Thermal interface materials (TIMs) play a vital role in the performance of electronic packages by enabling improved heat dissipation. These materials typically have high thermal conductivity and are designed to offer a lower thermal resistance path for efficient heat transfer. For some semiconductor components, thermal solutions are attached directly to the bare silicon die using TIM materials, while other components use an integrated heat spreader (IHS) attached on top of the die(s) and the thermal solution attached on top of the IHS. For cases with an IHS, two TIM materials are used—TIM1 is applied between the silicon die and IHS and TIM2 is used between IHS and thermal solution. TIM materials are usually comprised of a polymer matrix with thermally conductive fillers such as silica, aluminum, alumina, boron nitride, zinc oxide, etc. The polymer matrix wets the contact surface to lower the contact resistance, while the fillers help reduce the bulk resistance by increasing the bulk thermal conductivity. TIM thickness varies by application but is typically between 25 μm and around 250 μm. Selection of appropriate TIM1 and TIM2 materials is necessary for the reliable thermal performance of a product over its life and end-use conditions. It has been observed that during reliability testing, TIM materials are prone to degradation which in turn leads to a reduction in the thermal performance of the product. Typical material degradation is in the form of hardening, compression set, interfacial delamination, voiding, or excessive bleed-out. Therefore, in order to identify viable TIM materials, characterization of the thermomechanical behavior of these materials becomes important. However, developing effective metrologies for TIM characterization is difficult for two reasons: TIM materials are very soft, and the sample thickness is very small. Therefore, a well-designed test setup and a repeatable sample preparation and test procedure are needed to overcome these challenges and to obtain reliable data. In this paper, we will share some of the TIM characterization techniques developed for TIM material down-selection. The focus will be on mechanical characterization of TIM materials—including modulus, compression set, coefficient of thermal expansion (CTE), adhesion strength, and pump-out/bleed-out measurement techniques. Also, results from several TIM formulations, such as polymer TIMs and thermal gap pads, will be shared.

Thin-film evaporation of heptane in a V-groove geom try is experimentally investigated. The groove is made of fused quartz, and electrical heating of a thin layer of t itanium coated on the backside of the quartz substrate provides a constant heat flux. The effects of liquid feeding rate on t he emperature suppression in the thin-film region and on the meni scus shape are explored. High resolution (~6.3 μm) infrared thermography is employed to investigate the tempera ture profile in the thin-film region, while a goniometer is used to image the meniscus shape. An approximate heat bala nce analysis is used to estimate the fraction of total meniscus heat transfer which takes place in the contact line regi on.

Multi-chip packages (MCPs) based solutions are becoming increasingly adopted as it results in higher signal count, density and enables increasing bandwidth demands and allows for heterogeneous integration [1,2]. However, manufacturing tolerances impose a variability in these stacks which results in new requirements for thermal interface materials. This paper describes the thermal, mechanical, and reliability challenges associated with MCP packages, and highlights need for novel thermal interface materials.

Accurate estimation of the thermal conductivity of logic memory and memory-memory interfaces, between stacked die in 3D microelectronic packages, is key to effective design and early estimates of performance and reliability. Typically, interconnect layers contain hundreds to a few thousands of bumps. Hence lumped/compact modeling of this interfacial layer is essential to reduce computational time and complexity. The typical approach to this lumped modeling is to estimate the effective conductivity of the layer by assuming the bumps and underfill regions can be modelled as parallel thermal resistances (referred to as the volumetric method). This work demonstrates that the volumetric method can significantly underpredict 3D stack thermal resistance and junction temperatures. An alternative method-referred to as the single bump method-of estimation of the thermal conductivity of interconnect layers in 3D stacked-die packages is presented. Studies demonstrate that the proposed single bump method captures the heat transfer in these interfaces accurately. Validation of the single bump modeling is presented by comparing the single bump and volumetric methods with fully discretized models. This comparison also demonstrates that the prevalent volumetric method overestimates the effective thermal conductivity of the interface, while the single bump approach results in more accurate assessment of 3D stack resistance.

A detailed numerical model is developed that describes heat and mass transfer from a meniscus to open air. The model accounts for the effects of evaporation at the interface, vapor transport through air, thermocapillary convection, and natural convection in air. Evaporation at the interface is modeled using kinetic theory, while vapor transport in air is computed by solving the complete species transport equation. Since the vapor pressure at the liquid–gas interface depends on both evaporation and the vapor transport in air, the equations are solved in an iterative manner. Evaporation is strongest at the triple line due to the highest local vapor diffusion gradient in this region. This differential evaporation, coupled with the low thermal resistance near the triple line, results in a temperature gradient along the interface that creates thermocapillary convection. The numerical results obtained show satisfactory agreement with experimental data for the evaporation rate and the temperature profile. Additionally, results from a simplified model neglecting thermocapillary convection are compared with the full solution, thus delineating the importance of thermocapillary convection-induced mixing in the energy transfer process. The present generalized model may easily be extended to other geometries and hence may be used in the design of two-phase cooling devices.

Evaporation from a meniscus of heptane liquid in a V-groove geometry is experimentally investigated. A thin layer of titanium coated on the backside of the fused quartz groove is electrically heated to provide a constant heat flux. The temperature profile in the evaporating thin film region of the extended meniscus is measured using high-resolution infrared thermography and the temperature suppression in this region is obtained as a function of liquid feeding rate. The meniscus shape is captured using a goniometer. A temperature suppression of ∼0.2 K in the 150 μm region surrounding the contact line on each side indicates the efficacy of evaporation in the extended meniscus. At a given axial location, the fraction of total meniscus heat transfer which takes place in a 50 μm sub-region measured from the contact line is estimated by an approximate heat balance analysis to be ∼45% for the range of liquid feeding rates explored.

Evaporating sessile drops remain pinned at the contact line during much of the evaporation process, and leave a ring of residue on the surface upon dryout. The intensive mass loss near the contact line causes solute particles to flow to the edge of the droplet and deposit at the contact line. The high vapor diffusion gradient and the low thermal resistance of the film near the contact line are responsible for very efficient mass transfer in this region. Although heat and mass transfer at the contact line have been extensively studied, well-characterized experiments remain scarce. The local mass transport in a 100-400 microm region near the contact line of a water droplet of radius 1810 microm on a glass substrate is experimentally quantified in the present work. Microparticle image velocimetry measurements of the three-dimensional flow field near the contact line are conducted to map the velocity field. Combined with high-resolution transient liquid profile shapes, the measured velocity field yields transient local evaporative mass fluxes near the contact line. The spatial and temporal distribution of the local evaporative flux is also documented. The temperature distribution in the droplet near the contact line is deduced from the local evaporative fluxes and interface mass transport theory.

Thin-film evaporation from a meniscus in a confined space, which is the basis for many two-phase cooling devices, is experimentally investigated. The meniscus formed by heptane, a highly wetting liquid, on a heated fused quartz wafer is studied. Microscale infrared temperature measurements performed near the thin-film region of the evaporating meniscus reveal the temperature suppression caused by the intensive evaporation in this region. The high spatial resolution (∼6.3 μm) and high temperature sensitivity (∼20 mK) of the infrared camera allow for improved accuracy in the measurements. The effects of evaporation rate, applied heat flux, and channel width on the thin-film heat transfer distribution are also explored.

Micro-particle image velocimetry measurements of the three-dimensional (3D) convection patterns generated near an evaporating meniscus in horizontally oriented capillary tubes are presented. Analysis of the vapor diffusion away from the meniscus reveals a zone of intense heat flux near the solid-liquid-vapor junction that creates a temperature gradient along the meniscus. This results in a surface tension gradient which, coupled with buoyancy effects, causes buoyant-thermocapillary convection in the liquid film. The relative influence of buoyancy and thermocapillarity on the flow was investigated for tube diameters ranging from 75 to 1575μm. A transition from a pure two-dimensional thermocapillary flow to a 3D buoyant-thermocapillary flow is observed with an increase in tube diameter. For the 75μm tube, a symmetrical toroidal vortex is observed near the meniscus. For larger tubes, buoyancy effects become apparent as they dominate the flow field. The high mass fluxes in smaller-diameter tubes drive stronger vortices. Particle streaks and micro-particle image velocimetry images obtained in multiple horizontal and vertical planes provide an understanding of this three-dimensional flow behavior. A scaling analysis shows the importance of thermocapillary convection in evaporating menisci.

Experimental visualizations of the three dimensional (3D) convection patterns generated near an evaporating meniscus in horizontally oriented capillary tubes are presented. These patterns are caused due to the differential evaporation along the meniscus. In this study, transparent capillary tubes with refractive index close to that of the evaporating liquid were used to minimize refraction effects and obtain velocity vectors near the walls. Polystyrene fluorescent particles of 0.5 μm diameter suspended in methanol were used to make the measurements in tubes of 75, 200 and 400 μm diameter. For the 75 μm tube, gravity was observed to have no effect on the flow patterns and an axisymmetric counter-rotating vortex pair was present along the horizontal and vertical center planes, suggesting the presence of a toroidal vortex near the meniscus. With an increase in tube size, buoyancy effects became apparent as the axisymmetric pattern broke down. A counter-rotating symmetric vortex pair was observed in the horizontal center plane, whereas in the vertical center plane, a single vortex dominated the flow and pushed the secondary vortex to a corner. Particle streak and μPIV images were obtained in multiple horizontal planes and a vertical center plane to understand this 3D flow behavior.

Experimental visualizations of the 3D convection patterns generated near an evaporating meniscus in a capillary tube are presented. Epi-fluorescent micro-particle image velocimetry (μPIV) using a two-cavity frequency doubled ND-YAG laser as the illumination source is employed to map the small-scale spatial flow fields near the meniscus. Methanol seeded with 0.5 μm polystyrene fluorescent particles is used as the experimental fluid. These fluorescent particles absorb light from the laser beam (λabs~542 nm, green) and emit a longer wavelength light ((λem~612 nm, red). Images obtained at a specified time delay (~20 ms) were interrogated to obtain vector flow fields.