Silicones are ubiquitous materials used in a multitude of industries. They are, in fact, so common that designers often overlook their shortcomings. This paper discusses some of the common failure mechanisms of silicones, in particular with regards to the electronics industry that have been investigated by the Raytheon Failure Analysis Labs, McKinney, TX.

A single row ball bearing was presented to the Failure Analysis Lab because of noise and perceived roughness during low speed operation. The bearing was analyzed using a heterodyning ultrasound detector. Discrete sources of intermittent noise were detected. A quasi-static determination of the friction coefficient of the bearing was made. Contamination by hard foreign objects was suspected. The bearings were solvent washed, and particles of aluminum were recovered. The size of these particles was related to the static friction coefficient of the bearing. The cleaned bearings were retested and found to have residual friction related to microscopic defects caused by inclusions and voids in the races and balls. A generalized theory was formulated to account for the relation between noise and static friction.

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A ceramic substrate bonded to an electronic sensor was reported to crack during thermal testing between À46 and ?71°C. Initially, this was assumed to occur at high temperature because of the thermal expansion of a polymer pad under the sensor. However, further investigations indicated that fracture was also occurring at low temperature. This was surprising since the suspected cause of fracture was pressure exerted by the polymer pad under the ceramic. This polymer pad has a coefficient of thermal expansion much greater than any other component. Conventional wisdom suggests that this failure would happen only during expansion of the pad during high temperature phases of temperature cycling. Subsequent dynamic thermal analysis revealed that differential contraction of the steel clamp fasteners was causing pressure to be exerted on the ceramic during the initial phase of the cold cycle. In addition, lateral constraint of the polymer pad caused pressure to rise to counterintuitive levels. An alteration of the geometry of the pad proved to be the simplest and most economical solution. This was confirmed by subsequent testing.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.