The Characterization of the Stannous Chloride/ Palladium Chloride Catalysts for Electroless Plating
E. Matijević,1 A.M. Poskanzer2 and P. Zuman1
1Institute of Colloid and Surface Science, Clarkson College of Technology, Potsdam, NY
2Shipley Co., Inc.. Newton, Mass.
The 1976 Carl E. Huessner Gold Medal Award for Best Paper appearing in Plating and Surface Finishing in 1975
Originally published as E. Matijević, A.M. Poskanzer and P. Zuman, Plating and Surface Finishing, 61 (10), 958-965 (1975).
Editor's Note: This paper is part of a series on the AES/AESF/NASF Best Paper Awards. In 1976, Dr. Egon Matijević and co-workers received the Carl E. Huessner Gold Medal Award for Best Paper appearing in Plating and Surface Finishing in 1975. A printable PDF version is available by clicking HERE.
Sixteen mixed PdCl2/SnCl2 catalyst compositions for electroless plating have been studied by electron microscopy, ultracentrifugation, polarography, light scattering and tested for catalytic functionality. A number of these were commercial samples, whereas the others were prepared as described in the patent literature. According to the authors, whose procedures were followed, some of the catalysts were claimed to be colloidal sols and some others were claimed to be true complex solutions. It is shown that all active catalyst compositions contained colloidal particles which could be separated by sufficiently long ultracentrifugation, and depicted by the electron microscope after direct deposition on grids. Functionality tests proved that all bottom layers separated by ultracentrifugation, containing the colloidal particles, were exceedingly efficient catalysts in electroless plating. None of the supernatant liquids, regardless of their coloration, showed catalytic activity.
One of the prerequisites for the electroless deposition of metals is the application of a catalyst to metallic (conductive) or non-metallic (dielectric) surfaces which are desired to be plated. The most commonly used catalyst systems consist of PdCl2 and SnCl2 in acidic (HCl) solutions. Originally, a two-step procedure was employed in which the substrate was first sensitized by immersion in an acidic tin (II) chloride solution followed by activation in a palladium chloride solution.1 Actually, Pearlstein2 showed that PdCl2 solutions alone applied at 125°F can catalyze a variety of substrates for electroless nickel deposition over the pH range 3.8-4.8, but a pre-dip in SnCl2 broadened the pH range (0.9-4.2) and allowed plating at room temperature.2 The two-step process had its deficiencies. For example, the copper clad substrate when immersed in a PdCl2 solution produced a flash coating of palladium metal which proved to be very expensive. Also, the flash coating was only weakly bonded to the substrate and, therefore, the bulk of it had to be removed by sanding or buffing to achieve a better bond in electroless plating.
A significant improvement in the catalytic process was the development of mixed PdCl2/SnCl2 catalytic systems which allowed the combination of the sensitization and the activation steps by immersion of the substrates in a single bath (one step catalyst). Essentially, all of these catalysts consist of dark liquids, containing high concentrations of HCl, in which various amounts of PdCl2 and SnCl2 were dissolved using different procedures. The first of these systems was described in a patent by Shipley3 in which it was claimed that the catalytic solution contains colloidal particles, which, upon adsorption on the substrates provide the catalytic sites for the reduction of metals, such as nickel or copper, to be plated out. Additional patents have been granted for PdCl2/SnCl2 catalyst systems which are supposed to contain exceedingly uniform colloidal particles of great stability.4
Interestingly, several other patents, aimed at improving Shipley's process, make the claim that the palladium/tin catalysts are true solutions and that their activity in electroless plating is due to the presence of complexes of various stoichiometric compositions.5,6
In view of the fact that all of the mentioned catalytic preparations3-6 contain essentially the same starting materials (PdCl2, excess SnCl2, HCl), and often differ little in the methods of preparation, a number of investigators have devoted a considerable amount of effort in order to decide whether the catalytic activity in electroless plating is due to colloidal particles or to some kind of Pd-Sn solute complexes. However, this has not resolved the "colloid" vs. “complex solutions” controversy, as advocates for either concept can be found. Thus, ultracentrifugation, electron microscopy and Mossbauer spectroscopy work by Cohen, et al.7-9 led these authors to conclude that the PdCl2/SnCl2 acid catalyst systems indeed are colloidal. A number of Japanese investigators arrived at the same conclusion.10,11
Contrary to this, Rantell and Holtzman claim that mixed PdCl2/SnCl2 catalysts are non-colloidal, and that the active components are solute complexes of the type SnPd7Cl16,12,13 although these findings have been challenged by Feldstein, et al.14 In their most recent papers, Rantell and Holtzman concede that commercial catalysts are indeed mixtures of solute complexes and colloids, but they maintain that the complexes are the more active components.15,16 The catalytic activity of soluble tin-palladium complexes has also been suggested by deMinjer and Boom.17
Thus the question as to whether the active catalytic component in the PdCl2/SnCl2 activator systems is colloidal or in the form of soluble complexes, or if it varies depending on the method of preparation, has not been resolved as yet. The ambiguity of the seemingly "simple" question as to whether a system is colloidal or non-colloidal* is caused by the fact that the catalysts under consideration are exceedingly dark colored solutions containing high concentrations of electrolytes. Furthermore, the techniques employed have not always been appropriate, and as such may have resulted in erroneous conclusions. For example, light scattering (more specifically the presence or the absence of a "Tyndall cone") has been commonly used as a test for the presence of colloidal particle. The appearance of an intense Tyndall cone certainly indicates the existence of colloidal particles, but colloidal systems may look optically clear, if there is no refractive index difference between the particles and the medium, and in this case a wrong conclusion may be drawn from light scattering effects.
Another common shortcoming of the cited studies is that most of them dealt with catalyst systems after their application to the substrate; a few investigations only concerned the catalyst liquid itself. By using appropriate techniques (e.g., ultracentrifugation), Cohen, et al. did establish the presence of colloidal particles in catalytically active solutions.7-9
Finally, in some instances the question of colloidal vs. non-colloidal nature of the catalyst has led to "compromise'' statements which have no scientific foundation. For example, D’Ottavio talks about a "semi-colloidal" part of the catalyst.4 Obviously the "semi-colloidal" state has no physical meaning, and its introduction only adds to the confusion.
This work has been carried out to establish whether there are any differences between various formulations of mixed PdCl2/SnCl2 catalyst systems for electroless plating, and whether these differences are related to their state of dispersion, i.e., colloidal vs. true solution. Furthermore, it was intended to determine whether the colloidal fraction (if such existed) is catalytically active, or if the true solution components play the role of sensitizers and activators. For this purpose a variety of catalysts was studied; some of these were commercial samples whereas others were prepared by procedures which should either give "colloidal" or "true solution" systems. Special care was taken that the methods employed did not cause any changes in the systems while these were analyzed, so that no artifacts were introduced.
Table 1 - Results of tests with various PdCl2/SnCl2 catalysts.