<p>With the extreme increases in power density of electronic
devices, the contact thermal resistance imposed at interfaces between mating solids
becomes a major challenge in thermal management. This contact thermal
resistance is mainly caused by micro-scale surface asperities (roughness) and
wavy profile of surface (nonflatness) which severely reduce the contact area
available for heat conduction. High contact pressures (1~100 MPa) can be used
to deform the surface asperities to increase contact area. Besides, a variety
of conventional thermal interface materials (TIM), such as greases and pastes,
are used to improve the contact thermal conductance by filling the remaining
air gaps. However, there are still some applications where such TIMs are
disallowed for reworkability concerns. For example, heat must be transferred
across dry interfaces to a heat sink in pluggable opto-electronic transceivers
which needs to repeatedly slide into / out of contact with the heat sink. Dry
contact and low contact pressures are required for this sliding application.</p>
<p>This dissertation presents a metallized micro-spring array
as a surface coating to enhance dry contact thermal conductance under ultra-low
interfacial contact pressure. The shape of the micro-springs is designed to be
mechanically compliant to achieve conformal contact between nonflat surfaces.
The polymer scaffolds of the micro-structured TIMs are fabricated by using a
custom projection micro-stereolithography (μSL) system. By applying the
projection scheme, this method is more cost-effective and high-throughput than
other 3D micro-fabrication methods using a scanning scheme. The thermal
conductance of polymer micro-springs is further enhanced by metallization using
plating and surface polishing on their top surfaces. The measured mechanical
compliance of TIMs indicates that they can deform ~10s μm under ~10s kPa
contact pressures over their footprint area, which is large enough to
accommodate most of surface nonflatness of electronic packages. The measured
thermal resistances of the TIM at different fabrication stages confirms the
enhanced thermal conductance by applying metallization and surface polishing.
Thermal resistances of the TIMs are compared to direct metal-to-metal contact
thermal resistance for flat and nonflat mating surfaces, which confirms that
the TIM outperforms direct contact. A thin layer of soft polymer is coated on
the top surfaces of the TIMs to accommodate surface roughness that has a
smaller spatial period than the micro-springs. For rough surfaces, the
polymer-coated TIM has reduced thermal resistance which is comparable to a
benchmark case where the top surfaces of the TIM are glued to the mating
surface. A polymer base is
designed under the micro-spring array which can provide the advantages for
handling as a standalone material or integration convenience, at the toll of an
increased insertion resistance. Through-holes are designed in the base
layer and coated with thermally conductive metal after metallization to enhance
thermal conductance of the base layer; a thin layer of epoxy is applied between
the base layer and the working surface to reduce contact thermal resistance exposed
on the base layer. Cycling tests are conducted on the TIMs; the results show
good early-stage reliability of the TIM under normal pressure, sliding contact,
and temperature cycles. The TIM is thermally demonstrated on a pluggable
application, namely, a CFP4 module, which shows enhanced thermal conductance by
applying the TIM. </p>
To further enhance the potential mechanical
compliance of microstructured surfaces, a stable double curved beam structure
with near-zero stiffness composed of intrinsic negative and positive stiffness
elastic elements is designed and fabricated by introducing residual stresses.
Stiffness measurements shows that the positive-stiffness single curved beam,
which is the same as the top beam in the double curved beam, is stiffer than the
double curved beam, which confirms the negative stiffness of the bottom beam in
the double curved beam. Layered near zero-stiffness materials made of these
structures are built to demonstrate the scalability of the zero-stiffness zone.
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12739730 |
| Date | 04 August 2020 |
| Creators | Jin Cui (9187607) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Relation | https://figshare.com/articles/thesis/COMPLIANT_MICROSTRUCTURES_FOR_ENHANCED_THERMAL_CONDUCTANCE_ACROSS_INTERFACES/12739730 |
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