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74 DESIGN007 MAGAZINE I JULY 2019 Flex007 Feature by Weifeng Liu, Ph.D., Alex Chan, William Uy, Dennis Willie, Dongkai Shangguan, Ph.D. FLEX, MILPITAS, CA Introduction Flexible hybrid electronics (FHE) refers to a category of flexible electronics that are made through a combination of traditional assembly processes of electronic components with high- precision ink printing technologies [1] . By inte- grating silicon components with printed inks and flexible substrates, FHE will revolutionize the IoT and wearable industries. With FHE, designers can create a heterogeneous electron- ic system that can be fully integrated with dif- ferent sensors, lighter in weight, more cost ef- fective, more flexible and conforming to the curves of a human body or even stretchable across the shape of an object or structure—all while preserving the full functionality of tradi- tional electronic systems. The FHE industry is still in the early stages of development, and a variety of design, ma- terial, assembly and reliability issues need to be addressed. For example, electrical intercon- nections formed with conductive adhesives may not be as conductive or reliable as com- pared to conventional solder assembly. Typi- cal polymer-based conductive inks are not as conductive as the etched copper used to make circuit boards and they are mostly not readily solderable. Additionally, commercially avail- able stretchable thermoplastic-based film sub- strates have relatively low heat resistance and cannot withstand the current lead-free reflow process temperatures. This article will present a hybrid manufac- turing process to manufacture FHE systems with a two-layer interconnect structure utiliz- ing screen printing of silver conductive ink, filled microvias to connect ink traces at the dif- ferent layers, and use of the traditional reflow process to attach the semiconductor chips to the printed substrates. Experimental This study is to convert a rigid multilayer wearable development platform into a flexi- ble one using printed conductive ink and flex- ible substrate. The current rigid platform con- tains two active semiconductor components

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