The Importance of Impedance Control in PCB Design
Camptech II Circuits Inc.
PRECISION, QUALITY, DURABILITY AND CUSTOMER FOCUSED ELECTRONICS MANUFACTURING SOLUTIONS SINCE 1980.
1. What is Impedance Control and Signal Matching?
The continual increase in device switching speeds confronts engineers with signal integrity (SI) problems. Eventually, most devices are going to have to deal with SI issues. So,?Printed Circuit Board (PCB)?traces can no longer be treated as a simple point-to-point connection. Traces need to be considered as transmission lines and impedance matching becomes necessary to lessen or eliminate the impact on SI.?
Impedance control matches PCB trace dimensions and locations with the properties of the substrate material to ensure that the strength of a signal travelling along a trace is within a required range. Many potential signal integrity issues can be averted or mitigated by following good design practices and approaches.
So, here we’ll talk about the importance of impedance control, the causes of signal integrity issues, and ways to avoid them.
2. Why is impedance matching needed?
The function of a PCB trace is to transfer the signal power from the driver device to the receiving device.
Power propagation happens throughout the length of the trace. But achieving the maximum signal power can only be done with matching impedances on the PCB.
So, that is why there is a need for impedance matching. We want as much of the power from the driver to end up at the receiver.
If special care is not taken in the PCB layout stage, then high-frequency signals will degrade as they propagate from the driver to the receiver. When viewing this result on an eye diagram, you can see the signals will be very distorted and power levels will be different as the signal propagates from the start to the end.
3.?What is a high-speed signal or circuit?
IPC defines a high-speed signal as one when the rise and fall time of a signal is fast enough that the signal can change from one logic state to the other in less time than it takes for it to travel the length of the conductor and back. So, as the signal propagates from the source to the receiver and back – if the rise and fall time is faster than that, you are dealing with a high-speed design and have to consider high-speed issues.
A misconception is that the clock speed of the circuit determines whether the circuit is operating at high speed.
4. What are some of the factors affecting impedance?
The PCB manufacturer more or less fixes trace impedance; however, the PCB designer can define some variables affecting trace impedance.?
Power is transmitted uniformly across the length of the trace across the PCB when there is uniform impedance. Therefore, constructing a trace with a very constant cross-section is necessary for uniform impedance.
In other words, the shape and size of a trace should be as even as possible, running across a consistent dielectric constant of the material that does not extend the length of a given routing layer. And, the more uniform the trace is, the more balanced the dielectric constant is, which results in a more consistent impedance and less power degradation.
Failure to perform impedance matching can result in critical issues.
This diagram shows a microstrip.
H represents the distance between the trace and the adjacent plane. T is the trace height or copper thickness. Typically, this will be 35 or 70 microns, depending on how the stack-up is defined.
“W” is the width of the trace. Among these 3 variables, the trace width I the one that is within the complete control of the designer.
A targeted impedance on a PCB trace can be attained by varying its width.
It’s important to note that the voltage may vary significantly as it is propagated along the trace.
The presence of an impedance change or discontinuity will certainly cause a reflection back to the source of that signal.
A significant issue with boards that don’t have impedance matching is the presence of reflections. So, some of the signal's energy will reflect toward the driver and the remaining signal will continue onward. A pure square wave will not be seen when you look at the waveform for this. Instead, a distorted waveform with overshoots and undershoots and some ringing will be observed. Any unmatched impedances on the PCB will result in some electromagnetic interference (EMI). As a result, the miss-match of the impedance will cause some electromagnetic radiation in that localized area where this transition occurs and where these reflections appear.
The radiation can couple its energy to neighboring traces or affect some sensitive components on the board.
This graph shows a net with potential signal integrity issues.
This graph is the same net with a theoretical series termination resistor of approximately 40 ohms added.?
(Image from?altium.com)
Consider a trace where the impedance measures 40 ohms. When this trace enters another layer, the impedance goes up to 50 ohms due to the geometry of the PCB. We will have some energy reflected at the transition point in this case. So, we can see reflection problems at that point and encounter serious signal integrity issues in high-speed designs.
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When routing the PCB, one needs to pay special attention to any mismatches in impedance. Efforts should be made to ensure that impedances are maintained and possible throughout the part of a routed signal. Special consideration to this point is needed for products requiring CE and EMC approval.
5. How to achieve impedance matching?
Well-controlled impedance means that the trace impedance is constant at every point along the path on the PCB. This means that wherever the trace travels through the part, the impedance is the same, from the source to the destination, even if it changes layers. We don’t have much control over the impedance in the driver or the load, but we can control the impedance on the PCB. So, we want to have matching circuitry on the PCB that matches the impedance of the source and load. Therefore, we can ensure a consistent appearance throughout the entire path.
There are a few important design criteria that we need to consider. Keep in mind that good PCB design techniques can prevent many of the problems relating to EMI reflections. Another crucial point is the?choice of materials.
In the past, typically, FR4 was specified. But, with high-speed designs, the use of the?correct laminate?is critical. Using a material with a lower dielectric constant (Dk) is advisable and preferred. This ensures the best signal performance and will also minimize any signal distortion or phase jitter of the signal. Otherwise, you might see an inconsistency in operation between board batches: one batch of boards might work, and the next batch of boards you order might not.
Another important criterion is the loss tangent or dissipation factor. This is a measure of the signal loss as the signal propagates down the transmission line on the PCB. You would want to select the lowest loss material for very high-frequency designs.
From the table, you can see that the different laminates have varying loss tangents.
So, you would need to select the most suitable material for your application and specify this in your manufacturing notes. Ensure that the bare laminate used in the PCB fabrication process complies with IPC4101 grade. It is important to ensure good dielectric spacing between the copper and the laminate that it is bonded to achieve a consistent electrical performance of the trace running across the PCB.
Another point to consider when selecting the laminate material is the fibreglass weave pattern. A typical PCB core and prepreg substrates are constructed from various woven fibreglass fabrics bound together with epoxy resin. The glass and epoxy each have different Er/Dk values, resulting in an inhomogeneous medium for signal propagation. A loose weave pattern produces less uniform dielectric constants in the PCB laminate that can cause trace impedance variation and propagation skews.
The higher the frequency, the more evident this problem will be. The tighter the weave pattern is, the more uniform the dielectric constant. So, it is better to practice choosing a tighter weave, so the signal moves over more glass than anything else. The outcome of this will be a very consistent dielectric constant throughout the PCB.
Including power planes that can supply a signal return path below each signal path is an essential step in controlling impedance. Preferably, these planes should be dispersed through the board stack up and designed so a minimum of one plane adjacent to each signal layer carries controlled impedance routing.
By avoiding discontinuities (such as a split or blowout in the power plane, underneath any critical routing), the return path current flowing through the plane will seek to follow the same physical path as the route on the signal layer.
In addition to choosing the appropriate order for signal and plane layers, the material properties of each layer need to be determined. This includes:
7. Other design considerations
? Trace lines should be kept as short as possible and reduced lengths wherever possible. Using terminations can prevent reflection if trace lengths are relatively long.
? Avoid routing stubs and discontinuities, which can add to reflections and degradation of the signal quality.
? For differential pair routing, try and ensure that the signal pairs have the same length.
? Use of back drilling – for a thick backplane where the signal goes from the top layer to one of the inner layers, the remainder of the copper barrel of the via or the pin of the press-fit connector will be a stub, resulting in reflection. Back drilling removes the unwanted copper. It is a technique used to remove the copper barrel's unused portion or stub from a thru-hole in a printed circuit board.
? Consider using?immersion silver?as a surface finish rather than ENIG. Immersion silver results in less insertion loss (lossy) than ENIG purely because the nickel content in ENIG is very lossy, and due to the skin effect, it is not very good for high-speed designs. The flatness of the pad is just as good as ENIG, and it is more workable than ENIG.
? Reduce the size of antipads on plane layers. Antipads are where pads or copper are removed on plane layers where the pad should not or does not connect to that plane. Sometimes the anti-pad size is too large, creating unnecessary voids in the plane. Making the anti-pad a little bit smaller allows for more plane continuity resulting in a cleaner signal and return path.
? Specify the solder mask thickness. Solder mask also has a dielectric constant. Even thickness across the board can prevent unpleasant surprises.
? It is always a good idea to do some post-design simulation or signal integrity analysis. It's always less expensive to fix the design before manufacturing your board. There are some tools available to do signal integrity analysis. Hyperlynx is the industry standard and is very well known. Polar Instruments and Altium Designer offer signal integrity analysis functionality.
8. Impedance Control Verification
You can verify Impedance control after the PCB manufacturing by using test coupons (a test coupon is a PCB used to test the quality of a PCB fabrication process. Test coupons are fabricated on the same panel as the PCBs, typically at the edges. Coupons are then inspected to ensure proper layer alignment, electrical connectivity and cross-sectioned to inspect internal structures. Coupons can be designed custom for a PCB or selected from a vendor library).
You can ask your PCB manufacturer to design a test coupon or ask them to place test coupons on your working panels. Typically, the PCB manufacturer places test coupons at different locations on the working panel, representing a good cross-section of the PCB. Then, using a Time-Domain Reflectometer (TDR), the impedance can be tested. Subsequently, a report is generated to indicate if the characteristic impedance was achieved on your PCB.
The overall performance and EMC behavior of electronic equipment is determined by the design of the circuitry and geometry of the layout and the power distribution network.
Pay careful attention to:
Conclusion
With the increasing use of high-speed devices, board designers need to consider multiple factors impacting PCB performance. One of these considerations is impedance control, which has serious significance on signal integrity and the board's operation. By understanding the causal factors for impedance mismatch and acquiring the knowledge of design practices that can mitigate or remove impedance issues, the PCB designer can create a truly engineered solution. So, a robust design will allow the manufacture of a reliable and high-performing printed circuit board.