Randy Bancroft - Microstrip and Printed Antenna Design

Generally, the metal cladding attached to a dielectric substrate material is copper. Two types of copper foil are used as cladding, rolled foil and electrodeposited foil. Rolled foil is passed through a rolling mill a number of times until the desired physical dimensions are obtained and is then bonded to the substrate. Rolled copper has a polished mirror-like appearance. It is generally used for flexible circuitry. Electrodeposited foil is created by electrodeposition of copper onto an inert form, which is generally a rolling titanium or steel drum. A thin layer of copper is continuously removed from the form and then bonded to a substrate [].

Oxidation of copper traces is the primary cause of copper bond failure over time. The copper traces oxidize on both sides (top and bottom) of the trace. The diffusion of oxygen in the substrate material causes the bottom side of the trace to oxidize. Continuous ambient temperatures above 250 F will ultimately degrade the copper bonds on the surface of the board. The most effective way to mitigate this oxidation is to use an effective high temperature conformal coating. This coating will provide a barrier between the air and the surface of the resin and copper traces [].

The computation of characteristic impedance and losses of a microstrip transmission line (see Appendix C) depend on the copper foil thickness. The copper cladding is not generally presented as a thickness, but is indirectly derived from a provided mass over a given area. In Table ]. Thinner copper traces have smaller etching undercut than thicker traces. This allows for fine (narrow) traces as compared to thicker copper. Thicker copper has higher power handling capability. This is important where the small cross-section of a trace could produce heat build-up and eventual failure.

Table A.1 Metric copper foil mass and foil thickness

Table A.2 US copper foil weight and foil thickness

The surface roughness of copper can produce considerable loss at higher frequencies. The effect becomes significant when the skin depth of copper becomes less than or equal to the surface roughness of the copper cladding. Often a rough surface is used to improve the adhesion of the copper cladding to a dielectric substrate. The surface roughness of commonly available copper cladding is given in Table ].

Table A.3 Copper foil surface roughness

The skin depth of copper for selected frequencies is given in Table ].

Table A.4 Skin depth for copper with frequency

Dielectric substrates used to construct microstrip antennas (and radomes) may be grouped into plastics like fluoropolymers, such as Teflon [polytetrafluoroethylene (PTFE)] and cross-linked polystyrene (Rexolite 1422), ceramics, such as Al2O3 ( alumina) or silicon nitride Si3N4, and glasses, such as E-Glass, which are used in most Teflonfiberglass microwave substrates.

In some commercial applications, a planar antenna may be enclosed within an injection molded radome. The dielectric properties of the resins used are a very important consideration in an injection molded radome design. The resin should have known moisture absorption characteristics. Low moisture absorption is important in maintaining a stable resonance frequency and loss characteristics in environments which have humidity changes.