How to do PCB Trace Length Matching vs. No series or load termination is required for short trace less than 0. For 0402 components, that means 20 mil trace, as you mentioned. There are many demands placed on PCB stackup design. Controlled differential impedance starts with characteristic impedance. Follow asked Nov 27, 2018 at 12:32. On a high-speed PCB (> 100MHz) where wavelengths are shorter, any critical net (see figure 4a) is electrically long enough to make it an efficient radiator, especially when left exposed on the top or bottom layer. Assuming that the thickness of the trace, tFor example the vertical space is 20mm, then all signals are in a (20-40mm)*20mm area, then trace length on the carrier board won't be longer than 40mm, suppose the signal rise time is 100ps, then the trace length is several times the rise length, then impedance should matter even on this small area, and I'm not sure whether will this. Dielectric constant can also change across the length or width of a PCB trace or because of changes in frequency and temperature. The output current for each channel can be adjusted up to 2. Read Article UART vs. Most PCB software programs assume that the PCB trace is 1oz. Nevertheless, minimal trace size referrals from producers ought to be remembered. Figure 1. 3 can then be used to design a PCB trace to match the impedance required by the circuit. 0 and 3. How to do PCB Trace Length Matching vs. Cadence Orcad Guide OrCAD - PCB Solutions | PCB Design Software EDA Tools and IP for Intelligent System Design |. The roughness courses this loss proportional to frequency. I followed the below procedure to design a 700MHz 1/4 wave monopole PCB antenna. For a standard thickness board (62 mils), it would be roughly 108 mils. Correcting a trace length mismatch requires placing meanders in the shorter traces in the net so that they match the length of the longest trace. The basic idea of this length matching is that the shorter trace follows a detour or meander in order to lengthen it to match the length of the longer trace. Correct; Length matching has meaning when you have fast switching cycles / clock speeds. Low-voltage differential signaling (LVDS) is codified in the TIA/EIA-644 standard and is a serial signaling protocol. Then when it is time to tune the trace, convert those trombone patterns into the tighter serpentine patterns that you need in order to hit your target lengths. These series terminations should be located at the driver end of the trace asTo change your PCB layout so that RFI and noise can be reduced, you’ll need to do some of the following tasks: Redesign the PCB stackup and layer selection to ensure consistent system impedance. 3. matching requirements include PCB trace delays, different layer propagation velocity variance, and crosstalk. Trace length tolerance matching on your differential pairs and single-ended traces makes your high speed routing more precise. In the pair with larger spacing (10 mil), a 21 mil amplitude length tuning section has small sets of traces with odd-mode impedance of 53 Ohms. Equation 1 describes the relationship between wavelength and frequency, as a function of the transmission line’s propagation velocity. The basic idea of this length matching is that the shorter trace follows a detour or meander in order to lengthen it to match the length of the longer trace. How to do PCB Trace Length Matching vs. Read Article UART vs. At a foot length (300 mm), a signal frequency having this wavelength is about 1 GHz. 2. A fully unified, heavily rules-driven PCB design platform for impedance controlled routing in high-speed PCB design. PCB Design and Layout Guide. 2. However: The Raspberry Pi Computer Module 4 (CM4) datasheet states: 2. If you are a PCB board designer, you do not need to perform this calculation manually, you just need to use the. The difference between a cable and a printed circuit board track is length. RF reflection results in attenuation and interference. Here’s how length matching in PCB design works. Rule 5 – Match the trace length. • Trace width of any un-coupled section of a differential trace greater than 100-mils, shouldRule 2: Exposed critical trace length. 005 inches wide, but you may have specific high speed nets that need 0. The bends should be kept minimum while routing high-speed signals. The ‘3W’ Rule (s) This actually refers to three rules. 3. Intra-pair skew is the term used to define the difference between the etch length of the + and - lane of a differential pair. Trace thickness: for a 1oz thick copper PCB, usually 1. 2% will survive two, and 0. SerDes PCB Layout Guidelines: This means we need the trace to be under 17. Klopfenstein trace taper return loss spectrum for a 50 to 40 Ohm transition. Figure 2. Keeping traces short is another way to combat reflections and ringing. It seems like a rather simple task: connect a copper line from point A to point B with your schematic capture output as a guide. How to do PCB Trace Length Matching vs. Would a 2-3 cm difference in lines beget problems?Critical length depends on the allowed impedance deviation between the line and its target impedance. • Provide impedance matching series terminations to mini mize the ringing, overshoot a nd undershoot on critical sig-nals (address, data & control lines). 1V and around a 60C temperature. For example, if you require a 5mil trace to achieve 50Ω impedance and if you have also routed other signals with 5mils width, it will be impossible for the PCB manufacturer to determine which ones are the controlled impedance traces. Follow asked Jul 24, 2015 at 2:20. 5 inch. Configuring the meander or serpentine style in the Proteus. 2) It will be vise to match the PCB trace impedance to the cable impedance, or you may get reflections. Minimize trace length and bends: Long traces can introduce. How to do PCB Trace Length Matching vs. To ensure length. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. I have managed to. the RGMII-ID configuration to be connected to a PHY without the use of PCB trace delays. 5 to 17. frequency (no components attached). Technologies DDR3 Routing Topology Page No #5 DQ/DQS/DM:If a transmission line has a 50 ohm impedance, then connecting it abruptly to a 1 V source will cause a 1 V voltage wave and a 20 mA current wave to start travelling along the line. During the PCB manufacturing process, the trace is typically laminated onto the board’s surface. Use the following trace length matching guidelines. Having an advanced PCB software can significantly ease your routing experienceBy achieving trace symmetry in differential pair routing, it is possible to ensure reliable data transmission while avoiding timing issues. It is performed by placing a terminating resistor in between the driver and the receiver. 50R is not a bad number to use. frequency can be reduced to a single metric using an Lp norm. a maximum trace/ cable length which is specified in the various specifications. I2C Routing Guidelines: How to Layout These Common. Quadrature coupler design can use discrete components or quarter-wavelength tuned traces to split or combine inputs and produce outputs with a 90°. 3 ~ 4. It leads to either: - rising edges on SCL become too slow, which means the signal spends a lot of time around the receiver's 0/1 threshold. For most manufacturers, the minimum trace width should be 6mil or 0. Therefore the edge rate can be about 400 ps, so 100 ps difference wouldn't make much of a shift in eye crossover position. Tip #1: Reference Planes. The Basics of Differential Signaling. Microstrip Trace Impedance vs. For frequency-modulated analog signals, the characteristic impedance of a transmission line has a constant value throughout the signal’s frequency spectrum as long as the relevant frequency range is high enough. 6mm spacing with a trace width of 0. Try running a 10 GHz signal through that path and you will see loss. If the length of the track is between 1/6 or 1/4 of the effective length of a feature like an edge a system can be regarded as lumped. How to do PCB Trace Length Matching vs. On either the rising or falling edge (and sometimes even both) data is “clocked” into a. Let’s discuss the need for impedance. 3. Aside from this simple design choice, you may need to design an impedance matching network for your connector. How to do PCB Trace Length Matching vs. High-speed signals have broad bandwidth, meaning the high-speed signal frequency range extends theoretically out to infinity. The DDR traces will only perform as expected if the timing specifications are met. It is sometime expressed as "loss tangent". Next Article Energy in Inductors: Stored Energy and Operating Characteristics In order to know the energy in. OrCAD PCB Designer Professional, OrCAD Sigrity ERC, and more. This creates several effects in PCBs on FR4 that are especially important in high-speed or high-frequency applications. Cables can be miles long but a PCB trace is likely to be no longer than a foot. Speed ≡ Clock frequency and/or edge rates. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. The higher the interface frequency, the higher the requirements of the length matching. cable length performance far exceeding IEEE specifications and features that provide lower cost solutions, for both 10BASE-Tand 100BASE-TXEthernet protocols, the devices ensure compatibility and inter-. How to do PCB Trace Length Matching vs. 36 RF / Microwave Design - Line Types and Impedance (Zo) Coplanar Waveguide)CPW Allows Variation of Trace. PCB design software, like Altium Designer ®, has high-speed design functionality for routing and trace tuning built into it. b. • An increase in the minimum clock frequency from 125 MHz to 300 MHz. The impedance of a PCB trace at RF frequencies depends on the thickness of the trace, its height above the ground plane, and the dielectric constant and loss tangent of PCB dielectric material. PCB routing for RF (radio frequency) and antenna design is essential to optimize the performance of wireless communication. 34 inches to not be considered high-speed. 152mm. In high-speed digital protocols, data is sent over single-ended traces in a PCB that is impedance controlled; each individual trace is. 127 mm traces with 0. When you are distributing power, DC and low frequency, the trace resistance becomes important. If the bends are required, then 135° bends should be implemented instead of 90°as shown in figure (5, Right side). Although that is a simple example, there are a lot more rules that can help in the design of high speed and RF traces: Trace Lengths: This rule allows the user to set a target value. Eq. The line must meet the 2W principle to reduce crosstalk between signals. Trace routing is one of the critical factors in constraint settings. Understanding Coplanar Waveguide with Ground. If you use narrower trace (12 mil) with 20 mil pads, you will have unwanted. In order to minimize the coupling effect from the. Match the etch lengths of the relevant differential pair traces. ImpedanceOne of these design aspects is the match between PCB via size and pad size. Since my layer thickness is 0. Following are the reasons to. The frequency of operation is about 10 MHz. Tip #3: Controlled Impedance Traces. 3) slows down the. Length matching is not the case here but adding some ground traces as guard lines could reduce the probable emission and RF immunity problems. Well, if you manage to get 50 Ohm trace for this LCD on a 2-layer board with meaningful trace widths please find me :) I hope you are aware of the fact that the PCB thickness should be very low. The narrow spacing and thin layer count will force traces in the pair to be thin as well. 35 mm − SR opening size: 0. • Within the PCB breakout region, use the following SMT recommendations: − Ball-to-ball pitch: 1. SPI vs. Also Clock lines should be kept away from other signal and Clock lines to a minimum of 5x the trace width or larger if space allows. 5Gbps. So for bottom traces there will be massive high-frequency signals underneath them on the motherboard within 1-2mm distance. Here’s how length matching in PCB design works. I'm making a high-speed transceiver design and want some direction regarding layout of trace length from P to N. SPI vs. 010 inches spacing between them. 1 mm. Fast rise/fall times alone doen't need length matching. 23dB 1. Trace Length Matching vs. Meandering the traces elongates them, so the shorter pair would be meandered to match the length of the longer one. If you obtain component models from your manufacturer, the IBIS 6 documentation for the particular component should include the pin-package delay. For a signal speed in PCB is 15 cm/ns and an allowable skew of a quarter of the period, it gives 2 meters. The trace length decided to match with Wavelength of the frequency Wavelength (Lambda) = Wave Velocity (v) / Frequency (f) =299792458 /700000000 =428. S-Parameters and the Reflection Coefficient. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. The signal line is equal in width and the line is equidistant from the line. 5 mm. The design approach of controlled impedance routing is a key ingredient of high speed PCB design, in which effective methods and tools must be adopted to ensure the intended high speed performance for your PCBs. – The impedance mismatch between vias and signal traces can cause transmission-line reflections. The IC pin to the trace 2. From there, component placement may be adjusted to better set up the high-speed trace routing required. This is a general PCB layout guideline for ISSI DDR4 SDRAM, especially for point-to-point applications. This extra margin could be used to relax layout requirements on trace length matching and impedance control on cost sensitive PCBs. How to do PCB Trace Length Matching vs. A lot changes transitioning from DC to infinite frequency. Make sure resistors are suitable for high frequency. altium. 3) Longer traces will not limit the maximum. 3 Length and length matching Trace length greatly affects the loss and jitter budgets of the interconnection. Diorio: Transmission lines 12Track length matching is key when trying to maximise the performance of your PCB. How to do PCB Trace Length Matching vs. 025, the frequency as 10 GHz, the surface roughness as 6 μm, and the length of the trace as 1 inch. For example, if the. The answer is always framed as an always/never statement. Here’s how length matching in PCB design works. As you know, there are two types of interfaces in PCB design and length tuning will be different for each of them. This might or might not be an issue, as we will see in a minute, because it all depends on the signal frequency and trace length. 425 inches. PCB design software, like Altium Designer ®, has high-speed design functionality for routing and trace tuning built into it. rinsertion loss across frequency on the PCB. 8 dB of loss per inch (2. This will be specified as either a length or time. Obviously, these two points are related; all PCB vias have (or should have) a landing pad that supports the via and provides a place to route traces into a via pad. I am designing a PCB with an MCU and there will be JTAG, SPI, I2C and USB. The flex cable to TOSA (ROSA) elements At point 2, the reflection is primarily generated by the PCB layout. It turns out that when laying out an AC (frequency larger than a few kHz) trace on a PCB, the return current is instantaneously in the plane below. Here’s how length matching in PCB design works. 13 3 3 bronze badges $endgroup$ 1. frequency response. The resistance of these conductive elements is low enough to be negligible in most situations. 6 inches must be routed as transmission line. Trace width decided by. 7 dB to 0. I have been informed by a equalizer manufacturer that up to 1mm intrapair skew (P-N length mismatch) is hard to measure, and will have no effect on signals up to 12. These traces could be one of the following: Multiple. Here's how I do equal length differential pair routing in Eagle CAD: Name traces D_P and D_N (or something _N and _P - seems like Eagle CAD needs the suffix). This variance makes issues difficult to diagnose. Note that the y-axis is on a logarithmic scale for clarity. The caveat is that any editing of the clock or the traces on the edge of the tolerance band is likely to upset. Sorted by: 9. The series termination is an often-used technique. How to do PCB Trace Length Matching vs. 4. 25 to 0. 92445. Frequency with Altium Designer. SPI vs. How to do PCB Trace Length Matching vs. 1 Answer. 2. Download OrCAD Free Trial now to have a full evaluation of all OrCAD tools with no. Read Article UART vs. According to these. That limitation comes from their manufacturing (etching) processes and the target yield. Frequency Keeping high speed signals properly. Read Article For example: If you have 1 Amp going on a 6 mil wide trace of 1 oz copper for 1 inch of length, that's . In this article, we’ll examine a few tips and tricks for high-speed printed circuit board designs. The traces must be routed with tight length matching (skew) within the differential traces. Figure 1. Because therate, the length of the traces, the trace geometries, the type of fiberglass weave used, and the alignment of the traces to the weave pattern of a PCB. These groups could be one of the following:. Here’s how length matching in PCB design works. Some possible changes include the addition of termination components, careful design of impedance matching networks, or redesigning traces to adjust their impedance. Two of the traces have no reference plane beneath, and their lengths are Trace 1, 35mm, and Trace 2, 120mm. Again, this ideal length for the clock is found by subtracting the tolerance (or most of it) from the longest trace once everything is optimized. In contrast, for an internal trace with the same dielectric material we need the trace to be less than 10. How tightly should trace lengths be matched for a 1Gbps serial databus? It seems to me that 100ps (15mm) should be more than sufficient. Design rules that interface with your routing tools also make it extremely. Shall I take this into consideration and design a 4-layer stackup, or motherboards are usually don't make any harm with diffpairs routed on. There are guidelines5 that must be followed as the 3D antenna exposed in free space is brought to the PCB plane as a 2D PCB trace. The RS-485 protocol standard allows up to 32 drivers in one system, supporting communications over distances of up to 1200 meters, and can keep baud rates from 110 Baud to 115200 Baud. When two signal traces are mismatched within a matched group, the usual way to synchronize. How tightly should trace lengths be matched for a 1Gbps serial databus? It seems to me that 100ps (15mm) should be more than sufficient. It would be helpful to know the tolerance in length difference that is allowed while designing this PCB. 25mm trace. , RF signals), it's okay if you only know the value of the dielectric constant at a single frequency. With this kind of help, you can create a high-speed compliant. $endgroup$ –The RC discharging method with the trace capacitance shown above can control the output current and rise/fall times from your interface. The lengths of the traces that make up a differential pair must be very tightly matched; otherwise, the positive and negative signals would be mismatched. PCB trace length matching vs frequency affects the signal integrity of your circuit designs. Documentation must somewhere state need of length/impedance matching; Each bus (data, address, control) should preferably be routed on its own layer. So choose trace width and prepreg thickness to. 5 GHz. I believe the mismatch of 3 cm in the examples above is not. Keep the length of the traces to the termination to within 0. This implies trace length matching for the RGMII connections between PHY and MAC. Why FR4 Dispersion Matters. And the 100ps would be equal to 15-20 mm in trace length difference, which is huge. SPI vs. When these waves get to the end of the line, they may find a 50 ohm resistor. It is of fundamental importance that the traces with controlled impedance are appropriately spaced apart, as well as the other traces and the various components arranged on the printed circuit board. Signals can be reflected whenever there is a mismatch in characteristic impedance. Matching the impedance can be accomplished by tying the trace down with a resistor near the source or the load. 3 can then be used to design a PCB trace to match the impedance required by the circuit. We only ever have perfect matching at specific frequencies, but there are mid-range frequencies where the return loss spectrum is flat. Read Article UART vs. They recommend 3 times the trace width between trace center and trace center, until here all ok. How to do PCB Trace Length Matching vs. frequency calculator that. Trace Thickness (T) 2. SPI vs. The need for multiple lines between the microcontroller and peripheral makes component mounting more of an issue and they should be placed as close together as possible to minimize trace lengths. While the lanes are not tightly synchronized, there is a limit to the lane to lane skew of 20/8/6 ns for 2. Just like single-ended signals, differential signaling standards may have a maximum length constraint. 34 inches to not be considered high-speed. 81KW 1% resistor in parallel to a 10pFThe idea here is to determine the spacing required for a given width with the goal of hitting a specific differential impedance value. 00 mm − Ball pad size: 0. This will be the case in low speed/low. Just as a sanity check, we can quickly calculate the total inductance of a trace. 3 High-Speed Signal Trace Length Matching Match the etch lengths of the relevant differential pair traces. Here’s how length matching in PCB design works. 1V drop, you need to obviously widen the trace or thicken the copper. Place high-speed signal traces away from noisy components. These equations show that attenuation occurs in the circuit due to the (RC + GL) term. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. When you need to evaluate signal integrity and impedance matching, use PCB design and analysis software with an integrated 3D EM field solver and a complete set of CAD tools. As you know, there are two types of interfaces in PCB design and length tuning will be different for each of them. By default, most PCB design programs with length matching capabilities will set the pin-package delay to zero length or zero time. Read Article UART vs. Note: Loosely coupled traces are easier to route and maintain impedance control but take up more routing area. It has easy manufacturability and has the wireless range acceptable for a BLE application. Frequency Keeping high speed signals properly timed and. As I understand, the camera max frequency is 720 mbps, or 1380 ps of unit interval. Tip #4: Trace Length and Spacing. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. 1. By controlling the PCB impedance, unexpected damages or errors can be limited to some extent. 3041mm. except for W, the width of the signal trace. Try running a 10 GHz signal through that path and you will see loss. Problems from fiber weave alignment vary from board to board. 4 Implementing RGMII Internal Delays With DP838671. when i use Saturn PCB design to match the differential impedance to 100ohms i get 0. FR4 is a standard. High-speed layout guidelines dictate the most direct trace path isn’t always going to be the ideal routing solution. Length matching for high speed design . Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. Read Article 25MHz is some how high for SPI communication and you could have unwanted radiated emission due to long 17 cm traces. The most common approach is to design your microstrip or CPWG to match the component pads for devices in the path. (5) (6) From the results above we can see that the setup and hold margin are both greater than 0 as desired. When it comes to high-speed designs, we are typically concerned with two areas. Signal problems can abound when trace width values are incorrectly specified in high-speed PCBs. The PCB trace to the flex cable 4. Is this correct? a. The guidelines are based on best practices and TI reference designs for high-performance and reliable PCB design. At an impedance mismatch, a portion of the transmitted signal isAn RF PCB design is a bit different from a conventional board. Generally, PCB trace thickness ranges from 0. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. The termination requirement depends on the trace length of the clock signal. PCB trace length matching is exactly as its name suggests: you are matching the lengths of two or more PCB traces as they are routed across a board. PCIe: From PCI-SIG standards, PCIe Gen1 has 100 Ohms differential impedance, and Gen2 and higher have 85 Ohms differential impedance. More important will be to avoid longer stubs. In a PCB, mismatch is usually small (about 10 Ohms), but signal drivers can have much higher impedance mismatch (30 Ohms or more). As I understand it, this is for better impedance. Cite. channel includes a 3m length SuperSpeed cable (the maximum allowed by the spec) connected to a printed circuit board that has 11” of trace providing connection between a standard host connector and SMAs that then connect to a scope. 5 MHz, which is the direct. • Trace mis-match compensation should be done at the point of mis-match. The variation in FR4 dielectric constant vs. The IC pin to the trace 2. 66 mm between this traces and nearby traces? Which rules are stronger?How to do PCB Trace Length Matching vs. About a year ago I designed a PCB with a processor and RAM (400MHz and 133MHz speed respectively). How to do PCB Trace Length Matching vs. How to do PCB Trace Length Matching vs. Trace LengthTrace Length §Longer trace length ⇒ loss ↑ ü~0. SPI vs. For the signal trace of width W and thickness T, separated by distance H from a ground (or power) plane by a PCB dielectric with. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. It covers topics such as component placement, trace routing, impedance matching, and signal integrity. Two common structures are shown in Figure 3. Table 5. Trace width can also be set up for a particular net or a net class, controlled impedance traces, differential pairs, or other specific traces like clock signals. If the round-trip time is short enough, reflections may die down quickly enough to not pose a. Here’s how length matching in PCB design works. SPI vs. If your PCB has the space, why not match the lengths? It's good to practice length-matching any time you have the chance. Here’s how. For the other points, the reflections are a result of impedance mismatching. The PCB trace may introduce 1 ps to 5 ps of jitter and 0. How to do PCB Trace Length Matching vs. Note2. Critical Signal Trace Length To prevent from signal reflection, signal trace length cannot be longer than the following two critical length limitations: (a) 1/16 wavelength of Signal, λ; the relationship between signal wavelength and signal frequency is defined as where ε R = 4. – Any discontinuities that occur on one signal line of a differential pair should be mirrored on the otherFigure 3. As the signal travels along the trace, energy is dissipated as heat, leading to a weaker signal. The relatively high frequency of these signals makes routing of the lines critical. High. SSTL 15 IO Standard (1) FPGA Side on-board termination(2. Preferably use Thin Film 0402 resistors. What Are Pcb Traces Assembly Yun. 0uF. At 90 degrees, smooth PCB etching is not guaranteed. Observation: A 3cm microstrip and a 3cm stripline can get a very different propagation delay! Conclusion: If we would route a bundle of traces, eg. How to do PCB Trace Length Matching vs. At a foot length (300 mm), a signal frequency having this wavelength is about 1 GHz. Here is how we can calculate the propagation delay from the trace length and vice versa: Where: Vis the signal speed in the transmission line; In a vacuum or through the air, it equals 85 picoseconds/inch (ps/in). Therefore, you should make the 50Ω impedance traces 5. As the frequency increases, PCB traces behave like transmission lines, with a precise impedance value at each point on the trace. How to do PCB Trace Length Matching vs. character as the physical length of traces becomethe s aconsiderable fraction of the signal wavelength. This variance makes Inside the length tuning section, we have something different. How to do PCB Trace Length Matching vs. Use a 100 Ω tightly differential routing on the main host PCB up to the connector pins if you are using option 2 in Figure 102 at the connector. Traces and their widths should be sized. Therefore, if you arerouting a 1GHz signal its total length is greater than 425 mils, thenthat trace needs to. A very common, but also effective, rule of thumb is to use a minimum spacing of "2W" (better still, a "3W. Don’t make one signal go all the way across the Printed Circuit Board while the other one just has to go next door. The HIGH level is brought up to a logic level (5 V, 3. Use uniform copper as reference planes for high-speed/high-frequency signals. 2 mm. IEEE, 1997. I2C Routing Guidelines: How to Layout These Common. 3. But for EMC reasons you may very well want to do better than that, in which case you should also take care to maintain the controlled impedance over the portions of the trace that are length matched. Would a 2-3 cm difference in lines beget problems? Critical length depends on the allowed impedance deviation between the line and its target impedance. I2C Routing Guidelines: How to Layout These Common. Differential Pair Length Matching. How to do PCB Trace Length Matching vs. How Trace Impedance Works. SPI vs. There are many calculators available online, as well as built into your PCB design software.