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44 SMT Magazine • June 2014 Table 4 show the percent change in each property measured for sample 1 when exposed to 300°C at different times. Elongation and ad- hesion were found to be the most rapidly chang- ing property during thermal aging. This is some- thing typically observed in silicone products. High-temperature degradation promotes poly- mer rearrangement and oxidation (as discussed earlier) that creates additional chemical bonds between polymers, forming a stiffer structure. As the new bonds are formed, it is common for silicones to become physically stronger exhibit- ing a higher tensile strength (Figure 2), higher durometer (Figure 3), and higher modulus (Fig- ure 4). However, tensile strength will eventually begin to decrease as the material continues ag- ing at high temperature, becoming somewhat brittle. As the new polymeric structure loses some of its flexibility, which is clearly shown in Figure 1, elongation is dramatically reduced by thermal aging. For adhesive 1, the original elongation for the product reduces to 50% after an effective ex- posure of approximately six minutes at 300°C, marking this point the life expectancy limit at this temperature following the guidelines of UL Standard 746B10 previously mentioned. Simi- lar behavior is observed when the adhesive is exposed to the other temperatures (Figure 1). feaTure eXPLORIng HIgH-TeMPeRATuRe ReLIABILITy LIMITS FOR SILICOn ADHeSIveS continues A summary of results for the other properties (durometer, tensile strength, lap shear adhesion and modulus) are presented in the following figures. The test procedures used in this study al- lows to obtain data in a period of weeks to a couple of months at very high exposure tem- peratures that could be used to confidently predict longer term property degradation at much lower temperatures. The 10-year life ex- pectancy to lose no more than 50% of its ini- tial elongation was predicted to be 149°C for adhesive 1 (one-part, self-leveling heat cure adhesive). For this study working with silicone adhe- sives, elongation was probably the best property to track since it was found to degrade the fastest and had by far the most consistent results. Ten- sile strength also generally lowered over expo- sure time, but there was more scatter in the data and it is entirely possible that at early points in a given high-temperature aging process the tensile strength may increase and durometers may decrease for a short period before going in the directions most normally observed in ther- mal oxidative degradation mechanisms. At the beginning of the work, there was concern that lap shear adhesion may be the property to de- grade the fastest. The data showed that while figure 5: Lap shear adhesion during thermal aging.