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June 2014 • SMT Magazine 37 eXPLORIng HIgH-TeMPeRATuRe ReLIABILITy LIMITS FOR SILICOn ADHeSIveS continues feaTure ject the materials to a few seconds or minutes of high temperature, and materials are not ex- pected to significantly change in performance even though the exposure temperatures may be very high. Many applications may have transient tem- perature "spikes" that considerably exceed their standard operating temperatures. These spikes could have durations ranging from a few sec- onds to a few minutes or even an hour or more. While it may be assumed that a few seconds at relatively high temperature may have insig- nificant consequences on material properties, it is unrealistic not to recognize that even a few minutes at very high temperatures may change certain material characteristics. For example, for thermoplastics such changes may be read- ily apparent if the material begins to melt and lose dimensional stability. For thermoset prod- ucts however, degradation may not be as easy to observe. Silicone adhesives and sealants are widely used to bond, seal and sometimes protect elec- tronic components or modules for high-tem- perature applications. The stability of silicone adhesives and sealants to high temperature in the order of 200°C is well accepted, and typi- cally is higher than most other polymeric mate- rials used in electronics. However, there is very little relevant data detailing long-term high- temperature exposure (>200°C) or even short- term to the temperatures experienced in today's electronic applications (Table 1). Test Methodology Determining the thermal stability of a mate- rial is not always a straightforward task. There are many important properties of a given mate- rial, and many or most will change with heat induced degradation. All important properties should be evaluated with a test methodology that ideally enables short, mid and long-term durability estimates to be made. Thermogravimetric analysis (TGA) has been extensively used to determine the thermal sta- bility/degradation of different materials. The ba- sic principle involved in TGA is simply to weigh a sample of the material under study in a con- trolled atmosphere while the temperature of the sample is varied in a known manner. Thermal degradation is related then to the weight loss of the sample. This method has proven poor effi- cacy in determining the thermal degradation of silicone products as silicones when exposed to high temperatures undergo several transforma- tions by a variety of processes, including oxi- dation, siloxane rearrangement and hydrolysis. Siloxane rearrangement occurs without the for- mation of volatiles where extensive depolymer- ization may take place before fragments small enough to evaporate without further decompo- sition can be produced. This rearrangement will lead to the loss of physical properties, not pre- cisely linked to weight loss. At the same time, the degradation of sili- cones at high temperature by oxidation slow- ly transforms the material into more quartz- Table 1: Typical application temperatures and durations.