Abstract:
A heat spreading cap is placed over a chip or integrated circuit. The cap is shaped or sized to provide a distinct heat spreading and/or stiffness characteristic that differs as it extends into different regions of the module. The areas of differing stiffness or CTE reduce the warpage (or bending) of the module, thereby reducing the overall stress in the BGA.

Description:
FIELD OF THE INVENTION 
     The invention relates to flip-chip, ball grid array (BGA) packages whose improved thermal and mechanical design improves their reliability. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of plastic ball grid array (PBGA) packages, a chip is electrically and mechanically attached to a laminated, organic substrate. The thermal expansion of the material of the chip is not usually compatible with that of the organic laminate material. The mismatch in coefficients of thermal expansion (CTE) and contraction causes the module to warp, and leads to fatigue failure of the BGA. 
     In order to maintain the flatness of a PBGA package, especially those with a flip-chip, the chip may be overlaid with a heat spreading cap, which has been designed to balance the CTE and stiffness of the substrate that is disposed on the opposite side of the chip. However, the heat spreading cap has been only partially successful in maintaining flatness because it correctly balances the substrate only directly above the chip. 
     Another source of package warping has been determined to be the epoxy adhesive that cements the chip and cap to the substrate. The epoxy adhesive, which extends beyond the periphery of the cap, may not match the CTE of the substrate. Also, it has been observed that the epoxy tends to absorb and desorb moisture, causing additional expansion and contraction and consequent warpage of the module. 
     The present invention represents means by which the combined thermal mismatches between the chip, cap, adhesive, and laminated substrate can be overcome to maintain flatness over the operating temperature range, thereby increasing the fatigue life of the BGA. 
     The current inventors seek to vary the thickness, shape, or materials of the cap where it extends beyond the periphery of the chip. This variation in thickness, material, or shape resolves the mismatch problem by changing either the stiffness, the thermal expansion, or both, in the affected regions. The objective can also be achieved by the introduction of holes, grooves, and multiple materials in the cap(s). 
     Holes, fins, or separating borders may be fabricated by drilling, punching, shaping, etching, etc. to vary the stiffness. 
     The methods of the invention have been found to significantly reduce module warpage and to increase the fatigue life of the BGA while maintaining good heat dissipation, thus providing a structurally reliable module. 
     DISCUSSION OF RELATED ART 
     In U.S. Pat. No. 5,287,248, issued to Montesano, on Feb. 15, 1994 for METAL MATRIX COMPOSITE HEAT TRANSFER DEVICE AND METHOD, a construction is shown that conducts heat from a heat source to a heat sink. Thermal conduction is improved for mounting boards that support heat generating components. 
     In U.S. Pat. No. 5,247,426, issued on Sep. 21, 1993 to Hamburgen et al for SEMICONDUCTOR HEAT REMOVAL APPARATUS WITH NON-UNIFORM CONDUCTANCE, an apparatus for removing heat from a semiconductor is illustrated. The apparatus features different regions of high and low thermal conductance. 
     In U.S. Pat. No. 5,537,342, issued to Gainey on Jul. 16, 1996 for ENCAPSULATION OF ELECTRONIC COMPONENTS, an electronic device is shown for reducing stress. The device features an encapsulating member comprising a sandwich of two premolded parts. 
     In U.S. Pat. No. 5,736,785, issued on Apr. 7, 1998 to Chiang et al for SEMICONDUCTOR PACKAGE FOR IMPROVING THE CAPABILITY OF SPREADING HEAT, a heat spreading device is shown that is attached to a die. The die is supported by a substrate comprising a conductive pad for connection to a solder ball. 
     In U.S. Pat. No. 5,777,847, issued on Jul. 7, 1998 to Tokuno et al for MULTICHIP MODULE HAVING A COVER WITH SUPPORT PILLAR, a multichip is shown that comprises a substrate supporting a plurality of circuit chips. A cover plate covers semiconductor chips that are attached to a substrate. 
     In U.S. Pat. No. 5,831,826, issued to Van Ryswyk on Nov. 3, 1998 for HEAT TRANSFER APPARATUS SUITABLE FOR USE IN A CIRCUIT BOARD ASSEMBLY, a heat transfer apparatus is illustrated. The heat transfer apparatus is carried upon a substrate supporting a plurality of pads that connect to electronic components. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a module having a substrate upon which at least one chip or integrated circuit is mounted. A heat spreading cap is disposed over each chip or integrated circuit. The cap is shaped or sized to provide distinct combinations of stiffness and CTE in at least two areas. The configuration of the cap changes as it extends beyond the periphery of the chip. 
     It is an object of this invent ion to provide an electronic module with longer BGA fatigue life and less warpage. 
     It is another object of the invention to provide an electronic module having relatively low thermal resistance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which: 
     FIG. 1 illustrates a sectional view of a prior art electronic package; 
     FIG. 2 shows a sectional view of a first embodiment of the improved electronic package of this invention; 
     FIG. 3 depicts a sectional view of a second embodiment of the electronic package shown in FIG. 2; 
     FIG. 4 illustrates a sectional view of a third embodiment of the electronic package shown in FIG. 2; 
     FIG. 5 shows a sectional view of a fourth embodiment of the electronic package depicted in FIG. 2; 
     FIG. 6 depicts a sectional view of a second embodiment of the electronic package shown in FIG. 5; 
     FIG. 7 illustrates a sectional view of a third embodiment of the electronic package depicted in FIG. 5; 
     FIG. 8 shows a sectional view of a fifth embodiment of the electronic package illustrated in FIG. 2; 
     FIG. 9 depicts a plan view of a sixth embodiment of the electronic package shown in FIG. 2; 
     FIG. 10 illustrates a sectional view of a seventh embodiment of the electronic package depicted in FIG. 2; 
     FIG. 11 shows a sectional view of a second embodiment of the electronic package illustrated in FIG. 10; 
     FIG. 12 illustrates a sectional view of a third embodiment of the electronic package shown in FIG. 10; 
     FIG. 13 depicts a sectional view of a second embodiment of the electronic package shown in FIG. 12; 
     FIG. 14 shows a sectional view of a fourth embodiment of the electronic package depicted in FIG. 5; 
     FIG. 15 illustrates a sectional view of a fifth embodiment of the electronic package depicted in FIG. 5; 
     FIG. 16 depicts a sectional view of a sixth embodiment of the electronic package shown in FIG. 5; and 
     FIG. 17 illustrates a sectional view of a second embodiment of the electronic package depicted in FIG.  8 . 
     For purposes of clarity and brevity, like components and elements will bear the same numbering and designations throughout the figures. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Generally speaking, the invention features a module having a substrate upon which a chip or integrated circuit is mounted. A heat spreading cap is disposed over the chip or integrated circuit on the surface opposite to that of the substrate. The cap is shaped or sized to provide distinct combinations of CTE and stiffness in different regions of the module. 
     Now referring to FIG. 1, a sectional view of a prior art electronic circuit module  10  is illustrated. The module  10  comprises a substrate  12  upon which a chip  14  is mounted by means of solder balls  16  that are disposed within the underfill material  15 . The module  10  is itself mountable by solder balls  17 . 
     A heat spreader or cap  18  is mounted on top of the chip  14  in order to dissipate heat and counter-balance the forces exerted by the thermal mismatch between the chip  14  and the substrate  12 . The cap  18  is attached to the chip and substrate by means of encapsulant  19 , which is usually an epoxy. The cap  18  is designed to counter-balance the forces that arise from the thermal mismatch between the substrate  12  and the chip  14 . However, the counter-balancing cannot be achieved in entire module  20  with a homogeneous and uniform thick cap  18 . 
     Referring to FIG. 2, a sectional view of a first embodiment of the module  20  of this invention is shown. The inventive device is shaped and sized to reduce the aforementioned warpage inherent in the prior art design shown in FIG.  1 . The heat spreading cap  22  is designed with a first thickness above the chip that balances the stiffness and expansion of the substrate disposed on the opposite side of the chip  14 . A second thickness is used beyond the periphery of the chip  14  to balance the composite structure in this region. An upper, substantially flat surface is obtained, to which a heat sink, not shown, may be attached. It should be understood that the heat spreader can be disposed above the chip, covering a portion of the chip, extending beyond the periphery of the chip, or disposed entirely beyond the periphery of the chip, as variously shown in the remaining FIGURES. 
     Referring to FIG. 3, a second embodiment  20   a  of the module  20  shown in FIG. 2 is illustrated. The heat spreading cap  22  is configured with a mid-section  25  and a peripheral section  26 . 
     Referring to FIG. 4, a third embodiment  20   b  of the module  20  depicted in FIG. 2 is shown. It will be observed that the peripheral cap section  26  is built up above and below the mid-section  25 . All of the foregoing three embodiments shown, respectively, in FIGS. 2,  3  and  4  achieve flatness by varying the stiffness with a thickness change alone. 
     Referring to FIG. 5, a fourth embodiment  20   c  of the module  20  illustrated in FIG. 2 is shown. It will be observed that the cap  22  is now composed of two different material layers  28  and  29 , respectively. Thus, both stiffness and CTE are adjusted in this embodiment as well as in the embodiments depicted in FIGS. 6 and 7, described below. 
     Referring to FIG. 6, a second embodiment  21  of the module  20   c  shown in FIG. 5 is illustrated. The cap  22  is now shown having a mid-section  25  and a peripheral section  26 , which is similar to the cap  22  shown in FIG.  3 . However, the cap  22  now comprises two different materials,  28  and  29 , as shown in FIG.  5 . It should be understood that the heat spreader can be disposed above the chip, covering a portion of the chip, extending beyond the periphery of the chip, or disposed entirely beyond the periphery of the chip, as variously shown in the remaining FIGURES. 
     Referring to FIG. 7, a third embodiment  21   a  of the module  20   c  shown in FIG. 5 is depicted. It will be observed that the cap  22  can now comprise three different materials  29 ,  28 , and  30 , respectively, as shown on the righthand side of the FIGURE. Alternatively, two material layers  29  can sandwich the middle material  28 , as shown on the lefthand side of the FIGURE. 
     Referring to FIG. 8, a fifth embodiment  20   d  is shown of the module  20  illustrated in FIG.  2 . In this embodiment, the heat spreading layer  22  comprises a plurality of holes or grooves  32 . The holes or grooves  32  are filled with encapsulant  24  during assembly, thereby aiding adhesion. 
     Referring to FIG. 9, a plan view of a sixth embodiment  20   e  of the electronic package  20  illustrated in FIG. 2 is shown. It will be observed that the heat spreading layer  22  is configured with a staggered border  33 . The staggered border  33  comprises holes  34  that form a discontinuous rectangle in the spreading layer  22 . 
     Referring to FIG. 10, a sectional view of a seventh embodiment  20   f  of the electronic package shown in FIG. 2 is illustrated. It will be noted that the heat spreading layer  22  is now tapered upwardly in the outer region  35  that extends beyond the chip periphery  11 . 
     Referring to FIG. 11, a sectional view of a second embodiment  20   ff  of the electronic package  20   f  illustrated in FIG. 10 is shown. It will be noted that the heat spreading layer  22  is now tapered downwardly in the outer region  35  that extends beyond the chip periphery  11 . 
     Referring to FIG. 12, a sectional view of a third embodiment  20   fff  of the electronic package  20   f  shown in FIG. 10 is illustrated. It will be noted that the heat spreading layer  22  is now tapered both upwardly and downwardly in region  35 , as it extends beyond the chip periphery  11 . 
     Referring to FIG. 13, a sectional view of a second embodiment  20   fg  of the electronic package  20   fff  shown in FIG. 12 is shown. It will be observed that the heat spreading layer  22  has a number of heat dissipating fins  38  disposed upon its upper surface  39 . 
     Referring to FIG. 14, a sectional view of a fourth embodiment  21   b  of the electronic package  20   c  depicted in FIG. 5 is illustrated. Heat spreading cap  22  is shown comprising two different layers  40  and  41 , respectively. Layer  41  is disposed directly over the chip  14 , and comprises a different material than that of the extended layer  40 . 
     Referring to FIG. 15, a sectional view of a fifth embodiment  21   c  of the electronic package  20   c  depicted in FIG. 5 is illustrated. Heat spreading cap  22  is shown comprising two different layers  40  and  41 , respectively. Layer  41  is disposed directly over the chip  14  and above layer  40 , which extends beyond the chip region. 
     Referring to FIG. 16, a sectional view of a sixth embodiment  21   d  of the electronic package  20   c  depicted in FIG. 5 is illustrated. Heat spreading cap  22  is shown comprised of three different layers. Two of the three layers comprise upper and lower layers  40  that sandwich layer  41 . Layers  41  are disposed directly over the chip  14 . Layer  40  extends beyond the chip region. Layers  41  can be of the same material or of different materials. The material(s) of layer(s)  41  are different than the material in layer  40 . 
     Referring to FIG. 17, a sectional view of a second embodiment  20   df  of the electronic package  20   d  depicted in FIG. 8 is shown. It will be observed that the plurality of holes  32  is now disposed directly over chip  14 . 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.