Hybrid Electroless-Electrolytic Process for Metallization of 3D Printed Functional Microdevices
Authors: Bernasconi Roberto , Cuneo Federico , Carrara Elena , Credi Caterina , Levi Marinella , Magagnin Luca 
Published: February 2020

3D printing is currently finding an increasing number of applications in many different fields. This technique, relegated for years to high-end rapid prototyping due to its high costs, is now acquiring industrial relevance for the direct fabrication of objects. The well-known advantages of scalability, production rate and the possibility to produce shapes otherwise impossible to manufacture with conventional techniques are at the base of the exponential market growth of additive manufacturing technologies. Such advantages apply not only to the production of macroscopic structures, a practice already well implemented in the current practice, but also to the manufacturing of microscopic objects.
3D printing of functional devices in the millimetric and sub-millimetric dimensional range can be challenging due to the high resolution needed, in the order of tens of micrometers. This result is possible using only some of the technologies available: stereolithography (SLA), two photons lithography (2PL) or selective laser sintering (SLS). However, all these techniques are characterized by the same drawback: the difficulty to work with metals. Metallic parts are used when the functional structures produced must interact with magnetic fields or must be electrically/thermally conductive. Only selective laser sintering (SLS) is able to work on metallic parts with sufficient precision, but typically the objects does not have the same properties of a bulk counterpart, either forged or machined.
A possible solution to get metallic properties on 3d printed objects is to impart them only to the surface via external metallization. Particularly advantageous from this point of view are the wet metallization techniques, thanks to their low cost and high productivity rates. The handling of very small parts is however difficult and, for this reason, suitable approaches must be developed.
An approach to the plating of millimetric objects have already been investigated [1]. In that case, Cu and a CoNiP alloy were applied to freestanding 3D printed microdevices using a fully electroless process. The approach presented is characterized however by significant drawbacks, the first one being that only electroless alloys can be deposited. As an example, high performances hard magnetic alloys like CoPt are impossible to deposit. A second disadvantage is connected to the plating rate: electroless deposition must be carried out at low temperatures due to the physical characteristics of the 3D printing resins, and this strongly limits plating rate.
Another possible approach to apply a metallic layer on a 3D printed microstructure is the application of an hybrid electroless/electrolytic metallization route. A first layer of a metal is electroless applied, according to previous literature [2], to make the surface of the resin conductive. Once a first layer of metal covers the surface, electrolytic deposition can be used to deposit the functional coating. This technique is described in the present work, where two methods are used to contact the object with the current generator. The first one is the exploitation of the 3D printing supports, generally used during the 3D printing steps, as electrical contacts. The second one is the use of a modified version of barrel plating. Functionality and performances of the two approaches are studied on prototypical microdevices printed via SLS. Finally, an applicative example is presented.
The method described can find application in many sectors, including the production of magnetically controlled micromanipulators [3] or the additive manufacturing of MEMS.
[1] ECS Meeting Abstract MA2016-01 1105
[2] R. Bernasconi, C. Credi, G. Natale, M. Tironi, F. Cuneo, M. Levi and L. Magagnin; ECS Trans. 72, issue 21 (2016) 9-21
[3] S. Kim, F. Qiu, S. Kim, A. Ghanbari, C. Moon, L. Zhang, B. J. Nelson and H. Choi; Adv. Mater.25 (2013) 5863-5868