Abstract:
A method for forming sets of tri-metal material involving the use of cladding mills. When multiple sets of tri-metal material are formed, the outside surfaces of each set is prepared by oxidation to prevent each set from adhering to the set above or below. An alternative to oxidation is to provide a removable layer on the outside surface of the tri-metal material. Alternatively bonding materials may be used on the intermediate surfaces; such bonding materials can be selected from a group consisting of tin, nickel, titanium, chromium, silver and zinc.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to circuit boards and more particularly to a material construction for use in Etched-Tri-Metal, “ETM”, multi-layer circuit boards. 
     2. Description of the Related Art 
     In the field of electronics manufacturing, various additive and subtractive processes that are known for constructing printed circuit boards (PCBs). Among these are the processes disclosed in U.S. Pat. No. 3,801,388 issued Apr. 2, 1974 to Akiyama et al. which is a subtractive process. U.S. Pat. No. 4,404,059 issued Sep. 13, 1983 to Livshits et al is an additive process. U.S. Pat. No. 5,738,797 issued Apr. 14, 1998 to Belke, Jr. et al is assigned to a common assignee. All three of which are incorporated herein by reference. These patents describe various additive and subtractive plating and chemical etching processes for constructing multi-layer PCBs having air bridges. 
     SUMMARY OF THE INVENTION 
     A method for cladding material for etched-tri-metal circuits to maintain the dimensional tolerance and reducing the stresses in the cladded material having the steps of first forming a stacked set of tri-metal material having three layers of metal. The central layer is one material, which is aluminum, and the top and bottom layers are a second material, which are copper. 
     Then each set of the tri-metal material have the outside surfaces of the top and bottom layers oxidized to prevent the sets from adhering together when they are stacked. Prior to being stacked, each set is fed through a cladding mill having a predetermined pressure and temperature. After exiting the cladding mill, the individual sets of the tri-metal material are stacked with the oxidized surfaces abutting each other. 
     Then the stacked sets are processed through a rolling mill at a predetermined pressure and temperature to reduce the thickness of the multiple set of tri-metal cladded material. After finishing rolling, the multiple set of tri-metal cladded material is separated into the individual sets of tri-metal material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic of cladding multiple sets of tri-metals; 
     FIG. 2 is a schematic of cladding a tri-metal using removable layers; 
     FIG. 3 is a schematic of cladding a tri-metal using a thin coating on each side of the middle layer; 
     FIG. 4 is an expansive section view showing the washboard effect of tri-metals not processed according to this invention; and 
     FIG. 5 is an enlarged section of a preferred embodiment of a tri-metal sheet with the metals identified. 
    
    
     DETAILED DESCRIPTION 
     Cladded copper-aluminum-copper (Cu/Al/Cu) sheet or coil is used as a starting material for the subtractive process in Etched Trilayer metal, ETM, manufacturing. 
     The process disclosed in U.S. Pat. No. 3,801,388 issued Apr. 2, 1974 to Akiyama et al. is a subtractive process. Uniformity in thickness of each layer and good adhesion between the layers is important for the manufacture of high quality and reliable ETM substrates. Currently available Cu/Al/Cu clad materials have a large variation, about 50%, in the thickness of the individual Cu and Al layers. This is illustrated in FIG.  4 . In the manufacture of PCBs, an acceptable variation in individual layers thickness is a maximum of 10%. 
     To achieve this goal, uniform pressure on the clad material during the cladding process and control of the Cu—Al intermetallics growth is important. This invention provides new methods to manufacture high quality Cu/Al/Cu clad material sheets. These methods provide uniform pressure and at the same time control intermetallic growth during the cladding process. 
     Referring to the FIG.  1 . there is illustrated a schematic of cladding multiple sets  10 ,  12 ,  14  of tri-metals in one or more cladding rolls  16 ,  18 ,  20  and then combining all of the sets in a rolling mill  22 . The first set of tri-metal  10  is illustrated as being driven through a first pair of cladding rollers  16 . The three sheets, a pair of outside or second metal layers  24  and a central or first metal layer  26  of the metals, are shown enlarged for the purposes of illustration. Aluminum is the central layer  26  and a pair of sheets or outside layer  24  of copper is cladded to both broadside surfaces of the aluminum. 
     As illustrated, there are three pairs of cladding rollers  16 ,  18 ,  20 , each pair forming a cladding mill, as illustrated in FIG.  1 . It is understood that the number of pairs of cladding rollers comprising a cladding mill is at the option of the fabricator. The cladding process applies a heavy pressure to the tri-metal  10  and causes a bonding of the three metals. Each of the sheets  24 ,  26  of metal is smooth prior to being fed into the cladding rollers  16 ,  18 ,  20 . The heavy pressure of the cladding rollers not only reduces the thickness of the tri-metal set, but also introduces more waviness into the middle layer of the tri-metal. It is for this reason that the cladding rollers are of a very large diameter so as to have a large surface contact with the tri-metal set thereby reducing the waviness. 
     The top and bottom surface  24  of the tri-metal set as previously indicated is copper. The outside layers of the copper are oxidized to prevent bonding between the sets  10 ,  12 ,  14  in the rolling mill  22  thereby allowing an increase in the throughput of the cladding mills. By so doing, the initial material stock is closer to the desired finish tolerances of the ETM sheet construction. The stresses in the material that are normally introduced by the cladding process are reduced. After the tri-metal sets  10 ,  12 ,  14  are removed from the rolling mill  22 , the sets are separated and the copper oxide is removed. 
     Thus the method of FIG. 1 comprises the steps of first forming a stacked set of tri-metal material  10 ,  12 ,  14  having three layers of metal wherein the central layer  26  is one material and the top and bottom layers  24  are a second material. 
     The outside surfaces of the top and bottom layers  24  of each set of tri-metal material  10 ,  12 ,  14  are oxidized. In the preferred embodiment, the oxidizing layer is a copper oxidizing layer that will not adhere to the outside layer of another individual set of tri-metal materials. 
     Next each individual set of the tri-metal material is fed to a cladding mill  16 ,  18 ,  20  or roller to form a tri-metal cladded material. As each set of the tri-metal material leaves the cladding mill, they are positioned in an overlying manner on another set so that the oxidizing surfaces are adjacent to each other to create a multiple set of tri-metal cladded material sets. 
     The multiple sets are then supplied to a rolling mill  22  at a predetermined pressure and temperature for reducing the thickness of the multiple set of tri-metal cladded material. 
     After the multiple sets leave the rolling mill each individual set  10 ,  12 ,  14  of tri-metal cladded material is then separated from each other. 
     Referring to FIG. 2, there is illustrated another embodiment of forming a set of tri-metal cladded material. As in the previous method, the individual sheets  24 ,  26  are supplied via rollers  28  to the clad mill  30 . However, on the outside surface of both the top and bottom layer  24  is a removable layer  32 . Again the tri-metal material  10  is formed of copper on the outside and aluminum in the middle. 
     The method illustrated in FIG. 2 for cladding material for etched-tri-metal circuits to maintain the dimensional tolerance and reducing the stresses in the cladded material. The method has the steps of individually rolling three layers  24 ,  26 ,  24  of metal wherein a central layer  26  is one material, such as aluminum, and the top and bottom layers  24  are a second material, such as copper, forming a tri-metal material  10 . 
     On the outside surface of the top and bottom layer  24 , a removable layer  32 , such as tungsten, is added. The tri-metal material  10  with the two removable layers  32  forms a set of five-layer material  34  with the removable layers on the outside. The set of five-layer material  34  is supplied to a cladding mill  30  to clad the material at a predetermined pressure and temperature for cladding of the tri-metal material  10 . 
     After the cladding, the set of five-layer material  34  is rolled at a predetermined pressure and temperature to a predetermined thickness and then the removable layers  32  are removed resulting in a tri-metal cladded material  10 . 
     Another embodiment is illustrated in FIG.  3 . In this embodiment, the outer broadside surfaces of the central material  26  are coated with a thin layer  36  of a bonding metal such as silver or tin or an aluminum alloy before the material is supplied to the cladding mill  38 . After applying the bonding layer  36 , the outer layers  24  are placed on the bonding layer  36  for bonding to the central layer  26 . In the alternative, the inside surfaces of the outer layers  24  may have the bonding material layer applied thereto. The tri-metal material set is then supplied to the cladding mill  38  to complete the bonding and to reduce the thickness of the layers. 
     This bonding material reduces or eliminates a brittle Cu/Al intermetallic formation. Thick Cu/Al intermetallics at the interface make the copper and aluminum layers debond after cladding. The thickness of the bonding layer  36  is on the order of microns and typically is less than three microns. 
     Other bonding metal layers that can be used to enhance the copper-aluminum bonding and to prevent or reduce intermetallic formation include tin, nickel, titanium, chromium, aluminum and zinc and any of their alloys. Any of these metals, including silver, can be deposited by any of the standard deposition techniques such as plating, cladding, sputtering, etc. These bonding metals amalgamate into the aluminum and are not recoverable. 
     As previously indicated, FIG. 4 illustrates the washboard effect with unacceptable variations in thickness of the copper and aluminum currently available. 
     As an example, the waviness of the outside surface  40  may be 0.0015 inches (0.0381 mm), the thickness of the copper layer  26  between 0.002 inches (0.580 mm) and 0.0035 inches (0.889 mm) and the peak to peak dimension  42  may be 0.100 inches (25.4 mm). An example of the material used in the method of FIGS. 1-3 is 3003 aluminum and 110 copper. 
     Referring to FIG. 5 there is illustrated an etched-tri-metal sheet  44  having a central first metal layer  26  of aluminum having two broadside surfaces  46 ,  48 . A pair of second metal layers  24  of copper each having two broadside surfaces  50 - 53  that are substantially identical in shape to the broadside surfaces  46 ,  48  of the first metal layer  26  overlie each of the broadside surfaces of the central layer forming a sandwich. A bonding material is interposed the broadside surfaces  46 ,  51  and  48 ,  52 , of the first  26  and second  24  metal layers. 
     In one embodiment, the bonding material is coated on the broadside surfaces of the central metal layer  26 . In another embodiment a removable layer  32  is positioned on the top  50  and bottom  53  outside broadside surface of the second metal layers  24 . This removable layer  32  is fabricated from tin, tungsten forming a five-layer sheet  34 . 
     The bonding material is selected from a group consisting of tin, nickel, titanium, chromium, silver and zinc. In addition the outside broadside surfaces  50 ,  53  of the copper is covered with a copper oxide for preventing stacked tri-metal sheet sets  10 ,  12 ,  14  from adhering to each other. 
     There has thus been shown and described a method for processing cladding material for etched-tri-metal circuits to maintain the dimensional tolerance and reducing the stresses in the cladded material and the resulting article of manufacture.