Abstract:
In the case of using a photosensitive insulating resin for a surface protective layer of a circuit wiring pattern, or for an insulating layer between circuit wiring conductor layers in a circuit board, Na ions adsorbed on the photosensitive insulating resin are replaced with a polyvalent metal through a treatment step containing Na ions which is executed after a heat curing step of the photosensitive insulating resin. For the polyvalent metal, a II group including Mg or Ca can be selected.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the national phase of International Application No. PCT/JP2004/009667 which claims the benefit of Japanese Patent Application No. 2003-195114, filed Jul. 10, 2003, the contents of each of which are incorporated herein by reference in their entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to a circuit board and its manufacturing method, and more particularly to a circuit board having a surface protective layer and/or an interlayer insulating layer and its manufacturing method. 
       BACKGROUND OF THE INVENTION 
       [0003]    Miniaturization of the mounted pad portion of circuit boards, for example flexible circuit boards, by miniaturizing mounted components and/or by the high mounting density of components has been a conventional goal of the circuit board industry. These miniaturization efforts have encountered difficulties in the area of, for example, miniaturization by a method of punching and sticking a cover film on the surface of circuit board wiring pattern. Cover film has conventionally been used as a surface protective layer on the surface of the wiring patterns. This cover film is not compatible with the use of small sized components or in conjunction with high mounting density. 
         [0004]    An attempt to address the difficulties associated with the cover film has been through the use of a photosensitive insulating resin. This photosensitive insulating resin is applied to a board and is typically subjected to exposure, development, water-washing, and curing steps in order to form a surface protective layer. The use of photosensitive insulating resin has typically resulted in achieving fine opening shapes and complex opening shapes in comparison to methods utilizing the punching and sticking of cover film. 
         [0005]    In addition, because of the advantages associated with photosensitive insulating resin, the conventional manufacture of multi-layered circuit boards has made increasing use of photosensitive insulating resin as an interlayer insulating layer. 
         [0006]    However, the associated reduction in the weight and thickness of the circuit boards, due to miniaturization, has been accompanied by the problem of substrate bending due to the mismatching of material types in the mainly epoxy or acrylic photosolder resist. This is because the photosolder resist conventionally becomes a hard film, which is easily cracked in the handling of a substrate during the manufacturing process. To avoid these problems, a photosolder resin having a bulky molecular structure and a polymer component such as urethane has been used to form a flexible film. 
         [0007]    Conventionally the manufacture of circuit boards having a photosensitive insulating resin surface protective layer and/or a photosensitive insulating resin interlayer comprises forming an insulating film through the steps of coating or laminating a photosensitive insulating resin film on an insulating material base material on which a wiring pattern is present; an exposing step; a developing step; a UV curing; and a heat curing step. Subsequently, a surface treatment, such as plating, may be executed on a mounted pad portion of the circuit board on which the insulating film has been formed. 
         [0008]    The conventional development step includes applying an alkali developing solution to the photosensitive insulating resin and then washing the photosensitive insulating resin with water. The developing solution, used in the developing step, is an aqueous solution containing sodium carbonate. The water-washing step, used after the development step, has conventionally been executed by washing with a combination of tap water and purified water. 
         [0009]    During the conventional water-washing step Na ions of the sodium carbonate are combined with a carboxyl group present in the insulating resin film in the developing step. The Na ions, among other things, accelerate water absorption into the insulating film during electric conduction tests under high temperature and high humidity. The Na ions then work as an electrolyte thereby reducing insulation performance of the insulating film. 
         [0010]    In addition, short-circuiting of the wiring pattern occurs because of dendrite formation. Dendrite formation is caused by the migration and precipitation of ionized copper from an anode side to a cathode side of portions of the board. Furthermore, electrical characteristics and mechanical physical properties may be reduced because the Na ions hydrolyze the molecular chain of the resin under the high temperature and high humidity conditions. This hydrolization of the resin results in a deterioration of the film quality. 
         [0011]    Conventionally, Na ions have been removed from the insulating film by washing a circuit board with water containing Ca ions or Mg ions immediately after treatment with the alkali development solution and prior to curing. For example, Japanese Patent Applications Laid-Open No. 2000-208904, 2002-162739, or 2002-305368 disclose treatment immediately after alkali development and prior to curing. 
         [0012]    However, during subsequent manufacture of the circuit boards, surface treatments, for example plating, are often carried out following the formation and curing of the insulating resin film. Such treatments typically include an alkali treatment which results in the addition of Na ions to the surfaces. These surface treatments add Na ions by first causing a removal of the Ca ions and/or the Mg ions which had previously been in the water-washing step and second by replacing the removed ions with Na ions present in the surface treatment. For example, Japanese Patent Applications Laid-Open No. 2000-208904, 2002-162739, or 2002-305368, identify problems caused by the replacement of Ca and/or Mg ions with Na ions as described above. 
       SUMMARY OF THE INVENTION 
       [0013]    Briefly stated, in a preferred form of the invention, a circuit board includes a heat cured photosensitive insulating resin layer. The heat cured layer has a polyvalent metal adsorbed to the heat cured layer at a substantial majority of the sodium adsorption locations. The photosensitive insulating resin is included as a surface protective layer for a circuit wiring pattern and/or as an insulating layer between circuit wiring conductor layers of a multi-layer circuit board. The photosensitive insulating resin, after curing, has adsorption sites for Na ions which are adsorbed to at least one species of polyvalent metal. 
         [0014]    The invention also encompasses a method of manufacturing a circuit board which uses a photosensitive insulating resin for a surface protective layer of a circuit wiring pattern and/or an insulating layer between circuit wiring conductor layers of a multilayer circuit board, and includes replacing the adsorbed Na ions on the photosensitive insulating resin with at least one polyvalent metal through a treatment step. The replacement of the Na ions is executed after a heat curing step of the photosensitive insulating resin. 
         [0015]    An object of the invention is to alleviate the problem associated with a reduction in environmental resistance caused by water absorption and/or hydrolysis of the insulating resin film, and also to rectify the fragility of the insulating resin film caused by hydrolysis of the resin. 
         [0016]    Another object of the invention is to reduce or eliminate the need to use high cost special devices or methods for circuit board manufacture, and to maintain stable developing conditions in order to provide a circuit board which is stable and inexpensive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Other objects and advantages of the invention will be evident to one of ordinary skill in the art from the following detailed description made with reference to the accompanying drawings in which: 
           [0018]      FIG. 1  shows a portion of a multi-layer circuit board having a cured photosensitive resin interlayer insulating film consistent with the present invention; 
           [0019]      FIG. 2  shows a portion of a circuit board having a cured photosensitive resin protective surface film consistent with the present invention; and 
           [0020]      FIG. 3  is a flow chart depicting the steps of creating and treating a cured photosensitive resin protective surface film or interlayer insulating film consistent with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    With reference to the drawings wherein like numerals represent like components throughout the figures, a circuit board in accordance with the present invention is designated by the numeral  10 . As shown in  FIG. 2 , the circuit board  10  includes a cured photosensitive layer  12  disposed on the surface of an insulating member  14 . The cured photosensitive layer  12  forms a surface protective layer over the insulating member  14  and over a wiring pattern  16 . The cured photosensitive layer has a plurality of sodium ion (Na) adsorption sites. The Na adsorption sites include carboxyl groups. A substantial majority of the Na adsorption sites are adsorbed to at least one species of polyvalent metal ion. For example, in one embodiment of the invention the polyvalent metal ion is a group II element. Group II elements have two electrons in their outer electronic shell. For example, group II elements include beryllium, magnesium, calcium, strontium, barium and radium. 
         [0022]    In one embodiment of the invention, the photosensitive insulating layer is an interlayer insulating layer  12   a , as shown in  FIG. 1 . The interlayer insulating layer  12   a  includes a plurality of Na ion adsorption sites which are substantially adsorbed to at least one, polyvalent metal ion. The interlayer insulating layer  12   a  is positioned between a first insulating member  18  and a second insulating member  20  to form a multi-layer circuit board  10   a.    
         [0023]    The present invention also encompasses the method of manufacture for a circuit board  10  of the type shown in  FIG. 1 . In one embodiment of the invention, a wiring pattern  16  is formed on a surface of an insulating member  14 . A surface protective layer  12  is then formed on the insulating member  14  and over a surface of the wiring pattern  16 . The surface protective layer  12  is formed by first applying, as shown in  FIG. 3 , a photosensitive insulating layer to the insulating member  22 . For example, a photosensitive resin may be coated onto the insulating member or a photosensitive resin material may be laminated to the insulating member. The photosensitive layer is then exposed  24 . 
         [0024]    After the exposure step  24 , a developing step  26  is carried out. The developing step  26  may include applying an alkali developing solution to the coated insulating member. A UV curing step  28  is then carried out, wherein the photosensitive resin is cured with ultraviolet light. A heat curing step  30  is then carried out to form a cured photosensitive insulating layer. After formation of an insulating layer, the circuit board may be subjected to a surface treatment step  31 , for example, the circuit board may be plated. 
         [0025]    In one embodiment of the present invention, an ion replacement step  32  is carried out. For example, an aqueous ion replacement solution containing Mg ions and/or Ca ions is applied to the circuit board. The aqueous ion replacement solution contains a concentration of Ca and/or Mg ions in the range of about 0.1% by weight to about 10% by weight. Preferably about 0.5 wt % to about 5 wt % of Mg ions and/or Ca ions is present in the ion replacement solution. It should be understood that the ion replacement solution may be used to treat the insulating film after either the heat curing treatment  30  of either the insulating resin film or after the surface treatment  31  of the circuit board. During the ion replacement step  32 , Na ions that have adsorbed to the photosensitive insulating resin during the development step  26  and/or the surface treatment step  31  are replaced with Mg and/or Ca ions from the aqueous ion replacement solution. Thus, the Na ions which had combined with a carboxyl group of the Na ions adsorption site during the circuit board manufacturing process are replaced with Mg ions and/or Ca ions. Replacement of the Na ions with the Ca and/or Mg ions, among other things, suppresses water absorption of the photosensitive resin film and hydrolysis of the resin. This allows for an inexpensive and stable flexible circuit board to be produced which has sufficient environmental resistance as provided by a surface protective and/or interlayer insulating layer. 
         [0026]    In one embodiment of the invention the photosensitive layer is a negative type ultraviolet cured resin having a film-like photosensitive insulating resin similar in shape to a dry film type resist. 
         [0027]    In one embodiment of the invention, the replacement of Na ions with the Mg ions or the Ca ions is carried out after the heat curing step  30  and/or the surface treatment step  31 , and/or in conjunction with, a method such as disclosed in Japanese Patent Application Laid-Open No. 2000-208904, 2002-162739, or 2002-305368, used to wash the circuit board with a solution containing Ca ions or Mg ions immediately after alkali development  26 . 
         [0028]    Various experimental studies have been made with regard to the advantageous concentration of Mg ions or Ca ions in the aqueous ion replacement solution. Experimental results show that a range of 0.1 wt % to 10 wt %, and advantageously a range of 0.5 wt % to 5 wt % of Mg ions or Ca ions advantageously enhance insulating performance; flexibility of the insulating resin film under a high temperature and high humidity; and allow creation of an inexpensive circuit board. 
         [0029]    In one experimental test, a circuit board, after a surface treatment that included electrolytic gold plating, was treated with an aqueous ion replacement solution containing 1 wt % of Mg ions. The experimental result after a 1000 hour electrical conduction test showed no reduction in the insulating resistance value of the insulating film as a surface protective layer. There was also no short-circuiting of the wiring pattern in environmental conditions which included a temperature of 85° C. and a relative humidity of 85%. Furthermore, after the plated circuit board was treated with the aqueous ion replacement solution having 1 wt % of Mg ions, a portion of isolated insulating resin film exhibited greater elongation when compared with untreated insulating resin film having no Mg or Ca replacement. The isolated insulating resin film also had a greater comparative flexibility. 
         [0030]    While preferred embodiments of the foregoing invention have been set forth for the purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present invention.