Abstract:
A tri-metallic material for use in the manufacture of printed circuit boards is described, and the process for its manufacture is described. The tri-metallic material is a sandwich wherein a copper layer is essential the “bread” of the sandwich and an aluminum layer is the filling between both slices of bread. A metallic bonding and/or barrier layer is spread on the aluminum and is selected for its highly non-corrosive properties as well as its bonding, and diffusion inhibiting capabilities.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to circuit boards and more particularly to a tri-metallic material construction for use in Etched-Tri-Metal, “ETM”, multi-layer circuit boards. 
     DESCRIPTION OF THE RELATED ART 
     In the field of electronics manufacturing, various additive and subtractive processes 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. 
     Currently tri-metallic materials as used in the manufacture of PCBs are a sandwich of aluminum as the middle layer and copper as the top and bottom layers. The layers are typically bonded together with a zinc bonding layer. Due to the anodic nature of zinc relative to copper and its position in the Electromotive Force Series, EMF series, the zinc layer is prone to corrosion. 
     SUMMARY OF THE INVENTION 
     It is a principal advantage of the tri-metallic material of the present invention to provide a bonding layer between the middle or first layer and each of the top and bottom or second and third layers that is substantially resistant to corrosion. 
     It is still another advantage to provide a bonding layer that has excellent adhesion properties between the middle or first layer and each of the top and bottom or second and third layers. 
     It is still another advantage to provide a barrier layer that has excellent properties to prevent diffusion between the middle and either or both the second and third layers. 
     These and other advantages will become apparent from the tri-metallic material for use in multi-layer printed circuit boards having a first metallic layer with spaced-apart top and bottom broadsides. A second and third metallic layers having spaced-apart top and bottom broadsides for overlying the first layer. A metallic bonding layer is deposited on the top and bottom broadsides of the first metallic layer that is sandwiched between the second and third metallic layers. The bonding layer bonds the second and third metallic layers to the top and bottom broadsides respectively of the first metallic layer. 
     In the preferred embodiment the first metallic layer is aluminum and the second and third metallic layers are copper with the bonding layer being a noble metal. In particular the bonding layer in one embodiment is immersion tin deposited on the broadside surfaces of the aluminum. A thin layer of nickel is deposited on the tin, and a copper layer is plated on the nickel layer. 
     If a barrier layer is desired, to prevent diffusion between layers, the process is the same as is the material of the layer. However some metals function better as a diffusion barrier than they do as a bonding layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 is a cross-section of one embodiment of an etched tri-metal material fabricated according to the present invention; 
     FIG. 2 is a cross-section of another embodiment; and 
     FIG. 3 is a cross-section of yet another embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the Figs by characters of reference there is illustrated in FIG. 1 a cross-section of the tri-metallic material  10 , or sandwich, as used in multi-layer printed circuit boards. The tri-metallic material  10  has a first metallic layer  12  with spaced-apart top  14  and bottom  16  broadsides. Second  18  and third  20  metallic layers having spaced-apart top  22 ,  24  and bottom  26 ,  28  broadsides for overlying the first layer. A metallic seed  30  or barrier  32  layer is deposited on the top  14  and bottom  16  broadsides of the first metallic layer  12  that is sandwiched between the second  18  and third  20  metallic layers. The bonding layer  30  bonds the second and third metallic layers to the top  14  and bottom  16  broadsides respectively of the first metallic layer  12 . 
     In the present embodiments, the term “barrier layer” is a layer  32  that prevents diffusion from one layer to the other and the term “bonding layer” is a layer  30  that helps in the boding between the layers on either side. Both seed and barrier layers  30 ,  32  are on the order of less than ten microns thick, while the first metal layer  12  in the tri-metallic material  10  is typically approximately one hundred fifty microns thick and the second  18  and third  20  layers are approximately fifty microns thick. The phrases bonding layer and barrier layer are used herein interchangeably. 
     In the preferred embodiment the first metallic layer  12  is aluminum and the second  18  and third  20  metallic layers are copper with the bonding layer  30  being a noble metal. In one embodiment the bonding layer  30  is immersion tin deposited on the broadside surfaces  14 ,  16  of the aluminum. A thin layer or strike of nickel is deposited on the tin, and the second  18  or third  20  layer, a copper layer is plated on the nickel layer. 
     In the alternative the bonding layer  30  can be tin (Sn), silver (Ag), rhodium, palladium, platinum, (Pt) or gold (Au). Each of these materials is highly resistant to corrosion as indicated on the Electromotive Force Series, “EMF”. But the use of such materials is expensive in the manufacturing process, not only for the material itself but also the process. 
     Various metal alloys such as bronze (copper-tin) and brass can be used as the bonding layer  30 . Each of these alloys is immersion coated onto the aluminum, which would be the first metallic layer. These alloys exhibit good corrosion resistance and have good adhesion with the aluminum layer. 
     As a barrier layer  32 , the preferred metals are chromium, rhodium, palladium and platinum. The preferred alloys are zinc-nickel, cobalt-zinc and nickel-tin. 
     However it is to be understood that any of the previous metals, be it used for a bonding layer  30  or a barrier layer  32  contains properties that will function both as a bonding layer and a barrier layer. However, the lists in the previous paragraphs indicate those preferred metals to use if only a seed  30  or barrier  32  layer is wanted. 
     In the alternative, the bonding layer  30  may be coated on the second  18  and third  20  metallic layers. In an alternative embodiment, all of the metal layers are sheets of metal that have substantially the same broadside area. 
     A method for bonding aluminum and copper in the etched tri-metallic material has the steps of initially securing first  12 , second  18  and third  20  metallic strips having spaced-apart top  14 ,  22 , or  24  and bottom  16 ,  26 , or  28  broadsides. The first, second and third strip broadsides have substantially identical planar areas. 
     In FIG. 1 a metallic bonding layer  30  or a barrier layer  32  is deposited on both, top  14  and bottom  16  broadside surfaces of the first strip  12 . In the alternative the metallic bonding layer  30  is deposited on one broadside surface  26 ,  24  of each of the second  18  and third  20  strips. 
     In FIG. 2, typically a barrier layer  32  is deposited on the bottom surface  26  of the second  18  layer  18  and the top surface  24  of the third  20  layer and over the barrier layer  32  is a bonding layer is deposited. As illustrated the bonding layer is directly in contact with the first layer  12 , however, either the seed or barrier layer can be in direct contact with the first layer. 
     In FIG. 3 a barrier layer  32  is sandwiched between two bonding layers  30 . In turn this sandwich  32 - 30 - 32 , is located between the first strip  12  and either the second  18  or their strip  20 . 
     The three strips  12 ,  18 ,  20  are then formed into a sandwich wherein the second  18  and third  20  strip overlie respectively the top  14  and bottom  16  broadsides of the first strip  12 , such that the bonding layer  30  or barrier layer  32  are not exposed. The sandwich is then activated by a heat treatment or cladding for bonding the mating broadside surfaces of the three strips forming a unitary structure. 
     The first strip  12  is aluminum and the second  18  and third  20  strips are copper. Typically, the metallic bonding layer  30  or barrier layer  32 , are deposited on the both  14 , 16  broadsides of the aluminum strip. Depositing is by the process of immersion of both of the broadsides of the aluminum strip or one of the broadside surfaces of the second  18  or third  20  strips which are both copper, with the metallic bonding layer  30 . 
     The step of activating is by means of heat-treating the sandwich  10  at predetermined temperatures and pressures. In the alternative the step of activating is by means of cladding the sandwich  10  at predetermined pressures and temperatures. 
     There has thus been illustrated a tri-metallic material for use in the manufacture of printed circuit boards. Of particular interest are the various materials is and processes for used in the metallic bonding layer between the first and the second and third strips.