Vias are a fundamental component of PCB design, allowing designers to route vertically and connect layers of the board without any wiring or assembly. Inherent to this ability to tie points on multiple layers together, vias can also offer a highly flexible method of improving performance relating to EMI and current density that can be grounds for design failure during testing for safety reasons. While the terms are sometimes used interchangeably, via stitching vs. shielding are two methods to improve board performance when other design methods prove fruitless.
|Via Stitching vs. Shielding – An Overview|
An Introduction to Via Stitching vs. Shielding
Via stitching and via shielding are similar techniques from the design side, yet their purpose is distinct:
- Stitching – Via stitching ties the copper at the board position to some internal net.
- Shielding – Shielding acts as a tool to resolve EMI in the design. An array of shielding vias can isolate sections of the design from the environment by creating a gap too small for the emitted wave to traverse. For this reason, via shielding is also known as via fencing.
While the downside to incorporating either method via pattern is relatively small for the benefits available, these via patterns should not be added frivolously. First, HDI designs will likely preclude via patterns as they require significant routing and layout space. Second, these via patterns can become significant cost adders on high-volume production, where the addition of tens to hundreds of vias per board may get multiplied out thousands of times. Beyond cost, an unnecessary via array can significantly increase the manufacturing work/time necessary to fabricate a design.
Cautions With Via Shielding
Via shielding is a more complex application and requires a thoughtful approach from the designers. The technique aims to tie ground signals surrounding outer layer signal traces with an internal ground layer, effectively isolating the signal from nearby surroundings. The ground-encapsulated signal, ideally, blocks coupling and interference from the fenced signal to nearby traces (and vice versa) by preventing signal propagation according to the wavelength of the signal’s frequency. Best practices (to be certain, check performance in simulation software) dictate that via spacing for fencing should be approximately equal to λ/20, where λ represents the wavelength of the highest frequency; spacing vias wider than this will allow for the signal to pass through the via fence, while spacing vias closer than this adds additional processing cost and time for no benefit. Improperly spaced via fences can amplify EMI if the spacing is at the resonant frequency of a nearby trace, resulting in a completely detrimental addition to the design.
Via shielding should ideally occur post-prototyping, after identifying any EMI concerns during preparation for off-site EMI testing or other similar design benchmarks. While measurement methods or a strong design eye may detect the need for shielding before a board is production-ready, the need for this corrective method often comes into view after production. Designers should be careful not to add unnecessary features to a board layout that, at best, provide marginal benefits (for a specific design) while potentially complicating fabrication or assembly.
Typically, EMI results are better addressed at the root whenever possible. For all things in PCB, this comes down to the design of the stackup. Ensure proper signal and ground/power plane layering, and avoid common pitfalls like routing over split power planes or gaps in ground layers that increase EMI due to a larger current return loop. Via shielding has its place, particularly in high-speed microwave circuits, but designers should first exhaust simpler methods (like separating signals in the plane) to avoid complicating bare board processing.
The Numerous Applications of Via Stitching
On the other hand, stitching has much less commitment on the design end, and designers may decide to implement stitching vias for many reasons. Primarily, vias can help avoid current density issues, like those arising from power-generating circuitry. Additional stitching vias here help provide additional conductor area to move charge carriers, improving electrical and thermal performance. Beyond arrayed configurations for power nets, stitching vias can also provide a continuous ground reference: this is most often the case when performing a layer transition with differential pairs (the two signal vias receive equidistant ground vias) or at the edges of ground pours to ensure the ground reference remains accurate. In both cases, the stitching via prevents an overly circuitous return current path (and associated EMI with a larger loop), instead substituting a short, direct current path of low impedance.
Your Contract Manufacturer Optimizes Performance and Board Features
Whether via stitching vs. shielding, arrays can exist simultaneously within a design, and layout designers need to understand the situations that warrant each rather than adding unnecessary cost with superfluous drilling – a significant cost increase in high-volume production. While using via stitching is more forgiving than shielding (there’s rarely a wrong opportunity for a short, low-impedance return path), carefully applying either method ensures fabrication costs remain competitive and designs lean. Whatever your designs entail, we at VSE are eager to collaborate to optimize performance and minimize cost. Our engineering team is committed to building electronics for our customers, and we’ve been bringing life-saving and life-changing devices to market with over 40 years of experience in NPI and mid-volume production.