Microstrip patch antennas and their arrays may be rare but are the most widely used antenna form. They have a simple structure and can be formed by a dielectric, a metal conductor patch on the dielectric layer, and a ground plane. Even the dielectric in the middle can be an air structure. A typical microstrip patch antenna is made on a printed circuit board (PCB), and a fine line structure is etched on the conductive metal layer using a photocopying method. Since PCB materials are an important part of microstrip patch antennas and antenna arrays, the characteristics of PCB materials, including the types of conductive metal copper foils for the patch and the ground, must be carefully considered when designing microstrip antennas. At the same time, the line tolerance in the PCB manufacturing process will also affect the performance of the microstrip patch antenna; the additional PCB components (such as surface treatment plating) that may be added during the manufacturing process will also affect the performance. Understanding how PCB materials work together and how the final design requirements are formed through the manufacturing process will help achieve the intended design purpose of microstrip patch antennas or arrays, especially in the millimeter wave frequency band.

The size of a microstrip patch antenna is inversely proportional to the frequency. The higher the frequency, the smaller the patch size. Due to this relationship, at lower frequencies, the wavelength is longer, and the microstrip patch antenna will be too large to be practical. In general, the rectangular length L of the microstrip patch is 1/2 of the wavelength of the operating frequency, that is, L= ~λ/2. When the frequency is below 500MHz, the length of the patch is very large. For example, at a frequency of 100MHz, the resonance is that the length of the microstrip patch is close to 1m, and a large PCB size is required at this time. The higher the frequency, the smaller the patch size will be, especially for applications at microwave/millimeter wave frequencies, it makes more sense to integrate it on a PCB board with other circuits. When the gain and directivity of the required antenna are higher than the performance of a single patch, various patch array antennas can be designed.

When selecting PCB materials for microstrip patch antenna design, the influence of PCB material “composition” should be considered. Material “composition” includes the dielectric material itself, the type of metal copper foil, the final surface plating, etc., which affect the performance and characteristics of the antenna and circuit. For example, the dielectric constant of the dielectric material can affect the size of the patch (~1/2 of the wavelength), and the thickness of the dielectric material also affects the electromagnetic (EM) field distribution radiated by the antenna.

The radiation mechanism of the microstrip patch antenna is actually high-frequency electromagnetic leakage. The leakage through the longitudinal edge or “L” direction of the rectangular patch generates a radiated EM field (there is no EM field in the width direction or “W” direction of the rectangular patch). The final surface plating of the PCB has a great influence on these EM fields radiated from the edge of the patch. For example, due to the conductivity of the final surface treatment plating, the final conductor conductivity characteristics of the PCB will become very complicated and become a comprehensive conductivity characteristic. For example, chemical nickel gold (ENIG) is a popular and proven final surface treatment method. It is a surface treatment that combines thick nickel with thin gold. However, nickel is a ferromagnetic material. On the one hand, it will also have magnetic losses when protecting copper foil conductors; on the other hand, its conductivity is only 1/4 of copper, especially at higher frequencies, it will increase the conductor loss of the circuit and affect the circuit phase response. On the microstrip patch antenna, the characteristics of ENIG will of course also affect the field strength generated by the microstrip patch antenna, especially at higher microwave and millimeter wave frequencies.

The choice of PCB conductor surface treatment is a factor that needs to be considered in microstrip patch antennas because the surface plating changes the conductor loss at the edge of the patch. For ENIG, the conductivity of the conductor edge is composed of copper, gold and nickel in different proportions. There are many other PCB final surface platings that can be completed by chemical immersion processes, such as chemical tin or chemical silver. The coating thickness obtained by this process is extremely thin, and it seems that the thickness variation over the entire circuit is not obvious, but the impact on high-frequency millimeter waves of small wavelength signals cannot be ignored.

Design for Manufacturability

High-performance microstrip patch antennas require high-precision manufacturing processes and high-grade circuit materials. Although microstrip antennas are not suitable at lower frequencies, they are very attractive for use at higher millimeter-wave frequencies. If the antenna design can be supported by the manufacturing process, repeatable hardware can be produced at a feasible cost. The size of the patch at millimeter-wave frequencies is very small, such as the patch antenna of a 77GHz automotive radar sensor. Under such high frequency and small wavelength conditions, changes in material properties and manufacturing processes will affect the antenna performance and may cause unacceptable deviations. Therefore, the design of microstrip patch antennas must not only consider the characteristics of the PCB material, but also the limitations and bottlenecks of the manufacturing process.

For example, the ideal conductor corner of a microstrip patch is a 90-degree right angle, but during processing, when etching the copper foil conductor into a microstrip patch according to the design parameters, a rounded corner is generally formed. A rounded corner with a radius of 2 to 5 mils has little effect at lower frequencies, but a rounded corner with a radius of 3 mils will affect the signal wavelength at millimeter wave frequencies, which may cause changes in the electromagnetic field distribution and the radiation direction of the microstrip patch antenna. Although computer-aided design software (CAD) can be used to simulate the impact of a rounded corner of 2 to 5 mils, if the tolerance of the manufactured antenna is different from that described in the CAD tool, the simulated performance may not be consistent with the actual performance.

At millimeter-wave frequencies such as 77 GHz, the quality of circuit materials and manufacturing processes need to ensure minimal changes without affecting the performance of microstrip patches. One of them is that circuit nodes and interconnections (such as the feed line of the microstrip patch) must achieve a high impedance match (usually 50Ω) to minimize signal reflections at the nodes and avoid energy loss caused by electromagnetic radiation. In addition, for the series-fed patch antenna of the 77 GHz radar, it usually contains multiple different conductor widths. Due to the small size of the patch antenna itself, a small change in the conductor width will significantly affect the antenna performance. PCB processing capabilities with an etching line width tolerance of ±0.5 mil are recommended, but millimeter-wave frequencies may place higher requirements on the precision and accuracy of this process. When the nominal conductor width is 4~5 mils, a change of ±0.5 mil (1 mil in total) may mean that the circuit size and performance may change by 20% or more.

Understanding the effects of conductor and dielectric materials on the radiation of microstrip patch antennas and their arrays can help predict the gain and directivity of antennas at a specific frequency. However, any PCB manufacturing process must also be as tightly controlled as possible to obtain fine patches and their associated circuits, otherwise the prediction will be biased. By measuring and taking into account the changes in material properties and manufacturing processes, high-quality, batch-produced microstrip patch antennas and antenna arrays at millimeter-wave frequencies can be produced repeatedly and cost-effectively.

Any microstrip patch antenna or any high-frequency material of PCB, even the circuit material of KKPCB with good performance and high reliability, requires a practical design that can be mass-produced using modern manufacturing processes. Although high-quality materials help achieve high performance, they must also be assembled correctly and efficiently into the final component and form a practical solution.

KKPCB provides global customers with one-stop services from PCB layout, prototype PCB proofing, PCB manufacturing, PCBA processing (including SMT and DIP), PCBA testing, PCBA product assembly and outbound packaging. You could provide a Gerber file or BOM list to us, we will offer the finished PCB products or PCB assembly which are satisfied with you.