This calculator computes the inductance of a microstrip etched on a PCB, which according to the ARRL handbook is: L= 0. Trace capacitance calculation. 008. Image courtesy of Analog Devices. 048 for the visible traces and 0. PCB Traces. How do you calculate PCB trace inductance? The inductance of a PCB trace can be approximated using various formulas, such as the Wheeler’s formula: L = 0. A copper Thickness of 1 oz/ft^2 = 0. The stripline impedance calculator provided below is useful for gaining an initial estimate of trace impedance for striplines. Inductance of Straight Wire – from Length Calculator. 035 mm, we have a PCB trace width of w = 0. The approximate formulas used to calculate the inductance are provided along with restrictions on the relative size of the various parameters. Calculates the current a conductor needs to raise its temperature over ambient per IPC-2152. This delay results in timing errors, data skew, clock, and data mismatches, and causes reliability issues. Current and heat considerations: Depending on which. For example, a 2. As an example, assume DLY is 12 ps. Take note that this assumes that the distance between the conductor and the planes is not the same for both reference planes; hence the asymmetric term. It's actually possible to determine these parasitic values using impedance calculator tools, as well as some analytical formulas in the literature or field solver utilities (Ansys, COMSOL. PCB trace calculator is a crucial part of PCB assembly. Where I is maximum current in Amps, k is a constant, dT is temperature rise above ambient in °C, & A is cross sectional area of trace mils². Graph Paper. Spread the love Trace Capacitance Calculator Trace Capacitance Calculator Trace Length (in meters): Trace Width (in meters): Dielectric Constant: Calculate Capacitance FAQs What is the capacitance per inch of a trace?. A A = coil area (m 2)How to find the inductance of a PCB trace? Trace inductance increases with trace length and lack of ground plane. 8mm (0. Don’t Use a Via Inductance Rule of Thumb. It is common to find the broadside coupled trace in printed circuit boards where differential pair signals are routed. The formula for capacitance of two planes is: Using 4. Inductance is a property of an electrical circuit that describes the amount of energy stored in the magnetic field created by the current flowing through the circuit. Stripline Impedance. Broadside coupled traces are on adjacent planes and the return is. Unit: Options are mm, mil, μm, and. For. The source for formulas used in this calculator (except where otherwise noted) is the Design Guide for Electronic Packaging Utilizing High-Speed Techniques (4th Working Draft, IPC-2251, February 2001. What is typical PCB trace inductance? Typical PCB trace inductance can vary depending on the trace geometry, but as a rough estimate, it might range from a few nanohenries (nH) to a few microhenries (µH) for typical PCB traces. 15 0. Calculations provided on this page are a simplified subset from EMI Analyst models. (1) where w = the width of the rectangle (wide dimension) h = the height of the rectangle (short dimension), and a = the wire radius. Inductance is the ability to store energy in a magnetic field, and coils are a common way to create inductance. A PCB structure shown in Figure 1, contains two copper traces on a 4-oz FR4 PCB square board and a 5 mils thickness copper Ground. Ohm’s Law provides the framework for solving network analysis problems; when the curtain gets pulled back, Ohm’s Law updates to become the relationship between voltage, current, and impedance, not resistance. 0: 0. The inductance of the wire loop. interference coefficient = 0. – Required inductance. The KiCad PCB Calculator is a set of utilities to help you find the values of components or other parameters of a layout. Get accurate measurements with Keysights premium oscilloscopes – Source: Keysight. The equation is universal to microstrips, striplines, or any other pair of coupled lines with matching geometry. The PCB Impedance Calculator in Altium Designer. 5x would be best, but 2x is acceptable. In other words, this is the impedance the signal actually experiences as it travels on an individual line. A + jX. Impedance Calculators. At lower frequencies, current flows in a path that reduces I^2*R losses. I can choose the capacitor value, but the inductance is determined by the PCB trace, via structure, stackup thickness, etc. Figure 3. o use this calculator, enter the parameters of the regulator . Precise on-line inductance calculator for PCB inductors with rectangular and round. . This procedure produces the result ( w/h) = 1. 9E-3 [1/°C], T TEMP is the trace temperature [°C]. = 1. This calculator computes the characteristic impedance of a twisted-pair cable based on its dimensions. 23) Reverse Series and Parallel resistor calculations. Now lets increase the width of the board outline from 0 to 0. Measuring the parasitic inductance and capacitance in a PCB is extremely difficult. PCB Trace Width Calculator. Take a 1. Via inductance refers to the amount of inductance introduced by a via in a PCB. Electromagnetic software to to simulate various parts and components used in hybrid and electric vehicles, such as battery, charger, antenna, and electric motor. In this PCB calculator, microstrip line of capacitance per unit length is expressed in F/m. 2H, the formula for the non-embedded microstrip should be. Note: µ0 = 4¶·10 -7 [Hm -1 ] (Absolute Permeability)The wavelength of the signal that can propagate effectively inside a microstrip is called the guided wavelength. Stick to these design rules when you’re designing your stack-up. This online electrical calculator helps you determine the microstrip crosstalk on PCB based on the trace spacing and substrate height. If PCB Designer can estimate the inductance of the traces between IC and capacitor, then I can calculate the resonance frequency, and make it the same as the clock frequency. By understanding the microstrip transmission line, designers can. Enter the relative permitivity, width of the trace, the ground plane spacing, and the substrate thickness to calculate the characteristic impedance. 949. Traces such as these are found on adjacent planes with the return being identical to the trace both in length and in. . If you care about the inductance of the inductor (rather than the inductance of the circuit) then do your best to keep those to a minimum, at least by using the shortest possible wires. Exaple reflection and crosstalk data in a routed net. Figure 9 Shorten the trace between gate and driver output 4. Trace Width and Trace Space to the minimum permitted. Figure 1 Cross section of a PCB 1 E. LOGIN. Single layer Planar spiral coil inductor calculator : The first approximation is based on a modification of an expression developed by Wheeler; the second is derived from electromagnetic principles by approximating the sides of the spirals as current-sheets; and the third is a monomial expression derived from fitting to a large database of inductors (and the exact inductance values). Description. For an embedded (internal) microstrip, the formulas provided in paragraph 4. When in doubt, use 1 for copper, . Boyd and T. Where, d = via barrel length in inches. 5 voltage drop in signal leads—"kelvin feedback" 12. Specific designs may require adjustments based on factors like temperature rise and copper weight. ) coils, by setting the number of turns per layer, NT to 1 and setting the. A). As the coil is suddenly demagnetized, the. 3 ohms. 5 nH, which is consistent with typical trace inductance values. This can be seen nicely in the frequency-domain results, which are calculated with a Fourier transform. 9 r r r0----- = ln 4. A given trace may behave as a transmission line under some conditions while behaving as a simple conductor in other conditions. It should also be noted that blind and buried vias involve a higher production cost, so it is advisable to plan their use in advance in order to respect the budget allocated to the PCB. Pokud zjistíte, že logaritmické hodnoty jsou menší než 1, bude indukčnost stopy PCB. and a PCB Trace. In English units, ( h, s, and r in inches, L in nH) the constant μ/2π equals 5. 22 PARASITIC EFFECTS IN INDUCTORS 12. The amplitude of this crosstalk is what this tool calculates. Inductance Calculator. Where, Area = Thickness*Width. Microstrip inductance calculator. As rise times increase, the resulting impedance becomes more noticeable. Place the ground plane 1 dielectric away from signal and power planes. This decreases inductance and should be as wide as possible to reduce resistance. The surface finish is neglected. This article provides an in-depth look […]Identify the coil's physical characteristics, including the number of turns (N), the length (ℓ), and the area (A). Use this calculator to find the inductance of a multi-layer coil. The characteristic impedance (Z o) of coplanar waveguide with ground or microstrip lines with signal side ground plane can be. A very quick and helpful tool. The PCB Designers specify the copper plating weight in oz. Eq. Different board materials, the dielectric layer thicknesses and constant (Dk), and the thickness of the metal traces all need to be part of the calculations. Usage example. At low frequencies, where the impact of stray capacitance C. 5+0. Calculate trace width values based using the IPC-2221 charts by providing inputs such as current thickness and temperature values. 8pF per cm ˜ 10nH and 2. Conductors are assumed to be separated by a homogeneous medium having permittivity and permeability . ESL effective. A = X. The inductance of a PCB trace determines the strength of any crosstalk it will receive. Introduction A coil is a common electrical component that provides inductance in circuits. E. h is the height of the via. Trace Inductance calculator for wide traces over a ground plane with trace width (W) much larger than substrate thickness (T). These traces will have different impedance if they are driven in odd-mode or even-mode. 3 can then be used to design a PCB trace to match the impedance required by the circuit. See moreThe inductance of a flat or ribbon wire or a track on a PCB can be calculated with the following equation. You now have a total inductance of x 2 x 2; i. Online calculators will generally use. 3 Calculate the PCB trace inductance, or "L," using the formula L = Zo * DLY. If you’re designing with a mechanical relay, back EMF is a concern when the relay is released. The velocity of pulse travel along the transmission path is r. The IPC-2141 trace Impedance calculator will help make initial design easier by allowing the user to input basic parameters and get a calculated impedance according to the IPC. Calculates the characteristic impedance and per-unit-length parameters. 5. Vias (plated holes) are used to route electrical signals among different layers of a multilayer. External traces: I = 0. The formula that is most commonly used to calculate propagation velocity of an electrical signal on a trace is: On typical PCB material we get the rule of thumb values at Er=4, we have about ~15cm. These L and C are inductance and capacitance per unit length. The capacitance calculated using this tool is the capacitance between a flat conductor and two equidistant ground planes. Calculate the PCB trace inductance, or “L,” using the formula L = Zo * DLY. This is the path of shortest distance and is. The default units for all given values, except the subtrate. If. How Police Trace Cell Phone Location by Mobile Number or IMEI number. Enter the relative permitivity, width of the trace, the ground plane spacing, and the substrate thickness to calculate the characteristic impedance. PCB Coil Calculator. There are many demands placed on PCB stackup design. • Circuit traces on a PCB with a ground and power plane will be about 1−3 pF/in. Even more elaborate approximations are included in Brian Wadell's book, 'Transmission Line Design Handbook' ISBN: 0-89006-436-9. A picosecond is 1 x 10^-12 seconds. External Layers in Air Required Trace Width: Resistance: Ω Ohms. An open-source tool that allows for a fast and precise analytical calculation of multi-layer planar coils self-inductance, without any geometry limitation is proposed here. is negligible, L. References. 24 pH, where pH is units of picohenries. Resistance: Ω Ohms. The calculator then uses a formula to calculate the width and spacing of the trace or transmission line required to achieve the desired characteristic impedance. The PCB antennas and the high-frequency circuit make up the complete RF system of the high-frequency PCB. edu;Table 1: Connection inductance for typical capacitor configurations. Calculations are based on formulae from IPC-2221. formulas to calculate the self inductance L. Layout designers will use a PCB impedance calculator to solve the electromagnetic field and deliver accurate trace variables for fabrication. Something you need to know before calculating trace width. 54. Here’s a simple guide to PCB trace inductance and sizing trace widths for your board. 002 * (w/d) * (ln(2w/d) + 0. To use it, first select the tolerance of the resistor: 10%, 5% or equal or smaller than 2%. Overview. 048 x dT0. Abstract. In this case, there will be both an IR component due the small trace resistance (RM) and an L*dI/dT term due to the trace inductance (LM). 2). Although it may sound simple to calculate the impedance of a PDN, there is no single equation that can be used to calculate the impedance of the PDN in a PCB based on geometry and material properties. If the line width is equal to the spacing from the ground plane, then the total inductance of a PCB trace is close to 10 nH/inch. In these PCBs, the PDN should be arranged with at least one power/ground plane pair placed closely to. This Javascript web calculator calculates the resistance, voltage drop, and power loss of printed circuit board vias. Stack Exchange Network. The self inductance of a single wire in free space is defined below. According to script, this inductor would have inductance of ~3. For very large current signals (i. Wire Microstrip Impedance Calculator. First, convert the cross-section area from [mils 2] to [cm 2]: The inductance of a PCB trace determines the strength of any crosstalk it will receive. Use our free calculators. 26%; Circuit amplitude = 62. inches. PCB Coplanar Waveguide Calculator RF PCBs can be complex boards that need careful interconnect design and layout. To use this calculator, just enter the required values and press the "calculate" button. 2235* (w+h)/b) where: w is width of the strip in inches, b is the length in inches, h is the distance between the strip and the ground plane, and. PCB Trace Width Calculator Current (I) A Thickness (t) Temperature Rise (TRise) Ambient Temperature Trace Length Formula First, calculate the Area: Then, calculate the. The inductance of a wire loop is a common textbook example of a circuit with inductance. There are many demands placed on PCB stackup design. 1. com If it. The loop inductance should be examined in long and short transmission lines to understand EMI, propagation behavior, and return paths. Contrast this with a high-speed multilayer PCB. Enter one of the three values and the calculator will output the other two. The passive spiral inductor can be realized in different structure like square, circular, hexagonal, and octagonal with multiturns. The real structure of any. μ. (2) Where, • N = Number of turns • w = Average width of the rectangle • h = Average height of the rectangle • a = PCB trace width • µ0 = Permeability of. NOTE: For the wire radius, enter the trace width of the PCB traces. If we plug the impedance into Ohms law, we see. Select the box with the geometry that you would like to calculate. H = Seperation between wires in in. With PCBWay impedance calculator, you can calculate the approximate impedance of your PCB / high frequency circuit board. The IC power via length, spacing between Vdd and ground vias, and the package pin length also contribute to this. Your breadboard wiring or PCB traces will act as additional inductances. 725 For IPC-2221 external layers: k = 0. 4, by the number of turns n, the width of the conductor w, the spacing between turns s, the thickness t of conductor, the conductor’s total length lt, the average diameter, the diameters outside and inside dout and din. This calculator calculates the characteristic impedance, capacitance per unit length, inductance per unit length using relative permittivity, height of dielectric, width of trace, trace thickness values. Designers need numerical tools and the correct analytical formulas to calculate the inductance of their PCB. Now I can easily recalculate the NFC circuit, I mean the caps and indutors. 2. In other words, for proper impedance control, the load impedance must be equal to the characteristic impedance of. How to Do PCB Modeling For a Power Converter Figure 2. 21 STRAY INDUCTANCE 12. Inductance (in a General sense) is a function of Wire/Trace length, cross-section-area…material properties, etc. Advanced Circuits is MIL-PRF-31032, MIL-PRF-55110G, AS9100C, ISO 9001:2008 Certified, IPC 6012 Class 2, 3 and 3A Qualified, and ITAR Registered. Where: T TEMP is the trace temperature [°C], T RISE is the maximum desired temperature rise [°C], T AMB is the ambient temperature [°C]. Voltage drop calculationMicrostrip Max Current Calculator. THESE FORMULAS ARE APPROXIMATIONS! They should not be used when a high degree of accuracy is required. 10. The answer to whether your traces act like transmission lines depends on the amount of time it takes a signal. The inductance of the wire loop. Calculate inductance, capacitance, and achieve impedance matching using L-match, Pi-match, or T-match configurations in. 54 mm. 2E-6 Ohm-cm. PCB Structures / Discrete Structures / Signal Management. 5. Hammerstad and O. 725 PCB Trace Impedance Calculator. If the area of the loop made by the current flow increases, the inductance will increase proportionally. Related Products: Magnetics, Transformer MiscNow add a second trace in parallel (connected at each end) of the same length and dimensions, such that it also has the same inductance x x. This parameter is used for the loss calculations. Microstrip construction consists of aFigure 4. Using wider traces with power routing is helpful to reduce inductance and crosstalk. Tech Consultant Zach Peterson explores a very common issue in PCB Design: Trace Inductance and when to make traces wider. 24 pH, where pH is units of picohenries. A PCB trace is a highly conductive track that is used to connect components on a printed circuit board. Just enter the given values for trace thickness, substrate heights, trace width and substrate dielectric in the calculator above and press the "calculate. Assumptions and Limitations. This calculation is for loop and self inductance they are the same for this example. External Copper Thickness: Copper weight for the top and bottom layers. Designers need numerical tools and the correct analytical formulas to calculate the inductance of their PCB. Resistance calculation. Approximations for the impedance, delay, inductance, and capacitance of microstrips and striplines, as a function of trace geometry, are reproduced in my book, High-Speed Digital Design ISBN:0-13-395724-1. via barrel diameter d. Component: Copper Traces Purpose: Interconnect two or more points Problem: Inductance and Capacitance x = length of trace (cm) w = width of trace (cm) h = height of trace (cm) t = thickness of trace (cm) e r = PCB Permeability 0. 5. Sometimes the PWM signal generated in a daughter board and the trace between the controller output andYou must calculate sensitive data lines such as RF/analog traces and high-speed traces. Max Current calculation. About the Author. t is the thickness in cm (PCB copper @ 1 oz/ft2 = 0. Single layer Planar spiral coil inductor calculator : The first approximation is based on a modification of an expression developed by Wheeler; the second is derived from electromagnetic principles by approximating the sides of the spirals as current-sheets; and the third is a monomial expression derived from fitting to a large database of inductors. T= Experimental temperature. Loop Inductance Model. At its worse, this combination has the potential to affect the signal and power integrity of the circuit. In terms of the characteristic impedance in line 1, mutual. 0pF per inchThe Impedance Matching Calculator is a valuable tool in electrical engineering used to determine the optimal values of inductance (L) and capacitance (C) for achieving impedance matching in a given circuit. Many conductor properties are frequency dependent and dynamically affect electromagnetic characteristics of circuitry and systems. The large step in voltage. Since the goal of trace design in a PCB is generally to get close to an impedance target, designers will need to use some equation to calculate the impedance of a trace or a transmission line impedance calculator to properly size a trace in a PCB layout. Inductance calculations; experimental investigations. 0pF per inchBlackCoffee August 12, 2022, 4:44pm 2. While this calculator will provide a baseline, any final design considerations should be made towards loss, dispersion, copper roughness, phase shift, etc. Calculate the PCB trace inductance, or "L," using the formula L = Zo * DLY. Turns per layer vs. Because they are not enclosed, these PCB microstrips have a lower power handling capability and higher loss, thus allowing you to calculate a microstrip’s height and. readily calculate the resistance of a linear trace, by effectively "stacking" a series of such squares end to end, to make up the line’s length. Trace over a ground plane: microstrip line. Capacitance = ϵ ∗ Area/DielectricThickness C a p a c i t a n c e = ϵ ∗ A r e a / D i e l e c t r i c T h i c k n e s s. The first step in using the PCB Trace Calculator is to enter all. 3mm trace width, 35 turns, inductance= 12uH and 0. “Inductance Calculations” , F. as L. Let's consider the impedance for copper at 1 MHz. S. One challenge in designing PCB interconnects is maintaining system impedance while reducing crosstalk, which requires reducing trace inductance. Critical Nets Shown in a Buck Converter For example, the matrix data in Figure 3 and Figure 4 show the capacitance and inductance between two nets. Click below button to get online PCB fabrication cost with impedance control. One commonly used formula for calculating the characteristic impedance of a PCB trace or transmission line is the capacitance per unit length formula: Z0 = 1 / (C / L) where Z0 is the. 5 resistance of conductors 12. The IPC-2141 equations provide a capacitance per unit length equation that can be used to calculate the PCB trace inductance. This calculator helps translating the color bars from the resistor to its value. Eventually, the impedance of your power delivery network will. Bandwidth Calculator & Max PCB Trace Length Calculator ; Calculates the bandwidth of a digital signal and maximum trace length, using the IPC-2251 method or the frequency domain method, for transmission line effects. Dimensions are in meters. Resistance calculation. 254mm / 0. Traces on PCBs are small and therefore have small inductance levels. Loop Inductance Model. Always, ensure symmetry of the build-up. 435A. Specifications of the Rogowski Coil. Mutual and Leakage Inductance. When calculating the number of turns in the “Coil” tab, it is assumed that. Mounting Inductance. The parameters of outer diameters, inner diameters, trace width, trace spacing are represented. Trace Width Calculator. e. Propagation delay (t pd) in PCBs is the time taken by a signal to travel through a unit length of a transmission line . l is the Length in cm. The inductance for a rectangle loop is: where. The overall trace temperature can be calculated as follows T TEMP = T RISE + T AMB. Simply put, PCB trace impedance is a measure of the resistance that a circuit opposes to a current once a voltage is applied. The capacitance of a parallel plate capacitor is given by C= (kA/11. 8mm (0. Saturn PCB Design is a free tool (sadly Windoze only). 877 H/m. Power Loss is Current^2 * Resistance. trace geometry, and use this to calculate. 36) Co Equation: IPC-D-317A (Eq. The spacing between traces is 45nm as well (and the thickness of the metal layer is 45nm). 2. In order to acquire inductance parameters of the coils on PCB, a method is. The dV/dt node can be responsible for noise coupling around the PCB layout. Example parasitics calculation between two microstrip traces on a Dk = 4. It is important to note that in industry we can find that some differential pairs are routed this way for a strong coupling, but the actual return for the signal is on a. Calculate Coplanar impedance for given PCB parameters. For high-frequency signals such as GHz bands, PCB traces don’t act like simple connections. 57255 uH. Sierra Circuits’ Via Impedance Calculator uses the physical dimensions of a via to calculate its capacitance, inductance, and impedance. Output voltage = 0. A broadside trace inductance calculator is a tool used to calculate the inductance of a broadside trace on a printed circuit board (PCB). Low. ant = R. R 50Ω R 50Ω RM VS LM VS VI V=L V=IR d dT Scope Input I Figure 7. 031”) trace on 0. Hershenson, S. The real structure of any. Bonding wires is the most universal and popular chip interconnection method because of its. PCB Trace Width Calculator Current (I) A Thickness (t) Temperature Rise (TRise) Ambient Temperature Trace Length Formula First, calculate the Area: Then, calculate the Width: For IPC-2221 internal layers: k = 0. R 50Ω R 50Ω RM VS LM VS VI V=L V=IR d dT Scope Input I Figure 7. If a 4 mils trace is spaced 4 mils from ground trace, then the inductance of 1 inch trace is 1 nH. 2 Busbars A collection of formulae for busbar structures such as traces in PCBs, is shown in Table A. e. And that's why the Rogowski coil's design comes with an integrated circuit and a large shared inductance. Ohm’s Law provides the framework for solving network analysis problems; when the curtain gets pulled back, Ohm’s Law updates to become the relationship between voltage, current, and impedance, not resistance. Use the 'tline' element in LTSpice instead. 0254m. Our impedance tolerance is from ±5% to ±10%. This Tables converts Cu Oz to mils. Can PCB. A coil inductance formula is based on the basic loop inductance. 1996. The edge couple differential microstrip transmission line is a common technique for routing differential traces. However, if you bring some copper near a trace, their will be some additional mutual capacitance and inductance that will modify the impedance of the trace. Is there a way I can calculate Impedance between any two given points on a trace for a given frequency ?By default, most PCB design programs with length matching capabilities will set the pin-package delay to zero length or zero time. Where W is the trace width, L is the trace length, and H is the trace thickness. This calculator calculates the characteristic impedance, capacitance per unit length, effective propagation delay, inductance per unit length using relative permittivity, height of dielectric, width of trace, trace thickness values. 3. The Calculator has the following tools: Regulators. 36) Co Equation: IPC-D-317A (Eq. Plug in the values into the inductance formula: L = N²/R. The double integral of. Coil Inductance Calculator. This transmission line calculator. The inductance represented by the combination of one signal via and one return via at distance s equals: In metric mks units ( h, s, and r in meters, L in Henries), the constant μ/2π works out to 2·10 -7 H/m (assuming a non-magnetic dielectric). PCB spiral Coil. 5 mm PCB trace width with 3 oz copper thickness are selected in the study. Modeling approximation can be used to understand the impedance of. Coplanar Waveguide With Ground Characteristic Impedance Calculator. For 60Hz, the wavelength of your signal in copper is about 5000 km, so i wouldn't worry about couple-inch tracks on a board. These values are calculated with the example of 7-8 mils from capacitor-to-mounting-pad-edge, 20 mils from capacitor-mounting. Radio Frequency Engineering Calculator. Parasitic inductance and capacitance can both influence these circuit layouts, and they are best avoided by using short trace lengths. 19 INDUCTANCE 12. Where: T TEMP is the trace temperature [°C], T RISE is the maximum desired temperature rise [°C], T AMB is the ambient temperature [°C]. Avago RF VMMK Devices Improve Performance by Reducing Parasitic Inductance and Capacitance. The inner and outer radius of the planar spiral coil is determined from the general equation of the filament of the wire. The yellow traces that you see, exist on 5 of the 6 layers. T is the trace thickness [oz/ft 2], W is the trace width [mils], R is the resistance [Ω], ρ is the resistivity parameter, whose value for copper is 1. PCB Impedance and Capacitance of Asymmetric Stripline. A = L. Inductance of different line widths for fixed thickness of substrate Application Note AN258, Rev. Embedded Microstrip. The approximate formulas are provided along with restrictions on the relative size of the various parameters.