Evaluation of the oven reflow bonding and flip chip bonder bonding of lead free Sn0.7Cu solder bumped die on low-cost FR-4 substrate for flip-chip applications

With the increased awareness of the lead free solder in the electronic packaging industry, the development of the lead free solder on the low-cost FR-4 substrate is a necessary for the flip-chip applications. However, it is found that the high reflow temperature of the lead free solders (normally >250°C) caused substrate burnt related defects in the FR-4 substrates (Tg <200°C) during bonding process in the traditional reflow oven. In order to solve this problem, bonding was performed on the flip-chip bonder. Using this approach, no substrate burned was observed. Using the flip chip bonder instead of the reflow oven, it is a possible to solve the low Tg of the FR-4 substrate for the lead free solders flip chip applications. However, the draw back is that solder reflow data provided by solder paste suppliers must be adjusted for the flip chip bonder. In this paper, the main factors (stage temperature, bonding pressure, head temperature, bonding time and moving distance) affecting the bonding process by flip chip bonder in the Sn0.7Cu lead free solder were evaluated. Sn0.7Cu solder is chosen due to its high melting temperature of up to 227°C. If we can demonstrate the solution for this high reflow temperature solder, we can apply the same approach to the others lower melting temperature solders such as SnAgCu, SnAg and SnBi on the FR-4 substrates. The experiments were designed based on the Taguchi method. Die shear test of the bonded samples is selected as the tool to verify the die shear strength of the samples under different experiments. The S/N ratio is used to determine the optimum conditions for the bonding by the flip chip bonder. Based on the analysis of the Variance (ANOVA), the degree of distribution of the five bonding factors to the die shear strength is determined.