Surface roughness of copper foil of 5G printed circuit board

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Surface roughness of copper foil of 5G printed circuit board [Copy link]

Measuring Roughness Beyond Rz (Maximum Height) Using Laser Microscope
Analyzing Surface Roughness of Copper Foil on 5G Printed Circuit Boards
The rapid spread of mobile phones has driven innovation in mobile communication systems, which undergo major technological changes approximately every ten years. This evolution began with the first mobile communication systems (1G) using analog mobile phones in the 1980s. The second generation (2G) brought email and the Internet to mobile phones, while the third generation (3G) provided high-speed, high-capacity communications. The fourth generation (4G) brought higher speeds and greater capacity, making mobile phones a platform for watching videos and playing games. In 2020, we entered the fifth generation, 5G.
5G technology achieves ultra-high speeds and ultra-large capacity. In addition, it can achieve multiple simultaneous connections of up to 1 million devices per square kilometer (4G has 100,000 devices per square kilometer) with latency as low as 0.001 seconds (4G has a latency of 0.01 seconds). 5G will have a wide variety of Internet of Things (IoT) applications, such as video distribution for high-capacity communications and autonomous driving that utilizes low latency, as well as traditional services for mobile devices such as smartphones.
5G network diagram
5G technology uses higher frequencies than 4G. As signal frequencies rise, the electronic devices that make up the network must be able to transmit high-frequency signals to the maximum extent possible. To this end, these devices have undergone various technical modifications. In this application note, we focus on one adjustment - the roughness of the copper foil used in the device's printed circuit board (PCB).
The copper foil on the printed circuit board is adhered to an insulator (resin board) after heating and pressurization, while its surface is roughened. As part of the quality assurance process, the adhesion of the roughened copper foil to the insulator is evaluated by analyzing Rz (maximum height) to check its surface roughness using a stylus-type surface roughness tester.
One problem with 5G's use of higher frequencies is transmission loss, which is the loss of signals as electromagnetic waves pass through the communication path. There are two types of transmission loss: dielectric loss, caused by the electric field generated in the raw materials, and conductor loss, caused by the resistance of the conductor to the component (such as the wire on the PCB) (Figure 1). For example, in the PCB installed in the 5G antenna, the higher frequency of the electrical signal causes it to be transmitted near the surface of the copper foil, resulting in a type of conductor loss called the skin effect.

Figure 1: Two types of transmission losses.
When transmitting alternating current in a conductor, the skin effect occurs. As the frequency of the current increases, most of the current flows near the surface of the conductor, resulting in transmission losses (Figure 2).

Figure 2: A cross-sectional view of the signal transmission area in a copper circuit (left) and the relationship between frequency and skin depth (right).
However, transmission loss can be reduced by controlling the roughness of the copper foil used in the circuit. Although rough copper foil increases transmission loss by scattering signals, using a circuit with asperities (roughness) less than the skin depth results in a shorter signal transmission path, thereby reducing signal loss (Figure 3).

Figure 3: Relationship between copper circuit surface roughness and transmission loss
Controlling the roughness of copper foil is extremely important in 5G applications using high-frequency bands. However, evaluating the roughness of foil using a stylus pen is difficult because the concave/convex surface is smaller than the diameter of the probe tip, making it difficult to capture accurate measurements. In addition, the stylus probe tip can also damage the surface of the foil. In 5G applications, even small Rz (height) differences can affect the effectiveness of the foil, so using a non-contact, high-resolution laser microscope capable of three-dimensional (planar) evaluation is beneficial.

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High-precision measurement of micron-level roughness
Olympus' LEXT microscope uses a 405 nm laser with a diameter of 0.4 μm for surface measurement. The microscope's laser is much smaller than the tip of a contact stylus, making it a more accurate and effective tool for measuring the surface roughness of copper foil. Since the laser does not touch the surface, the roughness of the foil can be analyzed without damaging the foil.
Stylus-type surface roughness tester
LEXT laser microscope

Stylus Surface Roughness Tester / LEXT Laser Microscope

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High-precision measurement of micron-level roughness
Olympus' LEXT microscope uses a 405 nm laser with a diameter of 0.4 μm for surface measurement. The microscope's laser is much smaller than the tip of a contact stylus, making it a more accurate and effective tool for measuring the surface roughness of copper foil. Since the laser does not touch the surface, the roughness of the foil can be analyzed without damaging the foil.
Stylus-type surface roughness tester
LEXT laser microscope

Stylus Surface Roughness Tester / LEXT Laser Microscope

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While a stylus roughness tester can only collect information by tracing a single line along the measurement plane, the LEXT microscope uses its laser to collect roughness data over an area. This enables you to capture more data than a single line of information alone.

A line cross-section data obtained by a stylus roughness tester

Surface roughness information provided by laser scanning

Using the LEXT microscope, we measured the line roughness and surface roughness of the copper foil surface. The roughness data obtained by the laser microscope contains more information than what we can obtain using a stylus.

Figure 4: Surface roughness provides much more information than a single-line measurement with a stylus.

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While a stylus roughness tester can only collect information by tracing a single line along the measurement plane, the LEXT microscope uses its laser to collect roughness data over an area. This enables you to capture more data than a single line of information alone.

A line cross-section data obtained by a stylus roughness tester

Surface roughness information provided by laser scanning

Using the LEXT microscope, we measured the line roughness and surface roughness of the copper foil surface. The roughness data obtained by the laser microscope contains more information than what we can obtain using a stylus.

Figure 4: Surface roughness provides much more information than a single-line measurement with a stylus.

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Copper Foil Roughness Measurement Parameters
Below, we examined the roughness measurement results of five areas on a piece of copper foil (Table 1).

First, when examining the differences (range) in the Sz (maximum height) measurement values, there was a 1.7μm difference between the maximum and minimum values of the data in the five regions. Since Sz is a parameter that indicates the difference between the maximum peak height and the maximum valley depth within the measurement range, this indicates that the difference between the maximum and minimum values in the measurement area is expressed numerically.

Other suggested roughness parameters - Arithmetic mean height (Sa)
Sa represents the average height difference of the average plane within the measurement area. It provides stable results because the parameter is not significantly affected by plane scratches, foreign matter or other interference. As a result, the difference between the measured values of the five areas is only about 0.1μm. Compared with Sz, Sa enables stable evaluation with a smaller difference between the maximum and minimum values.

Other recommended roughness parameters - developed interface area ratio (Sdr)
The surface shape of copper foil includes not only elements such as the height gap of the bumps, but also the size of the surface direction, the degree of dispersion, etc. The above Sz and Sa are parameters that focus on the surface height (depth). Sdr (developed interface area ratio) is a parameter that expresses the rate of increase of the surface area. The growth rate is calculated based on the surface area A1, which is obtained by projecting the surface area A0 onto the XY plane.
The Sdr value increases as the surface texture becomes finer and rougher.

We compared the surfaces of two copper foils with different roughness conditions. Tables 2 and 3 show the Sdr values and their average values for five areas on the foil surface. The Sdr value of the "high frequency" sample is smaller than that of the "medium and low frequency" sample, which means that the former has less unevenness and a smaller gap between the maximum peak height and the maximum valley depth. Since the smaller the Sdr value, the shorter the signal transmission path, the transmission loss of the "high frequency" copper foil is expected to be smaller.

Table 2: Sdr measurement values of high frequency copper foil (%)

Table 2: Sdr measurement values of medium and low frequency copper foil (%)

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Copper Foil Roughness Measurement Parameters
Below, we examined the roughness measurement results of five areas on a piece of copper foil (Table 1).

First, when examining the differences (range) in the Sz (maximum height) measurement values, there was a 1.7μm difference between the maximum and minimum values of the data in the five regions. Since Sz is a parameter that indicates the difference between the maximum peak height and the maximum valley depth within the measurement range, this indicates that the difference between the maximum and minimum values in the measurement area is expressed numerically.

Other suggested roughness parameters - Arithmetic mean height (Sa)
Sa represents the average height difference of the average plane within the measurement area. It provides stable results because the parameter is not significantly affected by plane scratches, foreign matter or other interference. As a result, the difference between the measured values of the five areas is only about 0.1μm. Compared with Sz, Sa enables stable evaluation with a smaller difference between the maximum and minimum values.

Other recommended roughness parameters - developed interface area ratio (Sdr)
The surface shape of copper foil includes not only elements such as the height gap of the bumps, but also the size of the surface direction, the degree of dispersion, etc. The above Sz and Sa are parameters that focus on the surface height (depth). Sdr (developed interface area ratio) is a parameter that expresses the rate of increase of the surface area. The growth rate is calculated based on the surface area A1, which is obtained by projecting the surface area A0 onto the XY plane.
The Sdr value increases as the surface texture becomes finer and rougher.

We compared the surfaces of two copper foils with different roughness conditions. Tables 2 and 3 show the Sdr values and their average values for five areas on the foil surface. The Sdr value of the "high frequency" sample is smaller than that of the "medium and low frequency" sample, which means that the former has less unevenness and a smaller gap between the maximum peak height and the maximum valley depth. Since the smaller the Sdr value, the shorter the signal transmission path, the transmission loss of the "high frequency" copper foil is expected to be smaller.

Table 2: Sdr measurement values of high frequency copper foil (%)

Table 2: Sdr measurement values of medium and low frequency copper foil (%)