Characterization of Stencil Printing Parameters for Fine Pitch Wafer Bumping

Wafer bumping is a key process for flip chip packaging. There are several bumping techniques such as ball drop, electroplating and stencil printing, in which stencil printing is thought to be an economical choice because it is compatible with conventional SMT technology. However, when bump pitch is less than 100 mu m, it becomes more difficult to control the deposition volume of the solder paste. In this paper, multiple printing parameters had been characterized by orthogonal experiment method to find the optimal setting for a 4" wafer with 90 mu m bump pitch. These parameters include printing pressure, printing speed, separation speed, separation distance and printing gap. Transfer ratio of the solder paste was calculated and effects of the printing parameters were analyzed. Results showed transfer ratio increased when reducing printing pressure or increasing printing speed, and printing pressure had the most significant effect over other parameters. Defects like bump bridge and missing ball were also measured to evaluate the printing quality. According to the printing results, proper ranges of the parameters were found. Bumps were reflowed after printed with the optimal parameter setting, and then bumped chips were assembled using flip chip bonder and SEM observation showed good bonding interface.