A new No Clean Flux (NCFx) material for large die packages was formulated, which has good wettability for area array lead-free solder bumps of 20 × 20 mm dies, and which doesn't leave any liquid residues under the die. In a reliability study on large die modules with a copper lid as a heat spreader, we confirmed no failures of electrical insulation during thermal and humidity tests. However, significant chip sidewall delamination was found at the early stage of a thermal cycle test. XPS analysis of the chip sidewall showed that an enhanced plasma treatment is necessary to make the sidewall clean compared with a typical water-cleaned reference sample. A CantiLever Beam (CLB) test to measure adhesion strength on chip sidewall also suggested that the SiO 2 surface (assumed to be the chip sidewall) treated with the enhanced plasma has stronger adhesion than the reference sample. © 2013 IEEE.