Development status of photoresist as mask material for Injection Molded Solder (IMS) technique

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. Currently, the electroplating method or the solder ball placement method have been widely adopted for the fabrication of solder bumps down to sub-hundred microns in diameter. However, there are some limitations with these current bumping methods. For example, in the case of an electroplating method, the solder bump composition fabricated in this method is limited. In addition, careful maintenance of the plating solution is necessary in order to prevent a gradual compositional shift of the plating solution. Recently, a novel bumping process called Injection Molded Solder (IMS) 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 prevented 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. The number of defects was drastically decreased by applying the improved photoresist to the IMS process. Currently, solder bump arrays with 50 μm diameter were perfectly formed with SAC305 as a solder on a 6 inch wafer with no defects. Also, excellent solder filling was observed at holes down to 20 μm in diameter. Some other advantages of IMS, such as solder height uniformity and thermal reflow-free processes, are also described. Even though there is some significant progress in materials and processing, we still have room to modify our material design to establish a much more stable and reliable IMS process. As example parameters, the effects of the elastic modulus and molten solder wettability are also described. Our goal is to realize perfect solder filling with 12 inch wafers in the near future.