Reliable filling of through vias with silver based conductive adhesives in flexible PEN substrates using low-cost optimized stencil printing methods

Systems-in-foil technologies address the current need to manufacture low-cost, low-complexity and high-reliabiltiy large area organic electronics in the field of OLEDs, OPVs and sensors. For substrates with multiple layers of conductive tracks, there is a need to realize contact positions on either side of the foil. Unfortunately, typical interconnection solutions such as electroplating used in the PCB industry, are not highly compatible with large area electronics processing technologies, such as roll-to-roll processing. In this paper, the authors present results on the development of a reliable front-to-backside interconnection technology by filling laser drilled through vias (TVs) in PEN foil with a silver-based conductive adhesive using optimized stencil printing methods. Typical initial resistances for 100 μm TVs are situated in a range of 0.4 to 0.5 Ω and TVs in a range of 0.15 to 0.35 Ω for 200 μm, respectively. Samples were subjected to various accelerated life time tests; 1,000 hours at 60 C/90%RH, 1,000 hours at 85 C/85% RH, 1,000 cycles at -40 C/120 C and 15,000 bending cycles using cylinders with 10 mm and 25 mm bending radius. Only the bending stress resulted in an increase in via resistance. Stress during bending was simulated using FE modeling and maximum stress positions were compared to SEM cross sections of the vias. To emphasize the robustness of this interconnection technology, a flexible demonstrator was successfully fabricated consisting of a 10 by 10 grid of LEDs on the front side, connected to photo diodes on the backside of the foil using 200 μm TVs for interconnection.