The point is to keep the spacing small in comparison to a wavelength in the substrate dielectric so as to make the fence appear solid to impinging waves. Suggested practice is therefore to stitch/shield ground planes on an RF board with vias spaced at a minimum 1/10th the wavelength of the highest frequency of interest. When signal frequencies get high enough that the wavelengths are of similar size to copper features on the PCB the two ground planes become helpful waveguides, bouncing electromagnetic waves between them on their way out through the side of the board. Sadly, insulators of current – including the FR4 dielectric core of the PCB - are good conductors of electromagnetic waves. The goal here is to fence in electromagnetic waves radiating from these tracks. Stitching of copper around tracks at high frequency is done for different reasons entirely. In this role, stitching vias tend to be used on high impedance, analog circuitry (ADCs, SMPS, etc.) where there is a lack of solid reference planes. It’s important therefore to consider whether the stitched copper is effectively grounded at the frequency of interest. Stitched copper between tracks therefore has the same effect as a smaller gap (more crosstalk!) unless the copper is well grounded. A wider gap means less capacitance and less crosstalk. Think of two adjacent traces on the board as the terminals of a capacitor with the gap between them as the dielectric. Stitching copper can also be used to reduce capacitive coupling between signals. This advantage would go away of course if the whole board is used to carry current and therefore there is no spot which is cooler for the heat to spread out into. Stitching this construction with vias produces quite a bit more through-plane conductivity which carries and dissipates heat to the core, reducing overall temperatures. If tracking is only on the outer layers then a lot of the heat is carried sideways and the core may well be cooler than the traces. A PCB is more conductive horizontally (across the layer) than it is vertically (through the board). On some PCBs stitching vias can be helpful in distributing heat. Via stitching can be an effective and low-effort way to more tightly couple ground across the PCB. As soon as any current starts to flow it will cause a voltage across the copper through which it is flowing, serving to both spread the current out but also cause the ground to 'bounce' around depending on which part of the PCB is being looked at. Probably the most common use for stitching planes with vias is to ensure short return paths for signals or to help maintain a constant ground. There are many different views on when and how to use stitching / shielding vias on your PCB, some of which we’ll summarize in this post. Via shielding (sometimes called a picket fence) is where one or two rows of vias connect copper pour together at the perimeter of some tracks or the copper pour areas. Via stitching on the PCB is where a large number of vias are used to connect copper areas on different layers together.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |