Flux study for ultra fine pitch flip chip packages

Flux used in 25um ultra fine pitch flip chip attachment process has been found to play an important role in the bump joint and performance. It is commonly used in cleaning the surfaces of solder balls and the surfaces to be soldered to ensure a good wetting of the solder bumps on the substrate pads. Here we use Cu pillar bump with solder cap and substrate pad surface finish is plating tin, and furthermore the standard reflow process was used to perform the evaluation. Due to higher bonding temperature requirement, a Pb-free compatible no clean flux is needed. This kind of flux is formulated to work well with small bump diameter and pitch which have low residual after reflow profile and low weight loss during the whole process. Of course they are achieving compatibility with the underfill. For this new generation flip chip products, how to select a suitable flux is the most challenge. This paper gives a detailed description of the challenges encountered during assembly, such as the flux selection process, flux height study. The characteristics of flux can be determined by using TGA, wet-ability test, and thermal resistance. For ultra fine pitch flip chip application, no clean and low residue flux is the basic demand. Besides, jetting type flux will be the best choice because the flux height is hard to control and the flux amount might not be enough to provide the solder joint ability. Finally, we conclude that the most rigid bonding method, and parameter for fine pitch Cu pillar bump flip chip package.