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64 The PCB Magazine • March 2014 laminates are plotted for S21 loss at 10GHz in Figure 3, which shows there are many competi- tive materials available with only incremental performance differences, and a nearly linear slope of options from top to bottom. FR-4 Class Laminates PCB material discussions often use FR-4 as a baseline for comparison of material behaviors and, especially, processability. FR-4 will contin- ue to persist in product builds for both its ther- mal-mechanical strength (a valuable attribute for HDI fabs) and, of course, for its relative low cost. To continue to be in play, FR-4 also needs to provide at least acceptable limits of electrical performance coherent with transmission rates of interest. This has not been a great attribute of FR-4 historically, and our experience points to a maximum usable data rate of around 5–6 Gbps, depending on the design trace widths and routing lengths. The major impediment of using FR-4 is usually its loss tangent (Df). Yet, there have been some industry trends mitigat - ing the loss aspect of FR-4, extending its poten- tial use. OEM pressure on laminate manufacturers, which began around the time of the then-new requirement to withstand lead-free assembly solder excursions, lead to some other concur- rent enhancements and opened up a sub- class within a class: FR-4 resins modified and/ or blended in a manner that resulted in some achieving as much as 1/3 reductions in rated Df compared to traditional FR-4s. These select of- ferings continue in the market, as they present Df improvements at measurably less cost than mid-Dk/Df class entrants, although these modi- fied FR-4s do not enjoy much, if any, of the di- electric constant enhancements of the mid-Dk/ Df class. Even more to this point is the impact of halogen-free on the Df of FR-4. The change is from brominated flame retardants to non-bro- minated replacements that incidentally require lower content within the resin for effectiveness, that being the ability to conform and certify to LAMINATE MATERIAL SELECTION FOR TODAY'S PCBS continues Figure 1: some efforts address reducing costs at current performance levels.