Keywords: flexible, electronics, metallization, contact electroplating
Summary:Contact Electroplating Technology (CET), a novel direct contact electroplating technology, enables electrochemical deposition of desired metals or alloys on patterned flexible substrates by direct uniform contact with the substrates. The capability to electroplate metals is quite wide-ranging in the sense it enables depositing selected metals over substrates having the conductivity in the broad range of 1e-5S to 1.59e+8S. The article describes general features of the technology and some examples of its application in flexible electronics. Metal pastes such as Silver or Copper paste and Carbon pastes are applied by various printing techniques such as screen printing or jet printing over plastic substrates such as PET (milar) and others to form flexible electronics. To achieve adequate conductivity, the Silver or Copper pastes must be applied in significantly thicker layers, tenfold or more, than a pure metal. This makes it quite unrealistic to achieve high resolution and/or high density interconnects. Recently efforts have been made in the area by application of suspensions of Ag nano particles followed by sintering of the same to establish better quality interconnects. While providing conductivity superior to pastes the process involves lengthy thermal treatments at temperatures of several hundreds of centigrade, thus limiting the practicality and simultaneously increasing demand for extending the limits of the polymer substrate performance. EPT technology offers the capability to electroplate desired pure metals on microstructures of limited conductivity. Thus, for example, in the presence of a lasered or toner/carbon printed low conductivity isolated patterned seed layers, e.g. several Kilo-ohm cm, EPT provides the capability to electroplate several microns of metals such as Silver or Copper to convert the high resistance seed layer to a highly conductive few microns electroplated pure metals instantly. An example of the EPT device is a sandwich comprised of three components; (1) Counter-electrode, perforated or expanded mesh Pt/Ti, (2) Open porosity wettable chemically stable polymer and, (3) Fine highly conductive Metallic Fiber Cloth, web or metal Mesh (FC/M) While in operation the FC/M of the EPT sandwich is pressed against the seed layer or any other isolated pattern on the substrate submerged in electroplating solution. Power supply provides the electric field between the Counter-electrode and the FC/M via electrolyte soaked porous polymer. Since the FC/M is negatively charged it will be exposed to deposition of the electroplated metal. Furthermore, since FC/M is in close uniform contact with the seed layer of the patterns it will impose the negative polarization of the pattern causing the electroplating process of desired metal. Naturally, in above process both the FC/M and the substrate are electroplated. If not limited this effect may cause accumulation of metal at the FC/M and thus reduce the efficiency of the device. In the article, we will introduce various applications of the EPT on PWB and flexible electronics substrates as well as several approaches that successfully eliminate the problem of metal accumulation on FC/M.