Issue link: https://iconnect007.uberflip.com/i/271756
March 2014 • The PCB Magazine 65 UL94VO flame. This flame-retardant change has a measurable impact on Df. Many have tak- en note of halogen-free FR-4s, with Df ratings in the range of 0.010–0.016 and at measurably lower cost than many of the mid-Dk/Df class materials. Ultimately, the mid-Dk/Df materials have loss tangents available that are lower still, well down into the 0.008 range, but at an ap- preciable cost premium. Financial budgets, as well as loss budgets, tend to dictate the want and feasibility of us- ing lowered Df FR-4s as opposed to opting into a mid Dk/Df material. As cost is always a fo- cal point, most designs opt for the least costly laminate that will deliver the performance attri- butes needed for that design application. Using over-engineered material for a design applica- tion is more often the fault of market offerings forcing designs to over-buy on unneeded attri- butes in order to acquire the needed ones. As the material market continues to fatten out its class offerings, using over-engineered materials becomes less necessary. This is further demon- strated by the limited but available option of acquiring select lead-free FR-4 items on spread weave glass styles. As the primary benefit of these styles has been mitigation of differential impedance timing skew, these FR-4 offerings are targeting a specific performance aspect, where- as before these premium glass styles were only available bundled with higher performance res- in systems. Lower-profile copper options, those smooth- er than reverse-treated foil (RTF), have not real- ly appeared in FR-4s. We believe this is because greater trace loss benefit can be realized by in- vesting in other lower-Df resin systems rather than using cost premiums to clad relatively lossy FR-4 with finely surfaced coppers. Lead-free FR- 4s also enjoy regular use in high-speed applica- tions as part of stackup hybridization, reduc- ing cost by being relegated to power, ground, analog and other non-critical layers, while of- ten improving the overall thermal-mechanical performance compared to all high-performance material stackups. LAMINATE MATERIAL SELECTION FOR TODAY'S PCBS continues Figure 2: other efforts focus on increasing performance while staying at today's price point.