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PCB-June2015

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42 The PCB Magazine • June 2015 4. Relevance to Space Applications The need for thin, flexible packaging and stretchable interconnection is driven by the de- sire for increased functional density in a reduced or application-specific form factor. Reliability requirements for the typical use environments are not of the highest concern during the tech- nology development phase. The improvements offered by these new technologies, such as in- creased functional density, form factor reduc- tion and the possibility to adapt to any shape, are also interesting for space applications. In this application field, however, excellent reli- ability is an absolute priority. From this point of view, it is interesting to evaluate what reliability improvements these new technologies could bring. Failure mecha- nisms for solder interconnections are well-doc- umented [8, 9] . Although no legislative obligation is in place, lead-free solder issues become more and more relevant for space as well [10, 11] . Based on the Pb-free experience in consumer applica- tions, failures related to the use of solder will only increase with this transition. Embedding of components in general, and of UTCPs in par- ticular, offer a promising alternative by using plated interconnections in the form of copper microvias or plated though-holes. Both from a dimensional as from a material point of view, these interconnections have a thermo-mechan- ical advantage over bulky and brittle solder joints. Embedding the components inside the board also offers increased mechanical protec- tion with respect to bending or shock. By its very nature, stretchable interconnec- tions offer an even larger potential for mechani- cal robustness. By dividing the large system into smaller sub modules that are interconnected by elastic interconnections which can absorb the mechanical stress from vibrations or shock, sen- sitive components can be protected without the need for bulky anti-vibration frames. The authors are well aware of the fact that these statements are oversimplified and that the proposed technologies will suffer from or even introduce new failure modes. Then again, it is thinking outside of the box that put a man on the moon. 5. Conclusion Two emerging technologies for flexible pack- aging and stretchable interconnections were discussed in this paper. The UTCP technology makes it possible to realize chip packages with a total thickness of less than 100 µm. Embed- ding these packages to exploit the third dimen- sion results in a volume reduction of more than 60%. Stretchable interconnections in the form of encapsulated meanders can survive tens of thousands of stretching cycles of up to 5% elon- gation. It has been illustrated that mechanical reliability is strongly enhanced by introducing a flexible support material for the meanders and by providing smooth mechanical transi- tions between the rigid solder assembled stan- dard components and the soft and conformable stretchable interconnects. Compared to common assembly and inter- connection technologies for space applications, the proposed technologies are still in their in- fancy. The possibilities for new form factors with increased functional density are obvious, but also a potentially improved reliability in harsh environments merits further investiga- tion into applying these technologies to space applications. PCB FLExIBLE AND STRETCHABLE CIRCUIT TECHNOLOGIES FOR SPACE APPLICATIONS continues FeAtuRe Figure 10: Failures at the top of meanders with polyimide support. upper picture shows complete fracture of the copper trace, while the bottom picture shows an initiated microcrack.

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