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40 SMT Magazine • June 2014 feaTure Results and Discussion Samples prepared as described in the previ- ous section of this report were aged in convec- tion ovens at five temperatures. Since one of the purposes of the study was to find a much faster means to predict 10 years of heat aging, very high temperatures were used to obtain fast deg- radation rates. At the very high temperatures the degra- dation of the silicone adhesive occurred very quickly. This led to a problem in how to ac- count for the time the samples required to reach the oven set temperature. It was found that in nearly all cases the small dog-bone samples required approximately six minutes to reach 95% of the oven set point temperature. These six minutes were discounted for the pur- poses of this study. Therefore a time recording of 0.1 hours would in actuality refer to six min- utes of residence time at the specified tempera- ture plus an additional six minutes to reach the oven temperature. While this method has its drawbacks especially at the highest exposure temperature, it was a practical and expedient method that appeared to fall within the experi- mental error of the data. For reporting purposes, the following exper- imental data presented in detail through next sections of this report will describe the perfor- mance of the adhesive 1 (one-part, self-leveling, heat cure adhesive) when exposed to high tem- perature as representative of the other two ad- hesives/sealants tested. Thermal degradation is often not cata- strophic but instead a process of steady prop- erty loss. Determining a suitable endpoint can therefore be somewhat subjective. One line of reasoning would say that when any property exceeds specification limits it has degraded past the part design and qualification standards for which it was chosen. However, many material specification limits are based on a standard set of properties that are more specific to identifying the main character- istics of the material vs. actual application-use requirements. eXPLORIng HIgH-TeMPeRATuRe ReLIABILITy LIMITS FOR SILICOn ADHeSIveS continues Table 3: Initial properties for adhesive 1 (one-part, self-leveling heat cure). figure 1: Change in elongation during thermal aging.