With ultrathin Electrolytic copper foil Market demand expansion, domestic thickness ≤ 18 μ M's electrolytic copper foil production capacity is expanding year by year. The production of ultra-thin electrolytic copper foil requires very strict equipment, process and environment. Copper foil products are prone to pinhole, warpage, mottle, cross section porosity and other defects. Warping is Production of ultra-thin electrolytic copper foil The problems often encountered in the process are generally caused by the production process, but on the double bright side Electrolytic copper foil production Less in the process. Copper foil warping, on the one hand, affects machine plate stacking and cutting; On the other hand, it affects the smooth and uniform adhesion between the copper foil and the substrate, which is easy to cause wrinkles and bubbles in the copper foil, and seriously affects the quality of copper clad boards and printed circuit boards.

1. Performance test and warpage analysis of electrolytic copper foil

18 thick for 5 manufacturers μ m. The samples of electrolytic copper foils with different warpage were sampled, and the internal stress, texture, rough surface morphology and cross section were analyzed.

1.1 Residual stress

Use X-350A X-ray stress tester, and use roll fixation φ Method, measure the residual stress. Before the test, the sample is electropolished with XF-1 electropolishing machine to eliminate the surface stress, oxide layer, surface treatment layer and pollution layer of the sample. It can be seen that the bright side and rough side of copper foil samples from all factories are in a state of compressive stress. Because the bright side grain is much smaller than the rough side, there are many bright side grain boundaries, and the residual stress mainly exists at the grain boundaries. Therefore, the bright side compressive stress is always greater than the rough side compressive stress, and the residual stress is shown as the compressive stress toward the rough side. The copper foil warps toward the rough side, which is consistent with the actual situation. In addition, it can be found that the greater the residual stress, the greater the copper foil warpage.

1.2 Texture, twin boundary and grain size

In the electrodeposition process of copper foil, due to the different growth speed of each crystal face, there will be preferential growth of crystal face. This will affect the performance of the copper foil.

The texture, twin boundary and grain size of samples were tested and analyzed by Oxford instruments HKLEBSD system. Technicians analyzed that the smaller the grain, the more obvious the warpage; The more twin boundaries, the lower the warpage, because twin boundaries are conducive to the release of residual stress in copper foil. The data shows that the copper foil sample mainly has (220) texture, while there are (111), (200), (311) and other textures. The (220) texture of the sample with larger warpage is obviously larger, and the twin boundaries are less, and the grain size is smaller.

1.3 Roughness

The surface particle size is 5~10 μ m. The shape of particles is mostly conical. It can be seen from Figure 1 that the warpage degree of other manufacturers except JT is relatively low, and the surface ripening treatment electroplating layer is generally grown on the top of the sharp cone of particles, and the particles are uniform and the sharp cone shape is obvious; The surface ripening treatment electroplating layer of JT samples not only grows on the top of copper particles, but also on the bottom of the valley with large particle gaps, which is obviously different from other samples. Then take the raw foil sample corresponding to the cooked foil of JT Factory for scanning electron microscope analysis. It is found that the top of the particles is relatively round, hilly, and the particle size is uneven. A large number of small particles grow at the bottom of the valley of large particles. It can be seen that JT Factory copper foil has a higher degree of grain refinement than other manufacturers.

1.4 Cross section analysis

The FEIQuanta200F environmental scanning electron microscope was used to analyze the cross section of samples from each factory, as shown in Figure 3. In the process of electrodeposition, some holes will be formed due to crystal collapse, grain dislocation, grain annexation, hydrogen evolution or additives, impurities and other inclusions. It can be seen from Figure 2 that the grains on the cross section of JT copper foil are well combined, with relatively few cavities and uneven distribution. The samples from other manufacturers have many cavities, which are distributed on the whole section. Cavities are conducive to the release of residual stress in copper foil.

1.5 Cause analysis of warpage

There are compressive stresses on the bright and rough surfaces of the copper foil, and the compressive stress on the bright surface is greater than that on the rough surface, so the overall performance is the residual stress towards the rough surface, which makes the copper foil warp towards the rough surface, which is consistent with the actual situation.

The residual stress of copper foil is related to texture, twin boundary, grain size and internal holes. From the product sampling analysis, the more texture of copper foil (220), the more obvious warpage. The twin boundary is conducive to the release of residual stress and can reduce the warpage of copper foil. The finer the grains are, the more the grain boundaries are, the greater the residual stress is and the more obvious the copper foil warpage is. The internal holes are conducive to the release of residual stress. The more and more evenly distributed the voids are, the smaller the warpage is. Through the process comparison analysis and test demonstration of different manufacturers, it is found that additives, electrodeposition parameters and cathode roll surface roughness have a great impact on the above factors, which will cause copper foil warping.

2 Effect of additives on copper foil warpage

The test shows that the use of some additives will cause copper foil warping. If the circulating flow rate of electrolyte is too fast, the concentration of additives in the electrolyte will be high, which will cause copper foil warping. The additives SP (sodium polysulfide dipropane sulfonate), HEC (hydroxyethyl cellulose) and PEG (polyethylene glycol) have significant effects on the copper foil warpage, and the warpage increases with the increase of the dosage.

With the increase of SP content, the (220) texture of copper foil increases, the (111) texture and twin boundary decrease, and the grain size gradually decreases; The residual stress and warpage of copper foil increased. Figure 6 shows the refining and leveling effect of SP on copper foil rough surface particles. When the amount of SP is 1.3mg/L, the rough surface of the copper foil shows the brightness of the bright surface, and the rough surface particles are greatly refined and leveled. SP adsorption on the cathode improves the polarization of the electrochemical reaction of the electrode, hinders the excessive growth of copper particles, improves the nucleation rate, and makes the grains fine. SP can form surface complexes with Cu ions, change the grain growth mode, promote the (220) texture growth of copper foil, and inhibit the (111) texture growth.

With the increase of HEC content, the (220) texture of copper foil increases, the (111) texture and twin boundary decrease, and the grain size has no certain effect; The residual stress and warpage of copper foil are increased. When HEC adsorbs Pb, Zn, Fe, As, Cr and other impurity ions in the electrolyte, it also complexs with Cu ions, enters the deposit layer, and then affects the grain texture growth.

With the increase of PEG content, the (220) texture of copper foil increases, and the grain size decreases. The (111) texture and twin boundary do not change regularly; However, the residual stress and warpage of copper foil have increased to a certain extent. Under the action of PEG, the nucleation rate can be increased. PEG can form complex complexes with Cu ions and adsorb on the electrode surface, thus increasing the inhibition effect. With the increase of overvoltage, the nucleation mechanism gradually changes from continuous nucleation to instantaneous nucleation, which further affects the grain texture growth, and can refine and level particles to prevent abnormal growth of particles.

It can be seen that the use of some additives will cause copper foil warping. Additives affect the internal stress (residual stress) of the copper foil by improving cathodic polarization, increasing inhibition, changing nucleation density, thereby affecting the growth of copper foil grains, promoting grain growth to (220) texture, or refining copper foil grains, and thus affecting the warpage degree of the copper foil.