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36 The PCB Magazine • June 2015 FLExIBLE AND STRETCHABLE CIRCUIT TECHNOLOGIES FOR SPACE APPLICATIONS continues FeAtuRe with a thickness of about 70 µm, mounted on a glass carrier. The design of the metal fan-out al- lows converting the fine pitch (currently down to 65 µm) of the IC to a larger pitch compatible with the intended application, such as embed- ding or stacking. Furthermore, it is possible to create interconnections between several con- tact pads of the IC, reducing the required num- ber of connections to the external substrate. The embedded die can be used as a package (e.g., solder balls can be placed on the contacts and the package can be solder assembled on interconnection substrates). Alternatively the ultra-thin package can be embedded in a stack of rigid or flexible PCB layers. The UTCP can be tested before embedding, presenting a clear advantage compared to the direct embedding of bare dies. Figure 3 shows a schematic process flow for UTCP embedding, in this case between two polyimide copper-clad laminates. The UTCP is aligned to the copper pattern on one of the in- ner layers and encapsulated by the acrylic ad- hesive that is generally used for building multi- layer flexible printed circuit boards. After lami- nation, holes are drilled though the stack and consequently metallized, realizing the inter- connection between the UTCP and the board without needing microvia interconnection schemes. The embedding of UTCPs has been demonstrated for a number of flexible and rigid materials, which need to offer good adhesion to the polyimide-based UTCP and be compatible with the through-hole metallization process. A cross-section of the resulting board with embedded UTCP is shown in Figure 4a. A UTCP of a RF transceiver (ZL70102 from Microsemi) is embedded in a three-layer flexible printed cir- cuit board (FCB). The plated through-hole inter- connecting layer 1 and 3 of the FCB to the UTCP can be seen on the right-hand side. The total thickness of the FCB averages around 250 µm. Embedding the UTCP package inside the board not only avoids the use of bulky surface- mount packages, but also frees up board space for mounting active or passive component. This three-dimensional packaging approach allows for a significant reduction in board size and thickness. As an example, Figure 4b shows a comparison between an embedded UTCP ra- dio module and the original module based on a chip-scale package (CSP). The total volume of the module is reduced to less than 60% of the original. Further miniaturization can be realized by integrating multiple ICs in a single package. This can be done by stacking multiple UTCP-pack- aged dies on top of each other using lamination Figure 3: Schematic process flow for uTcP embedding. Figure 4: a cross-section of a board with embed- ded uTcP of a rF transceiver (Zl70102 from Microsemi, top) and the comparison between the embedded uTcP radio module and the original module (bottom).