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October 2017 • The PCB Design Magazine 53 their resin systems are going to be compatible with each other. They lend themselves, actually, to that hybrid construction. Shaughnessy: And with hybrids, you don't want to have a lot of different warp and twist rates… Berry: The vast majority of the Isola's high-per- formance and high-speed digital products have very similar press cycles, which lend themselves to the hybrid package construction. Hunrath: If the materials are very different, you'll end up with a potato chip at the end of the process, which is impossible to run through assembly, so it's important that the materials do work well together. Berry: At that elevated temperature, CTE mis- match is going to give in-plane shear stress, for instance. Hunrath: One of the other things we're seeing with the square glass materials is that it's not just about signal integrity. There is a trend to use those materials on standard resin systems where signal loss or skew is not so important, but the people want to get higher reliability, single-ply constructions, or they want to have better, quicker laser drilling. The square glass eliminates some of the glass bundle knuckles and makes it easier to laser drill, faster to laser drill. Productivity and reliability in single-ply construction are also driving more square glass constructions. Shaughnessy: Is there anything else you would like to add? Berry: We're dedicated to working with our fab- ricator customers to get out and educate their customer base, so that they make the right choices, close in on their revenue goals, and that we're all successful together. Shaughnessy: I appreciate your time. Hunrath: Thank you, Andy. PCBDESIGN INSULECTRO TEAMS WITH ISOLA TO ADDRESS SIGNAL INTEGRITY NEEDS In 2015, researchers at the Harvard John A. Paul- son School of Engineering and Applied Sciences (SEAS) developed the first on-chip metamaterial with a refractive index of zero, meaning that the phase of light could be stretched infinitely long. Now, SEAS researchers have pushed that tech- nology further, developing a zero-index waveguide compatible with current silicon photonic technolo- gies. In doing so, the team observed a physical phe- nomenon that is usually unobservable—a standing wave of light. When a wavelength of light moves through a material, its crests and troughs get condensed or stretched, depending on the properties of the mate- rial. How much the crests of a light wave are con- densed is expressed as a ratio called the refractive index: the higher the index, the more squished the wavelength. When the refractive index is reduced to zero the light no longer behaves as a moving wave, traveling through space in a series of crests and troughs, other - wise known as phases. Instead, the wave is stretched infinitely long, creating a constant phase. The phase oscillates only as a variable of time, not space. Imagine strings on a guitar, pinned on either side. When a string is plucked, the wave travels through the string, hits the pin on the other side and gets reflected back, creating two waves moving in oppo- site directions with the same frequency. This kind of interference is called a standing wave. This may be the first time a standing wave with infinitely-long wavelengths has ever been seen. A Zero-Index Waveguide

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