Thermally Stable, Low Loss Optical Silicones: A Key Enabler for Electro-Optical Printed Circuit Boards

We report the development and characterization of a low loss polymer waveguide material with a polysiloxane backbone for enabling the new generation electrooptical printed circuit boards (PCBs). The polymer was designed to have low optical loss of <;0.05 dB/cm at 850 nm and low residual stress to withstand the harsh reliability requirements during PCB integration. The thermomechanical properties of the polymer were tuned to exhibit a residual stress of ~1 MPa over a temperature range of 25 °C to 200 °C in ambient air. Multimode polymer waveguides were fabricated using the polysiloxane polymer and embedded in a six-layer PCB architecture that was subjected to lamination, through-hole via drilling, plating, and IPC shock test (immersion) in solder bath at 288 °C. Eight channels of multimode polymer waveguide spirals of length 1.2 m were fabricated and the insertion loss measured after waveguide fabrication, lamination, and solder reflow. The deviation in insertion loss as a function of the PCB fabrication process was less than 3% with final insertion loss after solder reflow being 0.052 +/- 0.002 dB/cm. This finding presents an optical waveguide material which when embedded in a PCB fabricated using an industry standard process meets reliability requirements while maintaining optical performance.