Abstract:
A method for attaching an IC package to a circuit board, the IC package having a plurality of electrical contacts in an arrangement having a perimeter, first positions the IC package adjacent to the circuit board. Then, electrically connects the IC package to the circuit board through the plurality of electrical contacts. The method finally, disposes at least one anchor mechanically attaching the IC package to the circuit board, the anchor disposed at a location outside of the perimeter of the plurality of electrical contacts. The type, quantity, and exact geometry of the anchors depend on the specific design parameters of the IC package and circuit board.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 10/064,462, filed Jul. 17, 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an integrated circuit (IC) package, and more specifically to a method for attaching an IC package to a circuit board.  
         [0004]     2. Description of the Prior Art  
         [0005]     An integrated circuit is typically housed inside of a package made of ceramic or composite material. The package provides mechanical, electrical, and thermal protection to the IC chip. The package has contacts to which solder balls or pins are attached to allow the IC chip to be electrically connected to a circuit board, and at the same time to provide mechanical attachment of the package to the circuit board. The solder balls or pins serve the purpose of both electrical and mechanical connection.  
         [0006]      FIG. 1  shows a bottom view of a prior art IC package  10  having an IC chip (item  16  of  FIG. 2 ). A plurality of solder balls  12  is disposed on a bottom surface of the package  10  in a grid-like array for attaching the package to a circuit board.  
         [0007]     A prior art method for attaching the IC package  10  to a circuit board  14  to manufacture an assembly  18  is illustrated in a side view shown in  FIG. 2 . The IC package  10  holding the IC chip  16  is attached to the circuit board  14  by the plurality of solder balls  12 . During manufacture, the solder balls  12  are fused to both the package  10  and the circuit board  14  to form an electrical and mechanical connection.  
         [0008]     The solder balls  12  can fail under stress from thermally induced mechanical loading or direct mechanical loading of the assembly  18 . For instance, the package  10  and circuit board  14  typically have different coefficients of thermal expansion. As the assembly  18  is heated or cooled the circuit board  14  and package  10  expand or contract at different rates, and consequently the solder balls  12  can be subject to high stresses. Additionally, the circuit board  14  can be flexed by externally applied forces, such as those forces experienced during the manufacturing, assembly, and testing processes. However, because the mechanical stiffness of the IC package  10  is appreciably greater than that of the circuit board  14 , the solder balls  12  can be subject to high stresses that can also cause mechanical failure. While the principal stresses in the solder balls  12  under these loading conditions may not be enough to cause immediate failure, a cyclic mechanical or thermal load can cause a fatigue failure.  
         [0009]      FIG. 3  shows the assembly  18  bending about a single axis due to a differential thermal contraction of the circuit board  14  and the package  10 . This condition can be caused, for example, by the assembly  18  being cooled from a higher than ambient assembly temperature resulting in the circuit board  14  contracting more than the package  10 . As the assembly  18  cooled, solder balls in two rows of solder balls  12   a  and  12   b  were subject to excessive tensile stress, and consequently cracked as shown. After a solder ball has failed mechanically, the IC package  10  is no longer effectively electrically connected to the circuit board  14 . Furthermore, if the loading on the circuit board  14  is not somehow released, the package  10  may become completely mechanically separated from the circuit board  14 .  
         [0010]     The prior art method of using solder balls to provide a mechanical connection between an IC package and a circuit board results in a weak connection that is prone to mechanical failure. When the circuit board and package undergo a mechanical loading or thermally induced mechanical loading, solder balls can fail mechanically.  
       SUMMARY OF THE INVENTION  
       [0011]     It is therefore a primary objective of the claimed invention to provide a method for attaching an IC package to a circuit board, so that the mechanical attachment strength is increased and the problems of the prior art are solved.  
         [0012]     According to the claimed invention, a method for attaching a IC package to a circuit board, the IC package having a plurality of electrical contacts in an arrangement having a perimeter, first positions the IC package adjacent to the circuit board. Then, electrically connects the IC package to the circuit board through the plurality of electrical contacts. The method finally, disposes at least one anchor mechanically attaching the IC package to the circuit board, the anchor disposed at a location outside of the perimeter of the plurality of electrical contacts.  
         [0013]     The type, quantity, and exact geometry of the anchors depend on the specific design parameters of the IC package and circuit board.  
         [0014]     It is an advantage of the claimed invention that the anchors can prevent mechanical failure of the electrical connection of the IC package to the circuit board, caused by mechanical and thermally induced mechanical loading.  
         [0015]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a bottom view of a prior art IC package.  
         [0017]      FIG. 2  is a side view showing a prior art method for attaching the IC package of  FIG. 1  to a circuit board.  
         [0018]      FIG. 3  is a side view of the assembly of  FIG. 2  bending about a single axis.  
         [0019]      FIG. 4  is a perspective view of attaching an IC package to a circuit board according to the present invention.  
         [0020]      FIG. 5  is a bottom view of the IC package of  FIG. 4 .  
         [0021]      FIG. 6  is a side view of the assembly shown in  FIG. 4 .  
         [0022]      FIG. 7  is a side view of the assembly of  FIG. 4  bending about a single axis.  
         [0023]      FIG. 8  is a side view of a second embodiment of the present invention.  
         [0024]      FIG. 9  is a side view of a third embodiment of the present invention.  
         [0025]      FIG. 10  is a bottom view of a fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]     The present invention method will be described in a preferred embodiment where four pins are used to strengthen the mechanical attachment of an IC package to a circuit board. Three alternative embodiments will also be described, wherein leads and solder are used as means for strengthening the attachment. The teachings of the present invention can apply to many modern IC package attachment systems.  
         [0027]     A perspective view of the preferred embodiment method of attaching an IC package  20 , accommodating an IC chip  26 , to a circuit board  22  to manufacture an assembly  24  is shown in  FIG. 4 . The package  20  is first aligned with the circuit board  22 . The package  20  is then electrically connected to the circuit board  22  by solder balls (item  30  of  FIG. 5 ). The package  20  is further mechanically attached to the circuit board  22  by a group of four anchor pins  28 . As the assembly  24  is manufactured, the anchor pins  28  can be produced at the same time and by the same device, as the electrically connecting solder balls. The pins  28  provide for a strengthened mechanical attachment of the package  20  to the circuit board  22 .  
         [0028]      FIG. 5  is a bottom view of the IC package  20  showing a plurality of solder balls  30  disposed on a bottom surface of the package  20  in a grid-like array for electrically connecting the package to a circuit board  22 . Each anchor pin  28  is located at a corner of the package  20 . In practical application, the anchor pins  28  are fit into holes formed in the package  20 , but the anchor pins  28  could also easily be fused directly to the bottom of the package  20  to have the same anchoring effect. The anchor pins  28  are arranged at the four corners of the package  20  outside of a perimeter formed by the outmost solder balls  30 , as this placement ensures that the anchor pins  28  will support a significant portion of any loading applied to the package  20  or the circuit board  22 . Alternatively, the anchor pins  28  could be intermingled with the solder balls  30 , but this arrangement is less desirable since the solder balls  30  are typically clustered together quite closely. The present invention is not limited by the arrangement of the solder balls  30  shown in  FIG. 5 . Moreover, the solder balls  30  could be pins or other means for providing electrical connections between the package  20  and the circuit board  22 .  
         [0029]     The exact diameter, length, and material of the anchor pins  28  are determined based on the precise arrangement of and material used for the solder balls  30 , as well as on other design parameters. The anchor pins  28 , however, do not provide an electrical connection between the IC package  20  and the circuit board  22 . For the preferred embodiment described herein, the anchor pins  28  have a diameter about equal to an average diameter of the plurality of solder balls  30 , and are of a material having a considerably higher allowable stress and stiffness than the material of the solder balls  30 .  
         [0030]     A side view of the assembly  24  is shown in  FIG. 6 . The anchor pins  28  are illustrated penetrating both the IC package  20  and the circuit board  22 . In the preferred embodiment, each anchor pin  28  has a beveled head at the package end and a non-removable flattened fastener at the circuit board end. As mentioned previously, the exact shape, size, and end connections of the pins  28  are determined at design time considering all of the necessary factors. Essentially, the pins  28  are designed to anchor the package  20  to the circuit board  22 .  
         [0031]     Also shown in  FIG. 6  is the plurality of solder balls  30  providing the electrical connection of the IC package  20  to the circuit board  22 , the design of such connection being well known in the art.  
         [0032]     If the assembly  24  is subject to thermally induced mechanical loading or direct mechanical loading, both of which may be simultaneous and cyclic as previously described in detail for the prior art, the assembly  24  may bend about a single axis as shown in  FIG. 7 . Certainly, the assembly  24  may bend about multiple axes but for the sake of clarity, only single axis bending will be addressed in the description of the preferred embodiment herein. Consideration of multiple axes of bending can be accomplished by the well-known principle of superposition. Due to a difference in coefficient of expansion, if the assembly  24  undergoes thermally induced mechanical loading, or due to a difference in stiffness, if the assembly  24  undergoes direct mechanical loading, the circuit board  22  bends significantly more than the IC package  20 . Regardless of the cause of the bending, the solder balls  30  and anchor pins  28  are subject to a tensile loading. The mechanical properties of the anchor pins  28  allow them to support a substantial portion of the loading that would otherwise be taken by the solder balls  30 . Note the region  32  of the circuit board  22  that is held by the anchor pins  28  to conform to the shape of the package  20 . As a result, none of the solder balls  30  experiences a stress large enough to cause immediate or fatigue failure.  
         [0033]     A second embodiment of the present invention method is illustrated in  FIG. 8 , which shows a side view of the IC package  20  and the circuit board  22 . The package  20  is anchored to the circuit board  22  by a plurality of metal straps  32 . The metal straps  32  are disposed around the perimeter, on all four edges, of the IC package  20 . The metal straps  32  are fused to four edges of the package  20  and the circuit board  22 , and can be produced at the same time and by the same device as the electrically connecting solder balls  30 . It is important to note that the metal straps  32  do not provide any electrical connection between the package  20  and the circuit board  22 . The exact quantity, dimensions, placement, and material of the metal straps  32  are determined based on relevant design parameters, such as expected manufacturing or operating temperature range and external loading.  
         [0034]     A third embodiment of the present invention method is illustrated in  FIG. 9 , which shows a side view of the IC package  20  and the circuit board  22 . The package  20  is anchored to the circuit board  22  by a plurality of redundant solder balls  34 . The solder balls  34  are disposed at the four corners of the IC package  20 . These mechanically connecting solder balls  34  are fused to the bottom of the package  20  and the circuit board  22 , and can be produced at the same time and by the same device as the electrically connecting solder balls  30 . As similar with the previously described embodiments, the redundant solder balls  34  do not provide any electrical connection between the package  20  and the circuit board  22 . The exact quantity, diameter, placement, and material of the redundant solder balls  34  are determined based on relevant design parameters, such as expected manufacturing or operating temperature range and external loading.  
         [0035]     A fourth embodiment of the present invention method is illustrated in  FIG. 10 , which shows a bottom view of the IC package  20 . For clarity, the circuit board  22  is not shown in  FIG. 10 . A continuous strip of solder  36  is disposed along the perimeter of IC package  20  to mechanically connect the package  20  to the circuit board  22 . The solder strip  36  can be produced at the same time and by the same device as the electrically connecting solder balls  30 . No electrical connection between the package  20  and the circuit board  22  is provided by the solder strip  36 . The exact length, diameter, location, and material of the strip of solder  36  are determined based on relevant design parameters, such as expected manufacturing or operating temperature range and external loading. Alternatively, the solder strip  36  need not be continuous and may have interruptions in its length.  
         [0036]     In summary, the described method of attaching an IC package to a circuit board offers a strengthened mechanical connection. In contrast to the prior art, the present invention anchors support a sufficient share of a thermally induced or direct mechanical load, both of which may be simultaneous and cyclic, to prevent solder balls from becoming overstressed and cracking. This enhanced stiffness package attachment method will lead to less mechanical failures of solder balls.  
         [0037]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.