Dynamic Liquid Drop/Meniscus: A New Route to Electrodeposition
Authors: Balucani Marco , Quaranta Simone 
Published: February 2020

Dynamic Liquid Drop/Dynamic Liquid Meniscus (DLD/DLM) is a patented technology suitable for localized chemical and electrochemical surface treatments. Indeed, DLD/DLM is an original and highly innovative tool able to continually renew the chemical solution used to produce selective chemical processes and treatments on a wide range of substrates and materials allowing very fast electrodeposition.

High-speed electroplating is usually performed in the mass transport-controlled regime [1]. Therefore, the limiting current density (Jmax
)is given by: Jmax=nFD/(δCb)

Where D is the diffusion coefficient of the deposited species, n is the number of electrons per ion, F the Faraday constant, Cb the bulk concentration of the ions in the solution and δ the diffusion layer thickness. Under mass control conditions, metal ions are reduced as soon as they reach the electrode. Conversely, current densities higher than Jmax
result into hydrogen evolution and lead to "burnt" and porous deposits [1]. Multiple approaches are employed to decrease the diffusion layer thickness (δ) and, therefore raise the limiting current density [2]:

Improving the hydrodynamics (i.e. the flow velocity) of the electrolytic solution (e.g. jet plating, rotating electrode, ultrasonic agitation)

Exploiting thermal effects stemming from Brownian motion and laser plating

Modifying the electrodes’ current distribution through Pulse plating, and by tailoring the anode–cathode configuration

Introducing additives into the electroplating bath formulation.

Feasibility of copper electroplating with deposition rates as high as 50µm/s (corresponding to a 150A·cm-2 current density) have been proven by using laser jet-electroplating [3]. Furthermore, deposition rates of µm/s combined to high cathodic efficiency (e.g. > 80%) could be achieved through electrolyte flows in the m/s range[4].

The present work puts forward DLD/DLM systems, relying on special 3D nozzles, for two technologically-relevant electronics application:

Mask-free metallization of HJT (i.e. Heterojunction) solar cells [5] by PRP (Pulse Reverse Plating) techniques. Electrolyte flow rates ranging from 0.1 to 2 m/s and electrodeposition rates between 1 and 5 µm/s can be attained (see figure 1.A).

Deposition of highly uniform and homogeneous multicomponent alloys by taking advantage of sequentially arranged DLD/DLM plating heads (i.e. multilayer method). Lead-free solders (e.g. 305 and 405 SAC) were deposited at 0.8 micron/s from single-component electrochemical baths (see Figure 1.B) [6].

In addition, nanostructured composite materials comprised of multilayers containing up to 10-15% of nanoparticles can be fabricated by means of coalescent DLD/DLM (a variant of the original technique). Thus, new methods aiming at enhancing the performances of anti-corrosion coatings can be developed from the DLD/DLM. Furthermore, chemical treatments other than electrochemical deposition such as galvanic displacement and electroless coating can be easily implemented by the DLD/DLM approach.

The presentation will provide a detailed description of the new sheet-to-sheet automatic DLD/DLM prototype equipment (see figure 1.C) with different 3D nozzles. Moreover, applications including a three-step metallization process for solar cells, multilayer deposition of two, three or four components lead-free solders, and nanostructured composites incorporating nanoparticles (e.g. SiO2 or Teflon). Finally, DLD/DLM capabilities in terms of speed of plating, flexibility and versatility will be also illustrated. In fact, the relatively small amounts of plating bath (typically 2-5 L) in conjunction with the possibility to adapt the technique to the coating of a broad range of surface areas (from tens of square microns to thousands of square centimeters) may bring about remarkable cost reductions in the electronic devices manufacturing and open a new route to electrodeposition.

[1] F. C. Strong "Faraday's Laws in One Equation". Journal of Chemical Education. 38 (2) (1961) 98

[2] M. De Vogelaere, V. Sommer, H. Springborn, U. Michelsen-Mohammadein, "High-speed plating for electronic applications", Electrochimica Acta 47 (2001) pp. 109–116

[3] R. J. von Gutfeld and D. R. Vigliotti, "High-speed electroplating of copper using the laser jet technique" , Appl. Phys. Lett. 46, (1985), pp. 1003-1005

[4] L. J. J. Janssen "High-rate electrochemical copper deposition on bars" J. of Applied Electrochemistry 18 (1988) 339-346

[5] M. Balucani, S. Quaranta "A Breakthrough in Plating for Solar Cell Metallization" AiMES 2018, Tuesday, 2 October 2018: 14:40 Abstract MA2018-02 818

[6] M. Balucani, S. Quaranta "A Breakthrough in Pb-Free Solder Electroplating" AiMES 2018, Tuesday, 2 October 2018: 10:40 Abstract MA2018-02 1128

Figure 1