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Microstrip or stripline? That is the question.

IS IT BETTER to route signals in microstrip or stripline traces? Most designers would say "It depends."

The answer really depends upon your definition of "better." So much of the design process is balancing tradeoffs, usually between acceptable performance and on-schedule delivery with acceptable risk and lowest cost.

This is why it is so dangerous to base designs on rules. Every design is custom. Design guidelines and rules of thumb are powerful tools to help zero in on possible design options, but ultimately, as design margins tighten up Verb 1. tighten up - restrict; "Tighten the rules"; "stiffen the regulations"
constrain, stiffen, tighten

confine, limit, throttle, trammel, restrain, restrict, bound - place limits on (extent or access); "restrict the use of this parking lot"; "limit the
, every engineer needs to become proficient with the tools required to reach the optimum custom design as quickly as possible.

All we can do is point out the pros and cons pros and cons
Noun, pl

the advantages and disadvantages of a situation [Latin pro for + con(tra) against]
 of microstrip and stripline, illustrate the tools for exploring design space and ssuggest examples so you can decide what is "better."

Obviously, fewer traces can be routed in microstrip than in a stripline multilayer board, especially if the surface is already cluttered with components. This analysis applies to a situation where we have the opportunity to route a surface trace or a stripline trace, and we wish to evaluate which path is preferred.

For performance, what's important in a signal trace? In most high-speed designs, the first-order factors are controlled impedance, crosstalk and attenuation Loss of signal power in a transmission.

The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities.
, which influences interconnect bandwidth.

What's not on the list is EMI (ElectroMagnetic Interference) An electrical disturbance in a system due to natural phenomena, low-frequency waves from electromechanical devices or high-frequency waves (RFI) from chips and other electronic devices. Allowable limits are governed by the FCC. . While it is correct that a microstrip signal trace will radiate ra·di·ate
1. To spread out in all directions from a center.

2. To emit or be emitted as radiation.

 more than a stripline, due to the finite total inductance in the return plane, very rarely do products fail FCC (1) (Federal Communications Commission, Washington, DC, The U.S. government agency that regulates interstate and international communications including wire, cable, radio, TV and satellite. The FCC was created under the U.S.  certification tests because of microstrip traces. They fail because of common signals on external cables; either shielded cables or twisted pair A thin-diameter wire (22 to 26 gauge) commonly used for telephone and network cabling. The wires are twisted around each other to minimize interference from other twisted pairs in the cable (Alexander Graham Bell invented this and was awarded a patent for it in 1881). . If you have this problem nailed and do not have a shielded enclosure but still have an EMI issue, then consider stripline over microstrip.

Most fab houses don't do a very good job of controlling the impedance of outer surface layers, but there is nothing inherent about microstrip that would make it a poor controlled impedance interconnect. It is more an issue of risk with the fab house, which can be reduced by qualifying your supplier.

One advantage of microstrip is that to achieve an impedance of 50 [ohm ohm (ōm) [for G. S. Ohm], unit of electrical resistance, defined as the resistance in a circuit in which a potential difference of one volt creates a current of one ampere; hence, 1 ohm equals 1 volt/ampere. ] in a single-ended line, the dielectric thickness is about half the line width when using FR-4. For stripline, to achieve 50 Q for a single-ended line requires a total dielectric thickness between the two reference planes of about twice the line width.

In a differential pair Differential pair is a pair of conductors with special characteristics, used for differential signaling.

Examples of the differential pair include:
  • twisted-pair cables, shielded and unshielded
, the difference is even more dramatic. You can use a pair of traces with tighter coupling and thinner dielectric in microstrip than in stripline traces. For example, with .005" wide, half-ounce traces, in edge-coupled microstrip with a spacing equal to the line width, the dielectric thickness is .0035". In edge-coupled stripline, with the same traces, you can't even get as high as 100 [ohm] with a space as tight as the line width. If we go to a space twice the line width, the total dielectric thickness is .028".

Stripline traces will have very little far-end crosstalk, while microstrip can have enough to easily exceed noise budgets. This is a real issue in the decision process. If far-end noise is a problem, don't use microstrip.

But when the trace-to-trace spacing is less than twice the line width, stripline actually has more near-end noise than microstrip. For spacings larger than twice the line width, stripline has lower near-end noise.

Attenuation is subtler. For the same 100 [ohm] differential impedance pairs, with .005" wide lines, the conductor loss is almost comparable between microstrip and stripline. However, the dielectric loss is lower in microstrip than stripline for FR-4. FIGURE 1 illustrates why. Some of the field lines in microstrip are in air, where they see a Iower dissipation factor In physics, the dissipation factor (DF) is a measure of loss-rate of power of a mechanical mode, such as an oscillation, in a dissipative system.

For example, electric power is lost in all dielectric materials, usually in the form of heat.
 than the bulk laminate. This gives microstrip about 30% lower attenuation than stripline, which means, potentially, a higher interconnect bandwidth by 30%.


FIGURE 2 shows the attenuation per length for microstrip and stripline. Above about 1 GHz bandwidth, there is an attenuation advantage for microstrip lines.


No decision is based on one answer. There are too many factors to be balanced in the tradeoffs. PCD&M

DR. ERIC BOGATIN is the CTO (Chief Technical Officer) The executive responsible for the technical direction of an organization. See CIO and salary survey.  at IDI IDI ICC (International Cricket Conference) Development International Conference)
IDI Israel Democracy Institute
IDI I Doubt It
IDI Initial Domain Identifier
IDI In-Depth Interview
, and president of Bogatin Enterprises;
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Title Annotation:NO MYTHS ALLOWED
Author:Bogatin, Eric
Publication:Printed Circuit Design & Manufacture
Date:Feb 1, 2006
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