A detailed thermal analysis for the FSOI (Free Space Optical Interconnect) technology incorporating VCSEL (Vertical Cavity Surface Emitting Laser) devices is performed using commercially available software. FSOI is one of the latest technologies used to transmit information at high-speed to/from a microprocessor to memory device via photons. Due to large heat dissipation and compact packaging design, temperature and associated thermal strain/stress could reach high values in the FSOI assembly, causing serious reliability and quality problems. Several design options are investigated in order to provide optimal thermal management for the FSOI module, and maintain VCSEL temperatures within reasonable limits. Convective cooling results for both organic and ceramic boards are investigated.
For designs with organic boards and without any thermal enhancement, the VCSEL temperature is well above the acceptable limit of 85°C at an ambient temperature of 30°C. The sequential inclusion of pedestals, board thermal vias, conductive rings between the optical modules, and metallic (Al) rods will significantly enhance the module thermal performance and reduce VCSEL temperature to 46°C. The presence of thermal vias in the organic board is critical; however, if the copper area percentage in the via block vs. the die area is above 3%, the VCSEL temperatures will remain constant.
The ceramic boards provide a good thermal solution, as VCSEL temperatures remain below the upper limit without including any thermal vias in the board. The comparison between the effect of convective air speed on FSOI with ceramic versus organic boards reveals that the VCSEL temperature is slightly higher (less than 2°C) for the case incorporating a ceramic board. However, the ceramic board has no thermal vias, compared to the 100% copper via block in the organic board. Hence, the same results are accomplished with much less complexity in the ceramic substrate design alternative. This option is suggested for manufacturing purposes, with improved thermal performances.