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June 2015 • The PCB Magazine 51 version of the service temperature gives a much more realistic value for L than the official IPC service temperature test method for coverlays. The J coverlay appeared to fail mainly be- cause of brittleness, and this reduced bend performance. This confirms that brittleness is one of the possible failure modes with high- temperature aging, and this new method was able to capture the performance loss with this failure mode. This is probably also the case for the L, however, it is difficult to tell because of acrylic adhesive is covered with a polyimide film. The all-polyimide coverlay based on experi- mental film, X, shows very good performance, in part because there is an all-polyimide flexible circuit containing a polyimide clad and cover- lay. As long as good adhesion can be achieved between the polyimide and the copper foil, an all polyimide flexible circuit should have the best high-temperature performance. We have developed an all-polyimide cover- lay and bondply to use with the all-polyimide copper clad laminates based on the X film. The new polyimide coverlay will require lamina- tion temperatures of around 290–300°C (554– 572°F), which will limit the use to only certain fabricators. So far it is clear that this is the best approach to achieve flexible circuits that can survive high temperatures. We will continue to refine this new coverlay service temperature test method. If it continues to show results more consistent with field expe- rience, we will recommend that it be considered as an IPC test method. Summary To meet the increasing needs for flexible circuit materials for high-temperature appli- cations, new test methods will need to be de- veloped. These new methods will assign new ratings, and we believe ratings that are consis- tent with actual performance. The present IPC service temperature test seems to work well for testing copper clad laminates. It does not work well for bondplies and especially coverlays. We have demonstrated a new coverlay test based on bend testing. The overall results clearly show that all polyimide clads, bondplies and cover- lays will provide the highest service tempera- ture performance. PCB Sidney cox is a product development research scientist at DuPont circuit and Packaging Materials. Stanford electrical engineer Jelena Vuckovic wants to make computers faster and more ef- ficient by reinventing how they send data back and forth between chips, where the work is done. In computers today, data is pushed through wires as a stream of electrons. That takes a lot of power, which helps explain why laptops get so warm. In essence, the Stanford engineers want to miniaturize the proven tech- nology of the Internet, which moves data by beaming photons of light through fiber optic threads. The Stanford work relies on the well-known fact that infrared light will pass through silicon the way sunlight shines through glass. The Stanford algorithm designs silicon struc- tures so slender that more than 20 of them could sit side-by-side inside the diameter of a human hair. by automating the pro- cess of designing optical in- terconnects, they feel that they have set the stage for the next generation of even faster and far more energy- efficient computers that use light rather than electricity for internal data transport. Stanford Creates Super-efficient Light-based Computers FeAtuRe FLExIBLE CIRCUIT MATERIALS FOR HIGH-TEMPERATURE APPLICATIONS continues

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