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38 SMT Magazine • June 2014 like properties. Thus the electrical insulation properties of the product remain almost un- changed or even improve with such expo- sures. Under these bases, it is easy to under- stand that the thermal stability of silicones adhesives and sealants under the scope of this paper needs to be linked to the degradation of physical properties, such as: tensile strength, elongation, etc. UL Standard 746B10 offers a methodology that can be used to estimate a 10-year life ex- pectancy or half-life of polymeric materials. It proposes the useful life of a material to be de- fined as the time required to lose no more than 50% of any application-important property. The highest temperature at which 10 years of continuous exposure will retain 50% of the most sensitive important property is referred to as the relative temperature index, or RTI. This standardized method works quite well to study high-temperature exposures for silicon adhe- sives and sealants as there are quantifiable prop- erties such as tensile strength or hardness that can be easily measure to verify the performance of the material. The UL RTI type of standard gives a means of quantitative comparison between materials for long-term exposures, and the data can also readily provide short-term exposure expecta- tions as well. However, it is important to re- mark that sensitive applications may require additional validation with specific perfor- mance and exposure time parameters. UL RTI values are 10-year exposure estimates. To ob- tain data for this length of aging is, of course, not practical. Instead data is typically collected at a minimum of four high temperatures. The time to lose 50% of original properties is then plotted vs. temperature to obtain predictive graphs. Ten-year life estimates are then extrap- olated from these plots. For this study, five ex- posure temperatures were used: 200, 225, 250, 275 and 300°C. The performance and life expectancy of the silicone adhesives studied here when exposed to temperature higher than 200°C will then be expressed in a more useful way following the guidelines mentioned above. Proposed Products and Tests The group of products used for this study is formed by a one-part moisture cure silicone sealant and two heat cure adhesives (self-lev- eling and thixotropic). These products will be evaluated for thermal stability by measuring properties such as tensile strength, elongation, modulus of elasticity, durometer and lap shear adhesion (to unprimed aluminum) after expo- sure to the five different temperatures indicated above for up to 1,000 hours or less, depending on the time to lose no more than 50% of origi- nal value for the property measured. The typical properties for the products in- cluded in this study are shown in Table 2. Cured sheets of 2 mm nominal thickness were prepared and standard tensile dog bone samples were die cut from listed products. These samples were laid onto trays and placed in convection ovens for set time durations and then removed, cooled to room temperature and tested. Lap shear adhesion was evaluated onto bare aluminum. eXPLORIng HIgH-TeMPeRATuRe ReLIABILITy LIMITS FOR SILICOn ADHeSIveS continues feaTure Table 2: Typical properties for the products used for this study.