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70 The PCB Magazine • January 2016 laser diodes. The ability to dissipate heat also becomes an important property in this context. Ceramic materials show an advantage here over organic materials. Low dielectric constant materials play a role at high operating frequencies where con- trolled (matched) impedance is required and only small propagation delays can be tolerated. As layer thickness decreases one needs lower Dk dielectrics for controlled impedance. Figure 1 shows the interrelationships between capaci- tance, inductance, Dk, and the effect of Dk on the propagation speed in a medium. Compared to the propagation speed in a vacuum, the sig- nal speed in a conductor surrounded by a dielec- tric is inversely related to the square root of Dk. A low CTE dielectric, more specifically a di- electric with a CTE that approaches that of the chip (CTE of approximately four), is very impor- tant in direct chip attach (DCA) and becomes more important with increasing chip size. Ceramic materials have a more favorable CTE than organics. To reduce the large CTE of organ- ics in the x/y-plane, ceramic fillers have been used successfully (e.g., in PTFE/ceramic compos- ites) as well as aramid reinforcement fibers. Flame retardancy of electronic devices is a basic safety requirement which basically af- fects the dielectric resin since other components such as copper, inorganic fillers, and glass fibers don't present a problem. The widely used FR-4 laminate typically derived its flame retardancy, as qualified by UL's V-0 Rating, by modifying the epoxy resin with tetra-bromobisphenol A. Concerns that this flame retardant may convert to dioxins upon combustion or during disposal by incineration, triggered the search for alterna - tives. Phosphorus-based additives are used as replacements but are not universally accepted. Resins containing aromatic rings (e.g., LCPs, polyimides) are inherently flame retardant, and so are PTFEs. Selecting epoxy resins high in aro- matic moieties helps solve the flame retardancy problem, so does the selection and loading of inorganic fillers. The term "halogen-free lami- nate" has been used to describe flame retardant alternatives to standard FR-4, but it is a bit of a misnomer: The target specifically is the bro- minated bisphenol A, so bromine-free might be more accurate. PTFE is also not on the "undesir- ables" list even though fluorine is a halogen. The "free" in halogen-free is also relative, since trace amounts of chlorine or bromine that constitute natural impurities in the process of manufacture of dielectric resins are also not targeted. Chemical stability is typically described in terms of percentage of material loss at a certain time and temperature (3–5% may be the upper Figure 1: interrelationships of electrical and dimensional parameters. karl's tech talk HIGH-PERFORMANCE LAMINATES