Research on Forming Mechanism and Optimization Scheme of Acid-Etched Pinhole on High-Density Packaging Substrate

The new high-density non-core package substrate needs to carry out the ultra-deep etching and thinning process of more than 20 microns when drawing the conductive graphics circuit and requires the smooth and uniform surface of the substrate after thinning. However, after the thinning process of more than 10 microns, there will be several pinholes with diameters over 10 microns on the surface of the copper plating layer of the encapsulated substrate. We have already probed into the formation mechanism of pinhole on the surface of copper plating after acid etching, which refers to coating crystal defect. On this basis, this paper puts forward two reasonable process improvement schemes. One is to change the interface energy of copper surface, by adding cationic surfactant in corrosive liquid (hexadecyltrimethylammonium chloride, tetramethylammonium chloride), the surface tension of corrosive liquid decreases, the viscosity increases, and the rate of transmission and diffusion of reactive substances is reduced. Thus, the reaction rate is reduced, and the corrosion is more uniform. Another one is to change the chemical status of copper surface by adding cysteine, alanine and other copper ions in the corrosive liquid ligand compounds. It can reduce the capillary effect and make copper ions different crystal surface dissolution rate changes which leaves the slow soluble crystal surface and achieves uniform acid erosion to make the pinhole phenomenon not easy to occur. After acid etching, the number of pinholes of copper coating on the package substrate decreased significantly, and the mixed use will not impact. The reaction mechanism is further discussed and analyzed. The improved process can effectively improve the yield of the product and has high practical value.