Rogers Corporation’s latest low-loss RF material, Radix™, does not follow the traditional suite of the materials they sell in this class. Instead of being sold in fully cured, laminated sheet form and pre-metallized for subtractive PCB processing, this material is shipped as an uncured resin, with loaded additives to create the desired RF properties of a 2.8 dielectric constant and a 0.0043 dielectric loss tangent as measured at 10GHz, all while being 3D printable. Due to the high viscosity of this loaded photo-curable polymer and the tendency of the filler material to settle in the liquid resin, Fortify’s 3D print solution, Flux Core, is the only 3D printer on the market capable of printing this low-loss resin and can do so to high print resolution.
With the maturation of this printing process and several pure dielectric applications advancing to the commercial level, metallized parts are in the pipeline right behind these bare dielectrics. Fortify has partnered with Averatek to explore metallized parts and has performed several studies on the metallization of Radix material with very good results. With metallized parts, comes another portion of insertion loss on the device beyond just the dielectric loss of the material. Metal conductivity is the main driver of the ohmic portion of insertion loss. Averatek is capable of achieving near the conductivity of bulk copper with their copper deposition. This process is performed with traditional plating, using a proprietary selective seed layer, traditional electroless copper that becomes selective due to the seed, and electroplated copper to achieve traditional signal thicknesses, such as 1/4oz, 1/2oz, or 1oz copper, or thicker upon request. Surface roughness of the signal metal as facing the ground plane(s) of the transmission line is also a major driver of insertion loss. The first five skin depths of the copper against the laminate and how rough that copper is will play a role in insertion loss. Copper roughness is typically specified as Ra (absolute value of the average peak or valley height relative to the arithmetic mean of the roughness), Rz (highest peak – lowest valley), or Rq (RMS value of the peak and valley roughness). However, there is an additional parameter of importance. The periodicity of the roughness also plays a substantial role in how much the copper roughness affects insertion loss. The more stretched out the peaks and valleys of the roughness are, the less impact they will have on insertion loss as compared to perfectly smooth copper. The first step in characterizing the copper roughness as applied by Averatek to the 3D printed polymers from Fortify is to create simple transmission lines to characterize over frequency on a network analyzer. Microstrip structures were chosen for this purpose as a simple transmission line. The copper roughness to be characterized here will be mainly the copper surface facing inward to the dielectric. Since the copper is an additive process, it is expected that the roughness of the copper will be dictated by the roughness of the dielectric face that the copper is applied.