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A newly designed three dimensional (3-D) flexible circuit as a package with five IC chips has been established, and the prototype of the 3-D package using laser micromachining has been successfully demonstrated. Fabrication processes of the 3-D package consist of (1) preparation of printed wiring on the flexible substrate, (2) selective polyimide material removing on contact pads using UV laser (3) component placing and soldering, and (4) preparation of bending windows by laser micromachining. The production of the so-called bending window is a unique application of laser material processing. These windows can be used in flexible circuits to define the exact position of deformation. It is done by reducing the thickness of the flexible substrate in a well-defined, narrow line. The unique feature of this newly developed package is the 2-D design for a 3-D structure. According to this design, 70% area reduction can be obtained without any designing and overheating problems, which usually occur. Furthermore, the new 3-D package design can simplify processes such as I/O redistribution, chip cooling, and package formation. It is proven that the mechanical integrity of the prototype 3-D stacked package promises to meet the short-term requirements of a damp-heat test.
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