Abstract:
Printed circuit boards whose outerlayer circuit lines are inlaid into the dielectric layer thus have 3-side adhesion between circuitry and adjacent dielectric material. Method for fabricating said PCB, in particular method without using of copper clad laminate and/or copper foil to circumvent various difficulties arid shortcomings associated with using them. This invention employs optical engraving and pattern metallization techniques conducted on a metal transfer plate to form the circuit pattern, then transfer the pattern to a dielectric material via lamination technique, a printed circuit is emerged after removal of metal plate and etching away the covering copper foil. This invention is particularly useful in manufacturing very thin PCB with very fine line width and space.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates in general to methods for producing printed circuit boards and to a printed circuits board produced by the method of this invention. In particular, it relates to a new method of forming outerlayer circuit lines that are inlaid into the dielectric layer.  
         BACKGROUND OF THE INVENTION  
         [0002]    A printed circuit board (hereinafter referred to as PCB) typically consists of at least one layer of dielectric substrate, typically fabricated from epoxy coated glass fabric, and one or both sides of thin layer of predetermined pattern of metallic traces, such as copper. Interconnections between the layer of metallic circuitry are often required such that the various metallic layers may communicate with each other electrically. In accordance with prior art the production of printed circuit board, layers of copper foils are laminated to a dielectric substrate to form a copper clad laminate. That copper clad laminate is further processed to convert the copper foils into predetermined circuit patterns by selectively removing portions of the copper foils by chemical etching technique. A PCB produced by the conventional process is characterized by its outerlayer circuits are bonded on top of the dielectric substrate as shown in FIG. 1. This conventional approach has several shortcomings. Firstly, the etchant must remove large amount of copper, which increases manufacturing costs and waste disposal casts. A further disadvantage of the conventional method is that the etchant does not create vertical sides of the circuit lines. Instead, it tends to etch away too much copper at the top and too less copper at the bottom of the circuit lines, leaving a trapezoidal-shaped circuit lines, this phenomena is frequently described as etching factor. As a result, the minimum Width of the circuit lines and minimum spaces between circuit lines are limited by the need for allowing this uneven and inaccurate etching. An approach of improving the accuracy Of circuit lines is to use thinner copper foil as it can be etched quickly with less undercutting. However, such thin copper foils are not only expensive but also difficult to handle. For very thin copper foil a carrier layers is usually applied to improve operability, one example is formed in U.S. Pat. No. 3,998,601 in which a thin copper foil is deposit on a supportive thick copper foil and separated from the thick copper foil by a release layer.  
           [0003]    Another known alternative method for forming fine-line patterns is shown in U.S. Pat. No. 6,117,300 to Carbin et al. According to the technique of Carbin et al., a thin conductive layer is applied to a substrate using a foil carrier; a photoresist may then be applied, image, and cured. The uncured photoresist may be removed, thereby defining some exposed regions or “trenches” on the surface of the substrate in which the circuit lines are to be formed. Since the conductive layer is now exposed, it is possible to selectively apply the circuit lines in those exposed regions. Finally, the cured photoresist may be removed and then the exposed conductive metal layer beneath the photoresist is removed, leaving the finished circuit pattern. An earlier technique for fabricating printed circuit boards by pattern plating process is described in the U.S. Pat. No. 5,733,468 to Conway. In accordance with technique of Conway, a thin, first layer of copper foil is bonded to the surface of the board, then a photoresist layer is laminated over the copper layer, and then selectively exposed and developed the photoresist to define the desired pattern of traces. A thick, second layer of copper is electrodeposited on the traces, and the photoresist is then removed. The board is etched to remove those portions of the first copper layer that are not covered by the second copper layer, leaving the finished circuit pattern.  
           [0004]    An obvious disadvantage, as circuit lines become finer, of the aforementioned techniques is that the copper traces may not adhere sufficiently to the dielectric layer, which may be peeled off from the dielectric layer during surface conditioning, such as brushing prior to soldermask printing. An improved technique also known or “imprint patterning” is described in the U.S. Pat. No. 6,005,198 to Gregoire. In accordance with technique of Gregoire, a U-shape recessed pattern of circuitry is formed on the surface of compressible substrate via stamping process, then conducter is disposed on the surface of recessed pattern by electroplating or metal transfer. While it has advantage of forming circuit lines and micro-vias integrally, the necessity of new substrate material, fidelity and reliability of the delicate mold after repetitive stamping are its may in disadvantages. Another disadvantage of fine line printed circuit board is its irregular zigzag surface formed by the dense circuit lines, the difficulty in filling soldermask and adhesion it to narrow spaces of finer circuit pattern may entrap air, and cause bubbling, blister or de-lamination of soldermask upon passing the solder bath. A further disadvantage of the conventional techniques in forming very thin multilayer PCB is that the copper clad laminate tends to shrink and warp after exposure to chemicals and/or high temperature, resulting in mismatch of circuit lines of different layers. All these shortcomings result in numerous quality control problems and need of costly capital equipment to meet the manufacturing tolerance.  
           [0005]    Bearing the aforementioned shortcomings in mind, there exists consequently a need for PCB with inlaid outerlayer circuits and a need for an economical and reliable production method that overcome above-identified shortcomings.  
         OBJECT OF THE INVENTION  
         [0006]    A primary object of this invention is to provide a printed circuit board whose outerlayer circuits is inlaid into the adjacent dielectric substrate. Another object of this invention is to provide a printed circuit board whereby complex, irregular and intricate circuit lines are inlaid into the dielectric substrate; thus a flat surface is obtained. Yet another object of this invention is to provide a printed circuit board whereby the thickness of the board is sum of the thickness of dielectric layers and the thickness of protective surface coatings. A further object of this invention is to provide a method of fabricating a printed circuit board with inlaid outlayer circuits. Still another object of this invention is to provide a method of fabricating a printed circuit board that overcomes the above-identified shortcomings. Yet another object of this invention is to provide a method for fabricating a printed circuit board that eliminates usage of costly thin copper foil and thin copper clad laminate. These and other advantages, features, and objects of this invention will be more easily understood in view of the following detailed description and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 shows a cross-sectional view of circuit lines of a printed circuit fabricated in accordance with prior art.  
         [0008]    [0008]FIGS. 2 and 3 show a cross-sectional view of circuit fabricated in accordance with an embodiment of this invention.  
         [0009]    [0009]FIGS. 4A-4J are a series of cross-sectional views showing in series a printed circuit at various stages during fabrication in accordance with one method of this invention.  
         [0010]    FIGS. SA- 5 I are a series of cross-sectional views showing in series a printed circuit at various stages during fabrication in accordance with an alternate method of this invention.  
         [0011]    [0011]FIGS. 6A-6C are a series of cross-sectional views showing in series a multilayer printed circuit at various stages during fabrication in accordance with a method of this invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    With reference to the drawings, FIG. 1 represents a prior art double side printed circuit board. The prior art circuit board includes a core layer, having circuit lines bonded on the surfaces of both sides. Core layer comprises typically epoxy coated glass fabric for forming a rigid substrate. The board may be laminated to form a multi-layer, structure using a layer of copper foil and a bonding sheet known as prepreg. A circuit pattern is produced thereon by any suitable fabricating technique. Those skilled in the art will understand that the circuit lines may not adhere sufficiently to the substrate, this can lead to problem such as peel off of circuit lines.  
         [0013]    [0013]FIGS. 2 and 3 illustrate preferred embodiments of printed circuit boards in accordance with the present invention. The circuit lines of outmost layers are inlaid into the dielectric substrate, thus having 3-side adhesion between a circuit line and the substrate. The irregular zigzag surface associated with printed circuit board fabricated in accordance with prior art is eliminated and a flat and smooth surface is achieved.  
         [0014]    [0014]FIGS. 4A-4J show first method of producing a printed circuit board with inlaid circuits according to the present invention, the method is particularly advantageous when used to fabricate the outer circuit layers of multilayer circuit boards, but may also be used for inner layers. As shown in FIG. 4A, the process can begin by forming a thin conductive metal layer (CU), typically copper, but may be aluminum gold or the like, to a clean and smooth surface of a metal transfer plate (RMP). Simple methods of forming a thin conductive metal layer include electroplating or electroless plating. Other metallization techniques such as chemical vapor deposition, sputtering, or the like can be used as alternative to plating.  
         [0015]    A layer of photoresist (PR) is applied on top of the thin metal layer as shown in FIG. 413, selectively exposure the photoresist according to a predetermined pattern as shown in FIG. 4C, so that portions of photoresist are cured as shown in FIG. 4D. Removing the uncured of photoresist so that the cured photoresist forms some trenches that define the pattern of circuit lines, which is shown in FIG. 4E. Forming a second conductive metal layer (CU) to the required thickness as shown in FIG. 4, then removing the photoresist completely leaving the circuit pattern (CU) on said transfer plate as shown in FIG. 4G. Then stacking by interposing semi-cured dielectric substrate or named prepreg (RESIN) between two transfer plates with circuits facing to the prepreg as illustrated in FIG. 4H. Transfer the circuit patterns to said dielectric substrate via pin lamination and then removing said transfer plates as shown in FIG. 41. Etching away the covering first conductive metal layers, leaving circuits inlaid into the dielectric substrate as shown in FIG. 4J. Drilling and though-hole plating selectively on portions of the board to interconnect circuit lines on both sides of said substrate. The board is then cleaned and a soldermask may be applied. The finishing steps, such as gold tab printing for edge connector or legend printing, may be applied to complete the printed circuit board.  
         [0016]    Alternatively, in the above pattern metallization process, a positive type photoresist can be applied whereby the portions exposed to radiation will be decomposed and removed. After exposure, the developing process removed the decomposed photoresist thereby forming trenches that then are deposited with conductive metal to form circuits.  
         [0017]    FIGS. SA- 5 I show second method of producing a printed circuit board with inlaid circuits according to the present invention, the method is particularly advantageous when used to make the outer circuit layers of multilayer circuit boards, but may also be used for inner layers. As shown in FIG. SA, the process can begin by forming a conductive metal layer (CU), typically copper, but may be aluminum, gold or the like, to a clean and smooth surface of a metal transfer plate (RMP). Simple methods of forming a thin conductive layer include electroplating or electroless plating. Other metallization techniques such as chemical vapor deposition, sputtering, or the like can be used as alternative to plating. Applying a layer of tin or eutectic tin alloy (SN) over said first metal layer as shown in FIG. SB. Selectively removing portions of tin layer by laser ablation to expose portion of first metal layer outside the predetermined pattern of circuit as shown in FIG. SC. Then removing the portions of metal layer that not covered by tin layer as shown in FIG. SD and removing remained tin layer chemically, leaving the circuit pattern on said transfer plate as shown in FIG. SE.  
         [0018]    Forming a thin layer of conductive metal (CU) on the surface of transfer plate and circuits as shown in FIG. SF. Then stacking by interposing semi-cured dielectric substrate or named prepreg between two transfer plates with circuits facing to the prepreg as shown in FIG. SG, then transfer the circuit patterns to said dielectric substrate via pin lamination as shown in FIG. SH. Removing said transfer plates and etching away the covering thin conductive metal layers, leaving circuits inlaid into the dielectric substrate as shown in FIG. SI. Drilling and though-hole plating selectively on portions of the board to interconnect circuit lines on both sides of said substrate. The board is then cleaned and a soldermask may be applied. The finishing steps, such as gold tab printing for edge connector or legend printing, may be applied to complete the printed circuit board.  
         [0019]    [0019]FIGS. 6A-6C illustrate a method of fabricating a multilayer of printed circuit board in accordance with the present invention. The two outmost circuit layers may be prepared according to above-mentioned first method or second method of the present invention, they are then stacked with pre-fabricated inner layers and layers of prepreg as shown in FIG. 6A. Transfer the circuit patterns to the dielectric substrate via pin lamination, then removing the transfer plates as shown in FIG. 6B. Etching away the covering thin conductive metal layers, leaving circuits inlaid into the dielectric substrate as shown in FIG. 6C. Drilling and though-hole plating selectively on portions of the board to interconnect circuit lines with each other. The board is then cleaned and a soldermask may be applied. The finishing steps, such as gold tab printing for edge connector or legend printing, may be applied to complete the printed circuit board. The material of the dielectric substrate can be freely selected to comply with various requirements of electrical, physical or thermal properties.  
         [0020]    In accordance with the embodiments described above, all types of printed circuit boards have inlaid outerlayer circuits can be produced according to the present invention. The circuit board is characterized in that at least one of outmost wiring circuits is inlaid into the substrate. The method of the present invention make possible more accurate production of circuit lines and, thus, the circuit designer does not have to compensate for the inaccuracy inherent in the formation of circuit line by etching. This means the resulting circuit lines can be finer and more compact. The method provided by the present invention is capable of forming circuit line width of 0.001-0.002 inches, whereas prior art subtractive etching methods are generally limited to forming circuits having line width of 0.003-0,004 inches. As noted above, a further advantage of the preferred embodiments is improved bonding of circuit lines.  
         [0021]    Those skilled in the art should appreciate that there are a large number of types of printed circuit boards that can be fabricated using the method provided herein. The above disclosure sets forth a number of embodiments of the present invention. Other arrangements or embodiments, not precisely set forth, could be practiced under the teaching of the present invention. It is recognized, however, that many changes and modifications may be carried out without departing from the spirit and the scope of the present invention described, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.  
       SUMMARY OF THE INVENTION  
       [0022]    In view of the foregoing background, the present invention aims to provide a circuit board that enhances the bonding strength of circuit traces to the dielectric substrate and eliminates the irregularities of board surface. None of the prior art references uncovered in the search show a method for fabricating PCB in accordance with the sequence of steps in the present invention and produces PCB features with inlaid outerlayer circuits. In particular, the present invention eliminates many of the shortcomings associated with conventional etching and pattern plating methods.  
         [0023]    In accordance with a first aspect of the present invention, a printed circuit board having outerlayer circuit lines inlaid into the dielectric substrate is provided. The printed circuit board is fabricated by forming a predetermined pattern of circuitry on a metal transfer plate, then transfer the circuitry to a dielectric substrate through lamination technique. A drilling and through-hole metallization may be applied to interconnect circuit lines of various metallic layers.  
         [0024]    In accordance with a second aspect of the present invention, a first method of fabricating PCB with inlaid outerlayer circuit is provided. The method includes forming a thin conductive metal layer, typically copper, on a smooth surface of a metal transfer plate. A photoresist may then be applied, imaged, and cured (cross-linked). The uncured photoresist is removed; thereby defining some exposed regions on the surface of conductive metal layer or “trenches” in which circuit lines are to be found. Selectively deposit metal in the trenches to the required thickness by known metallization technique. Finally, the cured photoresist is removed, and layer of prepreg (semi-cured dielectric substrate) is applied between two metal transfer plates with circuits facing to prepreg, a pin lamination process may be applied to inlay the circuits into the dielectric substrate. After removing the metal transfer plates and etching away the covering thin conductive metal layers, leaving printed circuit board with predetermined patterns of circuits. A drilling and through-hole metallization may be applied to interconnect circuit lines of various metallic layers.  
         [0025]    A second method of fabricating PCB with inlaid outerlayer circuit according to the present invention is provided. The method includes forming a conductive metal layer, typically copper, on a smooth surface of a metal transfer plate. A layer of tin or eutectic alloy of tin may then be applied over the conductive metal layer. Selectively remove portions of tin or tin alloy by laser ablation so that required circuit pattern are covered by tin or tin alloy, then removing the exposed conductive metal layer. Afterward, an etching process may then applied to remove the layer of tin or tin alloy. A thin layer of copper is formed to cover the surface of metal transfer plate and circuit pattern. Finally, layer of prepreg is applied between two metal transfer plates with circuits facing to prepreg, a pin lamination process may be applied to inlay the circuits into the dielectric substrate. After removing the metal transfer plates and etching away the covering thin conductive metal layers, leaving a printed circuit board with predetermined patterns of circuits. A drilling and through-hole metallization may be applied to interconnect circuit lines of various metallic layers.  
         [0026]    It will be understood by those skilled in the art that the copper and other conductive metals may be formed to the respective surfaces by any conventional method including, but not limited to electroplating, electrolytic deposition, electroless deposition, chemical vapor deposition, sputtering or combination of them.