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Thermally resistant die attach for harsh environment applications
Keywords: Die attach, Extreme tempeatures, Shear strength
A thermally stable structure for die attachment has been found, which has been proven to be stable under testing to 600 C showing no reduction in shear strength. This could be useful for applications in the automotive, aerospace and oil/gas drilling industries where the electronic systems can be exposed to high temperatures of 200 C and ideally higher. By considering the rule of thumb that every 100 C degree increase in storage temperature is equal to 1000 times longer storage at the lower temperature, this 600 C storage for 100 hour showing no reduction to shear strength can be equivalent to 100 billion hours at 300 C with no sign of reduction to shear strength. In this technique solid-solid inter-diffusion of silver and gold has been utilized, which can enable very robust and flexible structures as the die attach. Here the interdiffusion identified originally as a problem is converted into an advantage by using mesh interposers to achieve the thermal stability and avoid the problems associated to it. The solid-solid interdiffusion utilised here has many advantages over conventional techniques such as solid-liquid interdiffusion bonding or unaided nanoparticle silver sintering. In addition, the mesh structure can be optimised and custom designed easily for increasing mechanical and thermal compliance of the die attach improving the thermal cycling capabilities to new levels by relaxing the coefficient of thermal expansion mismatch between the die and substrate. Furthermore, the easy assembly processing and the fact that no pressure is required on the die during thermal processing makes this an attractive proposition for industry. This new technique has the potential to benefit from very different combinations of materials allowing usage of the right materials for a vast variety of different applications. In addition to easy processing, high reliability, easy implementation, and cost effectiveness, this lead-free die attach system can now enable the industry to take a significant step towards environmentally friendly electronic packaging. While previously published work targeted only thermal stability of the die attach assemblies, future research are focused on the applicability and performance of cheaper mixture of materials and also simulation of the assemblies using COMSOL Multiphysics to investigate the optimal mesh structures for release of stress from coefficient of thermal expansion mismatch.
Seyed Amir Paknejad,
London, Greater London
United Kingdom

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