Hierarchical Nested Surface Channels for Reduced Particle Stacking and Low-Resistance Thermal Interfaces

Abstract

This paper reports on the improvement of thermal interfaces through the control of particle stacking during bondline formation. Particle stacking occurs in highly filled materials due to pressure gradients developing during squeeze flow over a rectangular surface, resulting in non-uniform interface properties and thick bondlines with a large thermal resistance. Nested surface channel designs are presented to create a uniform pressure drop as interface material flows across a rectangular surface. Reductions in thermal resistance of 2-3× compared with that of flat surfaces are demonstrated with similar reductions in bondline thickness and assembly pressure. We obtained thermal resistances as low as 2 Kmm 2 /W for thin bondlines (< 5 µm). Comparative powercycling results also demonstrate improved reliability against paste pump-out with nested channel interfaces. KeywordsThermal interface material, TIM, bondline, particle stacking, filtering, squeeze flow, nested channels, lid attach Particle-Filled Interface MaterialsParticle-filled materials are used extensively in the electronics industry to reduce thermal resistance between high-power devices and thermal-packaging components such as spreaders, heat sinks, and liquid cold plates. Thermal interface materials (TIMs) are usually dispensed onto a desired surface from syringes or applied as pre-forms with a subsequent squeezing step to mate the two surfaces and accommodate manufacturing tolerances. Because thermal interfaces consume 20-50% of the total thermal budget, a reduction in their resistance can extend the lifespan of current cooling solutions and further reduce junction temperatures.Thermal greases and adhesives are often made by mixing solid particles in bi-modal or tri-modal size distributions into a viscous matrix at high volumetric loading levelssometimes reaching 78% by volume [1]. At a critical percolation threshold loading of 35-45 v%, the effective conductivity increases significantly owing to random networks of particles in contact with each other [2]. Effective bulk thermal conductivities ranging from 2-5 W/mK are common in the industry, with the thermal performance influenced mainly by particle loading and size distribution. Depending on particle-size distributions and loading fractions, the resulting materials have highly nonlinear viscoelastic properties with a significant yield stress and a sheardependent viscosity that can require squeeze loads high enough to damage flip-chip solder ball arrays and crack chips and substrates.As particle-filled mixtures reach higher viscosities and yield stresses, an increase in bondline thermal resistance is observed because of the difficulty in achieving thin gaps close to the maximum particle sizes. In the industry often a tradeoff is made between thin gaps with low conductivity materials (low particle fill of small size range) or thick gaps with high conductivity materials (high particle fill with large size range) [3]. In addition to extremely high viscosities and yield stresses, another reason why highly fi...

High-performance thermal interface technology overview

“…The first hierarchy level is the larger corner-corner channels (black), the second is formed by the grey lines with the required hydraulic diameter enlargement for the diagonal channels (darker grey) as described by Linderman et al [21]. A third level nesting can be made within the cells of the second level also using the previously defined pressuredrop-balancing methodology (not used in the design shown in Fig.…”

In-situ measurement of various thin bond-line-thickness thermal interface materials with correlation to structural features

“…Furthermore, precision spacer particles have been used to keep the gap width fixed. It is therefore assumed that at low BLT there is a structural effect caused by a crowding and stacking of filler particles, similar to the phenomenon described in [1]. As the structural analysis is still in progess, results about a possible structure-property correlation are still to come and will be presented later.…”

Topological design of channels for squeeze flow optimization of thermal interface materials

“…Brunschwiler et al [11,16] expanded on the HNC concept and provided new designs that focused on the flow performance as well as the idea of controlling "particle stacking." This is the phenomenon where particles in the TIMs tend to "stack up" along flow stagnation and bifurcation lines in the flow pattern, and limit BLT due to the stacking.…”