Miniaturized Printed Wiring Board Consisting of Polyimide Layers and Three Embedded Integrated Circuit Chips in Stacked Configuration

In the history of electronics packaging, continuous efforts have been made to achieve seemingly conflicting goals of reducing the size of electronic devices and increasing their functionality. With more sophisticated packages, wearable and portable devices in particular are required to be more compact as well as reliable. To reduce the size of a package, embedding IC (integrated circuit) chips inside is an effective solution instead of placing them on the surface. Based on this thought, we have developed a technology to produce WABE packageTM (wafer and board level device embedded package). The packages are produced by combining thin polyimide film layers and backgrinded IC chips. WABE Package technology has an unmatched feature that it can reduce the footprint of a package drastically by stacking IC chips if multiple chips need to be embedded in the package. Even if the package has two or more chips, the increase in the thickness of the chip-stack structure is very limited because each layer is so thin. Another feature of the technology is parallel fabrication of each polyimide-based layer with vias filled with a conductive material for interconnection. The individual and simultaneous preparation of layers enables “one-step” colamination, which has an advantage that only one-time press is needed even though the number of layers increases due to the chip-stack structure. This report describes the fabrication of a prototype circuit board that consists of 14 copper layers and 3 IC chips embedded vertically in them. The board is less than 0.9 mm in thickness even though it includes a large number of layers. We also report the results of various reliability testing conducted on the package. These results were obtained by electrical measurements of daisy chain patterns formed between some of the layers. The prototype showed high reliability under moisture and heat test conditions simulating those in autoclave sterilization and heat-shock test conditions simulating those in the production process. These results show that the WABE Package technology that allows three chips to be embedded vertically in the package is the most promising packaging technology for extremely miniaturizing electronic circuits for medical and wearable electronics.