A solder ball attaching process for attaching solder balls to a wafer is provided. First, an under-ball-metallurgy layer is formed on the active surface of the wafer. Patterned masking layers are sequentially formed over the active surface of the wafer. The masking layers together form a step opening structure that exposes the under-ball-metallic layer. A solder ball is placed on the uppermost masking layer and allowed to roll so that the solder ball drops into the step opening structure by gravity. A reflow process is conducted to join the solder ball and the under-ball-metallurgy layer together. Finally, various masking layers are removed to expose the solder ball on the bonding pad of the wafer.