Microstrip vs. Stripline, Surface vs. Internal: Which is Better?By ZM Peterson • Mar 31, 2021
The two dominant routing styles for high speed PCB layouts are microstrips and striplines, both at low layer counts and in the HDI PCB design regime. HDI has become rather routine, as has high speed design with common protocols like Ethernet and USB. Choosing between microstrip vs. stripline routing styles for these protocols often focuses on the wrong metrics or no metrics at all. With that said, which of these is the best choice for PCB routing?
Like most engineering and design choices, there is no objectively "best" routing style for every situation. Deciding between microstrip vs. stripline traces and routing on your PCB requires balancing multiple design choices and signal integrity metrics, and the distinction may be irrelevant in some systems. Let’s briefly break down which routing styles are best in your circuit board for certain signals, and how experienced PCB designers make a decision to use either type of routing.
Microstrip vs. Stripline Tradeoffs
The tradeoffs between microstrips and striplines fall into a few key areas. In particular, the choice of which routing style to use spans beyond signal integrity. Other aspects like routing density in internal layers, the need for isolation, and available space in internal layers will all influence the decision as to which signals are routed as striplines and which are routed on a surface layer as microstrips.
The table below shows a brief comparison of microstrips vs. striplines. Some of the important aspects include routing geometry and density, as well as roughness, isolation, and losses seen by propagating signals.
The point around trace width for a given impedance is quite important, and it will determine the allowable routing density in the design. I’ve discussed this in one of my regular articles on Altium’s PCB Design Blog, and it should be a major consideration in high speed/RF design. While you have freedom to arrange a board in such a way that internal stripline trace routing is required, the width in each layer will be different, and the width needs to be enforced to ensure controlled impedance.
Aside from these points above, both types of routing will be affected by some common sources of signal degradation in any high speed/high frequency system:
- Fiber weave effects
- Crosstalk and coupling
- Dielectric dispersion in the substrate
- Skin effect and roughness effects
Aside from the point about losses in the PCB substrate, we can’t necessarily say in general that one routing style will be “worse” for signal integrity than any other. In some cases, you’ll use controlled impedance with both routing styles to create your PCB layout.
When to Use Microstrip vs. Stripline Routing
In general, you have a choice of using either microstrip or stripline routing in your design. I can’t think of a situation where anyone would tell you that you must use one or the other routing style. One major exception is in a high pin count BGA, where escape routing from interior rows happens through internal layers. A similar example is with backplane connectors, which will have multiple rows of pins that can only be reached through internal layers. I also know some designers that advocate doing all routing through internal layers as striplines to take advantage of shielding from the nearby ground planes.
Here are a few of the primary considerations that go into selecting stripline vs. microstrip routing, as well as some reasons you might select each routing style:
- Short routes between components: There’s no need to route through a via for short routes between neighboring components unless absolutely necessary. In this case, just go with microstrips if impedance control is needed.
- High isolation is required between layers: In this case, use striplines as the ground planes will provide interlayer isolation. Note that this does not guarantee high isolation between striplines on the same layer, and crosstalk needs to be considered in the design.
- High density controlled impedance is required: Striplines provide higher density for controlled impedance routing as a thinner stripline will have the same impedance as a wider microstrip.
- Any high pin count component: Controlled impedance routing into and out of these components will require a mix of both routing styles.
- Ultra-high frequencies: I would not prefer stripline routing here as the via used to access an inner layer will act like its own transmission line, and parasitic capacitance around the via creates an impedance discontinuity and signal loss.
If you need much greater isolation and mode control, or if you’re operating at mmWave frequencies, you can use a grounded coplanar waveguide. This waveguide routing style is simple to work with and design, and there are analytical formulas available in Brian C. Waddell’s Transmission Line Design Handbook, which is my favorite textbook on transmission line design and analysis. If you need extreme isolation due to high external noise or if you’re operating at very high frequencies, you should use a substrate integrated waveguide. Although this particular style appears very unwieldy, it’s a great choice for many RF applications that require interfacing with an antenna or another waveguide. The best PB design firm can help you weigh the tradeoffs involved in each routing style and make the best choice for your system.
Experienced designers can help you decide between microstrip vs. stripline routing to support your advanced products. If your company is pushing the limits of telecom, data center, and low power embedded systems, it pays to work with an experienced electronics design firm. NWES helps private companies, aerospace OEMs, and defense primes design modern PCBs and create cutting-edge embedded technology. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your next high speed digital system is fully manufacturable at scale. Contact NWES for a consultation.