PCB ECAD software is highly intelligent and customizable to meet the needs of modern electronic design and manufacturing. Perhaps the most helpful tool for designers is the robust electronic rule checks that actively determine whether users are violating the predefined parameters for manufacturability; instead of waiting for a design review or feedback from the manufacturer, designers can update the layout in real-time. One of these concerns is clearance for electrically active board elements: within the layer, users can set distance between traces, copper pours, and vias/plated through-holes that prevent shorting. Clearance rules also aid performance by limiting electromagnetic interference (EMI) with appropriate spacing to prevent coupling; however, these rules do not apply to interlayer signal coupling, also known as broadside coupling. Unintended broadside-coupled striplines can become a significant source of EMI, and increasing high-density interconnect (HDI) boards mean designers are more likely to encounter them in modern layouts.
| Comparing Broadside-Coupled Striplines to Edge-Coupled Striplines | |
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| Broadside | Edge |
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How Broadside-Coupled Striplines Can Imperil EMC
Broadside-coupled striplines can be an intentional or unintentional method of coupling signal traces. The structure occurs in any stackup with multiple signal layers where traces overlap significantly in the z-axis (i.e., normal to the routing plane); a perfectly broadside-coupled stripline would have two traces stacked on each other. The terminology “broadside” differentiates the coupling from edge coupling, which occurs when routing two parallel traces within the same layer (e.g., differential signal lines).
When dealing with unintentional broadside-coupled striplines, it’s vital to understand that coupling is most intense when the traces align in the z-axis of the stackup; reducing the crosstalk between these signal lines to an acceptable value may require spacing between the offending signal layers ten times that of standard layer separation. This much dielectric between the signal layers will significantly increase processing costs (in addition to higher material costs for more/thicker prepreg) and also can stymy enclosure/system integration with a substantially thicker board. Because it’s unreasonable to expect designers to compromise substantially on board thickness or crosstalk/EMI concerns, designers can instead offset the offending broadside-coupled striplines in their respective routing layers: achieving appreciable noise minimization takes only half the distance of z-axis spacing for overlapping traces. This distance is trivial for most board configurations as planar layer area is relatively massive compared to the board thickness.
Any circuit with dense pinouts is susceptible to broadside coupling in multilayer boards, with ball grid arrays (BGAs) being notorious. There are multiple routing strategies to prevent broadside coupling when breaking out from the pins, but consider an alternating horizontal/vertical routing strategy on signal layers. Broadside coupling is minimal in this configuration due to the limited overlap between the crisscrossing signals. When a criss-cross method is unsuitable for the layout, ensure adequate spacing between traces on different signal layers in the same direction; in other words, preventing broadside coupling is similar to maintaining clearance on same-layer signals.
Manufacturing Considerations for Broadside Coupling
Broadside coupling is a desired stackup structure in some uses where edge coupling is unavailable. However, some critical manufacturing considerations can affect circuit performance and reliability:
- Broadside coupling relies on two different substrates in the stackup. While these can be the same material (and, thus, have the same electrical properties), a stackup with different dielectrics would be a non-starter.
- Despite broadside-coupled striplines requiring straightforward processing during fabrication, it can be difficult to achieve targeted impedance values on controlled impedance lines.
- The ideal arrangement is a total overlay of striplines to maximize coupling. This configuration helps balance against fine-width traces where misregistration can significantly reduce coupling. In other words, edge-coupled striplines do not have to account for layer misregistration, which makes their impedance error less volatile relative to the targeted value.
What’s the advantage of broadside-coupled striplines? The very disadvantage of strong coupling in the z-axis that occurs when traces align in the direction of the board thickness can become a boon in very dense layouts. Instead of routing differential lines side-by-side on a single signal layer, designers can “divide and conquer” their coupled striplines across multiple signal layers. However, additional timing issues with via transitions can come into play; ultimately, most applications will benefit more from edge-coupled striplines than broadside-coupled striplines. For cases where the layout density seems insurmountable, designers can lean on broadside coupling when the situation dictates.
Your Contract Manufacturer Can Help Guide Layout Strategies
Broadside-coupled striplines are often an unintentional byproduct of the board layout that can significantly hamper EMI performance. However, preventing them can be as easy as a crisscross approach to routing on alternating signal layers that minimize the coupling between overlapping traces. Regardless of the challenges of an individual layout, VSE is here to help with thorough design reviews pre-production that diagnose potential manufacturing issues. Our engineers are committed to building electronics for our customers, including identifying and correcting runtime performance problems before they arise. We’ve been realizing life-saving and life-changing devices with our valued manufacturing partners for over forty years.
