PCB News - High-end PCB process, all need those conditions

In this paper, the supply chain pattern of special electrolytic copper foil, special resin and special fiberglass cloth used in new and high-end substrate materials in recent years, as well as the new performance requirements of these three materials are reviewed.

Since the beginning of 2020, the spread of global new crown disease has caused serious changes in the supply and demand chain pattern of copper clad raw materials in China.Since the development of 5G, there have been great changes in technology, performance and varieties of substrate materials used for high frequency and high speed circuits, high HDI and IC encapsulation loading plates.In the face of these two major changes, the supply chain pattern of electronic copper foil used in new and high-end substrate materials is discussed.

Differentiation of properties of low profile electrolytic copper foil in different application fields

The types and categories of low profile electrolytic copper foils used in high frequency and high speed circuits are divided into five categories according to their application fields.Low profile electrolytic copper foil for rigid RF/microwave circuits;Low profile electrolytic copper foil for high speed digital circuits;Low profile electrolytic copper foil for flexible PCB;Low profile electrolytic copper foil for seal loading plate;Low profile electrolytic copper foil for thick copper PCB.These five application fields have different characteristics in terms of performance requirements for low profile electrolytic copper foil used in high frequency and high speed circuits, that is, there are some emphasis on performance items, and there are differences in performance indicators.

The performance requirements and differences of low-profile electrolytic copper foils used in 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 foils for rigid RF/microwave circuits show more obvious relative differences at different application frequencies.In the copper foil performance on the base plate DK uniformity, signal transmission loss, the presence of no ferromagnetic elements in the processing layer, PIM(Passive inter-modulation, Passive intermodulation) and other factors, requirements are more stringent.

For this reason, the copper foil used in high-grade RF - microwave electric subgrade board (such as the substrate used for millimeter-wave vehicle radar) is generally required to adopt pure copper treatment process for surface treatment to support the reduction of passive intermodulation (PIM) and achieve low PIM property of copper clad board. The reference index is below -158DBC ~ -160DBC.The copper foil treatment layer is arsenic-free.

At the same time, due to the different resin substrates of this kind of copper foil, there is a great difference in the selection of different RZ copper foil varieties.

Rigid RF/microwave circuit with low profile electrolytic copper foil in the thickness of the copper foil gauge square, a general use: 18μm, 35μm, 70μm, and high-end very low or ultra-low profile copper foil, thickness specifications multi-purpose: 9μm, 12μm, 18μm varieties.

(2) low profile electrolytic copper foil for high speed digital circuits

Most of the low-profile copper foil applications for high-speed digital circuits are located in the frequency range of centimeter wave (3 ~ 30GHz).Its main application terminal is the high school end server.The performance of this kind of copper foil has a more important impact on the insertion loss of the substrate and the machinability of the substrate, so there are strict requirements for this.At the same time, the thin shape specification and low cost of copper foil are also important requirements.High-speed digital circuit with low profile electrolytic copper foil in the thickness of copper foil specifications, generally used: 18μm, 35μm, 70μm, and high-end very low or ultra-low profile copper foil, thickness specifications multi-purpose: 9μm, 12μm, 18μm varieties.

The author for many varieties of all kinds of copper foil low Rz (HVLP, varieties of VLP, RTF, and so on) of the pressing of the profile of the investigation, comparison, guide the conclusion is: in the same class varieties brand, whatever in SI sexual relation to better varieties, and it's not pressing surface profile (Rz or Ra) are generally low.For example, a foreign enterprise's RTF copper foil product, its RZ =3.0μm(typical value), and the non-pressing surface is RZ =3.5μm.Therefore, whether it is RF/microwave embankment board or high-speed digital embankment board, in order to pursue better Si property, they also need the non-pressing surface RZ (or Ra, RQ) of the contoured copper foil used, which also has a very low profile.

Currently, one of the most important categories of low-profile copper foil used in high-speed digital circuits is reverse copper foil (RTF).In recent years, resins in Japan, Taiwan and other copper foil enterprises in RTF copper foil technology progress, its RZ less than 2.5μm of many varieties have come out, and even RZ less than 2.0μm varieties have appeared.As a result, its application market, as well as the proportion of the global market size of low-profile copper foils used in high-speed digital circuits, has been rapidly expanded.

At present, the global copper foil low-profile electrolytic copper foil manufacturing industry tends to have the same performance in terms of the types of copper foils used in rigid RF/microwave circuits and those used in high-speed digital circuits.For example, CircuitFoil from Luxembourg achieved mass production of ultra-low contour copper foil BF-NN/BF-NN-HT in 2019.This variety has achieved "two compatible" : First, from the original BF-ANP copper foil only used in PTFE resin type substrate, developed to "including polyphenyl ether (PPE/PPO) resin system.Also suitable for pure or modified fluoropolymer (PTFE) resin systems."Secondly, it can be applied in RF and microwave embankment board, and also suitable for high speed digital embankment board.

(3) Flexible PCB with low profile electrolytic copper foil

Flexible PCB with low profile electrolytic copper foil, due to the manufacture of fine line needs, mostly use very thin copper foil (no carrier).At present, the minimum thickness specification of this kind of varieties has reached 6μm, such as Fuda Metal Foil Powder Co., CF-T4X-SV6, CF-T49A-DS-HD2;And Mitsui Metal's 3EC-MLS-VLP (thickness minimum 7μm).

Flexible PCB with low profile electrolytic copper foil also requires that the copper foil has high tensile strength, high elongation.The excellent transparency of the etched base film is also an important demand item in the copper foil market.

In recent years, low profile electrolytic copper foil used in high frequency flexible PCB has begun to move towards low profile.Now there are many varieties with RZ less than 1.0um in the industry.For example, Mitsui Metal TQ-M4-VSP RZ ≤0.6 (typical value);Foton Metal CF-T4X-SV Rz=1.0 (Typical, 9/12/18);Foton Metal CF-T49A-DS-HD2,Rz= 1.0μm (Typical) (6/9/12/18);Progressive ISP, RZ ≤0.55(typical).

(4) IC seal loading plate with low profile electrolytic copper foil

The low profile electrolytic copper foil required by the sealing loading plate (including the module substrate) should have high tensile strength, high thermal stability, high elastic modulus and high peel strength at high temperature (210 degree Celsius/1h after treatment).Its thickness specification is 5.0μm ~ 12μm.And in recent years, the thickness specification of copper foil for high-end IC sealing loading plate is developing towards the more extremely thin, that is, the thickness reaches 1.5μm ~ 3μm.

In recent years, the high frequency and high speed requirements of pack-loaded plates (including module substrates) also appear.Therefore, in recent years, more types of low profile electrolytic copper foil for sealing and loading plates have appeared.For example, Mitsui Metal's 3EC-M2S-VLP (no carrier), Rz≤ 1.8μm (typical value);Tensile strength 51kgf/mm 2 at 210 degree Celsius /1h;Elongation rate 4.6%;The thinnest size of copper foil is 9μm.Mitsui Metal MT18FL (with carrier), RZ ≤1.3μm, forming circuit copper foil specifications 1.5, 2, 3μm.LPF (without carrier) of RZ ≤1.72(typical value), tensile strength of 52.3 KGF /mm 2 at 210 degree Celsius /1h;Elongation rate 3.7%;The thinnest size of copper foil is 9μm.

(5) electrolytic copper foil with low profile for high current thick copper PCB

For high current thick copper PCB with thickness specification ≥105um (3oz), low profile is used for electrolysis of copper foil. The usual gauges are 105, 140, 175, 210μm.There are special thickness requirements of ultra-thick electrolytic copper foil, thickness specifications up to 350μm (10oz), 400μm (11.5oz).

High current thick copper PCB with low profile electrolytic copper foil, mainly used for high current, power supply substrate, high heat dissipation circuit board manufacturing.The manufactured thick copper PCB is mainly used in automotive electronics, power supply, high-power industrial control equipment, solar equipment and so on.In recent years, the thermal conductivity of PCB has become one of the most popular and important functions.The market for ultra-thick copper foil is expanding.At the same time, due to the development of the manufacturing technology and application of thin lines of thick copper PCB, the ultra-thick copper foil it needs to adopt also has low profile characteristics.Mitsui metal RTF type low profile, for example, thick copper foil: MLS - G (2 type), Rz = 2.5 mu m (typical) products.Luxembourg TW-B, RZ ≤ 4.2μm(Product Indicator)

Market Expansion and Performance Demand of Ultra-thin Electrolytic Copper Foils for IC Sealing Loading Plate

In a recent paper written by overseas PCB experts, the application market and performance requirements of ultra-thin, low-profile copper foil are described.The paper proposed: "Since 2017, HDI boards began to adopt a large number of IC board products have been widely used in the line plating process.This process is called the semi-addition process (SAP), is the use of line plating technology, to meet the IC board less than 15 μm line structure requirements, this process has become the mainstream HDI manufacturing process after the general HDI board has not been adopted, but the use of ultra-thin copper sheet semi-addition technology (MSAP) adjustment."

"The difference between the semi-addition method (SAP) used for IC and the semi-addition method with copper foil (MSAP) used for SLP lies in whether the processed plate is prepressed ultra-thin copper foil.At present, the mature SAP process is usually ABF film material and adopts full-plate copper precipitation process, which is not suitable for most existing production equipment.This led to an improved solution, a semi-addition process with ultra-thin copper foil."

A comparison diagram of the semi-addition process (MSAP) with ultra-thin copper foil (see Fig. 2) is given in this paper. The position of "pre-pressed ultra-thin copper foil" is marked here for readers to have a clear understanding.

"The key to the semi-addition process with copper foil is the use of carrier copper, which helps to stabilize the peel strength of the copper foil and strengthen the support of the fiber," the paper suggests.However, several important properties of ultra-thin copper foil, which is coated on the substrate by pressing method, are also mentioned in this paper.It mainly includes: high and stable copper foil peel strength, thickness uniformity of ultra-thin copper foil, low surface roughness, suitable anti-oxidation coating on copper foil smooth surface, etching of fine circuit, etc.Among them, copper foil peel strength is the most important performance item.