posted on 2022-11-10, 12:10authored byAlessio Munari
Contemporary electronic devices such as microprocessors feature high component-level
heat-fluxes and tight temperature constraints. Thermal interface materials (TIMs) are
a critical element in the path for heat transfer from devices to ambient. The objectives
of this thesis are: to design, build and commission a thermal test facility capable of
measuring the thermal properties of TIM pads with an acceptable level of uncertainty;
and to manufacture and characterise nanomaterial-polymer composites and novel nanos-
tructures as viable TIM pads for electronic cooling applications.
A bench top test facility was designed and commissioned according to ASTM D 5470
Standard Test Method in order to measure the thermal impedance and e ective thermal
conductivity of both commercially-available and newly developed TIM pads under a wide
range of clamping pressures and input powers. The calibration process for this facility
was carried out using stainless steel disks as benchmarks with a thermal conductivity
of 14.85 Wm 1K1. The discrepancy was found to be within 1.08%, which confirmed
the successful design and implementation of the experimental setup. Dry-joint thermal
impedance tests were also conducted under a wide range of pressures for averaged input
powers based on the energy balance between the two meter bars: the heat losses were
in all the cases within 4%. At 2.5 MPa, a thermal impedance value of 0.525 Ccm2W-1
was measured with a calculated uncertainty of approximately 4.18%. Repeatability tests
showed variations within 1.26% and reproducibility tests were shown to be in good
agreement with previous studies.
Two principal types of nanocomposites were manufactured and characterised:
Batches of carbon nanotubes (CNTs), Ag nanowires (AgNWs) and Ag
akes were
randomly dispersed in a silicone elastomer matrix by both mechanical and solution
mixing at di erent weight percentages of the llers. Thermal impedances between
12{38 Ccm2W-1 resulted in one order of magnitude above the state-of-the-art of
thermal pads. It is believed that an aligned orientation of high-aspect-ratio fillers,
coupled with a reduced bond line thickness (BLT) of the pads, could lead to an
improved performance in terms of thermal impedance.
AgNW arrays embedded inside polycarbonate (PC) templates were fabricated by
electrodeposition. The vertically-aligned nanowires' protrusion from the mem-
brane surface appeared to enable them to conform to the surface roughness of
the contacting surfaces, resulting in better thermal contact. By depositing a 30
nm Au layer on top of the as-received composite, a thermal impedance value
of 0.93 Ccm2 W-1 was measured with a calculated uncertainty of 3.4% at a
contact pressure of 1 MPa. In order to reduce further the thermal impedance
of the samples, double-sided, ultra-long, high-density AgNW arrays were fabri-
cated using a nanoporous anodised aluminium oxide (AAO) template and their
feasibility as TIM pads was assessed. The new composite showed a reduction of
thermal impedance of about 37% at 0.95 MPa when compared to that of AgNW-
PC. By wetting the double-sided AgNW array with mineral oil, a value of thermal
impedance as low as 0.245 Ccm2W-1 was measured at 1 MPa with an uncertainty
of 8.47%. The double-sided AgNW array matched well with the state-of-the-art
of commercially-available thermal pads.
The ndings of the thesis are of practical relevance to the development of AgNW
arrays as a viable solution for component-level thermal management applications, where
thermal interface impedance is a critical factor.