Abstract
A newly designed three-dimensional (3D) flexible circuit as a package with five IC chips has been invented, and the prototype of the 3D package using laser micromachining has been successfully demonstrated. Fabrication processes of the 3D 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 2D design for a 3D structure. According to this design, 55% area reduction can be obtained without any designing and overheating problems, which usually occurs. Furthermore, the new 3D 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 3D stacked package meets the requirements of the 85 °C/85% test.
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