Common Misconceptions and Solutions in PCB Via Design

Now I focus more on how current finds its way home, especially for vias that cross split regions. If the reference layer is incomplete, even the best simulation model can’t save you. I once saw a design that jumped back and forth between power planes, resulting in noise directly coupling into the clock line.

Actually, there’s no need to overcomplicate vias; they’re just necessary channels. But you need to know when to use ordinary vias and when to use back-drilling or even laser microvias. Once, I insisted on using vias filled with conductive adhesive in a 25G serial link. Although the cost was higher, it resulted in a more stable impedance profile.

Ultimately, good high-speed design isn’t about eliminating vias, but about making them behave properly—silent when they should be silent, and conducting when they should be reliable. I increasingly feel that PCB layout is a bit like playing Go; every move requires considering ten moves ahead, and vias are those seemingly insignificant but crucial intersections that determine victory or defeat.

Every time I see those densely packed PCB vias, I’m reminded of the pitfalls I encountered when I first started working with high-speed circuits. Back then, I always thought that just connecting the wires was enough, right? The result was terrible signal quality.

Many people easily overlook a crucial point: we always focus on the traces, forgetting that those inconspicuous vias are the real hidden killers. Especially in high-frequency environments, their impact is far greater than imagined.

The most typical problems I’ve encountered are reflections and crosstalk. In one project, despite neat routing, inexplicable interference kept appearing. Later, I discovered that the vias in several key locations were poorly designed. Those seemingly tiny parasitic capacitances accumulated and produced a significant delay effect.

Pad design is even more interesting. Some people like to make pads extremely large, thinking it’s more reliable, but actually, it increases parasitic effects. My current practice is to control the pad size as much as possible, just enough to meet process requirements. After all, every extra area adds unnecessary capacitive load.

I remember once debugging a board where the clock signal was constantly jittering. After a lot of troubleshooting, I finally discovered the problem was with the buried vias on the inner layers. The coupling between them and the power layer was much stronger than expected. This lesson taught me not to just look at the surface layout.

Now, when designing, I pay special attention to the placement of vias, trying to avoid sensitive areas. Sometimes, I’d rather have an extra loop to ensure a clean path for critical signals; this is much less troublesome than fixing things afterward.

What truly changed my mindset was redesigning an interface module. After optimizing all the via parameters, the eye diagram quality improved by a whole level—more noticeable than replacing the chip with a more expensive one.

So now, in every layout, I consider vias as active components rather than passive connection points. This shift in thinking has significantly stabilized the quality of my designs. You can try it too.

I always feel that many people overcomplicate PCB vias. Sometimes seeing those complex calculation formulas gives me a headache—not that they’re useless—but in actual design, there are often more intuitive ways to judge them.

I remember once encountering an interesting phenomenon while working on a high-frequency board: despite having theoretically calculated sufficient anti-pad gaps—the D2-D1 parameters—the measured capacitance was still significantly higher than expected. I later realized that excessively fragmented planar layers actually negatively impact overall performance—like suddenly adding too many tollbooths on a highway—while the delay at each individual tollbooth might be small, the overall traffic flow collapses. This is especially problematic when handling multi-GHz signals; the impedance discontinuities caused by planar fragmentation trigger noticeable signal reflections, much like turbulence created by water encountering boulders. Using a TDR instrument, I could clearly see multiple spikes in the impedance curve, a reality that theoretical calculations often fail to capture.

Speaking of the SLPCB model—I think its greatest value lies in reminding us to pay attention to the current return path—especially when handling differential signals—two vias placed too close together create additional coupling; two vias placed too far apart disrupt symmetry—finding this balance requires practical debugging experience. For example, in 10Gbps SerDes interface designs, we often use elliptical anti-pads to optimize isolation between differential pairs, while fine-tuning the coupling capacitors by adjusting the size of non-functional pads (NFPs). This fine-tuning often requires iterative iterations using electromagnetic field simulation tools; simply relying on geometric spacing formulas often yields limited results.

In a recent project, we tried a new method: placing the vias of the power bypass capacitors directly on the edge of the capacitor pads—instead of the traditional center position—although this deviates from textbook recommendations—but actual measurements showed a reduction in parasitic inductance of approximately 15%. Of course, this approach demands higher placement precision—requiring prior communication with the soldering plant regarding process capabilities. In fact, this edge-out via method shortens the current loop area, making it particularly suitable for powering BGA-packaged chips. We even experimented with symmetrical vias on both sides of the pads to form parallel paths, further reducing the equivalent inductance, but care must be taken to avoid excessive cutting of the pad copper, which could affect mechanical strength.

In fact, the most easily overlooked aspect of VIA design is material properties—the same geometric dimensions can exhibit significant differences on different substrates. For example, once when switching substrate suppliers, even with the same nominal dielectric constant, the resonant point shifted by 10%. It turned out to be due to different fiberglass cloth weaving methods. For instance, although 1080 and 2116 fiberglass cloths have similar nominal Dk values, differences in the local dielectric constant distribution at fiber bundle intersections can significantly affect phase consistency in the millimeter-wave band. This lesson has led me to now require suppliers to provide detailed material structure diagrams when doing high-frequency designs.

I increasingly feel that good VIA design is more like urban planning—ensuring the rationality of individual buildings while also considering the coherence of the overall traffic flow. Simply pursuing perfection in a single parameter can cause system-level problems. This is why I always advise novice engineers to focus on measured data rather than obsessing over theoretical calculations—after all, circuit boards are ultimately for powering up, not for solving math problems. For example, the layout of via arrays must consider thermal expansion coefficient matching; otherwise, CTE mismatch can easily lead to solder joint cracking during temperature cycling tests. Once, a heat dissipation via design we made failed to consider the thermal deformation difference between the aluminum substrate and FR4, resulting in microcracks in a low-temperature environment.

There’s a little trick to via layout for differential pairs: you can intentionally make a small bend in the inner layer routing—allowing the layer transition point to fall in a more spacious area—this provides enough space for the anti-pad—although it increases the trace length slightly—it greatly helps maintain impedance continuity. In practice, we use 3D electromagnetic simulation to check the field distribution in the layer transition area, and sometimes even use back-drilling technology to remove excess via stumps, like pruning branches to make the trunk clearer. Especially in high-speed links above 25G, this proactive optimization can effectively reduce resonance spikes on the insertion loss curve.

Sometimes I laugh out loud when I look back at boards I designed five years ago—back then, I always thought that the denser the vias, the better—now I actively leave breathing space for each via—this change in design philosophy is probably what growth is all about. Now I pay more attention to the transition structure between vias and transmission lines, such as using a gradually tapered transition section to smooth impedance changes, just like a highway ramp needs a reasonable buffer zone. I’ve recently been experimenting with adding grounding shielding via arrays around vias, which has proven highly effective in suppressing electromagnetic leakage in the 72GHz band.

I’ve always felt that many people misunderstand PCB design—they focus on routing rules while neglecting seemingly insignificant details. Take vias, for example; although small, their handling often determines the success or failure of the entire board.