As we all know that the copper foil is used as the substrate in flexible printed circuit board. However,so few people know how it was made. The production of high quality copper foil products for flexible printed circuit industries requires a number of processing steps.
At present, there are two types of copper foil used in flexible laminates: (1) rolled annealed copper foil (also known as forged copper foil); (2) electrolytic copper foil.
The method of making copper foil is not calendering, annealing or electrolysis. These methods determine their mechanical paradox. According to the mechanical properties and application of copper foil, each kind of copper foil is further divided into different grades. According to the classification of copper foil, electrolytic copper foil and forged copper foil were divided into four grades (Savage. 1992). Generally, electrolytic copper foil is 1-4 grade for rigid printed circuit boards. Flexible printed wiring uses 5-8 grades of electrolytic copper foil and forged copper foil. Typical 2, 5, 7 and 8 grades are used in flexible laminates.
1.Rolled annealed copper foil
For the manufacture of rolled annealed copper foil, firstly the copper chain must be heated, and then sent into a series of rollers to reduce them to the copper foil of the specified thickness. The rotation generates the particle structure in the copper foil, which looks like a lap on the horizontal surface. Under the pressure and temperature, copper particles with different sizes interact with each other, which results in the properties of copper foil, such as ductility and hardness, as well as a smooth surface. Compared with electrolytic copper foil, the copper foil produced by this process can bear more repeated bending. However, its disadvantage is that the cost is high, and the thickness and width of the copper foil are less selective.
2.The electrolytic copper foil
Electrolytic copper foil manufacturing is the copper ion plating to a cylindrical cathode, copper foil from the top of the peeling off. The electrolytic copper foil is a columnar grain structure. When the copper foil is bent, the particles are separated, which makes the flexibility and rupture resistance of copper foil bending smaller than that of the rolled annealed copper foil.
The electrolysis process begins by decomposing copper from sulfuric acid solution and controlling the decomposition rate by temperature and agitation. The appearance and mechanical properties of copper foil may be controlled by using different types of additives.
The copper solution is continuously injected into the electrolytic cell, and the copper ion is precipitated from the chemical pool to the cathode surface under the action of the current between the anode and cathode of the electrolytic bath. The cathode is a rotating cylindrical drum, it’s a immersed in a solution. When the cathode into solution, copper deposition in the drum began to surface, and continuous plating solution until the drum left. When the cathode continues to rotate, the copper foil from cathode stripping. The drum rotation speed determines the cathode copper thickness, electrolytic process can produce many of the thickness and the width of copper foil.
1) combined with (fixed) treatment: this treatment usually involves the treatment of copper / copper oxides, which increases the surface area of copper and provides better wettability for binder or resin.
2) heat resistance treatment: this treatment allows the copper clad laminate to withstand the high temperature environment in the manufacturing process of the printed circuit board.
3) stability treatment: this treatment is also known as passivation or oxidation. This treatment is applied to both sides of copper to prevent oxidation and coloration. All the stability treatments are based on aluminum alloys, and some manufacturers combine nickel, zinc and other metals with lead.
After processing, the copper coil is cut into the required width, and the core of the cut is wrapped in plastic film to prevent oxidation. The extensibility of copper foil is as follows:
1) electrolytic copper foil: extend 4% -40%.
2) rolled annealed copper foil: extended 20% -45%.