The Silicone Conundrum
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CASE HISTORY—PEER-REVIEWED
The Silicone Conundrum W. John Wolfgong • Kirk Wiggins
Submitted: 1 April 2010 / Published online: 11 May 2010 Ó ASM International 2010
Abstract 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. Keywords
Chemistry Failure analysis Separation
Most commonly both R groups are methyl (–CH3), and they are more formally referred to as polydimethylsiloxanes. As a result, siloxane is another common term used for silicones. In many cases, their use is not problematic. However, silicones present three significant challenges: bonding problems due to process contamination, their propensity to outgas, and their large coefficient of thermal expansion (CTE). These topics are the subject of this discussion and the source of many Raytheon system failures.
Introduction Silicone Residues Interfere with Bonding Silicones are ubiquitous materials owing to their many desirable qualities. As a result, one may expect their presence in a wide variety of products ranging from personal care, foods, and all manner of electronics components and assemblies. In this regard, they are used in a number of applications such as elastomers for O-rings and pottings, fluids for coolants and lubrication, and many others. These materials are so common, that many times designers fail to consider the potential for problems presented by their use; hence the silicone conundrum—when to apply and when to exercise extreme caution. The basic form of silicones is as follows: R R Si O n
W. J. Wolfgong (&) K. Wiggins Raytheon Failure Analysis Labs, 2501 W University Dr M/S 8011, McKinney, TX 75071, USA e-mail: [email protected]
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No matter the form, silicones leave oily deposits on surfaces they contact. This may result from silicone pads used on equipment that manipulates boards and assemblies, silicone in personal care products such as hand creams, etc. The most common manifestation of these deposits is bonding failures between substrates. Examples include inks, epoxies, adhesives, conformal coats, and others that are applied to circuit cards and assemblies. Within Raytheon, we have experienced particularly troublesome problems with regards to silicone contamination and urethane conformal coat adhesion. This has resulted in considerable time for failure analysis and mediation requiring unanticipated cleaning steps and screening of components for potential sources of silicone contamination. This has added unexpected cycle time which ultimately effects cost. This and other silicone related issues have resulted in a complete banning of all silicone products from some production areas including seemingly trivial sources such as silicone arm bands used by many charities.
J Fail. Anal. and Preven. (2010) 10:264–269
Sili
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