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PCBD-Feb2016

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40 The PCB Design Magazine • February 2016 has an Er = 3.3 at the same frequency. The di- electric material determines the velocity (v) of propagation of the electromagnetic energy: where the speed of light (c) is 3x10^8 m/s. Substituting the numbers, the optical fiber has a velocity of propagation of 154.6x10^6 m/s compared to 164.8x10^6 m/s for Megtron 7. So believe it or not, the PCB substrate, in this case, will actually transfer the signal faster than an optical fiber. However, the dissipation fac- tor (dielectric loss) of silica is 0.00002 whereas Megtron 7 is only 0.003, which limits the band- width. However, this is still fairly low loss com- pared to standard FR-4 of 0.02. Fibers have a number of advantages over copper, including higher bandwidth, lower loss, immunity to electromagnetic interference. There is also no crosstalk between signals in dif- ferent optical cables. Further, non-armored fiber cables do not conduct electricity, which makes them ideal in high-voltage environments or structures prone to lightning strikes and for pre- venting ground loops. Figure 2 illustrates a rigid PCB substrate with an embedded optical polymer waveguide. This novel structural design offers potential solu- tions for low-cost and high-performance semi- conductor circuits with optical devices to realize wide bandwidth and low-profile optoelectronic packaging for chip-to-chip optical interconnect applications. The vertical cavity surface emitted laser (VCSEL), driver, and serializer chip are 3D stacked and then attached to one end of the em- bedded optical polymer waveguide in the PCB. Similarly, the photo-diode detector, trans-im- pedance amplifier (TIA), and deserializer chips are also stacked and then attached on the other end of the waveguide. Although this system has the same communications speed as a typical trace based interconnect, it potentially exhibits wider bandwidth and much lower noise. The actual velocity of electrons through a conductor is measured at an average speed called the "drift speed." The charge carriers (electrons) move very slowly; however, the "knock-on" ef- fect is very fast as it follows the electromagnetic field. When one electron is forced to move, it bumps into its neighbor making it move and so on: the domino effect. The energy propagates as an electromagnetic wave. The electrical effects that we observe, such as lights coming on immediately when a switch Figure 2: Low-cost, high-performance optoelectronic system embedded into an organic laminated substrate. (Source: Lau, Zhang, Lee) faster than a speeding Bullet

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