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About the increase of I/O, from BGA to C‐4 joints, there is a RDL of Cu wires in the upper part of the polymer board. From C‐4 joints to μ‐bumps, there is an RDL of Cu wires at the lower part of the interposer chip. This second RDL is invisible in the figure, but it is new in 3D IC because it does not exist in 2D IC devices, where typically there are only two levels of solder joints. The failure of the new RDL is of concern.

ssss1a and b show synchrotron radiation tomographic images of a 3D IC and part of a 2.5D IC device, respectively. The latter has a length about 4 mm, and a thickness and a height of about 0.5 mm. Due to the weak absorption of X‐ray, the two Si chips and the polymer substrate become invisible. We can see the vertical TSV pillars. Also, the solder balls and the Cu wires are shown clearly. In ssss1a, by using a pair of the BGA balls as the cathode and the anode and by passing 50 mA at 100 °C, following the arrows which indicate the conduction path, we can study time‐dependent failures caused by electromigration and Joule heating, to be discussed in ssss1.