Abstract:
A method is provided for connecting two conductive layers in an electronic circuit package comprising the steps of forming dendrites on selected regions of a first conductive layer, forming dendrites on selected regions of a second conductive layer, applying an epoxy adhesive material over the first conductive layer, and compressively attaching the second conductive layer to the first conductive layer such that the dendrites on the first conductive layer contact the dendrites on the second conductive layer. Also claimed is an electronic circuit package incorporating the dendrites used for electrical interconnection manufactured in accordance with the present invention. An alternative embodiment of the invention utilizes an intermediate surface metal with dendrites in place of a “through via.”

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 08/918,084, filed Aug. 25, 1997. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to electronic circuit packages with dendrites connecting two conductive surfaces through a film and method for producing same. The dendrites electrically connect the two conductive surfaces and avoid the need for drilling or punching holes for vias or through-holes. 
     BACKGROUND OF THE INVENTION 
     Electronic circuits contain many (sometimes millions) of components such as resistors, capacitors, inductors, diodes, electro-mechanical switches, and transistors. High density packaging of electronic components is particularly important to allow fast access to large amounts of data in computers. High density electronic circuit packages also are important in high frequency devices and communicating devices. The components are connected to form circuits and circuits are connected to form functioning devices. The connections perform power and signal distribution. In a multi-layer electronic circuit package, some layers of the package serve as power planes and other layers serve as signal planes, depending on the operational requirements of the device. The devices require mechanical support and structural protection. The circuits themselves require electrical energy to function. The functioning devices, however, produce heat, or thermal energy which must be dissipated so that the devices do not stop functioning. Moreover, while high density packaging of a number of components can improve performance of the device, the heat produced by the power-consuming components can be such that performance and reliability of the devices is adversely impacted. The adverse impact arises from electrical problems such as increased resistivity and mechanical problems such as thermal stress caused by increased heat. 
     Electronic circuit packages, such as chips, modules, circuit cards, circuit boards, and combinations of these, thus must meet a number of requirements for optimum performance. The package must be structurally sturdy enough to support and protect the components and the wiring. In addition, the package must be capable of dissipating heat and must have a coefficient of thermal expansion that is compatible with that of the components. Finally, to be commercially useful, the package should be inexpensive to produce and easy to manufacture. 
     High density packages necessarily involve increased wiring density and thinner dielectric coatings between layers in a multi-layer electronic circuit package. The layers in a multi-layer package typically are electrically connected by vias and through-holes. The term “via” is used for a conductive pathway between adjacent layers in a multi-layer electronic circuit package. The term “through-hole” is used for a conductive pathway that extends to a non-adjacent layer. For high density packages the through-holes are increasingly narrow in diameter and the through-holes in each layer must be aligned precisely. This invention provides an alternative means of interconnection—namely electrical interconnection using dendrites. Dendrite interconnection avoids the need to drill or punch holes for vias or through-holes. 
     An object of this invention is to provide a means for connecting two conductors along the vertical or z-axis without using vias or through-holes, but still resulting in tight tolerances and high yield. Dendrite interconnections accomplish this purpose. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an electronic circuit package with dendrites forming electrical connections between a first conductive layer and second conductive layer. 
     A further object of this invention is to provide an electronic circuit package that provides electrical connection in the z-axis using dendrites, thereby eliminating the need for drilling or punching through-holes or vias in the manufacturing process of the electronic circuit package. 
     A third object of this invention is to provide a method of fabrication of electronic circuit package with dendrites forming electrical connections between a first conductive layer and a second conductive layer. 
     Accordingly, a method is provided for connecting two conductive layers in an electronic circuit package comprising the steps of forming dendrites on selected regions of a first conductive layer, forming dendrites on selected regions of a second conductive layer, applying an epoxy dielectric material over the first conductive layer, and compressively attaching the second conductive layer to the first conductive layer such that the dendrites on the first conductive layer contact the dendrites on the second conductive layer. Also claimed is an electronic circuit package incorporating the dendrites used for electrical interconnection manufactured in accordance with the present invention. An alternative embodiment of the invention utilizes an intermediate surface metal with dendrites in place of a “through via.” 
     It is an advantage of the present invention that the dendrites provide electrical connection between two conductive layers of the electronic circuit package. 
     It is further advantage that the dendrites provide electrical connection in the z-axis using dendrites, thereby eliminating the need for photodefining, laser ablating, drilling or punching through-holes or vias in the manufacturing process of the electronics circuit package. 
     Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment of the invention taken in conjunction with the accompanying drawings and examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a depiction of a single layer of a multi-layer electronic circuit packing using dendrites, rather than through-holes or vias, to electrically connect two conductive surfaces, in accordance with the present invention. 
     FIG. 2 is a depiction of two layers of a multi-layer electronic circuit package using dendrites to provide an electrical connection where a “through via” would exist in more traditional manufacturing. 
     FIG. 3 is a flow chart of the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is of an electronic circuit package using dendrites to provide connection between two conductive surfaces, thereby avoiding the need to photodefine, laser ablate, drill or punch through-holes or vias. The invention can best be understood by reference to the drawings. 
     FIG. 1 illustrates a sample layer  10  of an electronic circuit package in accordance with the present invention. Referring to FIG. 1, at the bottom of the layer  10  is a first substrate  12 . The first substrate  12  can be a printed circuit board core or a subcomposite. The first substrate  12  is preferably made of FR-4 type epoxy. A first surface metal  14  is situated on top of the first substrate  12  and covers some portion of the upper surface of first substrate  12 . The first surface metal  14  forms a first conductive surface. In the preferred embodiment of the invention, the first surface metal  14  is made of a copper material. 
     Lower dendrites  16  are formed at selected locations on the first surface metal  14 . The lower dendrites  16  preferably are made of palladium metal. Palladium metal possesses desired mechanical and physical properties. Other suitable metals for the lower dendrites include, but are not limited to, nickel or copper. 
     The lower dendrites may be formed by a variety of methods. One such method is to apply a photoresist material to the area of first surface metal  14  and then expose and develop the resist (not shown) by photolithographic techniques to provide an exposed area on which the dendrites are to be formed. 
     Typical photoresist materials are methacrylate polymeric resist compositions and electrohoretic resists such as those obtainable from Shipley or Nippon Paint. 
     According to a preferred method, an intermediate layer of nickel (not shown) is electroplated onto the first surface metal  14  followed by an intermediate layer of palladium, after applying resist material. 
     The nickel layer is typically about 1 to about 2.5 microns and more typically about 2 microns thick. The nickel covers the first surface metal  14  to prevent it from contaminating the palladium plating composition. 
     In addition this intermediate layer of palladium is typically about 1 to about 2.5 microns and more typically about 2 microns thick. 
     The lower dendrites  16  then are formed on the intermediate palladium layer by any known technique such as by ultrasonic plating of palladium typically at about 80 to 100 milliamps/cm 2  of surface area of first surface metal  14 . It is preferred that the lower dendrites  16  are about 0.1 to 1.5 mil. pref 1.0 mil in height. If desired, each of the lower dendrites  16  can be coated with a metal that could interface with or diffuse to form a metallic bond. For instance, the lower dendrites  16  can be coated with pure gold or with tin. 
     The photoresist is then removed by stripping in a suitable solvent such as propylene carbonate. 
     Once the lower dendrites  16  are formed, a layer of epoxy dielectric  18  is applied across the upper surface of substrate  12 . The dielectric  18  thus covers the substrate  12 , the first surface metal  14  and the lower dendrites  16 . The adhesive dielectric layer  18  typically is in the range of 1 mil. to 5 mil. in thickness. In the preferred embodiment of the invention, the epoxy based dielectric  18  is Morton LB 404. Other suitable adhesives include, but are not limited to, BT-resin, polyimide, Teflon, and polysiloxanes. The dielectric may be applied by vacuum lamination, screen coating, curtain coating, or roller coating. In the preferred embodiment of the invention, the adhesive is applied by vacuum lamination of a dry film. 
     The top of the layer  10  is a second substrate  20 . The second substrate  20  in the preferred embodiment of the invention, is made of epoxy glass. A second surface metal  22  is situated on the lower surface of second substrate  20  and covers some portion of second substrate  20 . The second surface metal  22  forms a second conductive surface. In the preferred embodiment of the invention, the second surface metal  22  is made of a copper material. 
     The first substrate  12  and the second substrate  20  may be made of the same material, but are not required to be. The first surface metal  14  may be made of the same material as the second surface metal  22 , but is not required to be. 
     Upper dendrites  24  are formed at selected locations on the second surface metal  22 . The upper dendrites  24  typically are made in the same manner and have the same composition as the lower dendrites  16 . However, this is not required. The upper dendrites preferably are formed to a height of 0.1 to 1.5 mil. If desired, the upper dendrites  24  also can be coated with a metal, as described above. 
     The second substrate  20 , having the second surface metal  22  and the upper dendrites  24 , then is attached compressively to the first substrate  12  that has the dielectric layer  18 . After compression, the separation  26  between the first surface metal  14  and the second surface metal  22  is about 0.2 to 2 mil. As a result of the compression, the upper dendrites  24  are in contact with the lower dendrites  16 , thereby forming an electrical connection between the first surface metal  14  and the second surface metal  22 . 
     The upper dendrites  24  penetrate the adhesive layer  18  without any need for pre-drilled holes. To facilitate the mating process of the upper dendrites  24  to the lower dendrites  16 , the adhesive  18  may be heated to facilitate flow. The dielectric  18  will cure by heating after the mating is completed. 
     To achieve optimum electrical connection, the upper dendrites  24  should be positioned on the upper surface metal  22  to correspond with the position of the lower dendrites  16  on the lower surface metal  14 . Registration tolerance is large. Overlap of 1 mil. is desirable as long as 1 mil. clearance to an adjacent conductor is maintained. 
     The advantages of the dendrites  16  and  24  shown in FIG. 1 are two-fold. First, the dendrites  16  and  24  provide an electrical connection between the first surface metal  14  and the second surface metal  22 . Second, the electrical connection is achieved without the need to drill or punch and plate through-holes or vias thereby simplifying the manufacturing process. 
     Dendrites also can be used as electrical connectors when there are more than two layers involved. In an alternative embodiment of the invention, a circuitized core with dendrites formed in selected areas on it is deposited wherever a “through via” would exist in more traditional manufacturing. A “through via” is one that is intended to make contact with a conductor on the corresponding layer. The via in the core is used only to provide electrical connection for the dendrites for the through connection from surface metal on one substrate to surface metal on another substrate. 
     FIG. 2 depicts the second embodiment of the invention. FIG. 2 shows a circuitized core  120  with a via  122 . Surface metal such as  114 ,  124 , and  134  is attached to certain areas of the circuit board core  120 , the upper surface  132  and the lower surface  112 . Dendrites  116 ,  126 , and  136  are formed on selected areas of the surface metal  114 ,  124  and  134 . A dielectric adhesive  118  holds the dendrites  116 ,  126 , and  136  in position relative to each other. Dendrites in contact with each other thus form an electrical connection between the upper surface  132  and lower surface  112 . 
     The entire process described with respect to the two embodiments can be repeated to create more layers interconnected by dendrites as described above. 
     FIG. 3 is a flow chart in accordance with the method of the present invention. 
     Although specific embodiments have been described herein for purposes of illustration, various modifications may be made without departing from the spirit or scope of the invention.