铜表面镍活化层的反置换沉积过程与微观结构研究

Palladium activation layer is usually prepared on copper through galvanic replacement deposition to realize the electroless nickel-phosphorus plating on copper circuits of printed circuit boards, but there is problem of over-plating and high cost in the industrial application of palladium activation. Although nickel activation layer can effectively avoid over-plating and has the advantage of low cost, it is difficult to obtain the nickel activation layer on copper circuits by galvanic replacement deposition as the standard potential of nickel is lower than that of copper. In this application, an anti-galvanic replacement deposition method to prepare low-cost nickel activation layers on copper is proposed based on the experimental phenomenon that the stable potential of copper is lower than that of nickel in sodium thiosulfate solutions. In this research project, the correlation between the components of nickel activation layers on copper and the composition of anti-galvanic replacement nickel deposition solutions will be investigated. The effect of grain sizes and surface defects of copper substrates on the nucleation sites of nickel activation layers will be analyzed, and the fine anti-galvanic replacement deposition process of nickel activation layers will be studied. The influencing mechanism of the copper substrates on the microstructure of nickel activation layers deposited by anti-galvanic replacement will be revealed. On this basis, the activation performance of nickel and palladium activation layers will be comparatively studied by analyzing the density, bonding force, deposition rate and corrosion resistance of nickel-phosphorus coatings, which are electroless plated on the nickel and palladium activation layers. Important theoretical foundation and technical support will be provided to make the anti-galvanic replacement deposited nickel activation layers replace the palladium activation layer and become a novel method to activate electroless nickel-phosphorus plating on copper circuits of printed circuit boards.

置换沉积钯活化层是印制电路铜线路表面化学镀镍磷前应用最广泛的活化方法，但其在实际应用中存在易溢镀及成本高等问题。虽然采用镍活化层可有效避免溢镀现象，且具有低成本优势，但由于镍的标准电位低于铜，难以利用置换沉积方法在铜线路表面获得镍活化层。本项目基于铜在硫代硫酸钠溶液中具有较镍更低的稳定电位，提出铜表面反置换沉积镍活化层新技术。通过研究反置换沉积液组成与铜表面镍活化层组分间的相关性，探讨铜基底晶粒大小和表面缺陷等特征对其表面反置换沉积镍形核位点的影响规律，分析镍活化层的反置换沉积精细过程，揭示铜基底对反置换沉积镍活化层微观结构的影响机制。并对比不同镍活化层和钯活化层表面化学镀镍磷层的致密度、结合力、沉积速率和抗腐蚀性能等，考察镍活化层的活化效果，为反置换沉积镍活化层替代钯活化层成为一种新颖的印制电路铜线路表面化学镀镍磷前的活化途径提供技术支持和理论依据。

虽然化学镍钯金工艺被认为是印制电路铜线路最理想的表面处理技术，但由于其中镍磷底层化学镀沉积时所用的还原剂次亚磷酸盐在铜表面的催化氧化具有较高的能量势垒，因此要在铜表面实现化学镀镍磷，需首先对铜进行钯活化处理。然而，钯的催化活性过高，易导致铜线路间的聚合物基材活化，造成镍磷在铜线路之外沉积。此外，钯活化成本越来越高。因此，发展一种低成本、高活性和易规模化应用的无钯活化处理方法，对于化学镍钯金在印制电路行业的持续应用具有重要意义。除钯之外，镍、钴均能够显著降低次亚磷酸钠的催化氧化势垒，对于化学镀镍磷具有较好的活化效果。但是如何在铜表面高效制备镍、钴活化层成为一个难题。基于此，本项目提出了采用反置换沉积方法在铜表面制备镍、钴活化层的技术方案。根据能斯特方程，金属的实际电位随溶液中金属离子浓度、络合剂种类及含量和温度等的变化而变化，实际电位的变化可能会逆转置换沉积的方向。本项目发现，高浓度的碘离子或低共溶溶剂均能够显著降低铜的实际电位，并使其低于镍和钴的电位，使镍、钴在铜表面的置换沉积成为可能。研究发现，反置换沉积液的组成和工艺对铜表面置换沉积的镍、钴薄膜组分结构有着明显的影响。随着反置换沉积液中碘离子含量的增加，镍薄膜的沉积速率加快；同样，反置换沉积液中镍、钴离子含量的增加能够导致镍、钴薄膜的快速沉积。此外，沉积温度也会显著影响镍薄膜的反置换沉积速率，沉积温度越高，沉积速率就越高。当沉积液温度提高至95℃时，10分钟内既可在铜表面获得镍薄膜。对比研究铜表面置换沉积的镍、钴薄膜和传统钯薄膜作为化学镀镍磷活化剂的活化效果发现，镍、钴薄膜和传统钯薄膜表面沉积的镍磷镀层的形貌结构、致密度、厚度、沉积速率和抗腐蚀性能等基本一致，表明反置换沉积的镍、钴薄膜同样具有优异的化学镀镍磷活化效果。反置换沉积镍、钴薄膜有望替代钯薄膜成为一种新颖的印制电路铜线路表面化学镀镍磷前的活化途径。