Application of a Silicon Oligomer Coating As an Alternative Treatment to Substrate Surface Activation By Etching
Authors: Sadohara Daisuke , Cordonier Christopher Ernest John 
Published: February 2020

In order to attain adhesion of plated metals to substrates, conventionally the substrate surface roughness is increased by etching, such that deposited metal can form physical anchors into the substrate surface. Traditionally, chromic acid has been used to roughen organic resin substrates, however, due to restrictions on hexavalent chromium use, permanganate etching methods have been developed. For roughening glass and ceramic substrates, hazardous chemicals like hydrofluoric acid or molten sodium hydroxide are employed. Once surfaces have been sufficiently roughened, ionic or colloidal palladium is adsorbed onto the surface, then activated by treatment with a reducing agent, followed by electroless plating to deposit a conductive layer. With the demand for finer circuit patterns in the printed circuit board industry, smoother surfaces are needed, and due to transmission loss by the skin effect, plating adhesion to smooth surfaces is required in high frequency applications. To resolve these problems for resin substrates, UV irradiation methods and ozone treatment have been investigated where reasonable adhesion to nanometer scale roughened surfaces have been reported. An alternative method for plating on glass by coating glass with Pd catalyst containing siloxane solutions have been investigated, however instability of Pd in the presence of siloxane and other alcohol solvents stifle industrial application.

Our group has been developing silicon oligomer compounds by cross-linking tetraethoxy silane with ethylene glycol (instead of water) for use in combination with acrylic resin as a coating agent for various materials with a variety of purposes such as corrosion prevention for bolts and nuts, PC case coloring, and solar panel cover glass anti-reflective coating. The cross-linking reaction is catalyzed by a metal salt, and in the ethylene glycol-silicone oligomer case, when catalyzed by PdCl2, a stable composition of Pd ion containing ethylene glycol-silicone oligomer solution was obtained. In this solution, the ethylene glycol cross-linker was believed to form coordination bonds with the Pd ion to stabilize it. However, since ethylene glycol can be oxidized to generate aldehydes, carboxylic acids and release electrons, then heating the coated film should result in reduction of the Pd ions present. Analysis of the Pd/Si-oligomer films revealed simultaneous formation of aldehydes and carboxylic acids with reduction of Pd2+ to Pd metal at 200°C. Evaluation in application to electroless Ni plating revealed no catalytic activity after the Pd/Si-oligomer films were heated at 50°C, catalytic activity but poor adhesion at 100°C, catalytic activity and sufficient adhesion at 150 and 200°C. Next, semi-additive plating and adhesion strength on glass (PD200) and glass reinforced epoxy (Ajinomoto Build-up film GX-T31) substrates according to the process illustrated in the attached figure. Stripping of the residual Pd/Si-oligomer was accomplished by dry etching followed by treatment in dilute HCl solution to remove residual Pd. On the glass substrate, a 20 mm thick copper deposits with 90° peel strength of 0.3 kN/m, and 0.5 kN/m on the epoxy substrate were attained. By adjusting the electroless plating conditions 0.5 kN/m adhesion strength was also attained for alkali-free glass and alumina ceramics. Observation of the plated film cross-section suggested the adhesion mechanism to manifest from a physical anchor effect of the plated metal film to the Pd/Si-oligomer layer. Evaluation of the insertion loss at 50 GHz1) showed no significant difference compared to the conventional dry metallization method on glass and lower loss compared to electroless plating on roughened ABF. Using the Pd/Si-oligomer as a surface activation treatment enabled plating on glass and epoxy resin with no etching or surface roughening procedure, and no requirement for chemical reduction or activation of the Pd catalyst prior to electroless plating.

1) Daisuke Sadohara, Hiroyuki Aijima, Younggun Han, Yoshihisa Katoh; 33rd Electronics Packaging Soc. Japan Spring Meeting; 2019 March 11-13th; Takushoku University, Bunkyo Campus, Tokyo

Figure 1