Abstract:
A flipchip packaged integrated circuit comprising a substrate, a die and a heatspreader. The die may be configured to electrically attach to the substrate. The heat spreader may be rigidly attached to the die. The die and the heatspreader may have a combined coefficient of thermal expansion when attached. The heatspreader may be configured to match the combined coefficient of thermal expansion and a coefficient of thermal expansion of the substrate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and/or architecture for providing flipchip packaging generally and, more particularly, to an enhanced laminate flipchip package using a heatspreader with a high coefficient of thermal expansion (CTE). 
     BACKGROUND OF THE INVENTION 
     Flipchip interconnect technology supports an “area array interconnect,” in which a die (circuit) is mechanically and electrically connected through an array of solder bumps on the active face of the circuit. The flipchip technique increases the number of connections that can be made for a given die size and can also improve electrical performance. The die is attached to the substrate face down and can be reinforced with an epoxy underfill. 
     Referring to FIG. 1, a conventional laminate flipchip package  10  is shown. The flipchip package  10  includes a silicon die  12  attached to a laminate substrate  14  using flipchip solder bumps  16 . A gap between the die  12  and the substrate  14  is filled with an underfill  18 . A stiffener  20  is attached to the substrate  14  using a high modulus epoxy material  22  to flatten the substrate  14 . A heatspreader  24  is attached to (i) the die  12  using a thermally conductive low modulus material  26  such as thermal grease and (ii) the stiffener  20  using a high modulus epoxy material  28 . The substrate  14  has solder balls  30  on the side opposite the die  12 . 
     Component and board level reliability of conventional laminate flipchip packages can be a major concern. The die  12  can have a coefficient of thermal expansion (CTE) of approximately 3 ppm/° C. The laminate substrate  14  can have a CTE of approximately 17 ppm/° C. Because of the difference in expansion between the die  12  and the substrate  14  (i.e., the mismatched CTEs), the solder bumps  16  can fatigue and cause failure. To prevent fatigue of the solder bumps  16 , the conventional flipchip package  10  uses a high modulus underfill material  18  (i.e., a modulus in the range of 3 to 10 GPa). 
     In addition, a mismatch between the CTE of the die  12  and the CTE of the substrate  14  can warp the silicon die  12  causing tensile stress on the back side (non-active side) of the silicon die  12 . Tensile stress on the die  12  can lead to cracks in the silicon die  12  during processing and reliability testing. The warpage of the die  12  can contribute significantly to the overall warpage of the package  10 . Excess warpage in the region of the substrate  14  attached to the die  12  can cause cracks in the substrate  14  during stress testing. The stiffener  20  is needed to prevent excess warpage of the package  10 . Stress generated at the interface between the die  12  and the underfill material  18  due to the CTE mismatch can cause delamination of the underfill material  18  from the die  12 . The board level reliability of solder joints formed by the solder balls  30  can be compromised due to a CTE mismatch between the package  10  and the printed circuit board (PCB). 
     It would be desirable to have a laminate flipchip package that reduces and/or eliminates the component and board level reliability problems due to die and substrate CTE mismatch. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a flipchip packaged integrated circuit comprising a substrate, a die and a heatspreader. The die may be configured to electrically attach to the substrate. The heat spreader may be rigidly attached to the die. The die and the heatspreader may have a combined coefficient of thermal expansion when attached. The heatspreader may be configured to match the combined coefficient of thermal expansion and a coefficient of thermal expansion of the substrate. 
     The objects, features and advantages of the present invention include providing a method and/or architecture for an enhanced laminate flipchip package using a high CTE heatspreader that may (i) match the coefficient of thermal expansion (CTE) of the die and heatspreader combination and the CTE of the substrate, (ii) reduce or eliminate underfill material, (iii) reduce or eliminate die cracks, (iv) place the back side of the die in compression, (v) reduce or eliminate substrate warpage in the die region, (vi) reduce or eliminate substrate cracking due to excess warpage in the die region, (vii) make use of an optional stiffener, (viii) reduce stress at the die/underfill interface, (ix) increase the board level reliability of solder joints under the die, and/or (x) increase the component and board level reliability of the flipchip package. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
     FIG. 1 is a cross-sectional diagram of a conventional laminate board package; 
     FIG. 2 is a cross-sectional diagram of a preferred embodiment of the present invention; 
     FIG. 3 is a cross-sectional diagram of an alternate embodiment of the present invention; 
     FIG. 4 is a cross-sectional diagram of another alternate embodiment of the present invention; 
     FIG. 5 is a cross-sectional diagram of another alternate embodiment of the present invention; and 
     FIG. 6 is a cross-sectional diagram of another alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2, a cross-sectional diagram of a package  100  is shown in accordance with a preferred embodiment of the present invention. The package  100  may be implemented as an enhanced laminate flipchip package using a heatspreader with a high coefficient of thermal expansion (CTE). The package  100  generally comprises a die  102 , a heatspreader  104 , and a substrate  106 . The die  102  and the heatspreader  104  may be joined using a die attachment material  108  at any stage during an assembly process. In one example, the modulus of the die attachment material  108  may be greater than 100 MPa. The coefficient of thermal expansion (CTE) of the die  102  and the heatspreader  104  combination may be adjusted to match the CTE of the substrate  106 . 
     The die  102  may be implemented as a silicon die. However, other materials (e.g., germanium, etc.) may be implemented accordingly to meet the design requirements of a particular application. The heatspreader  104  may be implemented using a metallic material (e.g., aluminum). In one example, the heatspreader  104  may be implemented as a heat sink. However, other configurations may be implemented accordingly to meet the design criteria of a particular application. The heatspreader  104  may be coated (e.g., oxide coated, painted, etc.) or uncoated. The die  102  may have a CTE that is lower than a CTE of the substrate  106 . The heatspreader  104  generally has a CTE that is greater than a CTE of the substrate  106 . 
     The die attachment material  108  may be implemented as a high modulus, high glass transition temperature (Tg) adherent (e.g., an epoxy, a metal alloy, etc.). The die attachment material  108  may be configured to rigidly join the die  102  to the heatspreader  104 . The side of the die  102  that is attached to the heatspreader  104  may be in compression. Attaching the die  102  to the heatspreader  104  using the die attachment  108  may reduce the tendency of the die  102  to crack. 
     The die  102  may be mounted on the laminate substrate  106  via a plurality of flipchip solder bumps  110 . The solder bumps  110  may be configured to provide an area array interconnect such that the die  102  is mechanically and electrically connected to the substrate  106 . Optionally, an underfill  112  may be implemented to fill a gap between the die  102  and the substrate  106 . The present invention may allow the use of any appropriate underfill material. The substrate  106  generally comprises a plurality of solder balls  114  on a side opposite the side where the die  102  is attached. 
     The CTE of the die  102  is generally lower than the CTE of the substrate  106 . The heatspreader  104  generally has a CTE that is greater than the CTE of the substrate  106 . One or more characteristics of the heatspreader  104  may be adjusted to provide an effective CTE of the combination of the die  102  and the heatspreader  104  substantially the same as (matches) the CTE of the substrate  106 . In one example, a thickness of the heatspreader  104  may be adjusted to match the CTE of the die  102 /heatspreader  104  combination with the CTE of the substrate  106 . When the die (or wafer)  102  thickness is decreased, the thickness of the heatspreader  104  may be reduced. Similarly, in the example where the heatspreader  104  is implemented as a heat sink, thinning the die  102  may reduce the depth and/or number and/or change the shape of ribs on the heatspreader  104  required such that the CTE of the die  102 /heatspreader  104  combination matches the CTE of the substrate  106 . In another example, an area of the heatspreader  104  may be adjusted such that the CTE of the die  102 /heatspreader  104  combination matches the CTE of the substrate  106 . In one example, the area of the heatspreader  104  may be sized smaller than or equal to the area of the die  102 . 
     Referring to FIG. 3, a cross-sectional diagram of a package  100 ′ illustrating an alternative implementation of the package  100  is shown. The package  100 ′ may be implemented similarly to the package  100 . The area of heatspreader  104 ′ may be sized greater than the area of the die  102 . In one example, the heatspreader  104 ′ may be implemented as a heat sink. However, other configurations may be implemented accordingly to meet the design criteria of a particular application. 
     Referring to FIG. 4, a cross-sectional diagram of a package  100 ″ illustrating another alternative implementation of the package  100  is shown. The package  100 ″ may be implemented similarly to the package  100 . The package  100 ″ may comprise a stiffener  116 . The stiffener  116  may be joined to the substrate  106  using a stiffener attachment  118 . The stiffener  116  may have a CTE similar to the CTE of the substrate  106 . The stiffener attachment  118  may be a high modulus, high Tg adherent similar to the die attachment  108 . In one example, the modulus of the stifffener attachment  118  may be greater than 1 GPa. 
     Referring to FIG. 5, a cross-sectional diagram of a package  100 ′″ illustrating yet another alternative implementation of the package  100  is shown. The package  100 ′″ may be implemented similarly to the package  100 ″ except that the package  100 ′″ may comprise the heatspreader  104 ′ (e.g., the area of heatspreader  104 ′ may be sized greater than the area of the die  102 ). In general, the heatspreader  104 ′ may be unattached to the stiffener  116 . In one example, the heatspreader  104 ′ may be implemented as a heat sink. However, other configurations may be implemented accordingly to meet the design criteria of a particular application. 
     Referring to FIG. 6, a detailed sectional diagram of a package  100 ″″ illustrating still another alternative implementation of the package  100  is shown. The package  100 ″″ may be implemented similarly to the package  100 ′″ except that the heatspreader  104 ′ may be joined to the stiffener  116  using a heatspreader attachment  120 . The heatspreader attachment  120  may be implemented as a low modulus material (e.g., thermal grease, gel, etc.). The modulus of the heatspreader attachment  120  is generally less than 100 MPa. 
     The flipchip package  100  may have improved component and board level reliability when compared with conventional flipchip packages. The present invention may improve component and board level reliability of flipchip packages with laminate substrates. The present invention may match the effective CTE of the combination of a die and a heatspreader to the CTE of a laminate substrate. The present invention may reduce or eliminate warpage of the die  102 . The present invention may reduce (a) cracking of the die  102  due to warpage and (b) cracking the substrate  106  due to (i) warpage of the die  102  and/or (ii) warpage of the substrate  106 . The board level reliability of the solder joints between the package  100  and the printed circuit board (PCB) may be significantly increased. 
     The present invention may reduce or eliminate the underfill  112 . When the underfill  112  is present, the present invention may significantly reduce stress at the interface between the die  102  and the underfill  112 . The present invention may allow the stiffener  116  to have a reduced area and/or thickness or be eliminated. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.