Compressible thermally-conductive interface

Details Compressible thermally-conductive interface Compressible thermally-conductive interface

2000-10-12

2002-12-17

Tolin, Gerald (Department: 2835)

Electricity: electrical systems and devices

Housing or mounting assemblies with diverse electrical...

For electronic systems and devices

C156S306600, C165S185000, C174S016300, C428S040500

Reexamination Certificate

active

06496373

ABSTRACT:

The present invention relates to a thermal interface and, in particular, to a compressible thermally-conductive interface.
Management of the operating temperature of modern electronic components is often a substantial challenge in that as more and more functionality is provided in electronic components of seemingly ever-decreasing size, the removal of heat generated by such component becomes more difficult. In addition, these electronic components are being made to operate at faster operating rates which also increases the heat generated by such components. Thus, the problem of heat removal is compounded by the combined effects of smaller size and greater operating frequency.
Conventional approaches to removing heat from electronic components involve the transfer of the heat to a thermal dissipating element, such as a heat sink, cold plate or other suitable heat removal means. In the prior art arrangement of
FIG. 1
, for example, a semiconductor die
10
, which produces heat when operating, is coupled to a heat sink or heat spreader
20
, which dissipates heat aided by heat sink fins
22
. Die
10
is mounted to a circuit board
12
and is connected thereby to external apparatus (not shown) by solder ball or pin connections
14
. Typically, die
10
is protected by an encapsulant
16
, such as a molded plastic. To provide a heat transfer path between die
10
and heat sink
20
, an interface of a thermally-conductive material is placed between die
10
and heat sink
20
, and the collection of die
10
, circuit board
12
, heat sink
20
and interface
24
are mechanically fastened and held together, such as by clamps or clips
30
. Typical conventional interface pads
24
may be conformable to an irregularly-shaped component, but they are not compressible, which limits their utility.
Heat transfer is often aided if the interface pad
24
undergoes a phase change, i.e. melt flows, at suitably low temperature under the pressure of clamps
30
, thereby to better conform and intimately contact the surfaces of the heat generating component
10
and of the heat dissipating elements
20
. Such interface pads
24
have little or no bonding strength and so must be secured mechanically, as by clamps, clips and the like. Conductive interface pads such as type CP7508 available from AI Technology, Inc. of Princeton, N.J., will flex and melt-reflow at about 50° C. under normal clamping pressures, although they are not compressible and have little or no bonding strength to bond die
10
to heat sink
20
. While the melting helps to eliminate trapped air spaces and voids between the two interfacing surfaces, even if some bonding strength develops after melt-reflow, that bonding strength drops essentially to zero each time the melt-reflow temperature is reached.
The lack of compressibility, low-temperature phase change and/or substantial bonding strength of conventional interface pads tends to limit their effectiveness and/or convenience of use in improving heat transfer across an interface. The need for mechanical clamps and fasteners can add as much as US$0.50-US$1.00 to the installed cost of a typical integrated circuit, such as a CPU for a computer.
While the conventional interface pads may work satisfactorily for a single component, such as a CPU chip, they are less effective for modules including plural components that can have a planarity tolerance of the heights of the various components as large as about 0.5 mm (about 20 mils) as illustrated in
FIG. 2. A
plurality of heat-generating and non-heat generating components
10
mounted to a circuit board or substrate
12
have top surfaces that are either at different heights or are tilted. Such differences in height arise, for example, from differences in the height of the components
10
and differences in their mounting to and placement on substrate
12
. A conformable interface pad is flexible and so will tend to bend or drape across the differing height components
10
, but is inherently incapable of being in intimate thermal contact with the heat sink
20
and both the taller and shorter components
10
. A similar problem exists for a single component
10
that has an irregular or non-planar surface.
A compressible interface pad, if available, could accommodate such planarity tolerances. Prior art compressible interface materials include thermal greases and gels which are messy to use and require mechanical clips or clamps, and; elastomers that do not under go a phase change or melt-flow within the safe case operating temperatures of typical electronic components, and which often also require clips or clamps.
Accordingly, there is a need for a thermal interface material that provides the desirable characteristics of compressibility with a low temperature phase change or compressibility with low-temperature in situ curing to a substantial bonding strength.
To this end, the thermally conductive interface of the present invention comprises a blend of a compressible binder and at least one thermally conductive filler, wherein the compressible binder includes one of (a) a mixture of a thermoplastic rubber and a thermoplastic pressure sensitive adhesive, (b) a mixture of an epoxy blend and a curing agent for curing at less than 75° C. and a thermoplastic pressure sensitive adhesive and (c) a mixture of an epoxy blend and a curing agent for curing at less than 75° C. and a thermoplastic rubber or elastomer.


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