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54 The PCB Design Magazine • February 2014 The landscape of specialty materials chang- es so quickly that it can be hard for product de- velopers to keep up. As a result, PCB designers are inundated with data about microwave PCB materials. But very often, it's difficult to find useful information regarding these specialty substrates. So, this month, we present some of the most frequently asked questions about mi- crowave materials encountered at Rogers. Q: At what frequency is it necessary to tran- sition from FR-4 types of materials to high-fre- quency circuit materials? A: This is a challenging question to answer because different technologies can tolerate more or less performance from a laminate. I'll give a few examples and basic guidelines. Semiconductor technology has developed enhanced signal processing to the point where a FR-4 material could be used at higher speeds and frequencies than was once thought pos- sible. In most cases where high-speed digital applications reach 10 Gbps or more, you will need to use a high-frequency laminate. There are exceptions, and in some cases a lower data rate PCB will also demand a high-frequency laminate. High-frequency RF circuits, which are less concerned with insertion loss, could use FR-4 in some of these applications. However, high- frequency laminates offer more than just low loss; they provide very well-controlled dielec- tric constant. In many RF applications, the con- trol of dielectric constant for the material can be as critical as substrate thickness control. As a general statement, FR-4 materials are typically not used above 3 GHz in RF applications due to insertion loss concerns. However, when dielec- tric constant control is a critical concern, high- frequency materials should be used instead of FR-4 materials. by John Coonrod ROgeRS CORPORATiOn LIGHTNING SPEED LAMINATES FAQ: Microwave PCB Materials column