Warpage compensating heat spreader

[0002] The invention relates to flip-chip, ball grid array (BGA) packages whose improved thermal and mechanical design improves their reliability.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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).

[0008] Holes, fins, or separating borders may be fabricated by drilling, punching, shaping, etching, etc. to vary the stiffness.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] It is an object of this invent ion to provide an electronic module with longer BGA fatigue life and less warpage.

[0018] It is another object of the invention to provide an electronic module having relatively low thermal resistance.

[0019] 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:

[0020] FIG. 1 illustrates a sectional view of a prior art electronic package;

[0021] FIG. 2 shows a sectional view of a first embodiment of the improved electronic package of this invention;

[0022] FIG. 3 depicts a sectional view of a second embodiment of the electronic package shown in FIG. 2;

[0023] FIG. 4 illustrates a sectional view of a third embodiment of the electronic package shown in FIG. 2;

[0024] FIG. 5 shows a sectional view of a fourth embodiment of the electronic package depicted in FIG. 2;

[0025] FIG. 6 depicts a sectional view of a second embodiment of the electronic package shown in FIG. 5;

[0026] FIG. 7 illustrates a sectional view of a third embodiment of the electronic package depicted in FIG. 5;

[0027] FIG. 8 shows a sectional view of a fifth embodiment of the electronic package illustrated in FIG. 2;

[0028] FIG. 9 depicts a plan view of a sixth embodiment of the electronic package shown in FIG. 2;

[0029] FIG. 10 illustrates a sectional view of a seventh embodiment of the electronic package depicted in FIG. 2;

[0030] FIG. 11 shows a sectional view of a second embodiment of the electronic package illustrated in FIG. 10;

[0031] FIG. 12 illustrates a sectional view of a third embodiment of the electronic package shown in FIG. 10;

[0032] FIG. 13 depicts a sectional view of a second embodiment of the electronic package shown in FIG. 12;

[0033] FIG. 14 shows a sectional view of a fourth embodiment of the electronic package depicted in FIG. 5;

[0034] FIG. 15 illustrates a sectional view of a fifth embodiment of the electronic package depicted in FIG. 5;

[0035] FIG. 16 depicts a sectional view of a sixth embodiment of the electronic package shown in FIG. 5; and

[0036] FIG. 17 illustrates a sectional view of a second embodiment of the electronic package depicted in FIG. 8.

[0037] For purposes of clarity and brevity, like components and elements will bear the same numbering and designations throughout the figures.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] Referring to FIG. 3, a second embodiment 20aof 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.

[0043] Referring to FIG. 4, a third embodiment 20bof 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.

[0044] Referring to FIG. 5, a fourth embodiment 20cof 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.

[0045] Referring to FIG. 6, a second embodiment 21 of the module 20cshown 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.

[0046] Referring to FIG. 7, a third embodiment 21aof the module 20cshown 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.

[0047] Referring to FIG. 8, a fifth embodiment 20dis 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.

[0048] Referring to FIG. 9, a plan view of a sixth embodiment 20eof 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.

[0049] Referring to FIG. 10, a sectional view of a seventh embodiment 20fof 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.

[0050] Referring to FIG. 11, a sectional view of a second embodiment 20ffof the electronic package 20fillustrated 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.

[0051] Referring to FIG. 12, a sectional view of a third embodiment 20fffof the electronic package 20fshown 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.

[0052] Referring to FIG. 13, a sectional view of a second embodiment 20fgof the electronic package 20fffshown 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.

[0053] Referring to FIG. 14, a sectional view of a fourth embodiment 21bof the electronic package 20cdepicted 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.

[0054] Referring to FIG. 15, a sectional view of a fifth embodiment 21cof the electronic package 20cdepicted 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.

[0055] Referring to FIG. 16, a sectional view of a sixth embodiment 21dof the electronic package 20cdepicted 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.

[0056] Referring to FIG. 17, a sectional view of a second embodiment 20dfof the electronic package 20ddepicted in FIG. 8 is shown. It will be observed that the plurality of holes 32 is now disposed directly over chip 14.

[0057] 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.

[0058] Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.