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36 SMT Magazine • June 2014 management materials, even device packaging materials and wafer-level coatings. Silicones have a combination of properties which contribute to provide a proven long-term reliability and performance in electronics ap- plications. These features include: unmatched thermal stability, flexibility, moisture resistance, adhesion to many common substrates used in electronics, low ionic impu- rity and compatibility with common processing tech- niques. Among all these char- acteristics that are shared by the majority of silicones, one property is recognized as one of the most useful in electron- ics applications and that is their consistent performance over a very wide temperature range. The lower and upper op- erating temperature limits for silicones are not very well de- fined. Some approaches have placed these limits in the range from -40 to 150°C. Oth- ers, less conservative, have placed the limits between -50 and 200°C. Both approaches are correct; however, the ap- plicability of each will de- pend much on the product used. Operating temperatures for silicones have a great de- pendency on the formula- tion, filler type and content, additives, functionality of silicone polymer, etc. Silicone products can be formulated in different ways to provide higher thermal stability or lower temperature flexibility. There are silicone products that can be exposed for long periods of time to tempera- tures as high as 250°C or as low as -80°C. These would be considered, within the product line, as special materials. For general purpose adhe- sives/sealants, within the scope of this paper, the operating temperature limits will be defined as -45 to 200°C. The performance of silicone adhesive/seal- ants, such as the ones under the scope of this paper, when exposed to temperatures around 200°C has been very well documented and evaluated. However, there is very little relevant data detailing the performance of these prod- ucts when working above the operat- ing limits (200°C) for long-term exposures or even short-term periods. This missing data ac- quires great relevance for to- day's electronics applications, where electronic modules may be exposed to extreme high temperatures for short or long periods of time, identifying as extreme high temperatures any temperature above the limit exposed here, which is 200°C. It is the goal of this pa- per to review the performance of four different formulations of silicone adhesives/sealants when exposed to temperatures above 200°C as a way to pro- vide valuable guidance in the proper selection of these kinds of products for an electronics application. High-Temperature Requirements Nowadays a large number of industries require materials to be exposed to high temper- atures, usually for rather short time durations but some- times for extended periods that could go from a couple of months to several years. In electronics, the upper temperature limits are most frequently determined by the end use application. For some applications, such as automotive electronics, electronic parts may be exposed up to 175°C for extended pe- riods of time or even higher temperatures for short durations. Likewise, the time duration used to determine thermal stability is extremely dependant on the specific requirements of each application. Solder reflow ovens may only sub- eXPLORIng HIgH-TeMPeRATuRe ReLIABILITy LIMITS FOR SILICOn ADHeSIveS continues feaTure In electronics, the upper temperature limits are most frequently determined by the end use application. For some applications, such as automotive electronics, electronic parts may be exposed up to 175°C for extended periods of time or even higher temperatures for short durations. Likewise, the time duration used to determine thermal stability is extremely dependant on the specific requirements of each application. " "