Application of Copper Foil in Flexible Printed Circuit

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In flexible printed circuit boards, copper foil is used as a substrate, which is well known. However, few people know how it is made. The production of high quality copper foil products for the flexible printed circuit industry requires many processing steps.

At present, the flexible laminate used in two types of copper foil: ① calendering annealing copper foil (also known as forging copper); ② electrolytic copper foil.

Copper foil manufacturing methods, not calendering is electrolysis, these methods to determine their mechanical paradox. According to the mechanical properties and application of copper foil, each copper foil is further divided into different grades. Table 12-3 shows the classification of copper foil, electrolytic copper foil and forged copper foil were divided into four grades (Savage. 1992). Usually electrolytic copper foil is 1-4 grade, used in rigid printed circuit board. Flexible printed circuit using all the electrolytic copper foil and forged copper foil 5-8 level. Typical grades 2, 5, 7 and 8 are used in flexible laminates.

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1 calendered annealed copper foil

The production of calendered annealed copper foils is to heat the copper chains first and then feed them into a series of rollers to reduce them to copper foil of the specified thickness, as shown in Figure 12-3. The rotation produces the grain structure in the copper foil, which looks like a lap in the horizontal plane. Under the action of pressure and temperature, copper particles of different sizes interact, which acts as copper foil, such as ductility and hardness, and also produces a smooth surface. Compared with the electrolytic copper foil, this production process to produce copper foil can withstand greater bending.

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Figure 12-3 Copper foil products manufactured by calendering

However, its drawback is that the cost is higher and the thickness and width of the copper foil are less optional.

2 electrolytic copper foil

Electrolytic copper foil is made by copper ion plating to a cylindrical cathode, copper foil from the top of the stripped down. Electrolytic copper foil is a columnar grain structure, when the copper foil is bent when the particles separated, which makes the copper foil bending, its flexibility and resistance to cracking than the calender annealed copper foil is smaller. Figure 12-4 is a schematic diagram of the manufacturing process of an electrolytic copper foil.

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Figure 12-4 Schematic diagram of electrolytic copper foil manufacturing process (Savage, 1992)

The electrolysis process begins by decomposing copper from the sulfuric acid solution and controlling the decomposition rate by temperature and agitation. The appearance and mechanical properties of copper foils may be controlled using different types of additives.

The copper solution is continuously injected into the cell, and copper ions are precipitated from the chemical pool to the cathode surface under the action of the current between the anode and the cathode of the cell. The cathode is a rotating cylindrical drum, and it is immersed in the solution. When the cathode enters the solution, the copper begins to deposit on the surface of the drum and is continuously plated until the drum leaves the solution. When the cathode continues to rotate, the copper foil is peeled off from the cathode. The rotational speed of the cathode drum determines the thickness of the copper foil, and the electrolysis process can produce many thicknesses and widths of copper foil.

Whether forged or electrolyzed raw copper foil products, but also need to be processed in three processing stages, as shown in Figure 12-5.

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Figure 12-5 Copper foil processing stage and stability

1) Bonding (immobilization) Treatment: This treatment usually involves the treatment of metallic copper / copper oxide, which increases the surface area of the copper and provides better wetting properties for the binder or resin.

2) Heat treatment: This treatment allows the copper-clad laminate to withstand the high temperature environment in the manufacturing process of printed circuit boards.

3) Stability treatment: This treatment is also known as passivation or oxidation resistance, this treatment is applied to both sides of copper, used to prevent oxidation and coloring. All of the stability treatments are based on aluminum alloys, and some manufacturers use nickel, zinc and other metals in combination with lead.

After processing, the copper roll is cut to the desired width and the cut core is wrapped with plastic film to prevent oxidation. The ductility of the copper foil is as follows:

1) electrolytic copper foil: extended 4% -40%.

2) calendering annealed copper foil: 20% -45% extension.

The copper foil is usually covered with a film made of polyimide or liquid polymeric banyan liquid. After this treatment, the conductor film can be copper long-term protection, from corrosive environment and the impact of solder contamination.