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ssss1 is a scanning electron microscopy (SEM) image of the cross‐section of a 2.5D IC test device. It has only two pieces of Si chips stacking on a polymer board. Electrically, they are interconnected by three sets of solder joints. At the bottom or on the outside of the polymer board is the set of the largest solder balls of diameter up to 760 μm, which is called the ball‐grid‐array (BGA). These balls allow the test device to be connected to the circuits on a printed circuit board. Within the polymer board, there are Cu wirings, as well as Cu plated‐through‐holes, which are not shown in the image. On top of the polymer board, there is the second set of flip chip solder balls of diameter about 100 μm, the so‐called C‐4 (controlled collapse chip connection) solder balls, connecting the board to the first Si chip, which is the “interposer.” In this test device, there is no transistor on the interposer, which is passive and serves only as a substrate without introducing thermal stress to the active Si chip on the top. Often this test device is called 2.5D IC due to the fact that the interposer has no transistors. If the interposer has transistors, it becomes 3D IC.