Abstract:
A method of removing material from a surface, which includes the steps of providing a base layer, at least one layer attached to the base layer, an intermediate finish surface, and a bonding interface surface. The intermediate finish surface is formed by removing the at least one layer and a portion of the base layer during a material removal process. The bonding interface surface is formed by a polishing process applied to the intermediate finish surface. There is an oxidation layer which is part of the base layer, as well as a mold release layer and a contamination layer, both of which are part of the at least one layer. The material removal process involves laser etching the at least one layer to create the intermediate finish surface, and the polishing process includes applying a second laser etching to the intermediate finish surface, forming the bonding interface surface.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to a surface treatment for a casting surface, to make the casting surface more suitable for wire bonding and other processes. 
       BACKGROUND OF THE INVENTION 
       [0002]    Circuit assemblies have different components that are assembled using different manufacturing processes. One of the components often used in a circuit assembly is a metal casting, which is typically in the form of a heat sink. The heat sink is connected to other components, such as a printed circuit board (PCB). The connection between the heat sink and the PCB is typically accomplished using a wire bonding process, where the surface requirements are always high in regard to cleanliness, composition, roughness, and finish. 
         [0003]    Wire bonding is the method by which a length of small diameter soft metal wire is attached to a compatible metallic surface without the use of solder, flux, and in some cases, with use of heat above 150 degrees Celsius. Soft metals includes Gold (Au), Copper (Cu), Silver (Ag), Aluminum (Al), and alloys such as Palladium-Silver (PdAg) and others. 
         [0004]    Some current processes used for wire bonding on the PCB require a specific intermetallic interface between the aluminum bond and nickel phosphorus plating for solder and bonding. These processes also have various failure modes which limit the applications which the wire bonding process may be used for. 
         [0005]    Immersion gold is a robust surface for bonding. However, contamination or imperfections in the gold may result in a non-stick condition. The failures may be the result of supplier manufacturing, shipment, or production environment, or other factors. 
         [0006]    One type of process for polishing the surface is a diamond milling process. However, this process is expensive, and increases manufacturing costs significantly when used. 
         [0007]    Accordingly, there exists a need for a process which provides a surface that is sufficient for wire bonding, minimizes or eliminates failures, and is cost effective. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is a surface treatment process used to improve wire bonding between a PCB and a heat sink in a transmission control module, where the surface of the heatsink has been etched using a laser, thereby cleaning the surface before bonding. In one embodiment, the laser has a 1064 nm wavelength, but it is within the scope of the invention that other wavelengths may be used. This process minimizes the various failure modes that may occur at the bonding interface. 
         [0009]    In another embodiment, the process of the present invention may be used for as-cast base plates. As mentioned above, any contamination or surface faults between an aluminum bond and an aluminum base plate reduces the robustness of the connection. By using the laser process of the present invention, a bond is made with high strength, removing the need for post processing the casting progression, such as diamond milling or surface treatments. The laser process of the present invention involves two steps, the first step is a laser etch ablation process where material is removed, and the second step is a second laser etch to clean and smooth the surface. 
         [0010]    In one embodiment, the present invention involves bonding to a nickel phosphorous (NiP) and Au surface that has been exposed to a laser etch. In another embodiment, there is bonding to an as-cast surface (which has no post-casting surface treatment or finish) using laser ablation. The process of the present invention also involves no-clean flux from Au plated PCB bond pads. 
         [0011]    It is an object of this invention to provide a consistent surface finish due to the placement process, which allows for a very robust wire bonding connection. 
         [0012]    It is another object of this invention to remove contamination to provide a more robust surface for wire bonding post-cleaning. 
         [0013]    It is also an object of this invention to eliminate the need for post-casting surface treatment, providing a cost savings. 
         [0014]    In one embodiment, the present invention is a casting assembly having a bonding interface surface. In one embodiment, the casting assembly includes a base layer and an oxidation layer formed during the casting process. In other embodiments, the casting assembly may include a mold release layer. A contamination layer may form if the outer surface of the casting assembly is exposed to various debris and contaminants after being removed from the mold. There is also an intermediate finish surface, and a bonding interface surface. A material removal process is applied to the outer surface of the casting assembly to remove the contamination layer, the mold release layer, the oxidation layer, and a portion of the base layer to form the intermediate finish surface. Once the intermediate finish surface is formed, a material polish process is applied to the intermediate surface finish to form the bonding interface. 
         [0015]    In one embodiment, the mold release layer is a mold release material for facilitating the removal of the casting assembly from a mold. 
         [0016]    The base layer may be made from one of several materials, such as, but not limited to, aluminum, silver, gold, and copper. 
         [0017]    In an embodiment, the present invention is a method of removing material from a surface, which includes the steps of providing a base layer, providing at least one layer attached to the base layer, providing an intermediate finish surface, and providing a bonding interface surface. The intermediate finish surface is formed by removing the at least one layer and a portion of the base layer during a material removal process. The bonding interface surface is formed by a polishing process applied to the intermediate finish surface. 
         [0018]    In one embodiment, there is a base layer and an oxidation layer formed during the casting process. In other embodiments, the at least one layer may include a mold release layer. A contamination layer may form if the outer surface of the casting assembly is exposed to various debris and contaminants after being removed from the mold. 
         [0019]    The oxidation layer, mold release layer, and the contamination layer are removed during the material removal process. The material removal process involves laser etching the at least one layer to create the intermediate finish surface, but is it within the scope of the invention that other types of processes may be used. 
         [0020]    The polishing process includes the steps of applying a second laser etching to the intermediate finish surface, to form the bonding interface surface. The first laser etching process is different from the second laser etching process to form the bonding interface surface. 
         [0021]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0023]      FIG. 1  is a sectional view of a portion of a metal casting having undergone a surface finish process, according to embodiments of the present invention; and 
           [0024]      FIG. 2  is a perspective view of a first area of a metal casting having undergone a surface finish process, according to embodiments of the present invention; 
           [0025]      FIG. 3  is a perspective view of a second area of a metal casting having undergone a surface finish process, according to embodiments of the present invention; 
           [0026]      FIG. 4  is an enlarged view of a first area of a metal casting having undergone a surface finish process, according to embodiments of the present invention; 
           [0027]      FIG. 5  is a greatly enlarged topographical view of a first area of a metal casting having undergone a surface finish process, according to embodiments of the present invention; 
           [0028]      FIG. 6A  is a greatly enlarged view of first area of a metal casting having undergone a surface finish process, that includes a measurement line extending through both a bonding interface surface and a non-lasered area, according to embodiments of the present invention; 
           [0029]      FIG. 6B  is a first graph representing depth measurements of the measurement line in  FIG. 6A ; 
           [0030]      FIG. 7A  is a greatly enlarged view of first area of a metal casting having undergone a surface finish process, that includes a measurement line extending through part of a bonding interface surface in a first direction, according to embodiments of the present invention; 
           [0031]      FIG. 7B  is a graph representing roughness measurements of the measurement line in  FIG. 7A ; 
           [0032]      FIG. 8A  is a greatly enlarged view of first area of a metal casting having undergone a surface finish process, that includes a measurement line extending through part of a bonding interface surface in a second direction, according to embodiments of the present invention; 
           [0033]      FIG. 8B  is a graph representing roughness measurements of the measurement line in  FIG. 8A ; 
           [0034]      FIG. 9A  is a greatly enlarged view of first area of a metal casting having undergone a surface finish process, that includes a measurement line extending through part of an outer surface of a casting assembly in a first direction, outside of a bonding interface surface, according to embodiments of the present invention; 
           [0035]      FIG. 9B  is a graph representing roughness measurements of the measurement line in  FIG. 9A ; 
           [0036]      FIG. 10A  is a greatly enlarged view of first area of a metal casting having undergone a surface finish process, that includes a measurement line extending through part of an outer surface of a casting assembly in a second direction, outside of a bonding interface surface, according to embodiments of the present invention; and 
           [0037]      FIG. 10B  is a graph representing roughness measurements of the measurement line in  FIG. 10A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
         [0039]    A sectional view of a portion of a metal casting assembly is shown in  FIG. 1  generally at  10 . The assembly  10  includes a base layer  12  having an outer surface, shown generally at  12 A. When the casting assembly  10  is formed, the process typically involves a mold, where the material used to form the casting assembly  10  is injected into the mold. When the casting assembly  10  is removed from the mold, there is typically an oxidation layer  14  formed on the base layer  12 , a mold release layer  16  attached to the oxidation layer  14 , and some form of contamination material, which forms a contamination layer  18  on the mold release layer  16 . 
         [0040]    The oxidation layer  14  forms as part of the base layer  12  due to heat exposure during the casting process. The mold release layer  16  is made from a mold release material which is used to facilitate the removal of the casting assembly  10  from the mold. The contamination layer  18  is made of debris and other substances that may be located inside the mold during the casting process, or substances that could be anything contacting the casting assembly  10  after the molding process. Any one or a combination of the oxidation layer  14 , the mold release layer  16 , or the contamination layer  18  may be exposed on the outer surface  12 A, depending upon how the casting assembly  10  is made, and the environment the assembly  10  is exposed to after the casting process. In one embodiment, the oxidation layer  14  is formed such that the oxidation layer  14  is part of the outer surface  12 A, and the mold release layer  16  and contamination layer  18  are disposed on the surface  12 A, on top of the oxidation layer  14 . 
         [0041]    The first step of the process according to the present invention is a material removal process, where a laser etch is applied to the outer surface  12 A of the casting assembly  10  such that the contamination layer  18 , the mold release layer  16 , the oxidation layer  18 , and a portion of the base layer  12  are all removed, leaving a rough finish on the outside surface  12 A of the base layer  12 . This rough finish is an intermediate finish surface, shown generally at  20 . This material removal process functions to ablate and remove material. The laser may be any type of laser suitable for removing material from the casting assembly  10 . In one embodiment, during the material removal process, the laser has operating parameters of 38-42 Amps, 3000-10,000 Hertz, a speed range of 50-200 mm/sec, and passes over the surface 10-100 times to create the intermediate finish surface  20 . In yet another embodiment, during the material removal process, the laser has operating parameters of 40 Amps, 3600 Hertz, a speed of 120 mm/sec, and passes over the surface 54 times to create the intermediate finish surface  20 . The laser used during the material removal process has a wavelength of 1064 nm, but it is within the scope of the invention that other wavelengths may be used. 
         [0042]    The second step in the process according to the present invention is a polishing process. The polishing process is applied to the intermediate finish surface  20 , transforming the intermediate finish surface  20  into a bonding interface surface, shown generally at  22 . The polishing process may also be applied using a laser, and in one embodiment, during the polishing process, the laser may have operating parameters of 30-38 Amps, 20,000-40,000 Hertz, a speed of 100-400 mm/sec, and passes over the surface 5-20 times to create the bonding interface surface  22 . In another embodiment, during the polishing process, the laser is operated at 36 Amps, 36,000 Hertz at a speed of 300 mm/sec, and passes over the surface 7 times to create the bonding interface surface  22 . 
         [0043]    Once the bonding interface surface  22  is formed, the bonding interface surface  22  may be used for a wire bonding process, bonding the base layer  12  to a PCB board. In one embodiment, the bonding interface surface  22  has surface characteristics that are a result from undergoing both the material removal process and the polishing process. There is a total amount of material removed from the assembly  10  after undergoing both the material removal process and the polishing process, which places the bonding interface surface  22  at a “depth” relative to the original outer surface  12 A. In one embodiment, the depth  56  of the bonding interface surface  22  is about 3.7-37.0 microns, and the bonding interface surface  22  has a roughness Rz of less than 10 microns, but it is within the scope of the invention that other targets for the depth  56  and roughness may be used, depending on the materials used for the casing assembly  10 . 
         [0044]    Referring to  FIGS. 4-10B , various photos and graphs are shown depicting an example of a portion of a casting assembly  10 , with a portion of the outer surface  12 A having the bonding interface surface  22  placed under a digital microscope. There are areas of the outer surface  12 A that have not undergone the material removal process, shown generally at  12 B, along with the bonding interface surface  22  shown in  FIGS. 4-10B . 
         [0045]    Referring to  FIG. 6A , there is a photo  30  of an area of a casting assembly  10  which has the bonding interface surface  22 . The photo  30  includes a measurement line  32 , which is the distance measured taken along the surface  12 A of the casting assembly  10  and bonding interface surface  22  shown in  FIG. 6A . In  FIG. 6B , there is a first graph  34  depicting a surface measurement of the casting assembly  10 , including the bonding interface surface  22  (i.e., lasered area), and the areas of the casting assembly  10  that have not been exposed to either the material removal process, or the polishing process (i.e., non-lasered area). In  FIGS. 6A and 6B , the measurement line  32  is about 5.0 mm, as can be seen by the lower scale in  FIG. 6B , but it is within the scope of the invention that other lengths may be used. 
         [0046]    The first graph  34  from  FIG. 6B  has three sections, a first section, shown generally at  36 , a second section, shown generally at  38 , and a third section, shown generally at  40 . The first and third sections  36 , 40  represent areas of the outer surface  12 A in  FIG. 6A  that have not been exposed to either the material removal process, or the polishing process, and the second section  38  represents the area of the outer surface  12 A in  FIG. 6A  that has undergone both the material removal process and the polishing process. Also included in  FIG. 6B  is a first reference line  42 , the first reference line  42  is the average height of the non-lasered areas (sections  36 , 40 ) taken along the measurement line  32  in  FIG. 6A . 
         [0047]    The second section  38  has a width  44  of just over 2.0 mm, but it is within the scope of the invention that the width  44  may vary, depending upon the location of the measurement line  32  and the size of the bonding interface surface  22 . As is shown in  FIG. 6B , there is a maximum height  46 , which is the distance from the reference line  42  to the lowest point in the second section  38 . The second section  38  also has a mean height, which is the difference in the average height of the lasered surface (section  38 ) and the average height of the non-lasered surface (sections  36 , 40 ), which is shown in  FIG. 6B  as about 9.06 μm. 
         [0048]    Referring now to  FIG. 7A , there is the same photo  30  shown in  FIG. 6A , however, the measurement line  32  is at a different location and direction, and therefore represents a different surface measurement of the bonding interface surface  22 . The measurement line  32  in  FIG. 7A  is about 2.0 mm, and there is a second graph  48  in  FIG. 7B  showing the surface measurement of the measurement line  32  in  FIG. 7A . In  FIGS. 7A and 7B , the surface measurement taken along the measurement line  32  has a roughness measurement Rz of about 9.18 μm. 
         [0049]    Referring to  FIG. 8A , there is again the same photo  30  shown in  FIG. 6A , with the measurement line  32  at yet another location and direction, therefore representing a different surface measurement of the bonding interface surface  22 . Again, the measurement line in  FIG. 8A  is about 2.0 mm, and there is a third graph  50  in  FIG. 8B  showing the surface measurement line  32  in  FIG. 8A . In  FIGS. 8A and 8B , the surface measurement taken along the surface measurement line  32  has a roughness measurement Rz of about 9.15 μm. The measurements taken along the measurement lines  32  in both  FIG. 7A  and  FIG. 8A  produce similar measured results. 
         [0050]    Referring now to  FIG. 9A , again the same photo  30  is shown that is shown in  FIGS. 6A, 7A, and 8A . However, in  FIG. 9A , the measurement line  32  is taken at a location on the outer surface  12 A that is outside of the bonding interface surface  22 . Once again, the measurement line  32  is about 2.0 mm, and there is a fourth graph  52  in  FIG. 9B  showing the surface measurement represented by the measurement line  32  in  FIG. 9A . It can be seen in  FIGS. 9A and 9B  that the surface measurement taken along the surface measurement line  32  has a roughness measurement Rz of about 16.35 μm. 
         [0051]    Referring to  FIGS. 10A and 10B , again the same photo  30  is shown that is shown in  FIGS. 6A, 7A, 8A, and 9A . However, in  FIG. 10A , the measurement line  32  is again located outside the bonding interface surface  22 , but in a different location and direction compared to the measurement line  32  in  FIG. 9A . Once again, the measurement line  32  is about 2.0 mm, and there is a fifth graph  54  in  FIG. 10B  showing the surface measurement represented by the measurement line  32  in  FIG. 10A . It can be seen in  FIGS. 10A and 10B  that the surface measurement taken along the surface measurement line  32  has a roughness measurement Rz of about 14.73 μm. 
         [0052]    Furthermore, the roughness measurements (Rz) are about 9.18 μm and 9.15 μm in  FIGS. 7A and 8A , respectively, and the roughness measurements are about 16.35 μm and 14.73 μm in  FIGS. 9A and 10A , which is significantly larger than the roughness measurements in  FIGS. 7A and 8A . 
         [0053]    Examples of the completed wire bond are shown in  FIGS. 2 and 3 . The base layer  12  is shown having the outer surface  12 A exposed, and the bonding interface surface  22 , which is formed with both the material removal process and the polishing process. A wire  24  is shown bonded to a bond pad  26  of a PCB board  28 , and the wire  24  is also bonded to the bonding interface surface  22  formed as part of the base layer  12 . 
         [0054]    In one embodiment, the wire used for the wire bonding process is 203.2 μm in diameter, but it is within the scope of the invention that wires of other diameters may be used, such as, but not limited to, 125.0 μm to 508.0 μm, depending upon the type of material used for the process. In this embodiment, the material used for the wire bonding is aluminum, but it is within the scope of the invention that other materials may be used, such as, but not limited to, gold, silver, copper, and nickel. The target roughness value Rz for the bonding interface surface  22  is less than 10 μm, regardless of material or depth  56 . When aluminum is used, the bonding interface surface  22  has a target depth  56  of 3.7 to 37.0 μm. 
         [0055]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.