Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – Insulating material
Reexamination Certificate
1999-10-19
2001-04-24
Williams, Alexander O. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
Insulating material
C257S710000, C257S712000, C257S713000, C257S737000, C257S738000, C257S778000, C257S727000, C257S719000, C257S718000
Reexamination Certificate
active
06222263
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is generally directed to an electronic chip assembly (which is hereinafter also referred to as a module) in which structural support is provided for a lid. More particularly, the present invention is directed to a system and method for providing structural support between the lid and substrate through the use of a curable material, such as epoxy, as a mechanism for off-loading mechanical forces applied to the lid, particularly in those situations in which a compressive load is applied during chip operation. Even more particularly, the present invention is directed to an apparatus and method for supporting a lid through the utilization of holes in the lid, together with disks or pads which are affixed to a substrate beneath lid apertures, a curable material such as epoxy disposed being between the disks and the lid and bonding the two together.
The present invention relates to semiconductor packaging both for single chip and for multi-chip modules. One significant semiconductor packaging technology employs chips that are disposed in the so-called flip-chip configuration. In this configuration small solder balls are affixed to an electronic circuit chip device at appropriate points on the device and the chip is affixed to and disposed against a substrate which contains electrical interconnections which, in turn, provide electrical signal paths to and from the chip device. Typically the packaging of the devices in a flip-chip configuration employs the so-called C4 technology. In general, in flip-chip configurations, when chip cooling is necessary, which is typically the case, a lid or heat spreader is affixed to the back side of the chip. In flip-chip configurations, a majority of the chip cooling is provided by means of access to the back side of the chip. Such cooling may be active or passive in nature, but the present invention is applicable in either case.
The C4 packaging technology is typically employed for high end, high speed electronic circuit chip devices found in high end computer systems. However, the present invention is not limited to computer systems. Nonetheless, whenever they are used, because of the high speed and high power requirements for these chip devices, it is very desirable to ensure that there is a low resistance thermal path between the chip and its lid. In particular, two approaches that are useable for the interface between the chip and its lid include (1) adhesives and (2) solder.
High end flip-chip package design also frequently utilizes land grid array (LGA) interconnection techniques between the substrate and a card or board. In order to ensure reliable, low electrical resistivity connections between interconnection pads on the bottom of the substrate and corresponding interconnection pads on the top of the card via a compressible electrically conductive interposer (hereafter referred to as an LGA socket) the module is typically clamped against the card during normal chip operation (hereinafter, said clamping is referred to as the LGA socketing load). Accordingly, it is very desirable to employ lid attachment mechanisms and structures which not only provide a reliable, highly conductive thermal path but which also distribute this mechanical load across the lid and substrate in such a manner so as not to negatively impact the structural integrity of the thermal interface, the chip, its interconnections, or the substrate.
Accordingly, it is seen that for modules that have lids of Al, Cu, CuW, AlSiC, SiC, CuSiC, AiN, diamond, graphite, or other composite materials which are directly attached to a chip with adhesive or solder it is very desirable to employ structural support, at the module level, so as to prevent damage to the chip, its interconnections, the substrate, or the thermal interface. When the module uses LGA substrate-to-card interconnections and the LGA socketing load is applied through the lid, a lid support mechanism is desired so as to distribute the mechanical LGA socketing load through the supports, thus avoiding transmission of the full load through the chip itself and thus also avoiding high internal substrate stresses.
However, it is noted that there are several features that any solution to this problem should address. In particular, it is noted that whatever mechanism is provided for this load distribution, it should fit within a low profile region having a thickness of less than approximately 1.0 mm between the lid and the substrate. Furthermore, the structure provided should be rigid after the lid is attached. Additionally, it is noted that there are requirements for a certain degree of compliance in the lateral direction as result of mismatches of thermal expansion coefficients for the materials employed within the structure. Specifically, there is a thermal expansion matching goal with respect to the lid and the substrate. In addition, the module structure employed should be able to support heavy (in the present context) lids. Accordingly, it is desirable to provide a rigid lid and a support structure which is rigid in the axis of the LGA socketing load and compliant in the plane normal to this load.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention a metal disc or pad is attached to at least three spaced-apart portions of a substrate (and preferably to the four comers of a substrate) with a thin layer of a compliant silicone adhesive material. A corresponding lid is provided with a hole or aperture over each disc location. The lid is larger in horizontal extent than the chip and is attached to the back of the chip. An adhesive material is dispensed through the apertures in the lid, filling in the gap between the disc and the lid thus attaching the lid to the discs below their respective holes. In particular, preferred adhesives include epoxies and or any other hardenable materials which are dispensable, but which thereafter cure or harden so as to be able to withstand compressive loads without significant compression. In preferred embodiments of the present invention, the holes provided are countersunk on both sides of the lid to provide a larger anchoring surface for the adhesive which extends into the lid apertures so as to better bond to the lid.
In accordance with this embodiment of the present invention an electronic chip assembly, in which structural support is provided, includes a substrate which has electrical conductors thereon (and preferably also therein). An electronic circuit chip is affixed via solder bumps to the substrate so as to make electrical connection to the conductors on the substrate. A thermally conductive material is disposed on the back-side of the chip so as to be in thermal contact with a substantially flat thermally conductive lid which is disposed over the chip. The lid has a greater horizontal extent than the chip, thereby providing the lid with an overhanging portion which includes at least three holes. That is, the holes (or apertures) are in the overhanging portion of the lid. A hardenable bonding material, such as epoxy, is disposed within the holes and in contact with the disks beneath the holes so as to bond the lid to the substrate (via the disks) so as produce a structure which permits transmission of compression loads from the lid directly to the substrate. The disk-to-substrate compliant adhesive's bondline and modulus are such that the amount of compression due to the LGA socketing load is low, allowing the support structure to bear a significant portion of the compressive load.
Accordingly, it is an object of the present invention to provide structural support for a lid which is directly attached to an electronic integrated circuit chip device.
It is also and object of the present invention to provide a structure which is suitable for the utilization of thermal interface materials such as solder, adhesives and epoxies.
It is yet another object of the present invention to provide a structural support mechanism which is compliant in a lateral direction while at the same time being rigid in a di
Sherif Raed
Toy Hilton T.
Womac David J.
Anderson Jay H.
Cutter Lawrence D.
International Business Machines - Corporation
Williams Alexander O.
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