Session 18: FHE Chip Integration

Photonic Soldering to Enhance Manufacturability of Wearable Technology
Thursday, February 15, 2018
11:00 AM - 11:20 AM

Development of flexible hybrid electronics requires an inexpensive, reliable and high throughput attachment technique for functional components onto printed circuits. The challenge faced with the current reflow technology is that many of the desired substrates cannot withstand the necessary temperatures required to reach the liquidus point of common solders. To overcome this limitation, conductive adhesive attachment techniques were developed but they lack the reliability of soldering. To circumvent the stated challenge posed by the substrates, a photonic soldering technique has been developed that takes advantage of the selective absorption of light to reach the liquidus temperature of conventional solders without significant heating of the underlying substrate. The short time the substrate spends at elevated temperatures may not be sufficient to damage the underlying substrate. This technique could work with conventional lead-free solders (e.g. SAC-305) and commercial packages (e.g. 402MM) or bare thinned dies on a variety of substrates, including PET, PEN as well as metallic foils. The photonic soldering technique is feasible for integration into high throughput roll-to-roll or sheet-to-sheet setups. TPU is a common inlay for wearable applications with the functional circuit built on top of the inlay. TPU poses additional challenges as a substrate for soldering because of its high coefficient of thermal expansion and elasticity. For this work, photonic soldering was evaluated to overcome the challenges posed by the substrate. A large (10 mm x 10 mm) thinned bare die with over 400 bumps was used as the evaluation platform. Cross-sectional SEM and X-ray imaging of solder joints were used to compare the photonic soldering outcomes with the conventional reflow soldering results. The evaluated results indicated close correlation of outcomes between the two techniques. Acknowledgements: This work was partially funded through a NextFlex project in collaboration between NovaCentrix, Cal Poly, Jabil and DuPont to enable attachment of ultrathin bare die chips onto printed circuits for wearable applications. This material is partially based upon work supported by Air Force Research Laboratory under agreement number FA8650-15-2-5401. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of Air Force Research Laboratory or the U.S. Government.