Solder ball fabricating process

A solder ball fabricating process for producing at least one solder ball on a wafer, wherein the wafer has an active surface, a passivation layer and at least one solder ball pad, the passivation layer and the solder ball pad are on the active surface of the wafer with the passivation layer exposing the solder ball pad, the wafer further includes a stress buffer layer and an under-ball metallurgy layer, the under-ball-metallurgy layer is on the solder ball pad and the stress buffer layer is on the passivation layer exposing the under-ball-metallurgy layer, the solder ball production process at least comprising the steps of:

• forming a first patterned solder mask layer over the stress buffer layer, wherein the first patterned solder mask layer has at least a first opening that exposes the under-ball-metallurgy layer;
• depositing a first solder material into the first opening;
• conducting a first reflow process so that the first solder material melts to form a pre-solder body over the under-ball-metallurgy layer;
• forming a second patterned solder mask layer over the first patterned solder mask layer, wherein the second patterned solder mask layer has at least one second opening corresponding to the first opening expose the pre-solder body and the second opening has a diameter greater than the first opening;
• depositing a second solder material into the second opening;
• conducting a second reflow process so that the pre-solder body and the second solder material melt and fuse together to form a solder ball over the under-ball-metallurgy layer;
• and
• removing the first solder mask layer and the second solder mask layer.