Performance of Anisotropically Conductive Adhesive Attachments on Adhesiveless Polyimide Substrate during High-Temperatu
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Performance of Anisotropically Conductive Adhesive Attachments on Adhesiveless Polyimide Substrate during High-Temperature Storage Tests Sanna Lahokallio and Laura Frisk Tampere University of Technology, Department of Electrical Engineering P.O. Box 692, 33101 Tampere, Finland ABSTRACT The high-temperature performance of electronics packages was studied at 180°C, 200°C and 240°C for 3,000h. The structure tested consisted of a fairly large silicon chip attached with anisotropic conductive adhesive (ACA) onto an adhesiveless PI substrate. These structures showed good electrical reliability at 180°C. Several early failures were seen at the other temperatures. However, only 23% of the test samples failed at 200°C, while considerably more failures were seen at 240°C. The results showed that good high-temperature reliability can be achieved with polymer interconnections. However, the exposure time should be limited, especially at very high temperatures, to avoid failures caused by the materials becoming brittle. INTRODUCTION The widely accepted upper temperature limit for typical electronics structures and materials is 125°C [1, 2]. This limit may already be higher than the glass transition temperature (Tg) of thermoset polymer materials commonly used in electronics. The Tg is typically considered to be the upper temperature limit for use of thermosets, since above this temperature the mechanical properties of polymers often deteriorate and the coefficient of thermal expansion rapidly increases. For thermoplastics the upper temperature limit is either Tg or the melting temperature Tm. Thermoplastics cannot be used above Tm, but if used above Tg, they tend to regain their previous properties when cooled below Tg [3]. Thermosets, on the other hand, can be used above their upper temperature limit Tg without melting due to their cross-linked structure, but such exposure may lead to changes which are irreversible due to degradation of molecular chains [3], for example by chain scission. As polymers are increasingly used in electronics, many products including them must withstand rather high temperatures exceeding the accepted limits during their lifetimes. For example, industrial electronics and sensors are often used in environments where elevated temperatures occur, and the temperatures the product must at least temporarily withstand may be around 200°C. Numerous materials specially designed for use in high temperatures are available [1]. These are typically metals and ceramics having excellent thermal stability, even though polymers with improved thermal properties are also available. However, these materials are often expensive [4] and may be less readily obtainable and less versatile in terms, for example, of adhesion or fabrication methods. Therefore there is increasing interest in using organic, costeffective materials designed for lower temperature regions at higher temperatures. However, as polymers degrade more quickly at high temperatures, causing them to lose their mechanical stability, their reliable performance m
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