Copper foil selection guide for high-speed PCB—Outer layer pit avoidance design

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Copper foil selection guide for high-speed PCB—Outer layer pit avoidance design

Posted Date: 2024-01-31

Mr. Speedy member--Wang Huidong

Zhao Ligong said: The moment I saw you, there was a tsunami in my heart, and the waves surged into the sky, but I stood quietly and didn't let anyone know. Now you have a question on your board that I want to tell you. "

If Lin Ruyan says something wrong, please tell me quickly. There will be a surprise tonight.

Zhao Ligong said that the HVLP type cannot be used for this outer copper foil.

Lin Ruyan said that Professor Huang talked about the impact of copper foil roughness on high-speed PCB last time, and the customer said that HVLP should be used.

Classification of copper foil

HTE: Highly ductile electrolytic copper foil

RTF: reverse copper foil

HVLP: Ultra Low Profile Copper Foil

Roughness refers to the unevenness of the surface, usually expressed as Ra or Rz. RZ in the picture above represents the length of the copper teeth in the roughness of the copper foil.

Why low-profile copper foil is used at high speeds starts with something called the skin effect.

What is the skin effect?

When alternating current passes through a conductor, the distribution of the current on the cross-section of the conductor is uneven. The current density on the surface of the conductor is greater than the density in the center, and the higher the frequency of the alternating current, the more obvious this trend is. This phenomenon is called trend. Skin effect.

When there are microscopic bumps and depressions on the surface of a wire, the path of current along the wire becomes more circuitous, leading to an increase in resistance.

As the frequency of the signal becomes higher and higher, the current will readjust its distribution on the transmission path and be transmitted along the path of minimum resistance.

Because of the skin effect, the equivalent resistance of the wire increases, thereby increasing its conductor loss.

An extremely rough and uneven surface increases the length of the propagation path for current propagating near the surface.

Skin effect diagram

"Along the gentle curves of the ups and downs of the mountains and rivers. Letting horses love the Central Plains and loving the North and Jiangnan..."

Lin Ruyan said that some things can only be understood but cannot be expressed in words, just like the song "Borrow from Heaven for Another Five Hundred Years".

As long as the roughness of the copper foil is relatively large, just like the ups and downs of the road, the relative path will become longer, which will affect the vehicle speed.

High-speed products have copper foil roughness as a major consideration. In a link with high signal rate and strict loss requirements, low-roughness copper foil is necessary, and this is also an inevitable choice for high-speed products.

As long as the copper roughness is too large, the transmission path becomes longer, and the conductor loss is too large, it is inevitable to use low-profile copper foil.

Zhao Ligong said: Ruyan, everything you said makes sense, but there is one detail you ignored..."

Our conventional PCB lamination structure is foil+pp+core, copper foil+pp+core board.

As we can see from the picture above, the outermost layer is made of copper foil. During lamination, it is pressed together with PP through a press under high temperature, high pressure and vacuum conditions.

Looking at the laminated design in the picture below, it seems reasonable at first glance. In fact, if you look closely, there are some abnormalities. The outer layer uses HVLP copper foil.

HVLP is indeed good for signal integrity, but because its copper foil RZ is too smooth, the bonding force is poor during lamination, and delamination and blistering may occur.

It is also possible that during SMD assembly, the board may appear abnormal due to the influence of high temperature.

Therefore, when the customer requires HVLP or VLP copper foil laminate for the outer layer, we have to explain to the customer that the surface of the HVLP copper foil is very smooth and has poor adhesion. From the perspective of reliability, it is not recommended; in order to balance reliability and For signal requirements, it is recommended to use RTF or HTE copper foil lamination; some factories do not support RTF copper foil lamination.

Especially the design of high-speed multi-stage HDI requires multiple laminations and uses HVLP copper foil as the outer layer, so the risk of reliability problems is relatively high.

Therefore, when designing high-speed PCB, not only signal integrity must be considered, but also DFM manufacturability must be considered.

After listening to Zhao Ligong's words, Lin Ruyan's face rose with a blush. She took a deep breath, blinked her big eyes, her gaze swept across Zhao Ligong's handsome face, and said with admiration. : "Zhao Ligong, you are a good brother. After I change the board, let's go to Lele BBQ opposite the company for a drink after get off work in the evening."

Zhao Ligong, a word is final.

Review Editor Huang Yu

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