Automotive electronic safety systems are reaching higher in frequency, towards the use of 77 GHz for automotive radar safety systems that will one day make the highways safer. As explained in the previous ROG Blog, vehicular radars are already being designed and fabricated at millimeter-wave frequency bands such as 77 GHz. Specifiers of circuit materials for millimeter-wave frequencies (30 to 300 GHz) are faced with special requirements that are often different than those for circuits at microwave frequencies of 30 GHz and below. However, practice and experience of circuit designers working at millimeter-wave frequencies has shown that some circuit material parameters can be tightly linked to achieving high performance in millimeter-wave circuits, and that some circuit materials embody the material parameters as needed for excellent performance at 77 GHz and beyond.
As introduced in the previous ROG Blog, the six key circuit material properties for achieving low-loss circuits at millimeter-wave frequencies and at 77 GHz are:
- Dk tolerance
- Circuit material Df
- Copper conductor surface roughness
- Thermal coefficient of Dk and Df
- Moisture absorption
- Glass weave effect
When used as the guidelines for sorting circuit materials for 77 GHz PCB radar antennas for automotive applications and other millimeter-wave circuit applications, these six material properties very much point to RO3003 laminates as quite suitable substrates for low-loss circuits. In fact, because its characteristics are so well suited for the special needs of millimeter-wave circuits, RO3003 laminate is widely used for those applications. It is available with different types of copper, such as electrodeposited (ED) copper as well as smoother (and less-lossy) rolled copper, allowing specifiers to precisely specify the characteristics of their laminates to meet the most demanding requirements of millimeter-wave circuit applications.
Sorting Through the Six
Just how do those six key material parameters relate to electrical performance at 77 GHz and other millimeter-wave frequencies? At millimeter-wave frequencies, the short signal wavelengths can make having circuit material with tightly controlled Dk more important than simply having a material with low Dk. The tightly controlled Dk enables more consistent performance; on the other hand, variations in Dk (∆Dk) result in inconsistent phase angles at the short wavelengths of 77 GHz, which translate into poor radar performance at that frequency.
The Dk of a circuit material when it is extracted from circuit performance, or its design Dk, is influenced by the tolerance of the material’s Dk, and it is also affected by various other circuit material characteristics, including variations in the surface roughness of the copper conductor. Minimizing the variations in design Dk that can affect performance at 77 GHz involves controlling design Dk as well as those other circuit characteristics that contribute to design Dk. The best way to determine variations in design Dk is by means of accurate and repeatable measurements of reference circuits from many different samples of materials from different production lots.
Similarly, a circuit material’s dissipation factor (Df) must be tightly controlled to achieve repeatable, low-loss circuit performance at 77 GHz. Achieving low Df in a circuit material is a worthwhile goal but maintaining stable Df versus frequency is as important at millimeter-wave frequencies. Variations in Df, which is another contributor to design Dk, can make it difficult to maintain phase and frequency stability at the small signal wavelengths of millimeter-wave frequencies.
At 77 GHz, the amount of surface roughness of a circuit material’s copper conductor can make a significant difference in the conductor loss, with smoother copper yielding lower loss. Electrodeposited (ED) copper is a popular conductor type for circuits at millimeter-wave frequencies, although, due to its roughness, it exhibits higher loss than rolled copper conductor. Peel strength (both as-is and after thermal exposures) is not to be overlooked when assessing circuit material characteristics at 77 GHz and millimeter-wave frequencies since the adherence of the copper and dielectric layers influence the RF performance of the circuit. As with many material parameters, there can be a tradeoff between electrical performance and peel strength for smoother ED and rolled copper foils. Still, despite the expected tradeoff, when used with RO3003 circuit material, rolled copper provides good peel strength, while also delivering very low loss characteristics at 77 GHz.
Since millimeter wavelengths require such fine circuit features, the ways that a circuit material responds to changes in temperature can result in variations in performance at 77 GHz and other millimeter-wave frequencies. Excessive variations in thermal coefficient of Dk (TCDk) and thermal coefficient of Df (TCDf) will mean the same as variations in Dk and Df when a circuit material is exposed to a wide-enough temperature range or has a wide-enough TCDk or TCDf. By specifying circuit materials with relatively tightly controlled TCDk and TCDf, these temperature effects can be minimized. In general, a circuit material with a TCDk of ǀ 50 ǀ ppm/°C or less is considered good. As a real-world example, RO3003 laminate has a TCDk of 3 ppm/°C.
For most high-frequency circuit materials, low moisture absorption is a goal. For circuit materials to be used at millimeter-wave frequencies, where even small differences can affect performance, circuit materials that are guilty of excessive moisture absorption will suffer from increased loss as well as from variations and increases in Dk as a function of moisture. Performance that may have been acceptable under ideal operating conditions may fall short, for example, under operating conditions of high humidity, especially at the shorter wavelengths of millimeter-wave frequencies.
Finally, of the six circuit material parameters to consider for 77 GHz and other millimeter-wave circuits, the “glass weave effect,” as it is known, can result in variations in the Dk of a circuit material. Woven glass is used in many circuit materials to reinforce and strengthen the material. While it does this, it also results in patterns throughout the material where some spots have more glass than others, resulting in variations in the Dk. The use of glass reinforcement can strengthen a circuit material mechanically, but it will also impact the electrical performance of the material, especially at higher frequencies. Ideally, a material for higher frequencies does not need glass reinforcement.
Meeting the Requirements
Rogers RO3003 laminate has been found to meet these six key material requirements for 77 GHz and other millimeter-wave circuits quite well. RO3003 laminate is a very low loss material with typical Df at 10 GHz of 0.0010 and a tightly controlled Dk of ±0.04. Many designers of millimeter-wave circuits have relied on 5-mil-thick RO3003 laminates as their circuit material, either with ED copper or smoother (lower-loss) rolled copper. It has low moisture absorption, less than 0.04%, and very low TCDk of -3 ppm/°C.
Since it exhibits so many of the six key requirements for higher-frequency circuits, RO3003 laminate has become a popular choice of circuit materials for designers of 77 GHz and other millimeter-wave circuits. In addition, RO3003 laminate does not use or need woven-glass reinforcement, so it does not suffer from woven-glass issues. It is a durable, low-loss circuit material formulated for circuits where maintaining signal power is essential as frequencies go higher, especially for 77-GHz automotive radar systems focused on improving safety!
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