IC Substrate - Finding suitable PCB material for 77GHz automotive radar

The operating frequency of automotive electronic safety systems is getting higher and higher, and the use of 77 GHz automotive radar sensors for safety systems will one day make urban traffic safer. As introduced in the previous ROG blog, the 77 GHz millimeter wave band "vehicle radar" has been designed and manufactured. Circuit materials used for millimeter wave frequencies (30 GHz to 300 GHz) face special requirements, which are usually different from circuits at microwave frequencies of 30 GHz and below. However, according to the practice and experience of millimeter wave circuit designers, the material parameters of some circuits can be closely related to the high performance in millimeter wave circuits, and some circuit materials, such as RO3003™ circuit boards from Rogers, have the characteristics of millimeter wave circuits. The required material parameters can show excellent performance at 77 GHz and higher frequencies.

As described in the previous ROG blog, the characteristics of the six key circuit board materials used to implement low-loss circuits at millimeter wave frequencies and 77 GHz are:

Dk tolerance
Circuit material Df
Surface roughness of copper foil conductor
Thermal stability coefficient of Dk and Df
Water absorption
Glass weave effect
When used as a guide for the selection of circuit board materials for 77GHz automotive radar PCB antennas and other millimeter wave circuit applications, the performance of these six materials largely indicates that RO3003™ circuit boards are very suitable for low-loss circuits. In fact, it is precisely because of its characteristics that are very suitable for the special needs of millimeter wave circuits, RO3003™ circuit boards are widely used in millimeter wave circuits. It can provide different types of copper foil, such as electrolytic (ED) copper and smoother (and less loss) rolled copper, allowing designers to precisely specify the characteristics of their circuit boards to meet the most demanding requirements of millimeter wave circuit applications.

Six key properties of materials

What is the relationship between these six key material parameters and electrical performance at 77 GHz and other millimeter wave frequencies? At millimeter wave frequencies, the signal wavelength is shorter, and it is more important for the circuit board material to have a strictly controlled dielectric constant Dk value than to only select materials with a low Dk. On the one hand, tightly controlled Dk can achieve more consistent performance; on the other hand, changes in Dk (ΔDk) will result in inconsistent phase angles at 77 GHz, which will result in poor radar sensor performance at this frequency.

The Dk of the circuit material extracted from the circuit (or called the design Dk), its Dk value is affected by the material Dk tolerance, and also affected by the characteristics of other various circuit board materials, including changes in the surface roughness of copper foil conductors. The design Dk change that may affect the performance of the circuit at 77 GHz should be minimized, including the control design Dk change and other circuit characteristics that affect the design Dk change. The best way to determine the degree of design Dk variation is to make accurate and repeatable measurements on the reference circuit of multiple samples from different production batches.

Similarly, the loss factor (Df) of the circuit board material must be strictly controlled to achieve repeatable, low-loss circuit performance at 77 GHz. Choosing a low-Df circuit board material is a goal worthy of attention, but it is also important to maintain a stable Df variation with frequency at millimeter wave frequencies. The change of Df is another influencing factor of the design Dk change, which makes it difficult to maintain phase and frequency stability under small wavelength signals with millimeter wave frequencies.

At the frequency of 77GHz, the surface roughness of the copper foil conductor of the circuit board material has a significant influence on the conductor loss. The smoother the copper foil, the smaller the loss. Although electrolytic copper (ED) is a copper foil conductor type widely used in millimeter wave frequency circuits, it exhibits higher loss than rolled copper due to its roughness. When evaluating the properties of circuit board materials at 77GHz and millimeter wave frequencies, the peel strength (including initial and after heating) cannot be ignored, because the adhesion of the copper layer and the dielectric layer will affect the radio frequency performance of the circuit. Like many material parameters, a trade-off can be made between electrical performance and peel strength to select ED copper and rolled copper foil. However, despite the expected trade-offs, when RO3003™ circuit board materials are used, rolled copper has good peel strength while also providing very low loss characteristics at 77 GHz.

Because of the need for such fine circuit characteristics under millimeter waves, the influence of temperature changes on circuit board materials will also cause changes in its performance under 77GHz and other millimeter wave bands. When the circuit board material is exposed to a wide range of temperature changes, excessive changes in the thermal stability coefficient of Dk (TCDk) and the thermal stability coefficient of Df (TCDf) will be equivalent to changes in Dk and Df. These temperature effects can be minimized by selecting circuit board materials that relatively strictly control TCDk and TCDf. Generally, a circuit board material with a TCDk of 50 | ppm /°C or lower is considered to be a stable characteristic with good performance. Taking a real material as an example, the TCDk measured by the RO3003™ circuit board is 3 ppm/°C.

Low moisture absorption is one of the goals for most high-frequency circuit board materials. For circuit materials used in the millimeter wave frequency band, even a small difference will affect its performance. If the circuit material has excessive moisture absorption, its loss will increase, and Dk will increase with the change of moisture. The performance of the circuit under ideal working conditions may be acceptable, but may not meet the requirements in practical applications, such as under high humidity working conditions, especially at millimeter wave frequencies with shorter wavelengths.

Finally, among the six circuit material parameters that need to be considered in 77GHz and other millimeter wave circuits, it is well known that the "glass weaving effect" will also cause the circuit Dk to change. In many circuit materials, glass cloth is used to reinforce and reinforce the material. When doing so, it also causes some parts of the fiber pattern of the entire material to contain more glass fibers than others, which leads to changes in Dk. The use of glass-reinforced materials increases the mechanical properties of circuit materials, but it also affects the electrical properties of materials in high-frequency circuits. Ideally, materials selected at higher frequencies do not need to contain glass cloth or glass fibers.

Choose materials that meet your needs

Rogers RO3003™ circuit board materials have been proven to meet these six key material requirements for 77 GHz and other millimeter wave circuits. RO3003™ laminate is a material with extremely low loss, with a typical Df of 0.0010 at 10 GHz and a strictly controlled Dk tolerance of ±0.04. Many millimeter wave circuit design engineers choose 5mil thick ED copper or smoother (lower loss) rolled copper RO3003™ circuit board as the circuit material. It also has low moisture absorption of less than 0.04%, and TCDk is very low, with a typical value of -3 ppm/°C.

Because RO3003™ circuit board material reflects the six key requirements for high-frequency circuits, it has become a common choice for circuit board materials for 77 GHz and other millimeter wave circuit design engineers. In addition, RO3003™ circuit board does not use or need to be reinforced with glass cloth, so it does not have glass weaving problems. It is a durable, low-loss circuit board material, suitable for circuits that need to maintain signal power as the frequency becomes higher, especially for 77GHz automotive radar systems that focus on improving safety!