Void Detection in Dielectric Films Using a Floating Network of Substrate-Embedded Electrodes

Abstract

A sensor is developed for simple, in situ characterization of dielectric thermal interface materials (TIMs) at bond line thicknesses less than 100 lm. The working principle is based on the detection of regions of contrasting electric permittivity. An array of long, parallel electrodes is flush-mounted into each opposing substrate face of a narrow gap interface, and exposed to the gap formed between the two surfaces. Electrodes are oriented such that their lengthwise dimension in one substrate runs perpendicular to those in the other. A capacitance measurement taken between opposing electrodes is used to characterize the interface region in the vicinity of their crossing point (junction). The electric field associated with each electrode junction is numerically simulated and analyzed. Criteria are developed for the design of electrode junction geometries that localize the electric fields. The capacitances between floating-ground electrodes in the electrode sensor configuration employed give rise to a nontrivial network of interacting capacitances which strongly influence the measured response at any junction. A generalized solution for analyzing the floating network response is presented. The technique is used to experimentally detect thermal grease spots of 0.2 mm to 1.8 mm diameter within a 25 lm interface gap. It is necessary to use the generalized solution to the capacitance network developed in this work to properly delineate regions of contrasting permittivity in the interface gap region using capacitance measurements.

Capacitive sensing of local bond layer thickness and coverage in thermal interface materials

“…The efficacy of such an approach has been demonstrated for the detection of dielectric anomalies in a 25 μm interfacial gap [15]. While the orthogonal mesh concept cannot be readily implemented in a TTV, an alternative concept is proposed which is more conducive to this implementation.…”

“…With this demonstration, both bond layer thickness and void content have been successfully characterized using the algorithm. It is noted that in previous work, 15 grayscale void fraction maps such as Figure 6c have been used to generate a pseudo-high-resolution binary image of the estimated void shape, [18].…”

Near-field focusing sensor for characterization of void content in thin dielectric layers

“…When the gap is filled with a dielectric medium, any air voids present near a junction reduce capacitance. It has been shown that, for an electrode width of 640 m, femto-Farad measurement resolution is sufficient to resolve partial voiding of a junction region in a 25 m dielectric layer [29].…”

Level-set shape reconstruction of binary permittivity distributions using near-field focusing capacitance measurements

“…A near-field focusing sensor with very small separation between electrode planes (20 m -500 m) has been proposed for in situ characterization of thin dielectric layers [23]. For small electrode plane 4 separation, moderate electrode pitch is sufficient to guarantee independent detection zones at each electrode junction, even when inactive electrodes are allowed to float during measurement [24]. Virtual grounding of inactive electrodes during sequential actuation allows for detection zones to be approximately independent even with a smaller electrode pitch by damping the electric field outside the vicinity of the actuated junction [23].…”