Abstract:
A flexible circuit substrate includes a laminate which contains a polymer film, a VIB group metal layer sputtered on the polymer film, a nickel-chromium alloy layer sputtered on the VIB group metal layer, and a copper layer formed on the nickel-chromium alloy layer. The VIB group metal layer is selected from the group consisting of chromium, molybdenum, and tungsten. The VIB group metal layer is free of metal oxide. The VIB group metal layer and the nickel-chromium alloy layer are formed by a sputtering process using an oxygen-free inert atmosphere containing argon.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 10/022,267, filed on Dec. 20, 2001, and abandoned as of the filing date of this application. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to a flexible circuit substrate, more particularly to a flexible circuit substrate having an improved peel strength.  
           [0004]    2. Description of the Related Art  
           [0005]    Conventional flexible circuit substrates are essentially classified into two categories, i.e., one containing an adhesive layer and one free of an adhesive layer. The process for manufacturing the flexible circuit substrate containing the adhesive layer includes the steps of applying an adhesive layer onto a layer of polymer material, such as polyimide or polyester, and adhering a copper layer formed by calendering onto the adhesive layer so as to form a three-layer laminate. The adhesive layer has a thickness ranging from 12.5 μm to 25 μm, and is made of acrylate or epoxy resin. Since the flexible circuit substrate useful for supporting electronic elements thereon is usually processed at high temperatures, this type of flexible circuit substrate is susceptible to delaminating and bubbling.  
           [0006]    The process for manufacturing the flexible circuit substrate free of an adhesive layer includes the steps of directly applying a liquid polymer layer onto a copper layer formed by calendering, and drying the liquid polymer layer to form a polymer film on the copper layer. However, the adhesion between the polymer film and the copper layer is relatively poor.  
           [0007]    Moreover, since the calendering thickness of the copper layer for calendering in the aforesaid flexible circuit substrates is limited, it is difficult to produce a compact circuit by etching the flexible circuit substrate. Therefore, a manufacturing process including sputtering and plating techniques has been developed heretofore to produce a flexible circuit substrate. Referring to FIG. 1, the manufacturing process includes the steps of sputtering a chromium layer  12  on a polymer layer  11 , sputtering a first copper layer  13  on the chromium layer  12 , and plating a second copper layer  14  on the first copper layer  13 . The thickness of the chromium layer  12  and the first copper layer  13  can be smaller than 1 μm. The polymer layer  11  is made of polyimide, polyester or other plastics resistant to high temperature. The typical thickness of the product manufactured from this process is about 25-50 μm.  
           [0008]    In the laminate of the flexible circuit substrate shown in FIG. 1, the chromium layer  12  is interposed between the polymer layer  11  and the first copper layer  13 . Since the adhesion between the chromium layer  12  and the polymer layer  11  is superior to that between the first copper layer  13  and the polymer layer  11 , and since the adhesion between the chromium layer  12  and the first copper layer  13  is superior to that between the first copper layer  13  and the polymer layer  11 , the polymer layer  11 , the chromium layer  12 , the first copper layer  13 , and the second copper layer  14  are laminated in sequence to form the laminate shown in FIG. 1. In addition to improvements in the characteristics, such as thermal resistance, light weight and circuit density, the overall peel strength of the flexible circuit substrate shown in FIG. 1 is also increased by strengthening the bonding between the adjacent layers in the laminate of the flexible circuit substrate. The 90° peel strength of the flexible circuit substrate of FIG. 1 is about 0.5 kgf/cm.  
           [0009]    Additionally, the same effects can be achieved by substituting the chromium layer  12  with a nickel alloy (such as nickel-chromium alloy) layer.  
           [0010]    Furthermore, U.S. Pat. No. 4,917,963 discloses an assembly comprising a substrate (such as, polymer), a graded composition primer layer, and a conductor. The graded composition primer layer includes a first metal and a second metal different from the first metal, and having a composition continuously varying from a predominance of the first metal at the surface facing away from the substrate to a predominance of the second metal at the surface bonded to the substrate. The conductor is made of the first metal. This patent merely suggests that the primer layer intermediate the substrate and the conductor be a single layer which continuously varies in composition between two opposite surfaces thereof.  
           [0011]    U.S. Pat. No. 6,060,175 discloses a metal-film laminate, which comprises (a) a polyimide film having at least one surface bearing a non-continuous random distribution of metal-oxide, and (b) a metal surface including a first metal layer having a chromium metal composition, a second metal layer formed on the first metal layer, and a third metal layer formed on the second layer. This patent further discloses that the metal oxide layer may be produced via a film treatment using a chromium electrode and an oxygen plasma. The first metal layer may include an alloy of nickel and chromium, and the second and third layers may be copper. While the metal-film laminate disclosed in this patent can exhibit a high peel strength greater than 3 lbs/in, the laminate requires a sputtered metal oxide layer which must be produced by a reactive sputtering process using an oxygen plasma. The reactive sputtering process requires a high level of skill for control of the process, thereby complicating the manufacturing of the metal-film laminate.  
         SUMMARY OF THE INVENTION  
         [0012]    Therefore, the object of the present invention is to provide a flexible circuit substrate made of a metal-film laminate which has a high peel strength, and which can be produced without using a sputtered metal oxide layer.  
           [0013]    A flexible circuit substrate according to this invention includes a laminate which contains a polymer film, a VIB group metal layer sputtered on the polymer film, a nickel-chromium alloy layer sputtered on the VIB group metal layer, and a copper layer formed on the nickel-chromium alloy layer. The VIB group metal layer is selected from the group consisting of chromium, molybdenum, and tungsten. The VIB group metal layer is free of metal oxide. The VIB group metal layer and the nickel-chromium alloy layer are formed by a sputtering process using an oxygen-free inert atmosphere containing argon.  
           [0014]    The inventors of the present invention discovered that, when a laminate including an oxide-free VIB group metal layer and a layer of nickel-chromium alloy are formed between a polymer film substrate and a conductor layer, the laminate can exhibit a high peel strength of at least 1.0 kgf/cm. Since the oxide-free metal layer can be formed by a sputtering process using an inert atmosphere which can be carried out more easily than the oxygen plasma sputtering process, the manufacturing of the laminate according to the present invention can be facilitated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments with reference to the accompanying drawings, of which:  
         [0016]    [0016]FIG. 1 is a sectional view of a conventional flexible circuit substrate;  
         [0017]    [0017]FIG. 2 is a sectional view of the first preferred embodiment of the flexible circuit substrate according to this invention;  
         [0018]    [0018]FIG. 3 is a flow diagram of a process for manufacturing the preferred embodiment of FIG. 2;  
         [0019]    [0019]FIG. 4 is a sectional view of the third preferred embodiment of the flexible circuit substrate according to this invention;  
         [0020]    [0020]FIG. 5 is a flow diagram of a process for manufacturing the preferred embodiment of FIG. 4;  
         [0021]    [0021]FIG. 6 is a sectional view of the fourth preferred embodiment of the flexible circuit substrate according to this invention; and  
         [0022]    [0022]FIG. 7 is a flow diagram of a process for manufacturing the preferred embodiment of FIG. 6.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    Referring to FIG. 2, the first preferred embodiment of the flexible circuit substrate according to this invention is shown to include a polyimide film  21 , a chromium layer  22  sputtered on the polyimide film  21 , a nickel-chromium alloy layer  23  sputtered on the chromium layer  22 , a first copper layer  24  sputtered on the nickel-chromium alloy layer  23 , and a second copper layer 25 plated on the first copper layer  24 .  
         [0024]    Referring to FIG. 3 and Table 1, the process for manufacturing the first preferred embodiment of FIG. 2 includes the following steps:  
         [0025]    (1) The chromium layer  22  is sputtered on the polyimide film  21 . The polyimide film  21  is a square film that is 10 cm×10 cm in size. The chromium material for the sputtering step is a square material that is 20 cm×20 cm in size. The sputtering is performed for 1 minute in an argon atmosphere. The electric current applied during the sputtering step is 1.0 A. The pressure of the sputtering gas is 5×10 −3  torr. The flow of the sputtering gas is 2×10 −3  torr. The distance between the polyimide film  21  and the chromium material for the sputtering step is 20 cm. The thus-formed chromium layer  22  has a thickness smaller than 1 μm.  
         [0026]    (2) The nickel-chromium alloy layer  23  is sputtered on the chromium layer  22  in an argon atmosphere. The operating conditions for performing the sputtering process are identical to those used in the former step except that the material for the sputtering process is nickel-chromium alloy. The thus-formed nickel-chromium layer  23  has a thickness smaller than 1 μm.  
         [0027]    (3) The first copper layer  24  is sputtered on the nickel-chromium alloy layer  23  in an argon atmosphere. The material used for this sputtering process is copper. The thus-formed copper layer  24  has a thickness smaller than 1 μm.  
         [0028]    (4) The second copper layer  25  is plated on the first copper layer  24 . The material used for this plating step is copper. The plating step is performed for 1 hour with an electric current of 0.5 A. The thus-formed copper layer  25  has a thickness of about 18 μm.  
         [0029]    The expansion coefficients of chromium, nickel and copper are 6.5×10 −6 , 13.3×10 −6 , and 17.0×10 −6 , respectively. The expansion coefficient of the nickel-chromium alloy should be between 6.5×10 −6  and 13.3×10 −6 . In the laminate of the first preferred embodiment shown in FIG. 2, it is therefore apparent that the nickel-chromium alloy layer  23  has an expansion coefficient greater than that of the chromium layer  22  and smaller than that of the first copper layer  24 . The expansion coefficient gradients between adjacent layers of the laminate of this preferred embodiment are reduced as compared to those of the conventional flexible circuit substrate shown in FIG. 1. Therefore, the 90° peel strength of the laminate of this preferred embodiment is improved. According to a test conducted for the 90° peel strength, the 90° peel strength of the conventional flexible circuit substrate shown in FIG. 1 is 0.5 kgf/cm, whereas the 90° peel strength of the first preferred embodiment shown in FIG. 2 is greater than 1.0 kgf/cm, which is higher than that of the prior art.  
         [0030]    Referring to Table 1, in the second preferred embodiment, the operating conditions are identical to those used in the first preferred embodiment, except that the polyimide film is substituted with a polyester, such as PET. The thus-formed flexible circuit substrate also has a 90° peel strength of at least 1.0 kgf/cm.  
         [0031]    Referring to FIG. 4, the third preferred embodiment of the flexible circuit substrate according to this invention is shown to include a polyimide film  31 , a chromium layer  32  sputtered on the polyimide film  31 , a nickel-chromium alloy layer  33  sputtered on the chromium layer  32 , a nickel layer  34  sputtered on the nickel-chromium alloy layer  33 , a first copper layer  35  sputtered on the nickel layer  34 , and a second copper layer  36  plated on the first copper layer  35 .  
         [0032]    Referring to FIG. 5, the process for manufacturing the third preferred embodiment includes the steps of sputtering the chromium layer  32 , the nickel-chromium alloy layer  33 , the nickel layer  34 , and the first copper layer  35  in sequence, and the step of plating the second copper layer  36 . The relevant operating conditions for the sputtering process are identical to those applied in the manufacture of the first preferred embodiment, and the relevant operating conditions for the plating step are identical to those of the plating step in the manufacture of the first preferred embodiment.  
         [0033]    Since the expansion coefficient of nickel is between those of nickel-chromium alloy and copper, the expansion coefficient gradient between the adjacent layers of this preferred embodiment can be further reduced. According to the 90° peel strength test, the 90° peel strength of this preferred embodiment is greater than 1.0 kgf/cm.  
         [0034]    Referring to FIG. 6, the fourth preferred embodiment of the flexible circuit substrate according to this invention is shown to include a polyimide film  41 , a molybdenum layer  42  sputtered on the polyimide film  41 , a nickel-chromium alloy layer  43  sputtered on the molybdenum layer  42 , a gold layer  44  sputtered on the nickel-chromium alloy layer  43 , a first copper layer  45  sputtered on the gold layer  44 , and a second copper layer  46  plated on the first copper layer  45 .  
         [0035]    Referring to FIG. 7, in the process for manufacturing the fourth preferred embodiment, the process of FIG. 5 is repeated except that molybdenum material for the molybdenum layer  42  and gold material for the gold layer  44  (see FIG. 6) are substituted for chromium material for the chromium layer  32  and nickel material for the nickel layer  34  (See FIG. 4), respectively. According to the 90° peel strength test, the 90° peel strength of the fourth embodiment is also greater than 1.0 kgf/cm.  
         [0036]    Referring to Table 1, the fifth preferred embodiment of the flexible circuit substrate of this invention is identical to the first preferred embodiment, except that the sputtering process is performed for 2 minutes with an electric current of 0.5 A. According to the 90° peel strength test, the 90° peel strength of the fifth preferred embodiment is also greater than 1.0 kgf/cm.  
         [0037]    The sixth preferred embodiment of the flexible circuit substrate of this invention is identical to the first preferred embodiment, except that the pressure of sputtering gas is 5×10 −4  torr and that the flow of sputtering gas is 8×10 −5  torr. According to the 90° peel strength test, the 90° peel strength of the sixth preferred embodiment is also greater than 1.0 kgf/cm.  
         [0038]    The seventh preferred embodiment of the flexible circuit substrate of this invention is identical to the first preferred embodiment, except that the plating is performed for 0.5 hour with an electric current of 1.0 A. According to the 90° peel strength test, the 90 peel strength of this embodiment is also greater than 1.0 kgf/cm.  
         [0039]    Moreover, when tungsten, which has an expansion coefficient of 4.5×10 −6 , is used for the VIB group metal of the flexible circuit substrate of this invention, the 90° peel strength of the flexible circuit substrate is also at least 1.0 kgf/cm.  
         [0040]    While the present invention has been described in connection with what is considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.  
                                                                                                                                                                       TABLE 1                       Ex.   1   2   3   4   5   6   7                                Size of polymer   10 cm × 10 cm       film       Size of   20 cm × 20 cm       sputtering/       plating       material       distance   20 cm            Polymer   PI   PE   PI   PI   PI   PI   PI            Sputtering in an argon atmosphere            #1   Cr   Cr   Cr   Mo   Cr   Cr   Cr       (time)   (1 min)   (1 min)   (1 min)   (1 min)   (2 min)   (1 min)   (1 min)       #2   Ni—Cr   Ni—Cr   Ni—Cr   Ni—Cr   Ni—Cr   Ni—Cr   Ni—Cr       (time)   (1 min)   (1 min)   (1 min)   (1 min)   (2 min)   (1 min)   (1 min)       #3   Cu   Cu   Ni   Au   Cu   Cu   Cu       (time)   (1 min)   (1 min)   (1 min)   (1 min)   (2 min)   (1 min)   (1 min)       #4   —   —   Cu   Cu   —   —   —       (time)           (1 min)   (1 min)       Current   1.0   1.0   1.0   1.0   0.5   1.0   1.0       (A)       Pressure   5 × 10 −3     5 × 10 −3     5 × 10 −3     5 × 10 −3     5 × 10 −3     5 × 10 −4     5 × 10 −3         (torr)            Plating            material   Copper       thickness   18 μm            Current   0.5 A   1.0 A       (Time)   (1 hr)   (0.5 hr)            90° Peel   &gt;1.0 kgf/cm       strength