Effect of halogen-free direct coating process on free air ball morphology of palladium-coated copper wire

The characteristics of palladium layer on the surface of copper wire are important factors affecting the quality of free air ball （FAB） and bonding during chip packaging. In this paper， palladium-coated copper wires with different coating speeds and coating temperatures are prepared by halogen-free direct coating process. The effects of coating speed on the morphology of the coating and the characteristics of FAB are studied. The results show that with the increase of coating speed， the coating time decreases， and the uniformity of the distribution of palladium particles on the surface of the copper wire becomes worse. The agglomeration of palladium particles with uneven local distribution causes the concentration of palladium particles to be too high， and the agglomeration area of palladium particles on the surface of the coating increases. Under the plating speed of 50 m/min， the distribution of palladium on the surface of the coating is more uniform. With the increase of coating speed， the diameter of FAB decreases gradually. The difference of palladium content between the agglomeration area and the non-agglomeration area of Pd particles on the surface of Pd-coated copper wire increases， and the consistency of FAB sphere size gradually decreases. At a lower coating speed of 50 m/min， the distribution of palladium content on the surface of FAB is relatively uniform. At a higher coating speed of 100 m/min， a large number of agglomerated palladium particles on the surface of the coating are remelted to form a large area of continuous palladium-rich area on the surface of the FAB， and the uniformity of palladium redistribution is poor. Considering the uniformity of FAB size and surface palladium redistribution， the coating speed of 50 m/min and the coating temperature of 400 ℃ are the better process parameters for the coating of palladium-coated copper wire.