Embedding polymer optical waveguides (WGs) into printed circuit boards (PCBs) for intra-board or board-to-board high
speed data communications requires polymer materials that are compatible and inert when exposed to common PCB
manufacturing processes. Ensuring both WG functionality after chemical exposure and maintaining PCB manufacturing
integrities within the production process is crucial for successful implementation. The PCB manufacturing flow is
analyzed to expose major requirements that would be required for the successful implementation of polymer materials
for embedded WG development. Chemical testing and analysis were performed on Dow Corning ® OE-4140 UV-Cured
Optical Elastomer Core and Dow Corning® OE-4141 UV-Cured Optical Elastomer Cladding which are designed for low
loss embedded optical WGs. Contamination testing was conducted to demonstrate polymer compatibility in both cured
and uncured form. Various PCB chemicals were treated with uncured polymer material and tested for effective
contamination. Fully polymerized multimode WGs were fabricated and exposed to PCB chemicals at temperatures and
durations comparable to PCB manufacturing conditions. Chemical analysis shows that the chosen polymer is compatible
and inert with most common PCB manufacturing processes.
In order to maintain the overall optical performance in a step index rectangular waveguide, the complex index of
refraction of the core and cladding material must be maintained throughout the cycle of the lithographic fabrication
process. The percentage of the core and cladding material that is cured and the irradiance that cure took place directly
affects the complex index of refraction of these materials. Siloxanes produced by Dow Corning have been selected to
meet the requirements for embedded waveguides for circuit board applications due to their optical performance
characteristics and their compatibility with current manufacturing techniques. The required total dose for a 50 μm thick
layer of siloxane is 1200 mJ at an irradiance of 30 mW/cm2. In order to utilize lower irradiance levels the total dose of
the ultraviolet exposure must be characterized and calibrated. By measuring the changes in the absorption peaks of the
materials using transmission data from ellipsometric techniques it is possible to define the percentage cure of the
siloxane from different curing profiles. Ellipsometric techniques were also utilized to measure the complex refractive
index of the materials cured using different profiles. It was found that the total dose required for a complete cure and the
complex refractive index of these materials drastically changes with different irradiances and the profile for the total
dose compared to the curing of the siloxane materials at all irradiances is logarithmic.
In order to characterize and optimize the overall link budget for an optical communication channel, the absorption loss
of the waveguides must be well known, stable, and minimized. Research and characterization has been performed to
ascertain the impact of the use of halogen vs. halogen free FR-4 circuit boards. Halogen is utilized within glass resin
epoxy circuit boards as a flame retardant. An analysis of rectangular multi-mode polymer waveguide structures, with a
fixed core dimension of 50 μm × 50 μm, was done to characterize the effects of the halogen FR-4 on the absorption
loss. Thermal cycling times were varied in order to determine the relationship between heating of the polymer
material, halogen diffusion into the optical cladding and core layer, and optical losses.
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