Novel bumping technology that can realize high density assembly of IC chips and packages with a high number of I/O is required in the field of electronic packaging. Recently, a novel bumping process called IMS (Injection Molded Solder) was proposed, which enabled direct injection of molten solder into the holes of a photoresist patterned array. In this paper, the current status of photoresist development and recent achievements for obtaining excellent solder filling rates are described. One of the important factors to obtain a high solder filling rate is reducing the amount of outgases from the photoresists at high temperature, as they prevent smooth solder filling into the photoresist holes. In order to solve such an issue, novel photoresists with high thermal stability have been designed and developed.