Abstract:
A printed circuit board is disclosed having a substrate with an insulating layer, aluminum foil layers disposed on both sides of the insulating layer, and a through-hole formed in the insulating layer and aluminum foil layers. A metal layer is disposed over an exposed surface of the insulating layer positioned along an inner surface of the through-hole. A zinc film is positioned on a surface of the aluminum foil. A metal film is disposed over the zinc film. A plating film is disposed on a surface of the metal film. A circuit pattern is etched through the aluminum foil and the plating film.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of PCT Application No. PCT/KR2013/002746, filed Apr. 3, 2013, which claims priority to Korean Patent Application No. 10-2012-0035382, filed Apr. 5, 2012, and Korean Patent Application No. 10-2013-0035716, filed Apr. 2, 2013. 
    
    
     FIELD OF THE INVENTION 
     The invention is generally related to a printed circuit board, and more specifically to a printed circuit board having aluminum layers. 
     BACKGROUND 
     In general, a printed circuit board (“PCB”) refers to a component onto which various elements are mounted or electrically connected through integrated wiring. With technological development, the PCBs are being manufactured in various forms and with various functions including home appliances, communication devices, semiconductor devices, industrial devices, and electric vehicle control. As technological advances increasingly reduce the size of electronic components, PCBs are also becoming smaller, lighter, and higher value-added. Additionally, as the size of electronic components is reduced, PCBs are increasingly becoming multifunctional, with greater numbers of electronic components being connected on smaller and smaller PCBs. 
     A consequence of this multifunctional application is that the electronic devices consume more power, generating more heat. This heat generation can often determine user satisfaction and purchase criteria. 
     Conventionally, a multilayer PCB is manufactured by preparing a copper clad laminate (CCL) as a base substrate and accumulating a copper foil on which a circuit pattern is formed. The drawback of the convention copper (Cu) based multilayered PCB is that methods for increasing the efficiency of heat dissipation is limited. 
     Recently, aluminum (Al) based multilayer PCBs have been explored as a potential replacement to the Cu based multilayer PCBs, because Al has a higher thermal conductivity than Cu. 
     Japanese Patent Publication No. 2004-179291 discloses an example of an Al based PCB, which increases heat radiation efficiency and flexural strength by replacing the insulating layers with Al, and includes circuits disposed on both sides. Surface roughness is formed by through-hole processing. Also, the entire part is turned to a nonconductor. Al with both poles oxidized is brought into close contact with a coating layer of the through-hole, and circuits are formed on both sides of the Al. Both poles of the Al may be oxidized by depth of 10 to 100 μm. When the depth is 30 μm, the Al is not affected by electric shock and insulation efficiency is excellent. 
     A drawback of this approach is that a surface of the Al core may be corroded and undesired chemical reactions may occur during electrolytic reduction Cu plating, because Al has lower chemical resistance than Cu and is in an active ion state. The Cu layer generated by electrolytic reduction Cu plating may not completely achieve close contact with an Al core layer disposed under the Cu layer. Additionally, even after the Cu plating, a surface of the Cu layer may float due to a poor contacting force, accordingly reducing reliability of the product. 
     When the Al based PCB manufactured by the conventional method is used for office automation equipments, mobile terminals, and similar applications having an applied temperature range that is relatively narrow, the contacting force of the Cu layer may be stabilized by anodizing the Al core. However, when the Al based PCB is used in electronic control applications in vehicles, especially when mounting in an engine compartment, the applied temperature range is very large, resulting in degradation between an Al film and the Cu layer. 
     In addition, when the substrate is used for vehicles, the PCB may be damaged due to a difference in thermal expansion rate between the Al core and other layers disposed on the Al core. 
     SUMMARY 
     A printed circuit board has a substrate with an insulating layer, aluminum foil layers disposed on both sides of the insulating layer, and a through-hole formed in the insulating layer and aluminum foil layers. A metal layer is disposed over an exposed surface of the insulating layer positioned along an inner surface of the through-hole. A zinc film is positioned on a surface of the aluminum foil. A metal film is disposed over the zinc film. A plating film is disposed on a surface of the metal film. A circuit pattern is etched through the aluminum foil and the plating film. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will now be described by way of example, with reference to the accompanying Figures, of which: 
         FIGS. 1 to 11  show sectional views of a processes of manufacturing a PCB. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
     In the embodiment shown in  FIG. 1 , an aluminum (Al) foil  10  is prepared by introducing surface roughness on a surface of the Al foil  10  to increase bonding efficiency. In an exemplary embodiment, the surface roughness may be formed by etching with alkali-based or acid-based chemical processing methods, such as sulfate-based soft etching performed for about one minute. While general anodizing methods require about 6 to 10 hours, soft etching considerably reduces the time required for forming the surface roughness. 
     In the embodiment shown in  FIG. 2 , the Al foil  10  is bonded to an insulating layer  20  using polyimide-based or epoxy-based adhesive having high bonding capabilities and insulating properties. In an exemplary embodiment, the Al foil  10  shown in  FIG. 1  may be bonded using a polyimide-based insulating bonding sheet  30 . The polyimide-based bonding sheet may have a similar thermal expansion coefficient as the Al foil  10 . Since polyimide is stable at high temperatures of about 400° C. or more to a low temperature of about −269° C. or lower, variation according to the thermal expansion coefficient (23.03×10−6) of Al may be absorbed. Bonding of the Al foil  10  and the insulating layer  20  may be performed by heating or pressurizing an upper portion and a lower portion of the Al foil  10 . 
     In the embodiment shown in  FIG. 3 , a through-hole  40  is formed to pass through from the insulating layer  20  to the Al foil  10 . The through-hole  40  may be formed by drilling, punching by laser processing, chemical etching, or other techniques known to those of ordinary skill in the art. Although in one embodiment the through-hole  40  is formed after bonding of the Al foil  10  and the insulating layer  20  in the above description, in other embodiments the through-holes  40  may be formed separately in the Al foil  10  and the insulating layer  20 , prior to the Al foil  10  and the insulating layer  20  being bonded. In yet another embodiment, a hole may be formed in advance through the Al foil  10  and, after bonding of the insulating layer  20 , the through-hole  40  may be formed at the insulating layer  20  corresponding to the hole by a chemical method. For descriptive purposes herein, a structure in which the Al foil  10  is bonded to both sides of the insulating layer  20  using the insulating bonding sheet  30  and the through-hole  40  is formed will be referred to as a ‘substrate.’ A pattern may be formed on the substrate by the through-hole  40 . 
     Although the embodiment in  FIGS. 1-3  shows a monolayer substrate formed by bonding the Al foil  10  on both sides of a single insulating layer  20 , in other embodiments a multilayer substrate may be applied, formed by preparing a plurality of insulating layers  20  and bonding the Al foil  10  on both sides of each insulating layer  20 . 
     In the embodiment shown in  FIG. 4 , a metal layer  50  is formed on an inner surface of the through-hole  40  to cover the exposed insulating layer  20  and the insulating bonding sheet  30  within the through-hole  40 . The metal layer  50  may be formed by direct plating of a carbon plating layer, where the carbon serves to penetrate into the insulating layer  20  and the insulating bonding sheet  30 . The metal layer  50  bridges the Al foils  10  formed at the upper portion and the lower portion of the substrate to provide electrical conductivity therebetween. 
     In the embodiment shown in  FIG. 5 , zincate processing may be performed with reference to the metal layer  50 , thereby replacing a predetermined thickness of the surface of the Al foil  10  with a zinc (Zn) layer  60 . As the zincate processing is performed, a side surface of the Al foil  10  exposed through the inner surface of the through-hole  40  of the substrate, excluding the metal layer  50 , may be selectively replaced with the Zn film  60 . That is, during the zincate processing, part of the surface of the Al foil  10  is replaced with the Zn film  60  as shown in  FIG. 5 . The zincate processing refers to a method of replacing a surface of metal having a high oxidation potential with Zn, which has a lower oxidation potential. In  FIG. 5 , the surface of the Al is replaced with Zn. 
     Since the surface of the Al foil  10  is replaced with the Zn film  60  through the zincate processing, a contacting force of the surface of the Al foil  10  may be increased during electrolytic reduction plating or electroplating. 
     In the embodiment shown in  FIG. 6 , electrolytic reduction plating is performed to form a metal film having high chemical resistance with respect to the Zn film  60 . For example, a nickel (Ni) film  70  is formed on the metal layer  50  and on the Zn film  60  by performing electroplating using Ni. the advantage of the Ni film  70  is that the Ni film  70  may be formed by electrolyzing (restoration) at a relatively lower production cost compared to other electrolytic reduction plating methods. In  FIG. 6 , the Ni film  70  may be formed by performing Ni electrolyzing (restoration) with respect to the Zn film  60 . However, depending on conditions, the Zn film  60  may be replaced with the Ni film  70 , where an entire or only a partial thickness of a surface of the Zn film  60  may be replaced with the Ni film  70 . Therefore, even after the Ni displacement plating, part of the Zn film  60  may remain on the surface of the Al foil  10 . 
     When the plating of the Zn film  60  is performed, another metal having high chemical resistance may be used instead of Ni. For example, gold (Au) or silver (Ag) may be used. However, in consideration of material cost, it is exemplary to use Ni for the plating of the Zn film  60 . 
     In the exemplary embodiments, bonding between the Zn film  60  and a copper (Cu) plating film  80  is increased by forming the Ni film  70 . Since bonding between Zn and Ni and between Ni and Cu is higher than bonding between Zn and Cu, when the Ni film  70  is formed between the Zn film  60  and the Cu plating film  80 , corrosion of the Zn film  60  may be prevented while simultaneously increasing bonding and surface strength. 
     In the embodiment shown in  FIG. 7 , a plating film  80  may be formed by electroplating or electrolytic reduction plating on a surface of the Ni film  70 , including the surface of the through-hole  40 . The plating film  80  may be a Cu film formed by electroplating. The plating film  80  may be formed to a thickness of about 20 μm or more. The thickness of the plating film  80  may be controlled according to electroplating methods known to those of ordinary skill in the art. An advantage of using electroplating or electrolytic reduction plating is that production costs may be reduced while increasing quality and strength of the film. 
     According to an exemplary embodiment, since the surface of the Al foil  10  and the inner surface of the through-hole  40  include the Ni film  70 , the entire film is conductive, allowing electroplating to be performed. When electrolytic reduction plating is performed, replacement occurs between palladium (Pd) used as a catalyst and Zn, accordingly producing a large amount of hydrogen. Furthermore, a compound of Pd and Zn is generated, which contaminates the plating solvent. Conversely, when electro plating is performed, problems caused by hydrogen generation and Pd/Zn contamination are minimized in by use of electroless plating. 
     In the embodiment shown in  FIG. 8 , a dry film  90  is applied to a surface of the plating film  80 . In the embodiment shown in  FIG. 9 , an engraving film is matched and exposure light is supplied for a predetermined time, thereby developing a desired pattern  91 . 
     In the embodiment shown in  FIG. 10 , based on the dry film  90  remaining on the surface of the plating film  80 , acid etching may be performed with respect to the pattern  91 , using chloride based materials such as ferric chloride (FeC13), copper chloride (CuC12), sodium hypochlorite (NaOCl), and other similar materials known to those of ordinary skill in the art. Through the acid etching, the Zn film  60 , the Ni film  70 , the plating film  80 , and the Al foil  10  are removed, thereby forming a hole  92  in which a circuit desired by a user is to be formed. 
     In the embodiment shown in  FIG. 11 , the dry film  90  is stripped, and the plating film is formed by paneling and the desired circuit is formed. 
     Although exemplary embodiments of the present invention have been shown and described above, the present invention is not limited to the described embodiments. Instead, those of ordinary skill in the art would appreciate that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents. 
     The disclosure has been described with reference to particular exemplary embodiments, but variations within the spirit and scope of the disclosure will occur to those skilled in the art. For example, a proper result may be accomplished even when the described techniques are performed in different order and/or the described system, structure, apparatus, circuit and the like are connected or combined in different manners from the above or replaced with other components or equivalents. 
     Accordingly, other embodiments and equivalents to claims are within a scope of claims.