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High aspect ratio bumping process with solder bump included core pillar
Keywords: Fine pitch, Bumping, Transient liquid phase sintering, High aspect ratio
This study provides the core pillar bumping method, which can make high aspect ratio bumping with low cost and high production efficiency. For high performance and downsizing of the semiconductor package, high density bumping is a necessity. Bump volume and height become low for fine pitch bumping. If bump volume is too low, defective bonding will occur easily during the assembly process due to the shortage of bump height. On the other hand, if bump volume is too high, bridge bump will occur with the neighboring bump during the reflow process. One solution we propose is the high aspect ratio bump, which can resolve the issue mentioned above for fine pitch bumping. The major bumping process processes are paste printing, electroplating and the ball mount method. Paste printing method is the least costly while having the highest production efficiency. Unfortunately, if the normal paste printing method is used for fine pitch bumping, a bridge or defective bonding issue will occur. To resolve this issue, we propose a core pillar method. First, a core pillar paste is printed with a stencil mask and then first reflowed to make the core pillar. Then, solder paste is printed, covering the core pillar. After the second reflow, a solder bump including the core pillar is formed. Since the core pillar is used in the transient liquid phase sintering process, the core pillar can be made with the same reflow temperature as solder paste. Additionally, core pillar is not melted during the second reflow of the solder paste. The solder bump including core pillar has a high aspect ratio compared to the normal paste printing process. With the optimization of the core pillar paste, we achieved a 25% increase in bump height when compared to the bump formed by the normal paste printing process. In conclusion, we propose that the core pillar method is one of the solutions for fine pitch bumping.
Sho Nakagawa, R and D enginner
Mitsubishi materials corporation
Sanda-shi, Hyogo-ken

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