removed in a bath to expose the circuit pattern  40  and thickened terminal pads  44   a ,  46   a ,  48   a , and  50   a  (FIGS.  8  and  9 ). Terminal pads  44   a  and  46   a  are positioned at the ends of respective traces  44  and  46  adjacent to traces  42   b . Terminal pads  48   a  and  50   a  are positioned over widened regions  49  at the ends of respective traces  48  and  50  adjacent to traces  42   b . Finally, in Step  34 , an opening  52  is routed out from substrate  36  near and between terminal pads  44   a ,  46   a ,  48   a , and  50   a  (FIG.  10 ). The opening  52  removes the temporary bussing  42  including via holes  38 , rectangular region  42   a  and traces  42   b  without employing an etching step. As a result, there are no subsequent process steps to affect the quality and definition of the side edges of circuit pattern  40 . The opening  52  allows an electrical device to be positioned therein and the thickened terminal pads  44   a ,  46   a ,  48   a  and  50   a  allow the electrical device to be wire bonded thereto. Depending upon the application at hand, opening  52  may be a hole that extends completely through substrate  36  or may be merely a recess or pocket having a depth that is less than the thickness of substrate  36 . 
     Since the circuit pattern in the present invention is etched from a relatively thin layer of metal, the etching time is relatively short and fine or delicate trace definition can be achieved without significant lateral etching and/or undercutting of the side edges. Longer etching times tend to allow the etchant to attack the side edges of the circuit traces resulting in ragged or undercut side edges which can affect the quality and performance of the circuit. This is important especially when forming circuits with delicate traces. The protective metallic layer further insures that the definition of the traces is not affected by subsequent process steps. Forming the terminal pads on the circuit pattern only at the locations required is both cost and time effective in comparison to prior art processes where large areas are first thickened and then later require etching. Finally, routing out the temporary bussing mechanically removes the temporary bussing and eliminates another etching step. This is desirable because additional etching steps after the formation of the circuit pattern can affect the quality of the edges of the traces. Circuits made in accordance with the present invention not only are high precision and high quality, but also can be manufactured with higher tolerances and with higher yields than by prior art methods. 
     In one embodiment, circuit board substrate  36  (FIG. 2) is preferably made of low loss, low dielectrical circuit board material, but alternatively, may be fiberglass, teflon or multifunctional epoxy, etc. Substrate  36  is preferably about 0.003 to 0.070 inches thick, but alternatively, may be less than 0.003 inches or greater than 0.070 inches. The base layers  36   a  of copper are preferably about 350 to 700 micro-inches (0.00035 to 0.0007 inches) thick. Layers  36   a  are preferably formed from foil that is rolled onto the underlying board material, but alternatively, may be formed by electrolytic deposition. Although two layers  36   a  are preferred, there may be instances where one layer  36   a  is desired. 
     The via holes  38  are preferably 13 to 20 mils in diameter. In some applications, some via holes  38  may be kept in the final circuit board configuration if desired. Although metallized via holes  38  are preferred for providing electrical continuity, alternatively, conductive pathways may be provided by mechanically inserting a series of conductive members through the substrate  36  which are in contact with layers  36   a . In such a case, removal of the conductive members may be by routing or pushing the conductive members from the base substrate  36 . 
     The thin layer of copper  37  formed by electroless copper deposition in step  14  is typically about 50 microinches thick. The thicker layer of copper  39  formed by electrolytic copper deposition in step  16  is typically about 100-150 microinches thick but may be greater. Although Steps  14  and  16  (FIG. 1) are preferred for depositing layers  37  and  39  over layers  36   a , alternatively, Steps  14  and  16  can be replaced by a direct plating step which is an electroless process capable of depositing a thicker metallic layer than is possible with Step  14 . The layers of nickel  43  and gold  45  forming the protective metallic layer (steps  24  and  26 ) are each about 50 to 150 microinches thick. The layer of gold  54  formed by electrolytic gold deposition in step  30  to provide the terminal pads  44   a ,  46   a ,  48   a  and  50   a  is about 80 to 100 microinches thick. 
     Although layers  36   a ,  37  and  39  are preferably copper, layers  36   a ,  37  and  39  may be formed of other suitable materials such as aluminum, silver or gold. In addition, although nickel is preferred as the first layer  43  of the protective metallic layer on the circuit pattern  40  (FIG.  6 ), other suitable metals may be employed such as palladium, silver or tin. In such cases, the materials forming layers  36   a ,  37 ,  39 ,  43 ,  45  and  54  are appropriately selected for compatibility. Finally, depending upon the materials chosen, the protective metallic layer may be formed from a single layer of material instead of an inner barrier layer and an outer layer. 
     FIG. 11 depicts another method for forming a circuit in accordance with the present invention. Generally, instead of plating a whole panel as performed in Step  16  of FIG. 1, the method depicted in FIG. 11 plates a desired pattern defined by photoresist. Consequently, some of the process steps in FIG. 11 are performed in a different order than in FIG.  1 . For example, in FIG. 11, after depositing a thin layer of electroless copper in Step  14 , a first photoresist layer is deposited, exposed and developed in Step  18 . Then in Step  16 , a thick layer of copper is deposited by electrolytic copper deposition in a desired pattern defined by the photoresist. The thickened patterned layer of copper is in the configuration of the desired circuit pattern. Next, in Step  25 , a layer of metal (or metals) compatible with gold is deposited over the metallic pattern for providing a protective metallic layer similar to that provided in FIG. 1 by Steps  24  and  26 . This protective layer typically covers only the top surface. The first photoresist layer is stripped in Step  22  and the exposed copper is etched in Step  20  to form the circuit pattern. The second layer of photoresist may then be deposited, exposed, and developed in Step  28  in preparation for the formation of terminal pads as in FIG.  1 . 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 
     For example, although a particular circuit pattern  40  has been depicted in the figures, it is understood that any circuit pattern may be formed and that there may be multiple circuit patterns on one or both sides of circuit board substrate  36 . In addition, although via holes  38  are depicted in the figures and described above, the via holes  38  may be omitted in certain instances. In such cases, steps  14  and  16  of FIG. 1 may be omitted or altered to suit the situation at hand. It is understood that the configuration and locations of the temporary bussing  42  may vary between circuits. Although the temporary bussing  42  including the via holes  38  is preferably removed by routing, alternatively, such areas maybe removed by drilling, punching, another etching step or laser ablation. Furthermore, although specific dimensions have been provided for circuit pattern  40 , such dimensions may vary depending upon the situation at hand. Finally, various features of the fabrication methods depicted in the figures and described above may be omitted, substituted or combined, depending upon the situation at hand.