PCB007 Magazine

PCB-Jan2015

Issue link: http://iconnect007.uberflip.com/i/442619

Contents of this Issue

Navigation

Page 43 of 64

44 The PCB Magazine • January 2015 are impedance discontinuities). If a transmis- sion line runs parallel, and on top of a glass fi- ber strand, or along a resin-rich area between glass fibers, then the dielectric environments of these two lines are noticeably different, which unfortunately is the case most of the time as laminate is sheared parallel to the weave pat- tern, and conductor lines tend to run in the same direction. Angling the transmis- sion lines relative to the weave pattern will improve the situa- tion by randomizing the glass/ resin concentration differ- ences along the transmission path. References 2 and 3 have studied this problem area. Conductor Loss Metallic conductors are characterized by their ability to conduct current, or the re- verse—their resistivity. These resistivities can vary signifi- cantly. For example, the re- sistivity of copper is a low 1.7 μΩ.cm, whereas nickel has a resistivity of 7.4 μΩ.cm. Conductors with low conduc- tivity cause high conductor loss. Conductivity is propor- tional to the conductor cross- section area and is inversely proportional to the resistivity. However, this is only a first approxi- mation for the following reasons: • A D/C signal is uniformly distributed through the conductor, as is the signal of a low frequency A/C signal. However, as the frequen- cy of an A/C signal increases, the signal travels preferentially near the surface of the conduc- tor, a phenomenon known as the skin effect. Thus, the effective conductor cross section for high-frequency A/C signals is the "skin depth," not the entire cross-section area. Skin depth is inversely related to the square root of the fre- quency, the material permeability and the ma- terial conductivity. • Since the copper of the transmission lines are typically covered with a material other than copper (in the case of outer layers a final metal surface finish, and in the case of innerlayers a multilayer bonder), we must ask ourselves in which way do these materials affect conductor loss? This depends on their resistivity and the thickness of the coating. If we look at differ- ent final finishes that are in use, we note the following resistivities: Ag=1.6 μΩ.cm, Cu=1.7 μΩ.cm, Au=2.4 μΩ.cm, Ni=7.4 μΩ.cm, Sn=10.9 μΩ.cm, Sn/Pb=17 μΩ.cm, and electroless Ni (containing phosphorus)=55-90. This is- sue was addressed in the study of Reference 4. The author came to the conclusion that a thin deposit of immersion silver has no detrimental ef- fect on conductor loss. elec- troless nickel/immersion gold (ENIG), on the other hand, worsens conductor loss which can become an issue with long transmission lines that are found on backplanes. • A contributor to conduc- tor loss can be the topography of the conductor. Conductors with rough surfaces are lossi- er than smooth conductors. This effect becomes more pro- nounced at high frequencies. References 5 and 6 have inves- tigated this problem. Dielectric Loss An ideal insulator, or dielectric, would not have any conductivity and would not attenu- ate the signal strength. However, due to the material properties of the insulating material, there is some dielectric loss, or loss tangent, or dissipation of the material (dissipation fac- tor). The end result is a leakage current through the dielectric that contributes to signal loss. The bulk A/C conductivity of a dielectric, ac- cording to Ref. 1, is a function of the signal frequency, the permittivity of free space (E 0 ), the relative dielectric constant (E r ) or dielectric constant for short, and the dissipation factor (or loss tangent) of the material. The dielectric loss is basically caused by the dipoles present SIGNAL LOSS continues an ideal insulator, or dielectric, would not have any conductivity and would not attenuate the signal strength. however, due to the material properties of the insulating material, there is some dielectric loss, or loss tangent, or dissipation of the material (dissipation factor). " " karl'S tECh talk

Articles in this issue

Archives of this issue

view archives of PCB007 Magazine - PCB-Jan2015