PCB Tech - The current status and performance requirements of three key raw materials for high-end PCB copper clad laminates

PCB technology copper clad laminate raw material
The supply chain pattern of special electrolytic copper foil, special resin, and special glass fiber cloth used in new and high-end substrate materials, and the new performance requirements for these three major materials
This article describes the supply chain pattern of special electrolytic copper foil, special resin, and special glass fiber cloth used in new and high-end substrate materials in recent years, as well as the new performance requirements of these three materials.

Since the beginning of 2020, the global spread of the new crown epidemic has caused serious changes in the supply and demand chain of my country's copper clad laminate raw materials. Since the development of 5G, copper clad laminates for high frequency and high speed circuits, substrate materials for high HDI and IC packaging substrates have also evolved greatly in technology, performance, and variety. In the face of these two important changes, in-depth research on the supply chain pattern of electronic copper foil, special resin and special glass fiber cloth used in new and high-end substrate materials, as well as the new performance requirements for the three major materials, is regarded as very important, Urgently needed work. This article discusses these two aspects.

1. Electrolytic copper foil

1.1 The current supply of various low-profile electrolytic copper foils and the new characteristics of the market structure
The scale and pattern of the global high-frequency and high-speed electrolytic copper foil market in 2019 (market share of each country/region and major manufacturers) are shown in Figure 1 and Table 1.
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Figure 1 The scale and pattern of the global low-profile copper foil market in 2019
Table 1 Statistics and forecast of the market structure of low-profile copper foil for high-frequency and high-speed circuits in 2018 and 2019
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From Figure 1 and Table 1, it is known that the global production and sales volume of low-profile copper foil (ie market size) is estimated to increase by 49.8% in 2019, reaching 53,000 tons. It is estimated to account for 7.6% of the total global electrolytic copper foil. Among the global high-frequency and high-speed electrolytic copper foil production and sales in 2019, the ratio of RTF to VLP+HVLP production and sales is approximately 77:23. However, the proportion of VLP+HVLP will increase in the next few years.
In 2019, domestic and foreign-funded copper foil enterprises in mainland China produced 7,580 tons of low-profile copper foil, of which domestic-funded enterprises accounted for 51.2% (3880 tons). The production and sales of low-profile electrolytic copper foil by domestic-funded enterprises accounted for 2.7% of the total output (144,000 tons) of copper foil for electronic circuits in domestic-funded enterprises. In 2019, domestic domestic-funded enterprises achieved new breakthroughs in mass production of VLP+HVLP varieties, but the production and sales of such low-profile copper foils are very small, accounting for only 2.3% of the total global production and sales of such electrolytic copper foils.
1.2 New characteristics of differentiation of low-profile electrolytic copper foil varieties and performance requirements for high-frequency and high-speed circuits
1.2.1 Corresponding to different transmission loss grades of high-frequency and high-speed copper-clad electrolytic copper foil varieties and low profile
In order to pursue better signal integrity (Signal Integrity, abbreviated SI) for high-frequency and high-speed circuits, copper clad laminates must achieve (especially at high frequencies) lower signal transmission loss performance. This requires the conductor material used in the manufacture of the copper clad laminate-copper foil, which has the characteristics of low profile. That is, the copper foil used in the manufacture of copper clad laminates is of low Rz, low Rq and other varieties.
According to the four levels of signal transmission loss, corresponding to various low profile copper foil varieties, Rz requirements and major manufacturers' brands, as shown in Table 2. Table 2 also lists the ranking of various types of low-profile copper foils in the required amount of substrate transmission loss grade copper clad laminates.
Table 2 Rz index range of several electrolytic copper foils corresponding to different transmission loss grades of high-frequency and high-speed copper clad laminates

1.2.2 The performance difference of low profile electrolytic copper foil in different application fields
The varieties of low-profile electrolytic copper foils for high-frequency and high-speed circuits are divided into five categories according to application fields. That is, low-profile electrolytic copper foil for rigid radio frequency/microwave circuits; low-profile electrolytic copper foil for high-speed digital circuits; low-profile electrolytic copper foil for flexible PCBs; low-profile electrolytic copper foil for packaging substrates; low-profile electrolytic copper foils for thick copper PCB s Copper foil. In these five application areas, low-profile electrolytic copper foils for high-frequency and high-speed circuits have different characteristics in terms of performance requirements, that is, they are focused on performance items and different in performance indicators.
The performance requirements and differences of low-profile electrolytic copper foil varieties used in the five major application fields are shown in the following aspects:
(1) Low profile electrolytic copper foil for rigid RF/microwave circuits
Low-profile electrolytic copper foil for rigid RF/microwave circuits has more obvious relative differences under different application frequency conditions. The requirements are more stringent in terms of copper foil performance on the Dk uniformity of the substrate, signal transmission loss, no ferromagnetic elements in the processing layer, and PIM (Passive Inter-modulation).
For this reason, copper foil used in high-end RF-microwave circuit substrates (such as millimeter-wave automotive radar substrates) generally requires pure copper treatment to support the reduction of passive intermodulation (PIM) and realize the improvement of copper clad laminates. Low PIM performance, reference index: reach below -158dBc～-160dBc. The copper foil treatment layer is arsenic-free.
At the same time, due to the different resin substrates of this type of copper foil, there are great differences in the selection of different Rz copper foil varieties.
Low-profile electrolytic copper foil for rigid radio frequency/microwave circuits generally uses 18μm, 35μm, 70μm in terms of copper foil thickness specifications, while high-end ultra-low or ultra-low profile copper foils are widely used in thickness specifications: 9μm, 12μm, 18μm varieties .
(2) Low-profile electrolytic copper foil for high-speed digital circuits
In the application market of low-profile copper foil for high-speed digital circuits, most of them are positioned at frequencies generally in the centimeter wave (3～30GHz) range. Its main application terminal is high-end server and so on. The performance of this type of copper foil has a more important impact on the insertion loss of the substrate and the processability of the substrate, and there are strict requirements for this. At the same time, thin specifications and low cost of copper foil are also important requirements. Low-profile electrolytic copper foils for high-speed digital circuits generally use 18μm, 35μm, 70μm in terms of copper foil thickness specifications, while high-end ultra-low or ultra-low profile copper foils are mostly used in thickness specifications: 9μm, 12μm, and 18μm.
The author has investigated and compared the contours of the non-pressing surface of many low-Rz copper foil varieties (including HVLP, VLP, RTF, etc.), and the derived conclusion is: in the same grade, For varieties that perform better in SI properties, their non-compression surface profile (expressed in Rz or Ra) is generally low. For example, for an RTF copper foil product of a foreign-funded enterprise, its Rz=3.0μm (typical value), while the non-pressing surface is Rz=3.5μm. Therefore, whether it is RF/microwave circuit substrates or high-speed digital circuit substrates, in order to pursue better SI performance, they also need the Rz (or Ra, Rq) of the non-pressing surface of the first profile copper foil, which also has Very low profile.
Currently, an important category of low-profile copper foil for high-speed digital circuits is reversed copper foil (RTF). In recent years, with the advancement of RTF copper foil technology by copper foil companies such as Japan and Taiwan, many varieties with Rz less than 2.5μm have come out, and even varieties with Rz less than 2.0μm have appeared. In this way, its application market and the proportion of the global low-profile copper foil market for high-speed digital circuits have also been rapidly expanded.
At present, the global copper foil low-profile electrolytic copper foil manufacturing industry is more towards the same performance in terms of copper foil for rigid RF/microwave circuits and copper foil for high-speed digital circuits. For example, CircuitFoil (CircuitFoil) realized mass production of ultra-low profile copper foil BF-NN/BF-NN-HT in 2019. This variety has achieved "two compatibility": First, the original BF-ANP copper foil is only used for PTFE resin type substrates, and it has developed to "including polyphenylene ether (PPE/PPO)-based resin systems. It is also suitable for pure Or modified fluoropolymer (PTFE) resin system.” Second, it can be applied to radio frequency microwave circuit substrates and also suitable for high-speed digital circuit substrates.
(3) Low profile electrolytic copper foil for flexible PCB
Low profile electrolytic copper foil is used for flexible PCBs. Due to the need to manufacture fine circuits, ultra-thin copper foil (without carrier) is often used. The current minimum thickness specifications of this type of products have reached 6μm, such as CF-T4X-SV6 and CF-T49A-DS-HD2 of Fukuda Metal Foil Co., Ltd.; and 3EC-MLS-VLP of Mitsui Metals Co., Ltd. (the minimum thickness is 7μm) .
Low profile electrolytic copper foil for flexible PCB also requires copper foil to have high tensile strength and high elongation. The excellent transparency of the base film after etching is also an important demand item for this copper foil market.
High-frequency low-profile electrolytic copper foil for flexible PCBs has begun to become low-profile in recent years. Many varieties with Rz less than 1.0um have appeared in the industry. For example, Mitsui Metals TQ- M4- VSP Rz≤0.6 (typical value); Foton Metals CF- T4X- SV Rz=1.0 (typical value, specification 9/12/18); Foton Metals CF- T49A- DS- HD2, Rz =1.0 μm (typical value) (specification 6/9/12/18); Nissin ISP, Rz≤0.55 (typical value).
(4) Low profile electrolytic copper foil for IC package carrier board
The low-profile electrolytic copper foil required for the package carrier (including the module substrate) should have high tensile strength, high thermal stability, high elastic modulus, and high peel strength at high temperatures (after 210°C/1h treatment). Its thickness specification is 5.0μm～12μm. And in recent years, the thickness specifications of copper foils for high-end IC packaging substrates are developing to be even thinner, that is, the thickness reaches 1.5 μm to 3 μm.
Package carrier boards (including module substrates) have also seen demand for high frequency and high speed in recent years. Therefore, in recent years, more varieties of low-profile electrolytic copper foil for package carrier have appeared. For example: Mitsui Metals' 3EC-M2S-VLP (without carrier), Rz≤1.8 μm (typical value); after 210 degree Celsius/1h, the tensile strength is 51kgf/mm 2; the elongation rate is 4.6%; the thinnest specification of copper foil is 9μm. Mitsui Metals' MT18FL (with carrier), Rz≤1.3μm, and the specifications of the circuit copper foil are 1.5, 2, 3μm. The LPF (without carrier) of Nissin Material Co., Ltd., Rz≤1.72 (typical value), the tensile strength after 210 degree Celsius/1h is 52.3kgf/mm2; the elongation is 3.7%; the thinnest specification of copper foil is 9μm.
(5) Low-profile electrolytic copper foil for high-current thick copper PCB

Low-profile electrolytic copper foil for high-current thick copper PCB with thickness specifications ≥105um (3oz). Common specifications: 105, 140, 175, 210μm. There are also ultra-thick electrolytic copper foils with special thickness requirements, with thickness specifications up to 350μm (10oz) and 400μm (11.5oz).
Low-profile electrolytic copper foil for high-current thick copper PCBs is mainly used for the manufacture of high-current, power supply substrates, and high-heat dissipation circuit boards. The thick copper PCB produced is mainly used in automotive electronics, power supplies, high-power industrial control equipment, solar equipment, etc. In recent years, the thermal conductivity of PCB has become one of the most common and important functions. The market demand for ultra-thick copper foil is constantly expanding. At the same time, due to the development of the micro-circuit manufacturing technology and application of thick copper PCB, the ultra-thick copper foil it needs to use also has low profile characteristics. For example, Mitsui Metals RTF type low profile thick copper foil: MLS-G (Type II), Rz=2.5μm (product typical value). Luxembourg TW-B, Rz≤4.2μm (product index).
1.3 Application market expansion and performance requirements of ultra-thin electrolytic copper foil for IC package substrates
A recent paper written by an overseas PCB expert gave a more incisive explanation on the application market and application performance requirements of ultra-thin, low-profile copper foil. The article puts forward: "Since 2017, HDI boards have begun to use a large number of circuit electroplating processes that have been commonly used in IC substrate products. This process is called semi-additive process (SAP), which uses circuit electroplating technology. In order to meet the requirements of the circuit structure of IC carrier boards less than 15 μm, this process has not been adopted in general HDI boards. However, after the adjustment of semi-additive technology (mSAP) with ultra-thin copper skin, it has become the mainstream process of HDI manufacturing.