Abstract:
A laser system for micro via formation directly over a plated through hole (PTH). The laser system forms the micro via directly over the PTH with full dielectric removal from a capture pad while minimizing the dielectric removal from a center portion of the PTH.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to a via formation system and more particularly to a laser system used for forming a micro via.  
           [0003]    2. Related Art  
           [0004]    FIGS.  1 - 5  illustrate a related art method of forming a microvia over a plated through hole (PTH) of a substrate. In particular, FIG. 1 illustrates a cross sectional view of a related art substrate  12 , having a PTH  10  formed therein. The PTH  10  includes an opening  14  and a capture pad  16  formed using conventional techniques. As illustrated in FIG. 2, a dielectric material  18  is deposited within the opening  14  of the PTH  10 , filling the opening  14  of the PTH  10 , and coating a lower surface  20  of the substrate  12 . A plated stud  22  is attached to a capture pad  24  of the PTH  10 , as shown in FIG. 3. FIG. 4 illustrates an additional step after related art FIG. 3 wherein a dielectric material  18 B is added to a top surface  26  of the substrate  12 . A cavity  16  is formed in the dielectric material  18 B by chemical etching, drilling or other mechanical method. The plated stud  22  is required to protect the dielectric material  18 B from being removed by the chemical etching or drilling processes used during formation of the cavity  16 . FIG. 5 illustrates related art FIG. 4 after an object  28 , such as a microvia, is attached to the plated stud  22 .  
         SUMMARY OF THE INVENTION  
         [0005]    A first general aspect of the present invention provides a laser beam system comprising:  
           [0006]    a laser system providing a bimodal focused beam, and wherein an energy density at a centroid of the beam is at least 10 percent less than the average energy density of the beam.  
           [0007]    A second general aspect of this invention provides an apparatus comprising:  
           [0008]    a laser source producing a laser beam; and  
           [0009]    a beam shaping lens for forming the laser beam into a bimodal focused beam wherein an area of reduced energy density at a centroid of the bimodal focused beam is at least 10% less than an average energy density of the bimodal focused beam.  
           [0010]    A third general aspect of this invention provides a method comprising:  
           [0011]    applying a focused laser beam onto a circuit board wherein the focused laser beam removes a dielectric above a surface of a capture pad of a Plated Through Hole (PTH) while maintaining the dielectric within the PTH to a substantially flush level with the surface of the capture pad. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    For an understanding of the present invention, reference should be made to the following detailed description taken in connection with the accompanying drawings wherein:  
         [0013]    [0013]FIG. 1 illustrates a cross sectional view of a related art formation of a PTH in a substrate;  
         [0014]    [0014]FIG. 2 illustrates the related art substrate of FIG. 1 having a dielectric on a lower surface of the substrate and within the PTH;  
         [0015]    [0015]FIG. 3 illustrates the related art substrate of FIG. 2 having a plated stud over a capture pad of the PTH;  
         [0016]    [0016]FIG. 4 illustrates the related art substrate of FIG. 3 having a dielectric on a top surface of the substrate;  
         [0017]    [0017]FIG. 5 illustrates the related art substrate of FIG. 4 having a microvia over the plated stud;  
         [0018]    [0018]FIG. 6 illustrates a schematic view of a laser system for applying a bimodal focused laser beam to a surface of an object in accordance with the present invention;  
         [0019]    [0019]FIG. 7 illustrates a related art Gaussian laser beam energy distribution;  
         [0020]    [0020]FIG. 8 illustrates a related art uniform laser beam energy distribution;  
         [0021]    [0021]FIG. 9 illustrates a plan view of a bimodal focused laser beam of the present invention;  
         [0022]    [0022]FIG. 10 illustrates a laser beam energy distribution for the bimodal focused laser beam;  
         [0023]    [0023]FIG. 11 illustrates a cross sectional view of a circuit board including a substrate, a PTH, and a dieclectric in accordance with the present invention;  
         [0024]    [0024]FIG. 12 illustrates FIG. 11 including a cavity created by the bimodal focused laser beam; and  
         [0025]    [0025]FIG. 13 illustrates FIG. 12 including a microvia attached to a capture pad of the PTH. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    The present invention eliminates the related art steps that require a separate fill of the PTH (FIG. 2) and the overplate to form the plated stud  22  over the PTH  10  (FIG. 3). Also, the present invention eliminates the related art step of applying the dielectric material  18  over the top surface  26  of the substrate  12  (FIG. 4). Instead of using the related art mechanical methods to form the microvia, the present invention utilizes a laser system  30  (FIG. 6) to drill directly through a dielectric  18 A to a capture pad  24 A above a PTH  10 A (FIGS.  11 - 13 ).  
         [0027]    The laser system  30  includes a laser source  32 , a laser beam  34 , a beam expander  36 , a turning mirror  38 , a beam shaping lens  40 , a focusing lens  42 , a vacuum chuck  44 , and a motion system  46 . The laser source  32  may be any suitable source such as a Nd:YAG laser operating at 355 nm. The laser source  32  produces a pulsed laser beam  34  that travels through the beam expander  36 . The beam expander  36  creates a broadened laser beam  48  that is turned toward a substrate  12 A by the turning mirror  38 . After reflecting off of the turning mirror  38 , the broadened laser beam  48  travels through the beam shaping lens  40 . The beam shaping lens creates a bimodal laser beam  50  (FIGS. 9 and 10). The bimodal laser beam  50  passes through the focusing lens  42  that directs a focused bimodal laser beam  52  towards the substrate  12 A. The focused bimodal laser beam  52  strikes the dielectric material  18 A of the substrate  12 A. The focused bimodal laser beam  52  strikes the substrate  12 A, such that the focused bimodal laser beam  52  may be any suitable shape (e.g., circular, oval, etc.).  
         [0028]    The vacuum chuck  44 , attached to the motion system  46 , securely holds the substrate  12 A in place. The motion system  46  provides bidirectional movement to the substrate  12 A. Thus, the motion system  46  moves the substrate  12 A to a selected position where the focused bimodal laser beam  52  may be applied over each PTH ( 10 A,  10 B), as illustrated in FIG.  
         [0029]    [0029]FIG. 9 illustrates a plan view of the focused bimodal laser beam  52  of the present invention, and FIG. 10 illustrates a bimodal energy distribution  58  of the present invention. The energy density at a centroid  62  of the bimodal focused laser beam  52  is at least 10% less than the average energy intensity of the bimodal focused laser beam  52 . A high energy perimeter portion  68  of the bimodal focused laser beam  52  surrounds the centroid  62 , as illustrated in FIGS. 9 and 10. The centroid  62  containing an area of reduced energy comprises at least 25% of the total area of the bimodal focused laser beam  52  (FIG. 9).  
         [0030]    [0030]FIG. 11 illustrates the substrate  12 A including PTHs  10 A and  10 B A layer of dielectric material  18 A is deposited on a top portion  64 A and a bottom portion  64 B of the substrate  12 A using conventional deposition techniques, such as, lamination, coating, etc. Additionally, the dielectric material  18 A fills an opening  14 A and an opening  14 B in the PTHs  10 A and  10 B, respectively, during deposition. The focused bimodal laser beam  52  is applied above the PTH  14 A and than above the PTH  14 B. As illustrated in FIG. 12, a cavity  66 A and a cavity  66 B are formed over the PTHs  10 A and  10 B, respectively, by the focused bimodal laser beam  52 .  
         [0031]    For example, in a single step, the focused bimodal laser beam  52 , having the bimodal energy distribution (FIG. 10), forms the cavity  66 A in the dielectric material  18 A of the substrate  12 A. The high energy perimeter portion  68  of the focused bimodal laser beam  52  assures that all the dielectric material  18 A is removed over the capture pad  24 A of the PTH  10 A, while the area of reduced energy  62  of the laser beam  52  minimizes the dielectric material  18 A removed within the opening  14 A of the PTH  10 A. Pulsing of the focused bimodal laser beam  52  assists in the removal of the dielectric material  18 A. A surface  72 A of the capture pad  24 A is completely cleared of the dielectric material  18 A (FIG. 12), and the level  70 A of the dielectric material  18 A within the opening  14 A of the PTH  10 A is substantially flush with the surface  72 A of the capture pad  24 A.  
         [0032]    Similarly, the focused bimodal laser beam  52  forms a cavity  66 B over the PTH  10 B (FIG. 12). The focused bimodal laser beam  52  is positioned over the PTH OB, and a cavity  66 B is formed with a completely cleared surface  72 B of the capture pad  24 B, and leaving the level  70 B of the dielectric material  18 A within the opening  14 B flush with the surface  72 B of the capture pad  24 B.  
         [0033]    [0033]FIG. 13 illustrates the substrate  12 A of FIG. 12 after the addition of an object  28 A and an object  28 B to the capture pads  24 A and  24 B, respectively. The objects  28 A and  28 B may comprise microvias formed for providing electrical connection between, e.g., substrates, cards, boards, microchips, etc.  
         [0034]    Alternatively, conventional laser beams, such as a laser having a Gaussian energy distribution  54 , as illustrated in FIG. 7, or a laser having a uniform energy distribution  56 , as illustrated in FIG. 8, may be used to remove the dielectric material  18 A. (Note that the energy is plotted in a Y direction.) However, conventional Gaussian distribution and uniform distribution are less desirable because these energy distributions tend to remove dielectric material  18 A within the opening  14 A of the PTH  10 A (FIG. 12), which may lead to reliability problems caused by entrapped plating solution.  
         [0035]    While embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. For example, the focused bimodal laser beam  52  may drill through any suitable material (e.g., glass/epoxy, organic dielectric, etc.). Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.