Overview of the calendering copper foil production process

Calendered copper foil is made of copper ingots by continuous rolling, and its form is a flake structure. Compared with electrolytic copper foil, the calendered copper foil has better ductility, softness, bending resistance, and higher strength, so the calendered copper foil is often used in flexible copper cladding plates. At the same time, the surface roughness is low and the density is high, which is conducive to the transmission of high-frequency signals and greatly reduces signal loss. Therefore, in fine line, high frequency, high-speed transmission of PC and other high-end products is essential.

At present, in addition to the general calendering copper foil varieties, there are high flexural calendering copper foil, high strength (calendering copper alloy) calendering copper foil and thin calendering copper foil varieties. Calendered copper foil adopts different heat-resistant layer surface treatments and can be divided into several calendered copper foil varieties according to different heat-resistant layer surface treatment methods. There are three types of calendered copper foil with different surface treatments in common use, namely BHN, BHC, and BHY.

Copper foil rolling

• Calendering copper foils require smaller thickness deviations, better flatness, and less residual stress than copper strips. On the circuit board high-speed etching line, the copper foil is too thick or too thin, which will lead to an etching residual or over-etching phenomenon. Copper foil rolling is the key process of calendering copper foil production, and the thickness, speed, tension, and surface quality control of the copper foil rolling process are particularly important.

Thickness control

• According to the formula of minimum rolling thickness: Hmin≥i58 μm (1.155Rp-q)DE.
  Where Human is the minimum thickness of rolled piece; D is the diameter of the work roll; E is the elastic modulus of the roll material; μ is the friction coefficient; Rp is the yield strength of rolled material; q is the average tensile stress.
  Copper foil rolling belongs to limit calendering, and AGC control mode with no (negative) roll seam as the main body is used when the thickness of rolling is below 0.035mm. In the no (negative) roll seam state, the deformation of the roll is a non-circular profile, the contact arc length is equal to the roll flattening, the roll seam has been pressed, the pressure and the size of the rolling pressure has no absolute relationship, that is, the change of the strengthening of the rolling force on the changes of the thickness of the sheet has little effect, the rolling process is completely controlled by the size of the tension and rolling speed to complete. The thickness control of the copper foil mill is mainly realized by the tension control ring and speed control ring.
  According to the Stone equation, the thinner the rolled foil, the smaller the diameter of the work roll. The minimum rolling thickness is proportional to the diameter of the work roll. If the work roll diameter is too small, it is not suitable to obtain a higher rolling speed for thinning and obtaining higher rolling torque. Therefore, for copper foil rolling, the minimum rolling thickness, rolling torque, and rolling speed should be considered comprehensively to obtain the appropriate working roller diameter.

rolling speed and oil removal effect

• Rolling speed is an important parameter of copper foil rolling, and its size directly determines the production efficiency of the rolling mill. It is an important index to measure the level of rolling technology. It is also an important factor to achieve the thin rolling thickness of foil. The higher the rolling speed, the thicker the oil film thickness in the deformation zone, which is conducive to thinning. With the increase in rolling speed, the friction coefficient is reduced, the deformation resistance is reduced, and the energy consumption is reduced. Because the thickness of the copper foil is extremely thin and the length of a single roll of copper foil is extremely long, the high-speed and stable rolling of the copper foil mill is the premise of high-efficiency production. Taking copper foil with a mass of 4t, a width of 650mm, and a thickness of 18μm as an example, it still takes 1.5-2h for a single pass to be rolled at 400m/min. So how realizing high-speed and stable rolling of copper foil is one of the key technologies in copper foil production.

 rolling tension

• Rolling tension is an important means to control the thickness of copper foil rolling, and it is also an important way to eliminate rolling defects, obtain good shape and realize stable rolling. In copper foil rolling, a larger tension value can be selected to achieve stable rolling, conducive to plate shape control, effectively reduce rolling pressure, reduce energy consumption, and prevent strap slippage. Generally, the back tension is more effective in preventing the bite "deviation" and reducing the rolling force, while the front tension is obviously effective in eliminating the "wave" and "creasing" defects.
  The adjustment function of rolling tension to thickness is related to the friction coefficient of rolling. The smaller the friction coefficient is, the greater the regulating effect of the thickness of tension is. The thickness of the lubricating oil film and the roughness of the roll determine the rolling friction. The higher the speed, the more lubricating oil will enter the rolls. However, as the rolling process goes on, the thermodynamic conditions change, and the rolling friction will increase with the increase in temperature. In the production, it is also necessary to consider the influence of the viscosity change of lubricating oil on the lubrication conditions, to find the appropriate combination of speed - oil quantity - viscosity - tension relationship.

The surface quality of copper foil rolling

• Surface quality is one of the important indexes of calendered copper foil. Appearance requirements include cleanliness, pinhole, air gap, pitting, indentation, notch, tear, wrinkle, scratch, etc. Process lubrication, roll surface roughness, rolling oil filtration precision, and external environment are the three important factors affecting the surface quality of copper foil rolling.

 coarsening and curing

• The main reason for the coarsening treatment on the surface of calendered copper foil is that the surface of calendered copper foil is very smooth, and the surface roughness is generally only about 1μm. The surface of untreated copper foil cannot be pressed with resin basically. Therefore, in order to improve the bonding force between copper foil and substrate, a layer of nodular crystalline particles was electroplated on the surface of copper foil to increase the surface roughness of copper foil.

Heat-resistant layer treatment

• The main function of heat-resistant coating treatment is to form an insulating layer on the wool surface to isolate the copper foil from the substrate. When the copper foil is bonded with resin insulating substrate after heat-resistant layer treatment, it can inhibit the spread of copper ions to the resin layer, and prevent the reaction between copper foil and resin in the subsequent curing process, resulting in stains and peeling problems

Anti-oxidation treatment

• Copper foil is easy to oxidize and discolor in the air. The main function of the antirust layer is to prevent copper foil from oxidizing and discoloring in the storage, transportation, and pressing process. At present, the antirust treatment mainly adopts ZnNi and NiCr alloy layer electroplating and then soaks the coating and layer organosilane