Method for direct attachment of a chip to a cooling member

Details Method for direct attachment of a chip to a cooling member Method for direct attachment of a chip to a cooling member

1999-02-01

2002-07-02

Ball, Michael W. (Department: 1733)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C438S119000

Reexamination Certificate

active

06413353

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a semiconductor chip assembly and, in particular, to an apparatus that uses a flexible, silicone elastomer adhesive to directly join a chip or plurality of chips to one or several cooling members.
2. Description of Related Art
Semiconductor devices are continually becoming smaller and more dense with the evolution of new technology. However, increases in circuit density produce corresponding changes in overall chip packaging strategies in order to remain competitive. Chip and chip carrier manufacturers are therefore constantly challenged to improve the quality of their products by identifying and eliminating problems, reducing package size and weight, decreasing package costs and providing improved thermal efficiencies with new generations of advanced devices. Whereas significant improvements are being made to eliminate systematic problems by reducing process variability, process improvements alone are not sufficient to eliminate all the problems which affect both performance and reliability.
FIG. 1
illustrates a currently employed method and apparatus for sealing a metal cap
11
′, to a ceramic substrate
12
, by means of a sealant or adhesive bond
13
. Typically, chip
14
, is first secured to a substrate
12
, via a plurality of solder balls
15
on pads
16
that reside on the top surface of the substrate
12
. Substrate
12
could also have one or more electronic devices
17
such as a decoupling capacitor
17
that is also electrically connected to substrate
12
, via metallized pads
16
and either solder balls or surface mountable solder. For some applications solder balls
15
and pads
16
could be encapsulated with a polymeric underfill material
18
. A semi-liquid or paste type thermally conductive material
19
is usually applied over the exposed surface of chip
14
such that a direct thermal contact is made between the chip
14
and the cap
11
′ when cap
11
′ is placed so as to cover chip
14
. A cap adhesive sealant
13
is typically provided in order to secure cap
11
′ to the substrate or module
12
. Heatsink
20
can be secured to cap
11
′ using a heatsink adhesive
21
. Substrate
12
is typically secured electrically and mechanically to a mother board, card or socket
22
, via I/O (Input/Output) means
23
, such as pads, pins, solder balls, solder columns, etc.
Cap
11
′, typically fashioned from metals or ceramics having either high thermal conductivity and/or matched thermal expansivity to the chip carrier, is placed over chip
14
and is permanently secured to the surface of substrate
12
. As shown, cap
11
′ has sidewall portions
11
a
and usually completely seals the sides of the chip module from the surrounding environment. This is done primarily to prevent mechanical and chemical injury to chip
14
, solder balls
15
, decoupling capacitors
17
, underfill
18
and any exposed metallurgy or circuitry on the substrate
12
. It is well known that a leak in cap
11
′ may result in irrecoverable module yield losses and degrade expected reliability performance under actual use conditions. A picture-frame type area on the top surface of the substrate
12
is required to specifically seal cap
11
′ to substrate
12
using cap sealant
13
. The width of this frame type will vary as a function of overall substrate size, using experimentally derived data, but includes the actual seal area in addition to associated required clearances
16
,
17
around the seal to eliminate sealant runover or mechanical damage to devices during assembly. Therefore, the placement of all devices, such as, for example, chips
14
, decoupling capacitors
17
, is restricted to be within this picture frame area, which is typically 50 percent, of the area that would otherwise be available for additional or larger devices. Additionally, cap
11
′ typically adds between 30 percent and about 50 percent to the overall height of the module. Furthermore, the presence of cap
11
′ adds additional weight to the completed or assembled module.
The entirely enclosed internal cavity
28
created by the cap and seal will tend to trap moisture. This leads to high internal pressures and related damage during high temperature (above 100° C.) operations, such as joining the module to a mother board. This phenomenon, known in the industry as “moisture sensitivity,” drives added cost through special handling and preparation procedures, such as dry-bagging with dessicants and ambient exposure time limits to minimize the amount of moisture in the chip carrier prior to high temperature processes.
Thermal compound
19
must be placed between chip
14
and cap
11
′ to provide an efficient heat transfer path via the heatsink adhesive
21
to the heatsink
20
. The thermal compound is typically of semi-liquid or paste consistency to absorb the large thermally induced strains associated with this “doubly-connected” structure before they are transmitted to and damage the chip
14
.
In some cases, thermally conductive epoxies have been used to provide a better thermal contact between the chip and the heat sink, while others have used thermally conductive pastes, greases and/or oil films. For example, U.S. Pat. No. 5,367,193 discloses use of Dow Corning 340 heat sink compound, which is a grease-like silicone material filled with metal oxide filler, in conjunction with a multistructural mechanical support. All of these methods suffer from one or more drawbacks, such as poor thermal performance, brittleness at low temperatures, material flow or movement over time, or the need for secondary mechanical support.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus which has a thermally conductive mechanically robust path between at least one chip and at least one cooling member.
Yet another purpose of the invention is to increase the available area on the substrate or the chip carrier for device joining, for example, active devices, such as chips, or passive devices, such as capacitors, resistors, etc.
Another purpose of the invention is to provide a method and apparatus to support high cooling member weights, for example, lid plus heat sink assemblies of at least 100 grams.
Another purpose of the invention is to provide a method and apparatus for ensuring the mechanical and operational integrity of the bond between devices and the cooling member under typical use conditions such as gravity, mechanical shock, vibration, high temperature with humidity and repeated thermal expansion/contraction cycles due to temperature variation.
Still another purpose of the invention is to provide a method and apparatus that will minimize thermal performance degradation over the chip carrier life.
Still yet another purpose of the invention is to provide a method and apparatus that will absorb thermally inducted strain without damage to the chip carrier or associated devices.
Yet another purpose of the invention is the ability to rework or repair the completed or assembled module in a simple, efficient manner. still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
SUMMARY OF THE INVENTION
The above and other objects, which will be apparent to those skilled in the art, are achieved in the present invention which provides in one aspect a chip assembly comprising a substrate having an upper surface for carrying one or more semiconductor chips, at least one semiconductor chip mounted on the upper surface of the substrate and a cap covering the semiconductor chip and the upper surface of the substrate. A silicone adhesive is placed between the semiconductor chip and the cap. The adhesive has sufficient bond strength to secure the cap to the chip without additional mechanical constraint while providing a direct thermally conductive path and permitting sufficient heat flow from the chip to the cap to maintain

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