Application of copper foil in flexible printed circuit board

In flexible printed circuit boards, copper foil is used as a base material, which is well known. However, very few people know how it is made. The production of 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: calendering, annealed copper foil and electrolytic copper foil. The method of making copper foil is not rolling, annealing, or electrolysis. These methods determine their mechanical resistance. According to the mechanical properties and application of copper foil, each kind of copper foil is further divided into different grades.


For the manufacture of rolled annealed copper foil, the copper chain is first heated and then fed into a series of rollers to reduce them to the copper foil of the specified thickness. The rotation produces particles in the copper foil that appear to overlap on the horizontal surface. Under pressure and temperature, copper particles of different sizes interact with each other, which makes the properties of copper foil, such as ductility and hardness, and at the same time produce a smooth surface. Compared with electrolytic copper foil, the copper foil produced by the process can bear more repeated bending. However, its disadvantage is that the cost is higher, and the thickness and width of the copper foil are less selective.


Electrolytic copper foil is fabricated by electroplating copper ions onto a cylindrical cathode, and copper foil is continuously peeled off from the top. The electrolytic copper foil is a columnar grain structure. When the copper foil is bent, the particles are separated, which makes the bending and bending resistance of copper foil 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 shape and mechanical properties of copper foil may be controlled by different types of additives.


The copper solution is continuously injected into the electrolytic cell, and the copper ions are separated 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 is peeled off from the cathode. The drum rotation speed determines the cathode copper thickness, electrolytic process can produce many of the thickness and the width of copper foil.


Combining (fixed) treatment: this treatment usually involves the treatment of metal copper / copper oxides, increasing the surface area of copper and providing better wettability for binders or resins.


Heat resistance treatment: this process allows copper-clad laminates to withstand the high temperature environment in which printed circuit boards are manufactured.


Stability treatment: this process is also known as passivation or oxidation. This process 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 treatment, the copper coil is cut into the desired width, and the core of the cut is wrapped in plastic film to prevent oxidation. The ductility of copper foil is as follows:


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

2) rolled annealed copper foil: extended 20% -45%.


Copper foil is usually covered with a film made from polyimide or liquid polymer liquid. With this treatment, the conductor film enables the copper to be protected for a long time from the corrosive environment and the solder contamination.


 

(Source: internal information)

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