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As mobile electronics products become more compact and lighter and perform better, the need to decrease the number of large-scale integration (LSI) packages mounted on each board as well as decreasing their mounting area is increasing. This paper describes a newly developed ultrahigh-density three-dimensional (3-D) stacked package called a flexible carrier folded real chip size package (FFCSP). The FFCSP is constructed by stacking very thin single chip packages of real chip size on top of one another. Each single chip package consists of an LSI chip and a small piece of flexible printed circuit (FPC), which has an insulating layer made of thermoplastic resin. The FFCSP's advantages are its ultrasmall size, thinness, highly flexible assembly, and a good test yield compared with conventional 3–D stacked packages. The FFCSP will enable the future miniaturization and raise the functionality of the next wave of mobile electronics products.