PCB thickness has a major effect on controlled impedance, but that’s not the only influencer.
The electrical performance of a printed circuit board can be greatly impacted by how it is fabricated, especially at higher frequencies. High-frequency PCBs incorporate controlled-impedance circuit traces that require tight conductor etching tolerances and tight control of thickness. In addition, electrical losses must often be controlled at higher frequencies, and such losses can be influenced by fabrication issues, such as the application of solder mask and electroless-nickel-immersion-gold (ENIG) surface plating. Even cleanliness issues in PCB fabrication can have an impact on the electrical performance of the final fabricated board.
Higher-frequency circuits depend on tight control of transmission-line impedance, and a number of factors can contribute to variations in impedance. Some concerns may be more or less important depending on the circuit thickness. Many of these circuits are fabricated as multilayer PCBs, and a multilayer PCB will serve as a good example of how fabrication processes can affect the performance of a PCB. This example will use a multilayer PCB with the two outer-most layers formed as a high-frequency microstrip transmission line. In this example, it is a single-ended transmission line, with one layer containing the ground plane and one layer holding the conductive signal trace.
When fabricating a controlled-impedance circuit, such as the microstrip circuit example, the conductor width may not be as critical as other issues during fabrication, depending on the thickness of the circuit. Modern fabrication processes can control the conductor width within ±1 mil, and if the conductor width varies 1 mil for a thicker circuit, it will have less impact than the same variation on a thin circuit. Table 1 offers comparisons for a microstrip transmission-line circuit fabricated on two different thicknesses of a novel laminate. The top part of Table 1 is for 10-mils thick RO4350B laminate, while the bottom shows values for 4-mils thick laminate. A 1-mil conductor width difference for the thicker circuit makes a difference in characteristic impedance of about 2.8%, while the same difference in conductor width for the thinner circuit translates into a difference in characteristic impedance of 6.5%. If a 20-mil-thick version of the same type of laminate were used in this example, considering a change in conductor width of 1 mil, it would mean a 1.4% difference in characteristic impedance.
Table 1 covers much ground in comparing differences with two thicknesses of PCB materials, including a change of 10% in substrate thickness. Such a change has about the same effect on both the 4- and 10-mil laminates, about a 6.3% change in impedance. For the different circuit features being considered for their effects on impedance, the change in thickness, as a percentage, has some of the most significant effects regardless of the nominal thickness. Typically, if this portion of the circuit is made with a copper-clad laminate, there is better thickness control than if this portion of the circuit is made with prepreg and a raw foil lamination. For circuits requiring tightly controlled impedance, the use of the laminate may be a better choice than the prepreg-foil lamination.
Table 1 also details the effects of thickness in copper thickness, showing that it has a more significant effect for the thinner circuit than the thicker circuit. This is a trend that is followed by microstrip transmission-line circuits, as well as most other high-frequency PCB circuit types. For some circuit types with coupled features, such as grounded coplanar-waveguide circuits or differential-pair circuits, differences in copper thickness may have even greater impact on changes in impedance.
Table 1 also lists the effects of changes in dielectric constant (Dk) or, in the case of this table, in Design Dk, which is an average, practical value of Dk developed based on measurements under different operating conditions and meant to be accurate when used as the value for Dk in software simulation tools. Changes in Dk or in Design Dk often receive much more attention than necessary when troubleshooting circuit performance issues. For the thicker circuit, a change of 0.1 for the Dk value has the least impact on impedance, considering the effects of changes in the other circuit features, and the same can be said for the thinner circuit. In this example, a Dk difference of 0.1 is the total range possible for the studied materials; other PCB materials may exhibit a far wider range in Dk values and greater impact on circuit performance. (Information contained in Table 1 was generated from an impedance-loss-modeling software tool from Rogers Corp. called MWI-2013, available at rogerscorp.com/acm/technology/index.aspx.)
Insertion loss is the total electrical loss exhibited by a high-frequency circuit, resulting from a number of different loss components. These loss components include conductor loss, dielectric loss, radiation loss, and leakage loss. Of these losses, RF leakage loss is typically insignificant for most PCB materials and applications due to the high-volume resistivity of the material. Radiation loss can be difficult to evaluate, since it is related to the circuit design, thickness, frequency and material Dk. For the purposes of this discussion, radiation loss will not be considered as a significant loss component. But dielectric loss and conductor loss will be considered significant loss components, and will be covered in greater detail below.
Dielectric loss is related to a PCB material’s dissipation factor or tangent delta. Basically, a higher dielectric loss will cause higher circuit insertion loss, although that depends on the circuit thickness and the effects of conductor loss.
Conductor loss is based on many variables and is often the culprit of unexpected increases in insertion loss when troubleshooting RF circuit performance. Conductor loss depends on substrate thickness, copper surface roughness, conductor width, conductor thickness, circuit design, and finish. Assuming insignificant radiation loss, a simple way to consider insertion loss is to see how much dielectric loss and conductor loss contribute individually to insertion loss. In a thick microstrip circuit, say 60 mils thick, where most of the circuit is dielectric material, conductor losses will have very little impact. For a thinner microstrip circuit, however, when the dielectric material is less than 10 mils, conductor losses are more significant and dominate the overall insertion loss. In a yet thinner circuit, such as only 5 mils thick, conductor losses and electrical properties associated with conductor losses will dominate. In general, when considering a low-loss RF/microwave circuit material, a material with a circuit thickness of about 20 mils will offer an even contribution between dielectric loss and conductor loss.
Figure 1 compares the example circuits and plots how dielectric and conductor losses add to insertion losses. Figure 1 also shows the impact of using novel smooth and rough (standard electrodeposited) copper surface treatments on the 4-mil circuit. The rougher copper surface results in higher conductor losses.
Figures 1a and 1b compare different thicknesses of the same circuit substrate material, showing the impact of conductor loss. The 10-mil-thick circuit in Figure 1a shows that the conductor loss is slightly higher than the dielectric loss, with the two types of losses adding to yield the total insertion loss. The thinner, 4-mil-thick version of the same substrate material in Figure 1b shows how conductor loss dominates and is a much greater component of the insertion loss.
A comparison of Figures 1b and 1c shows the effects of copper surface roughness for the same 4-mil-thick substrate type. The same copper weight is used in both cases, but smoother copper is used in the 4-mil laminate of Figure 1c. The insertion loss for the 4-mil-thick laminate with smooth copper (Figure 1c) is much less than for the 4-mil-thick laminate with standard copper (Figure 1b). Since there is no significant difference in dielectric loss, the main component making the difference in insertion loss is the difference in the copper type or specifically the copper surface roughness.
Copper is a good conductor, although copper roughness can impact loss performance, but other issues can impact PCB conductor losses, such as surface finish. ENIG finish is often used in the PCB industry, and it is a very good finish for many reasons. However, this nickel layer has only about one-third the conductivity of copper, and thus will increase the conductor loss of a high-frequency circuit.
Even gold, which is considered a good conductor, is not as conductive as copper and can contribute to circuit conductor losses. The thickness of the nickel/gold layer will also play a role on which frequencies are affected by losses due to skin effects. Skin effects refer to differences in current density for conductors at different frequencies. As frequency increases, the current density will occupy less of the conductor. But when using a lower conductivity metal, the skin depth will increase, so a nickel layer will force the current to reside in this layer more than might be expected with a copper layer.
Since an ENIG finish covers the outer three surfaces of a microstrip conductor, as seen in a cross-sectional view, it does not have an impact on the electric fields directly between the signal and ground plane layers. But an ENIG finish does have an effect at the bottom edges of the conductor, an area that will typically have higher current density.
An exception to the circuit finishes typically used in the PCB industry is silver. Pure silver is more conductive than pure copper. Although the silver used in the PCB industry is not pure, it does offer very good conductivity, with very little impact on conductor losses when applied to copper circuits as a finish. Figure 2 shows microstrip transmission line circuits with different finishes, measured for insertion-loss performance. It also shows the effects of solder mask applied to bare copper on a 20-mil-thick circuit laminate.
The loss differences associated with the metal finishes are related to conductor loss. However, the loss due to the solder mask is related to dielectric loss. A microstrip circuit operates with electric fields in the substrate material and in air above the signal conductor; the loss in the air is lower than that in the material. When the signal conductor is covered with solder mask, air is used less, and the dissipation factor of the solder mask adds to the dielectric loss, which ultimately adds to the insertion loss.
As expected from the details of Figure 1, the impact of insertion loss due to increases in conductor loss with an applied finish will be greater for a thinner circuit than for a thicker circuit. This is reinforced by Figure 3, using a low-loss, 10-mil-thick material, where the insertion losses of circuits are compared using bare copper and with an ENIG finish.
There are other aspects of circuit fabrication that can be difficult to characterize regarding impedance and loss variation, and one of these issues is circuit cleaning. Circuits undergo many different wet chemistry processes during fabrication; having a thorough chemical cleaning of a circuit is necessary for many reasons. Circuit cleanliness is typically very difficult to detect because it generally concerns a residue that cannot be seen, but may have some free ions associated with it. If ions are concentrated around the edges of a conductor, they can cause the electric fields to extend beyond the edge of the conductor. Extension of these fields can cause a circuit to behave as if its conductor was wider, with a corresponding change in effective impedance. The ions can also cause an increase in loss, since the ion path will exhibit low conductivity compared to a copper path.
To demonstrate this concept, a simple experiment was performed, where circuits were processed through a typical sulfuric acid bath, and then one circuit was cleaned very well and the other circuit only partially cleaned. The partially cleaned circuit still had sulfuric-acid residue; however, this was not visually obvious. The circuits were 10-in.-long microstrip transmission-line circuits. Insertion loss measurements were taken, and the results are shown in Figure 4. As can be seen, cleanliness translates to lower loss.
In summary, many fabrication issues can affect the electrical performance of a high-frequency PCB. The thickness of the circuit board plays a major role in different PCB issues related to controlled impedance, with a thin circuit more susceptible to impedance variations caused by normal circuit fabrication processing than a thicker circuit. PCB material thickness can also influence the amount of circuit losses exhibited at higher frequencies. And circuit cleanliness can play a role in circuit performance, although this can be difficult to characterize.
Ed.: John Coonrod will present “Electrical Challenges for PCB Millimeter-Wave Applications” at PCB West at the Santa Clara (CA) Convention Center this month (pcbwest.com).
Have you registered for PCB West, the Silicon Valley's largest printed circuit board trade show? Sept. 24-26, at the Santa Clara Convention Center. www.pcbwest.com