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The flip-chip on organic substrate has relied on underfill to enhance the solder joint reliability. Recently, the no-flow underfill and wafer level underfill process was invented to improve the production efficiency of flip-chip process. These two underfill processes are designed for flip-chip in Surface Mount Technology (SMT) assembly. The elimination of post cure is desired although in many applications post cure is still required or recommended. Therefore, there is a need to understand how material properties change at the later stage of curing. In this paper, the post cure behavior of a particular no-flow underfill is studied and its material properties at late stage of cure were characterized. A Differential Scanning Calorimeter (DSC), a Fourier Transformed Infrared Spectrometer (FTIR), a Dielectric Analyzer (DEA), and a Dynamic Mechanical Analyzer (DMA) in Torsional Braid Analysis (TBA) were used to study the post cure behavior of the underfill. The study on the glass transition temperature (Tg) with respect to degree of cure (DOC) illustrated that the Tg was a sensitive probe of DOC at late stage of cure. The mechanical properties of the underfill in post cure were investigated using a dynamic mechanical test and a flexure test. The change of the modulus was found to be due to the room temperature free volume difference at different extents of cure. The moisture absorption experiment at 50°C/50% Relative Humidity (RH) was carried out to further investigate the change of the free volume with respect to the extent of cure. These results indicate that the post cure properties of the underfill need to be carefully studied and controlled.