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66 The PCB Magazine • March 2014 Mid-Dk/Df Class Laminates There is even a more dynamic mid-Dk/Df market at present. Mid-Dk/Df is defined here as having a dielectric constant measurably lower than FR-4, meaning at or below 4.0 and a Df rat- ing in the range of 0.007–0.013, with a number of entries rated at or below 0.010. This class of materials has existed since the issuance of GE's Getek circa 1988, which was followed by Park- Nelco's N4000-13 and its equivalent competitor materials. Nelco also added low-Dk/Df options to N4000-13, as did several of its competitors over time. Changes to this class came fast once RoHS dictates were being adopted. These are highly modified epoxy-based systems, some blended with cyanate esters, PPEs, PPOs and other pro- prietary content to achieve their performance targets. These now long, complex molecules were not proving very adaptable to the elevated temperatures of lead-free assembly and their im- pact on CTE-Z. Modifications were again needed and established mid Dk/Df players did just that, but only incrementally to protect their existing resin UL qualifications, which would be lost and require a new qualification cycle if UL Labs were to determine that a resin had changed to the degree of designating it as a new system. In many ways, this hamstrings the lamina- tors in industry-critical efforts to improve resin systems. Once a resin system formula has been locked down, it takes about a year for a lamina- tor to get that new product through laminate UL, and the multi-months-long fabricator UL qualification extends that time even further. So, pressures of time-to-market and return on investment can run very counter to the need to advance a resin system beyond some limited scope despite the need for a fuller reinvention. The market was able to manage for a few years, but HDI has become more complicated, BGA pitches smaller, and plated over via-in-pad and sequentially laminated build-up boards' de- signs are much more prevalent today. The need to cross that UL line has been made evident and several laminate suppliers have responded with new, more robust and performance-improved systems. Not only have they been addressing the thermal-mechanical requirements, but they have also been able to chip away at bettering LAMINATE MATERIAL SELECTION FOR TODAY'S PCBS continues Figure 3: a variety of competitive materials are available, with only incremental differences.