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With the increasing frequency of future use, the concept of high-frequency PCB design is also changing. For example, high-frequency PCBs are increasingly shifting from single and double-sided boards to multi-layer board structures, and complex metalized via structures (arbitrary interlayer interconnections) are replacing simple metalized or non-metal via structures. What are the suitable combinations of "dielectric+copper foil" for different high-frequency circuit application scenarios?
1. Transmission Line Form in High Frequency PCB Design
According to the theory of microwave technology, TEM (Transverse Electromagnetic) transmission lines and waveguides can both serve as carriers for high-frequency signal transmission. Microstrip lines, strip lines, and substrate integrated waveguides (SIW) in TEM transmission line structures are all used in high-frequency PCB design.
In the design of high-frequency PCB, in terms of the relationship between the attenuation of high-frequency signals in different transmission lines and copper foil, the influence of copper foil on microstrip lines and striplines is far greater than that of copper foil in SIW structure, or the dielectric loss in SIW structure contributes more to the insertion loss of the entire transmission line.
2. Skin Effect
In the design of microstrip or strip lines, when high-frequency signals are transmitted in the wire, most of the electromagnetic wave energy is confined in the dielectric layer between the wire and the shielding layer (ground), and the skin effect causes the transmission of high-frequency signals to gather in a thin layer on the surface of the wire, and the closer to the wire surface, the higher the alternating current density. For microstrip lines, the skin effect will occur at the contact point between the microstrip line and the dielectric, while for strip lines, the skin effect will occur at the contact point between the surface of the strip line and the dielectric. By using the formula for calculating skin depth, the trend of skin depth variation with frequency can be obtained.
The skin depth decreases significantly with increasing frequency. When the frequency is 5GHz, the skin depth drops to around 1um, while in the millimeter wave frequency band (>26GHz), the skin depth further decreases to below 0.5um. This indirectly illustrates that the roughness of copper foil in contact with the medium has a significant impact on the insertion loss of the product. The roughness of the copper foil referred to here can be the roughness of the treated side of the copper foil in contact with the substrate medium, or the roughness of the Untreated side of the copper foil surface after the PCB process, such as the roughness of the copper foil surface (line top and sidewall) caused by inner layer roughening before etching or lamination in strip line design.
3. The Influence of Different Types of Copper Foil on Electrical Properties in High Frequency Design
In the design of high-frequency circuit boards, designers usually pay more attention to the dielectric constant (Dk) and tangent loss (Df) of PCB materials when selecting materials. For the selection of copper foil, they often only focus on the thickness of the copper foil, which easily ignores the influence of the roughness of different types of copper foil on the electrical performance of the product.
What is the impact of copper foil type on electrical performance?
a、 The roughness of different types of copper foils varies greatly. In the design of microstrip lines, the roughness of the contact surface between copper foil and dielectric will directly affect the insertion loss of the entire transmission line.
For the design of strip lines, in addition to considering the roughness of the treated side of the copper foil, it is also necessary to consider the roughness of the untreated side of the copper foil and the sidewall of the line. The size of these two aspects of roughness is closely related to the processing technology and processing capacity of the PCB board factory, and it is necessary to control the selection of bottom copper thickness, etching solution or inner layer roughening solution. Otherwise, excessive roughness on the surface of the strip line or residual copper at the edge of the line can lead to deterioration of the electrical performance indicators of the transmission line, such as insertion loss, standing wave, intermodulation, etc.
b、 The influence of different types of copper foils on insertion loss
In high-frequency design, reducing the insertion loss of transmission lines has a positive significance for improving product gain and power efficiency. TACONIC low loss material TSM-DS3 (Dk 3.0, Df: 0.0011@10GHz ）As a medium, different types of copper foils were used to test the insertion loss of a 50 ohm microstrip line. The results showed that as the frequency increased, using ULP copper foil was of great help in reducing the insertion loss of the line. The insertion loss of TSM-DS3 with ULP copper foil tested at 45GHz was -0.24dB/10mm, which was about 77% lower than that with STD copper foil in the same frequency band. This forces us to consider how to reduce the insertion loss of transmission lines by combining low loss dielectric materials (such as TSM-DS3) with copper foil with the lowest possible roughness.
c、 The Influence of Different Types of Copper Foil on Intermodulation Performance
In the current application of sub-6GHz base station antennas, the use of low loss PTFE material combined with RTF copper foil to enhance intermodulation index has been widely accepted in the industry. For example, TACONIC's RF-30, RF-30-7H, RF-30A, TLX-8-P combined with RTF copper foil are used in the base station antenna market.
However, for some application scenarios with narrow design windows and strict requirements for intermodulation indicators, replacing RTF copper foil with ULP copper foil can increase the intermodulation indicator from -163dBc (mean) to -167dBc (mean). If the PCB unit can be improved by 4dB, it will help improve the overall intermodulation indicator of the antenna.
4. Comparison of Bonding Forces between Different Types of Copper Foil and Media
In high-frequency design, while using low roughness copper foil to improve the electrical performance of products, concerns about whether the bonding force between low roughness copper foil and the medium can meet the requirements are often mentioned by designers. In response to this concern, TACONIC PCB has optimized the process to maintain the same level of adhesion between ULP copper foil and the medium as STD copper foil.
5. High frequency application PCB material selection - optimal combination of "dielectric+copper foil"
For the selection of PCB materials for high-frequency applications, it is necessary to comprehensively consider various factors such as the basic performance indicators of the material medium (Dk, Df, CTE, TcK, dimensional stability, thermal conductivity, etc.), the type of copper foil to be used, processability (multi-layer board processing), stability (consistency), and cost. Based on the practical application experience of TACONIC's high-frequency materials, and according to different frequencies, provide the optimal solution.
At sub-6GHz frequency, the contribution of copper foil to line insertion loss is limited. Taking ordinary base station antennas as an example, PCB design is relatively simple (with a maximum of 6L layers, mainly single and double panels), but it is sensitive to the overall cost of PCB. TACONIC's RF-30A (Dk 2.97, Df0.0012 @ 1.9GHz) and TLX-8-P (Dk2.55, Df 0.0010 @ 1.9GHz) materials, when paired with RTF copper foil, can meet the actual needs of most customers, including insertion loss and intermodulation indicators. But if customers need further improvement in plug loss and intermodulation to meet certain demanding specifications, both of the above materials can be used with ULP copper foil, which will increase the cost to some extent.
For multi-layer antenna boards, low loss adhesive sheets must also be considered as one of the factors. TACONIC's fastRiseTM series of adhesive sheets (Dk2.43-2.76, Df: 0.0014 @ 10GHz) can help customers reduce line insertion loss and maintain intermodulation indicators in multi-layer antenna boards.
The high-frequency PCB design of Above 6GHz will be more complex, with higher requirements for "dielectric loss" and "copper foil loss", and the increasing number of multi-layer board structures, thus requiring higher requirements for the material's PCB processability and related reliability (metalized vias).
For Above 6GHz applications, TACONIC's TLY-5 (Dk2.2, Df: 0.0009@10GHz ）TLY-5Z (Dk 2.2, Df: 0.0013 @ 10GHz) and TSM-DS3 (Dk3.0, Df: 0.0011 @ 10GHz) combined with ULP copper foil can greatly reduce line insertion loss.
high-frequency PCB
In terms of applicable frequency and design feasibility, TLY-5, TLY-5Z, and TSM-DS3 have their own applicable ranges when paired with ULP copper foil:
1. TLY-5+ULP copper foil "is suitable for designs up to 77GHz. If there is a multi-layer structure, the number of layers should not exceed 6 and the total thickness of the PCB board should not exceed 40mil. It is not suitable for designs with too many metalized vias.
2. TLY-5Z+ULP copper foil "is suitable for designs with frequencies below 30GHz. If there is a multi-layer board structure, the number of layers should not exceed 12 and the total thickness of the PCB board should not exceed 120mil.
3. TSM-DS3+ULP copper foil "is suitable for designs up to 77GHz and can be used with fastRiseTM series adhesive sheets to produce high-frequency multilayer circuit boards, which can withstand multiple presses (excellent dimensional stability)
4. Complex PCB structures such as densely metalized vias. In addition, the stable TcK coefficient (temperature drift coefficient: 5.4ppm/C) of TSM-DS3 is also an important indicator for its application in the millimeter wave frequency band.
In high-frequency PCB design, the selection of the combination of "dielectric+copper foil" should not only consider the principle of optimal electrical performance, but also take into account various influencing factors such as PCB processability (implementation of complex structures) and cost, in order to find the best balance point.