| dc.description.abstract | The most commonly used surface finished treatment in recent years is electroless nickel, which is widely used in either the electroless nickel immersion gold process (ENIG) or the electroless nickel electroless palladium immersion gold process (ENEPIG). Electroless nickel often used in electronic products, as a protective layer on copper lines, owing to its good corrosion resistance. Although the electroless nickel plating process has been relatively matured, there are still difficult to prevent defects forming on the surface of the EN. The contamination of the plating solution may be the main cause of these surface defects, and the sources of the contamination may because of the solder mask dissolution. When the curing reaction of the solder mask is not complete, it will dissolve into plating solution during the ENIG process, which will affect the results of reliability of EN on the solder pad. Skip plating happened and exposed of the copper surface at the edge of the solder pad (close to the solder mask area) would be occurred.
A series of discussions on the corrosion resistance of EN affected by sulfur-containing molecules in solder mask would be presented in this study. First, in order to confirm whether the S content in the EN would affect the corrosion resistance, a simple test method is used to confirm the trace S content in the EN. Combined with the corrosion test result, it showed that the trace S content in the EN has no obvious effect on the corrosion resistance of the EN. The influence mechanism of sulfur-containing molecules in solder mask on the corrosion resistance of EN coating was further discussed. The sulfur-containing molecules dissolved in solder mask detected by GC-MS is 4-(Methylsulfanyl)benzalde. Thus, the EN process is completed with adding 4-(Methylsulfanyl)benzalde in the plating solution. According to the results of CV and OCV analysis, 0 to 30 ppm 4-(Methylsulfanyl)benzalde will enhance the oxidation reaction of H2PO2- during the EN process, so that H2PO2- is oxidized to H2PO3-, and release electrons to the reduce Ni2+ ions to Ni. At the same time, the oxidation reaction of H2PO2- would inhibit the reduction reaction of H2PO2-, thus, the P content in the Ni(P) layer will also decrease. The XRD analysis results also found that with the decrease of P content, the crystallinity of the Ni(P) layer gradually changed from amorphous to polycrystalline structure. However, when the addition amount of 4-(Methylsulfanyl)benzalde exceeds 30 ppm, 4-(Methylsulfanyl)benzalde will adsorb on the Cu surface to form a barrier layer, preventing the occurrence of Ni2+ reduction reaction. The Ni(P) specimens plated with different 4-(Methylsulfanyl)benzalde concentrations were subjected to electrochemical corrosion analysis. The results showed that with the increase of 4-(Methylsulfanyl)benzalde addition, the corrosion resistance of the Ni(P) layer decreased. From the results of SEM, it can be seen that the crack width on the surface of the Ni(P) coating becomes wider and the number of cracks increases. The reason is that the content of P in the Ni(P) layer decreases due to 4-(Methylsulfanyl)benzalde, and the crystallinity changes from amorphous to polycrystalline. The polycrystalline structure allows the etching solution to attack Ni(P) via grain boundary corrosion, which eventually causes Ni(P) to be severely corroded. | en_US |