PCBA Tech - ​Choosing circuit board materials for low PIM PCB antennas

Circuit board materials with various good characteristics can meet the needs of modern wireless communication systems and lay the foundation for PCB antennas with low distortion...

Although the antennas have different shapes and sizes, printed circuit board (PCB) antennas can keep their performance unchanged while greatly reducing their size. Of course, antennas (including PCB-based antennas) must be designed and manufactured to ensure minimum passive intermodulation (PIM) indicators in order to perform best in today's crowded signal environment.

For PCB antennas, although the low PIM index is mainly related to the antenna design, the circuit board material also has a great influence on the overall PIM performance of the PCB antenna, so it is also necessary to consider how to choose radio frequency (RF)/microwave circuit materials.

PIM is a nonlinear diode-like effect. When two or more signals are combined (for example, from different transmitters), unnecessary harmonic signals are generated. When the level of these additional harmonic signals is high enough and falls within the receiver's receivable frequency range, then it may cause problems and interfere with the receiver's normal detection of signals in the frequency band. Although PIM will not affect every application, it may interfere with the normal operation of the wireless communication system, especially when it tries to recover lower level signals.

PCB antenna

High-frequency antennas manufactured in the form of PCBs can have many different structures, from simple dipoles to complex structures based on ring resonators and Rotman lenses. One of the more popular PCB antennas is the microstrip patch antenna, which can design a simple and compact antenna structure within a specific frequency range (see Figure 1). Many products use multiple PCB patch antennas or resonant structures to realize beamforming network (BFN) or phased array antennas, and control the amplitude and phase of their thin PCB antenna structures for radars or communication systems through electronic adjustments. And direction.

At the millimeter wave (mmWave) frequency, compact planar PCB antennas are also attracting more and more attention. For example, the 77GHz advanced driver assistance system (ADAS) used in automotive electronic safety systems uses this antenna to achieve blind spot detection and automatic Braking system and anti-collision functions. Due to the low signal power of this system, ADAS receivers must rely on their high sensitivity to reliably detect radar echoes reflected from pedestrians and other vehicles.

The microstrip patch antenna unit radiates electromagnetic energy (EM) into free space when transmitting, and transmits the electromagnetic energy to a connected circuit (for example, a receiver) when receiving. But the patch is only a component of the PCB antenna, and the feeder constitutes another important part. The feeder acts as a bridge between the connected microstrip circuit and the radiating patch to transmit and receive electromagnetic energy. Ideally, the patch should exhibit high radiation, while the feeder has low radiation, so as to realize the effective transfer of energy from the circuit to the patch.

PIM strategy

Antennas with higher PIM may cause data loss in wireless communication systems (such as 4G LTE wireless networks). This kind of network relies on the Distributed Antenna System (DAS) to extend wireless coverage, and the emerging 5G wireless network, despite its higher frequency, is actually the same.

For the carrier signal frequencies f1 and f2 in the two frequency bands in the transceiver system, PIM is a mixed product of nf1-mf2 and nf2-mf1, where n and m are integers. Such derived PIM harmonics can be classified according to certain rules, and the order is determined by the sum of m and n, such as the third-order components of 2f1-f2 and 2f2-f1 (Figure 3). Third-order intermodulation products are worthy of attention, because they are the closest to the carrier signal and may fall within the receiver's frequency band, and if the components have higher power, they may cause reception blocking.

The amplitude of the PIM harmonic component is not only a function of the amplitude of f1 and f2, but also a function of its PIM order. The amplitude of PIM harmonic components decreases as the order increases. Therefore, the 5th, 7th and 9th order PIM harmonic power levels are usually small and will not affect the receiver performance.

How low power level can be considered low PIM? This value may vary from system to system. For 4G LTE systems using some passive components (such as connectors and cables) included in DAS equipment, -145dBc is usually low enough. Generally speaking, -140dBc or higher is considered poor PIM performance, while -150dBc is usually better, and -160dBc is even better.

When measuring the PIM level of antennas and other passive components in a specially designed microwave anechoic room, the noise level as low as -170dBc may exceed the noise level of the darkroom test environment. When two +43dBm single tone signals are used for measurement, the actual noise level of most PIM test darkrooms is -165dBc.

When the same antenna uses a common feeder to achieve both transmitting and receiving functions, low PIM is particularly important. Because both the transmitter and the receiver are located in the same system at the same time, the non-linear products of multiple transmitted signals will always lead to unwanted mutual tuning waves whose amplitude is often sufficient to degrade the performance of the receiver. Understanding the effects of different material properties on PIM can reduce the impact of PIM on PCB antennas.

Although in most cases PIM is caused by uneven materials in circuit nodes (such as solder joints or connectors), the characteristics of circuit board materials, such as rough copper foil surfaces and different types of electroplating surface treatments, may also be affected. Produce lower or higher PIM levels. Certain parameters in the circuit board materials can be used as a reference for designing low PIM PCB antennas.

Antennas and other passive components made of PCB materials will also have an impact on PIM performance after surface plating. Ferromagnetic materials (such as nickel) seriously affect the performance of PIM. Immersion tin plating generally has better PIM performance than bare copper circuits, while circuits using chemical nickel gold (ENIG) will have poorer PIM performance due to nickel.

The cleanliness of the circuit surface is conducive to reducing the PIM performance of microstrip antennas and other passive components. Circuits with solder masks usually have better PIM performance than bare copper circuits. Clean circuits and no residue wet chemical treatment are important foundations for reducing PIM performance. Any form of ionic contaminants or residues in the circuit may cause poor PIM performance.

Similarly, the etching quality of the circuit is also very important for improving the PIM performance. If the copper foil conductor is not sufficiently corroded, causing roughness and burrs on the edge of the circuit, this situation may also degrade the PIM performance.

As long as the material of the circuit board is carefully selected, it is possible to improve the PIM performance of passive components or circuits. However, even if low-PIM materials are used, certain types of circuits may not be able to improve their PIM performance due to their structure being more susceptible to PIM. For example, Rogers Corp. used 32.7 mil thick RO4534 circuit board material to conduct related experiments. The characteristics of this antenna laminate are: Dk of 3.4, tolerance of ±0.08, and a low loss factor (low loss) of 0.0027 at 10 GHz.