At one time, the choice of circuit laminates for high-frequency, thin-film circuits came down to almost an “either/or” decision for circuit designers: fabricate it on lower-cost FR-4 circuit material or on higher-performance (and higher-cost) PTFE-based laminates (or alumina ceramic substrates in the case of high-frequency thick-film circuits). FR-4 really refers to a family of circuit materials based on woven-glass-reinforced flame-retardant epoxy. The material is popular for its low cost and ease of circuit fabrication, but suffers degraded electrical performance at higher frequencies, typically above about 500 MHz, and many circuit designers had learned their own “cutoff frequency point” below which they could use FR-4 and above which required a PTFE-based circuit laminate.