Characterization and evaluation of the underfill encapsulants for flip chip assembly

Studies of solder-bumped flip chip on organic substrates reported in the literature so far suggested the necessity of a polymeric underfill to compensate for the large thermal mismatch between the silicon and the substrate. In our target applications of this process, the underfill not only has to meet the much published mechanical and chemical requirements, it also has to flow through a vertical clearance of 0.020 to 0.0375 mm quickly and also be cured in a relatively short time. Our evaluation of the underfill materials starts with some basic understanding of how the different ingredients in the underfill formulation might affect its physical and chemical properties. The flow characteristics of the underfills have been on first priority in the selection. Then a detailed thermal-mechanical analysis helped us to determine the optimal cure schedule with the desired physical properties. A simple test die/test board system has been designed to evaluate how the underfill might work in a quasi-production process and to allow for our subsequent reliability evaluation. This paper highlights the “pluses and minuses” of the currently available commercial underfills in relation to the optimization of a high-volume production process