Exploring the High Temperature Reliability Limits for Silicone Adhesives

The thermal stability of silicone polymers, fluids and resins has been well documented and studied extensively. The high temperature performance of silicone adhesives and sealants used for electronics applications has only moderately been investigated. This report documents the effects of very high temperature exposures to electronics-grade silicone adhesives and sealants for such properties as tensile strength, elongation, tensile modulus, weight loss, shrinkage, durometer, and lap shear adhesion. The goal of the work is to determine application “life expectancies” of the products as well as an extrapolated estimate of the Underwriter’s Laboratories’ “continuous use” temperature rating – the highest temperature at which a product is expected to lose no more than 50% of its original value for whatever key property degrades the fastest Four different formulations of silicone adhesives and sealants were evaluated for high temperature stability. For these products, elongation was found to be the fastest degrading property among those tested. The data was found to fit a power curve of exposure temperature vs. time to reach a 50% loss of initial tensile strength and elongation to an R-squared value of 0.99 and to a linear fit in an Arrhenius plot to the same very strong fit. These plots could be used to closely estimate the effects of heat aging on the material over a wide range of temperatures. INTRODUCTION Silicone is the generic name used for many to identify a family of products based on the polydimethyl siloxane (PDMS) molecule with unique characteristics. For electronics applications silicones can be used as adhesives, encapsulants, gels, protective coatings, thermal 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 applications. These features include: unmatched thermal stability, flexibility, moisture resistance, adhesion to many common substrates used in electronics, low ionic impurity and compatibility with common processing techniques. Among all these characteristics that are shared by the majority of silicones, one property is recognized as one of the most useful in electronics applications and that is their consistent performance over a very wide temperature range. The lower and upper operating temperature limits for silicones are not very well defined. Some approaches have placed these limits in the range from -40 to 150 °C. Others, less conservative, have placed the limits between -50 and 200 °C. Both approaches are correct; however, the applicability of each will depend much on the product used. Operating temperatures for silicones have a great dependency on the formulation, 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 temperatures as high as 250 °C or as low as -80 °C. These would be considered, within the product line, as especial materials. For general purpose adhesives/sealants, within the scope of this paper, the operating temperature limits will be defined as -45 to 200 °C. The performance of silicone adhesive/sealants, such as the ones under the scope of this paper, when exposed to temperatures around 200 oC has been very well documented and evaluated. However, there is very little relevant data detailing the performance of these products when working above the operating limits (200 oC) for long term exposures or even short term periods. This missing data acquires great relevance for today’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 oC. It is the goal of this paper to review the performance of four different formulations of silicone adhesives/sealants when exposed to temperatures above 200 C as a way to provide 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 temperatures, usually for rather short time durations but sometimes 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 oC 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. Solder reflow ovens may only subject the materials to a few seconds or minutes of high temperature – and materials are not expected to significantly change in performance even though the exposure temperatures may be very high. Many applications may have transient temperature “spikes” that considerably exceed their standard operating temperatures. These spikes could have durations ranging from a few seconds 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 insignificant consequences on material properties, it is unrealistic to not recognize that even a few minutes at very high temperatures may change certain material characteristics. For example, for thermoplastics such changes may be readily apparent if the material begins to melt and lose dimensional stability. For thermoset products however, degradation may not be as easy to observe. Table 1 Typical Application Temperatures and Durations Application Max Temp Duration Solder reflow ovens 225-260C 10-90 seconds Automotive on-engine modules (extended warranty) 175C 150C 150C 5 min. 1000 hrs 3000 hrs Industrial power devices 250C 200C 175C hours months years Silicone adhesives and sealants are widely used to bond, seal and sometimes protect electronic components or modules for high temperature applications. The stability of silicone adhesives and sealants to high temperature in the order of 200 oC is well accepted, and typically is higher than most other polymeric materials used in electronics. However, there is very little relevant data detailing long term high temperature exposure (> 200 oC) or even short term to the temperatures experienced in today’s electronic applications (see Table 1). Test Methodology Determining the thermal stability of a material is not always a straightforward task. There are many important properties of a given material, 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 stability/degradation of different materials. The basic principle involved in TGA is simply to weigh a sample of the material under study in a controlled 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 efficacy in determining the thermal degradation of silicone products as silicones when exposed to high temperatures undergo several transformations by a variety of processes, including oxidation, siloxane rearrangement and hydrolysis. Siloxane rearrangement occurs without the formation of volatiles where extensive depolymerization may take place before fragments small enough to evaporate without further decomposition can be produced. This rearrangement will lead to the loss of physical properties, not precisely linked to weight loss. At the same time, the degradation of silicones at high temperature by oxidation slowly transforms the material into more quartz-like properties. Thus the electrical insulation properties of the product remain almost unchanged or even improve with such exposures. Under these bases, it is easy to understand 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 ten year “life expectancy” or “half-life” of polymeric materials. It proposes the useful life of a material to be defined 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 adhesives and sealants as there are quantifiable properties 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 expectations as well. However, it is important to remark that sensitive applications may require additional validation with specific performance and exposure time parameters. UL RTI values are ten year exposure estimates. To obtain 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 extrapolated from these plots. For this study, five exposure temperatures were used: 200, 225, 250, 275 and 300 oC. The performance and life expectancy of the silicone adhesives studied here when exposed to temperature higher than 200 oC 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-leveling 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 exposure to the 5 different temperatures indicated above for up to 1000 hours or less, depending on the time to lose no more than 50% of original value for the property measured. The typical properties for the products included in this study are shown in the following chart (Table 2). Table 2 Typical Properties for the Products used for this study Silicone