Nowadays have witnessed wide applications of RF/Microwave PCBs in numerous handheld wireless devices and commercial industries including medical, communication, etc. Since RF (radio frequency)/Microwave circuits are distributed parameter circuits that tend to generate skin effect and coupling effect, interference and radiation in circuits are difficult to control in practical printed circuit board (PCB) design. The commonly occurred issues include cross-interference between digital circuit and analog circuit, noise interference caused by powers, and similar interference issues caused by absurd layout. As a result, how to balance advantages and disadvantages in PCB design and try to shrink interferences is a crucial aspect for RF/Microwave PCB design.
Every design differs, but experience plays an active role as a great teacher and the manufacturing engineer is capable of providing solutions to the major pitfalls. Detailed PCB design guidelines concerning RF/Microwave PCBs will be introduced and discussed in this article.
As an early stage in circuit design, PCB substrate material selection plays such a key role in RF/Microwave PCB design that optimal substrate material contributes to excellent performance and high reliability of end products. When considering substrate material in conformity with your PCB design, some aspects have to be focused like relative permittivity, loss tangent, thickness, environment etc. The following content will detail their significance and ideal selection approaches will be displayed.
Relative permittivity refers to the ratio between dielectric constant and vacuum permittivity. Relative permittivity of substrate materials applied for RF/Microwave PCB design must be sufficiently high to meet demands of space and weight. Other applications such as high-speed interconnect, however, call for extremely low relative permittivity to produce high-impedance circuits with acceptable line width and impedance tolerances.
In advance of final substrate materials determination, some parameters have to be confirmed including line width for a certain range of board thickness, wavelength of circuit working frequency and approximate dimensions of leading components. A sketch of circuit board diagram has to be drawn in order to establish acceptable maximum and minimum relative permittivity.
Moreover, relative permittivity deviation provided by substrate material manufacturer has to be low enough to make electric performance within a tolerance range.
Dielectric loss is a function concerning loss tangent and relative permittivity. As for some substrate materials, dielectric loss per unit length can possibly be offset by application of shorter lines that can reduce conductor loss as well, which is vitally important when conductor loss becomes obvious in high-frequency situation. Thus, when parameters of component loss in some circuits are being estimated, it is loss per unit length or frequency that is estimated instead of ordinary loss per unit line length under given frequency.
Within a certain frequency range, substrate material loss has to be low enough in order to meet input/output power requirements with heat dissipation issues avoided. Furthermore, power response of some circuit elements (such as filters) has to maintain a sharp frequency roll-off characteristic so that electric performance requirement can be met. Naturally, dielectric loss can impact this frequency characteristic.
Substrate material thickness is associated the following design elements:
Printed circuit board fabrication and operational environment keep constraints to substrate material selection. The main material performances that should be taken into consideration include:
Crucial high-frequency electrical characteristics contain characteristic impedance (Z 0), attenuation coefficient (α) and signal transmission speed (v). Characteristic impedance and signal transmission speed are determined by effective relative permittivity while signal loss by attenuation coefficient.
Among all possible transmission structures, such as stripline (definition of stripline will be introduced in Section a below), microstrip, bipolar pulse or groove, stripline and microstrip are most widely applied in microwave circuit design and generally depend on soft base material. For either stripline or microstrip, ratio between ground distance and conductor width, conductor thickness and distance between coupling conductors strongly influences characteristic impedance and attenuation coefficient. Within a certain frequency range and on a structure of transmission line, attenuation coefficient, relative permittivity and characteristic impedance may feature frequency dependability.
When cross-section size of stripline or mircrostrip is larger than wavelength in dielectric, other (higher) transmission mode becomes significant, which makes electrical performance of transmission lines weakened. As signal speed and frequency rise, dimensions of transmission lines must be proportionally decreased to avoid higher order modes, requiring application of thinner substrate materials with given characteristic impedance maintained.
Stripline is a transmission line structure, including a signal line and two wider grounds that are parallel to the signal line that is clamped between. The figure below demonstrates a typical stripline in a sectional view excerpted from IPC-2252.
Formula for characteristic impedance of stripline are classified into two aspects: narrow signal lines and wide signal lines.
Z 0 refers to characteristic impedance (ohms); ε r refers to relative permittivity; b refers to distance between grounds (m); w refers to signal line width (m). Value of Y caters to formula: In this formula, t refers to copper thickness (m).
In this formula, C f refers to fringing capacitance and conforms to the following formula:
When signal line is placed among grounds (or powers) but not at the central position, calculation formulae of stripline have to be modified. In the process of modification, it's necessary to coupling differences between signal lines and nearer and further grounds. If signal lines lie in the range of one third of center, deviations aroused by the assumption that signal lines lie in the center will be very small.
When coupling is highly required between signal lines, asymmetric stripline structure has to be depended, compromising two signal lines that are located in different surfaces and separated by dielectric. Coupling is carried out through parallel lines or crossing lines. When it comes to high-frequency circuit design, coupling isn't needed, the structure of vertically crossing signal lines doesn't work.
Microstrip is also a type of transmission line structure, including a signal line and ground that is parallel to the signal line.
Characteristic impedance formula of microstrip is based on a simple model of microstrip containing only one dielectric that is a conductor with none thickness. The formula goes like Formula 7 In this formula, formula 8, the second "0" and "1" after Z refers to denote zero conductor thickness and a type of dielectric. Thus, the accuracy of this model is better than 0.01% when the value of u is less than 1. When the value of u is less than 1000, the accuracy is better than 0.03%.
Among designing elements, dimension and tolerance design are vitally important. In field designing, bilateral tolerancing and true-position tolerancing are usually applied.
True-position dimensions and tolerances that are simply marked make manufacturers arrange deviations within positions and dimensions at any proportion, which usually leads to increased manufacturability. As a result, designers ensure functionality requirements and provide manufacturers sufficient freedom by which leading deviation can be arranged in the manufacturing process where accuracy is the lowest.
Position tolerance capacity mainly depends on material type, thickness and overall size of components. A true-position diameter of 0.254mm (0.01inch) is the most commonly seen and the easiest to be obtained. When tolerance requirement is required to be higher than 0.152mm (0.006inch), manufacturability will be compromised. When it is suitably required, however, the maximum material condition should be required to allow manufacturers to balance between aperture error and position error to increase manufacturability.
When a via is manufactured according to its minimum diameter, true-position tolerance is required to be used by maximum material condition that is simply marked. Nevertheless, via manufactured by a larger and acceptable diameter is usually positioned with lower accuracy, which still ensures to be fit and function. Thus, larger vias can get sufficient position tolerance, equal to acceptable adding value to diameter of minimum via. With extra tolerance added to true-position tolerance, inspection tolerance is generated.
When minimum material condition is applied, tolerance is established according to maximum diameter. "Regardless of feature size" refers to application of mark tolerance without extra tolerance and characteristic dimension tolerance is determined according to acceptable different manufacturabilities.
Although both true-position dimension and tolerance can be applied in any case that can be imagined, they are best applied to features similar to positions of hole, pockets and the other X and Y axi
