Preparation And Perfomance Of Pd-catalyzed Electroless Gold Deposition From The Sulfite Based Bath

Printed circuit board(PCB) finishing process is the final surface treatment in the manufacturing process. PCB finishing is not only providing a protective layer on the surface of the copper lines, but also is the key technology for connecting PCB to other electronic components. This article first presents general background information about PCB finishing, and then studies the problem in electroless Ni/immersion Au(ENIG) process. A novel substrate(Pd)-catalyzed electroless gold deposition method is reported in this paper. And the technique is used in electroless Ni/electroless Pd/immersion Au(ENEPIG) process.In the traditional ENIG process, gold deposition is carried out following a displacement reaction. The nodules edge on Ni-P surface may undergo severe corrosion during the displacement reaction, due to the penetration of cyanide ions and the uneven distribution of electric charge on the surface. The bonding is very weak due to the porosity of the corroded Ni-P, and the “Black Pad” occurs on the heavily corroded Ni-P surface. The sulfite-based, cyanide-free electroless gold bath is used for ENIG process. The optimal solution composition and processing conditions are determined by studying the effects of plating parameters on the stability of gold bath and the uniformity of the Au layer. After optimization, the obtained plating solution has a good stability and the relative standard deviation(RSD) of the coating thickness is less than 10%. For 10 minutes plating, the Au layer thickness is 0.05 μm, with surface roughness of 20.8 nm. The gold film growth model is established by studying the surface morphology change with different plating time. According to the growth model, the “Black Pad” defect of the ENIG plating is due to galvanic hyper-corrosion of the Ni-P film in the immersion gold process. At the same time, elemental sulfur is detected in the gold film obtained from the thiosulfate-sulfite mixed ligand bath. It is proposed that sulfur is generated on the Ni-P surface, mainly at the start of plating process. The coverage of Au on the Ni-P surface subsequently hinders the contact between S2O32- ion and Ni, and prevents further formation of sulfur.Based on the above study, palladium layer was added in ENIG process to solve the problem of Ni-P corrosion. Three methods were used to prepare the palladium layer, i.e., the displacement palladium plating, electroless palladium plating using hypophosphite and formate as reducing agents, respectively. These results indicated that there are pores, nodules or large grains on Pd surfaces. Therefore, these methods are not suitable for Pd plating. In addition, electroless Pd deposited on Ni-P surfaces using formic acid as reducing agent is found to be a two stage reactions. First, Pd plating proceeds via a galvanic displacement reaction between Ni and Pd2+. Subsequently, Pd deposition via autocatalytic reaction takes place on Pd surfaces. A novel method is further developed for electroless deposition of Pd using reducing agents by studying the effect of P content on structures and surface morphology. In this method, hypophosphite and formate are used as the two reducing agents. The P content of Pd-P layer can be controlled by adjusting the concentration of hypophosphite. The special Pd-P coating with P content less than 1 wt% can be obtained by the optimized method. The SEM images and AFM results indicated that the Pd-P coating surface is smooth and clean, and the nodules on the Ni-P surface disappeared. The presence of Pd-P layer significantly improves the corrosion resistance of the sample.Since a suitable Pd-P layer has been prepared, the sulfite-based electroless gold is used for gold plating on the Pd-P surface. Thus complete the entire ENEPIG process. Then, some electrochemical measurements, such as the potential change in reaction process, the change of the metal ion concentration in gold bath and the deposition rate measurement are employed to demonstrate the substrate(Pd)-catalyzed electroless gold deposition. During the reaction, the palladium is not replaced with Au. Gold deposition reaction consists of two parts: a substitution reaction with Ni and Pd-catalyzed electroless deposition. The proportion of the displacement reaction decreased as the thickness of the Pd-P layer increased. With a 0.1 μm thick Pd–P layer, 34.2% of the reaction was a displacement reaction.The addition of Pd-P layer in ENIG process is beneficial to inhibiting the “Black Pad” defect: First, the addition of a thin Pd layer between the Ni and Au layers prevented the dissolution of Ni during the immersion Au plating process. Second, the Pd-P films obtained via electroless plating are smooth, compact, and uniformly distributed on the entire Ni-P surface. Therefore, the nodules on the Ni-P surface are covered by Pd-P layer. There is no serious corrosion occurs in nodules edges. Last, there is no phosphorus-rich layer and sulfur impurities present in the ENEPIG layers. This is beneficial to the welding performance. Finally, the overall performance of ENEPIG layers is evidenced by testing the corrosion resistance, weldability and welding reliability.