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Pre-applied inter chip fill material for 3D chip stack integration – How to improve bonding quality and reliability
Keywords: Pre-applied underfill, Mechanical property and stress analysis, Reliability test
There has been considerable interest in studying thermal compression bonding (TCB) process with pre-applied underfill material. This interest is mainly due to the potential use for the next generation 2.5 and 3D packaging. The advantage of this material and process is that (i) it can reduce thermal resistance if material has high thermal conductivity (ii) it can apply for large chip and narrow gap bonding (iii) it can simplify process compared with conventional capillary underfill process. From this kind of background, we have developed and reported paste types of materials that can be used for pre-applied joining process, which we call “inter chip fill (ICF)”. However this kind of process has not been widely spread in the packaging industry, because of some technical issues such as filler entrapment and longtime reliability. To overcome this situation, it is necessary to develop highly reliable and easy-to-use pre-applied ICF and also to understand what kind of property govern reliability. In the present work, we are focusing our interest on relationship among material formulation, mechanical properties and reliability. We made many formulations changing base resin, curing agents, fillers, flux and their ratio. For these formulations, we evaluated mechanical properties like modulus, Tg and CTE (Coefficient of thermal expansion). We conducted joining test and reliability test, particularly thermal cycling test, using these formulations and test vehicles. As one example, some formulation passed several hundreds of thermal cycling test, while other formulation caused delamination problem just after pre-conditioning test. We also carried out stress and strain simulation for stacking chip using pre-applied ICF. These simulation results gave us insight into warpage problem and microbump protection. From this simulation, we can estimate suitable mechanical properties for longtime reliability. Detailed comparison between experimental results and simulation are reviewed in this paper.
Kan Takeshita, Senior Resercher
Mitsubishi Chemical Corporation
Kitakyushu, Fukuoka

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