PTH Via

Current Carrying Capacity in PCB Vias

Current Carrying Capacity of PCB Conductors and Vias

The current carrying capacity in PCB conductors is a common question from new designers. According to Rush PCB UK, along with the current carrying capacity of PCB conductors, the current carrying capacity of PCB vias is of equal importance, especially when the design is of a new board that must carry high currents. The designer must keep the conductor and via temperatures below an appropriate limit, and this in turn helps to keep components on the board cold enough.

Although the IPC 2152 standards deal extensively with the recommended current carrying capacity of traces, they focus much less on vias in multi-layered boards. However, there have been several investigations into current carrying capacity and temperature limits, while comparing them to the temperature excursions of typical traces carrying the same current.

Importance of Via Current Carrying Capacity

Designers typically specify the current carrying capacity of traces that will be carrying high current. They usually determine this using the copper weight and the allowable temperature rise, while using the nomograph in the IPC 2152 standards. They aim to size the traces such that the components and the board will remain within safe temperature limits while operating. If there are vias on the traces, it is important to compare the temperature rise in the vias with that of the traces and or planes to which they connect.

PTH Via

Figure 1: Various Types of PCB Vias

As the via connects, at each end, to a hot trace that is carrying a high current, it is reasonable to expect the via temperature to be at least as high as that of the traces that connect to it. Planes and traces connected to the via can get quite hot with the passage of high currents, especially for low copper weights carrying 5-10 Amperes. Therefore, it is natural to expect heat accumulation in vias. Also, with exposure to air for traces on the surface layers, it is natural to expect they will run cooler compared to traces buried in the interior layers of the board. These are significant considerations that relate to the via reliability, especially for microvias.

In reality, the situation is contrary to popular intuition. Traces on the surface run at temperatures higher than that of buried traces in internal layers. At 25 ℃, the thermal conductivity of air is approximately 0.026 W/mK, while that of FR-4 is about 0.25 W/mK. In addition, alternative substrate options are available that offer even higher thermal conductivity. That means the substrate acts more like a heat sink for conductors passing through it. As the substrate also surrounds vias, the above applies to vias as well.  This helps to explain the fact that vias tend to have a lower temperature compared to traces that connect to them. An article in the Signal Integrity journal, by Douglas Brooks and Johannes Adam, substantiates the above through measurements and results.

Read About: Make Your PCBs More Reliable with Vias

Rule of Thumb

Engineers often prefer to follow the 0.5 A rule of thumb. As this rule offers a conservative result, it is acceptable to follow in most cases. However, for higher DC currents, an excessive number of vias may do more harm than good when the designer is connecting them to planes. For instance, the designer may have set a limit of 1 A per via. If they must supply 5 A instantaneously, they can safely place 5 large vias with thick plating, as long as the via temperature is not too high near a component.

In practice, the danger in the above example is not about the high temperature in the vias. Rather, it is more about temperature cycling. If the temperature swings between very low and very high temperatures, it might lead to fatigue setting in, resulting in failure.

Analysis of Current Carrying Capacity of Vias

In practice, thin traces on outer layers exposed to air and carrying high currents often operate at higher temperatures as compared to the temperature of vias connecting to them, although the difference is only a few degrees. This is due to the thermal conductivity of air being lower than the thermal conductivity of the substrate material surrounding the via. The net effect is the via loses heat faster than the thin traces can dissipate it into air.

While the above is true for thin traces, the situation reverses itself for wider traces. For instance, traces with widths of 200 mils and above can operate at temperatures lower than that of connected vias, although, the temperature difference is only a few degrees. This is because the heat loss from the wide trace now has two components. While the trace loses heat to the surrounding air, it also loses heat through the higher thermal conductivity of the substrate in contact with it. As the heat loss depends on surface area, and the trace is wide, the exposure is now through a much larger surface area of the trace. This allows a wide trace to have a lower temperature at equilibrium. Therefore, it is safe to summarize as follows:

  • For thin traces, vias act as heat sinks for the trace
  • For wide traces, the trace acts as a heat sink for the via

Of course, there are additional factors in the above analysis—the contribution from planes in layers. In reality, large planes act as additional heat sinks, further lowering the operating temperature of the conductors. Moreover, the designer can use alternative substrates with even higher thermal conductivity than that of FR-4. This removes more heat from conductors and vias, leading to an even lower temperature at equilibrium.

Deciding on Via Size

The designer must size the conductors according to the IPC 2152 standard guidelines for carrying high currents. They must also provide thick-walled vias. As the temperature of the via will not rise above that of its conductor, the design will not require further considerations related to vias.

The heat from the via will dissipate into the substrate and the nearby planes and traces. If the traces already have wide surfaces, they will dissipate more heat from their larger surfaces as compared to that dissipated by the via. Therefore, as the heat leaves the traces faster than it does from the via, the entire system will operate at a lower equilibrium temperature.

The current carrying capacity of the via also depends on its thermal conductivity, which the designer can control by adjusting the copper weight, thickness of the prepreg material, and/or the material filling the via.

Conclusion

Rush PCB UK recommends using a reputed PCB CAD design software that allows creating professional via designs and building a stackup from a wide variety of standard substrates.

PCB

PCB Vias and Everything You Need to Know About Them

Understanding Different Types of Vias in PCBs

Looking at a complicated Printed Circuit Board (PCB) such as the motherboard of a computer, you are likely to find several tracks going nowhere, and terminated rather abruptly. However, a closer inspection, preferably with a magnifying glass, will reveal more details at the point of termination of the track. Most likely, you will see it ending in a small PCB pad, not much larger than the width of the track itself, with or without a hole in its center.

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Fig.1: Tracks on a PCB

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Fig.2: Close-Up of a Via

In reality, the track does not terminate, but rather continues to travel, albeit on a different layer, hidden under the outermost layer of the PCB. The pad at its end is actually a small pipe through the insulating material, electrically connecting the two parts of the track. In PCB terminology, such an arrangement that allows tracks to continue, but on a different layer, are known as a PCB vias.

Types of Via in PCB

Multilayer PCBs use different types of vias for various purposes. There might be through-hole vias, blind vias, and buried vias in the same PCB. Although the construction of all vias is same, their nomenclature depends on the layers of origin and termination.

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Fig.3: Type of Vias

For instance, a via originating from the outermost layer, traveling through the board, and terminating at the other outermost layer is a through-hole via. In its passage through the layers of the board, it may or may not connect to intermediate layers, depending on the necessity of the electrical circuit.

A blind via originates from one of the outermost layers, but terminates on an intermediate layer, and therefore, is visible only on the originating layer. It may or may not connect to other layers in between.

A buried via is not visible from either of the outermost layers, as it originates in one of the inner layers and terminates in another inner layer, possibly connecting other layers in between.

Construction of a Via

By design, a via consists of two outer pads and a copper tube electrically joining them. The two outer pads reside on the originating and the terminating layers of the PCB, while antipads on all intermediate layers allow electrical isolation of the copper tube from the electrical circuits on these layers as it passes through.

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Fig.4: Construction of a Via

While the two outer pads and antipads are part of the layout pattern a fabricator etches onto the PCB, an electrode position process forms the copper tube connecting the two. Although in regular multilayer PCBs, you may find through-hole vias, these are less likely in high density interconnect or HDI boards.

Difference Between Plated Through Hole and Via

The major difference between the two lies in their construction process. A fabricator can electroplate a through-hole only after assembling all the layers of a multilayer PCB, since a through-hole spans all the layers, while he can form a complete via, including electroplating it, when assembling each layer pair.

Another advantage with vias in multi layer PCBs is, the designer can either stack or stagger them to suit the requirements of the circuit layout, while he or she cannot do that with a through-hole. Therefore, vias help in increasing the layout density of a board, allowing the designer to reduce the size of and/or number of layers on a multilayer PCB.

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Filling a Via

The designer may decide to fill the vias in the PCB during manufacturing. While blind vias require filling to avoid surface dimpling, some designers may specify an additional epoxy filling after lamination to maintain better surface flatness.

You can ask your PCB fabricator to fill the vias with either epoxy or metal epoxy. The choice between epoxy and metal epoxy is that the latter is conductive. Therefore, if you have designed the via with a thermal application in mind, for instance, to disperse heat from one side to the other, filling the via with metal epoxy will be a better choice as opposed to epoxy filling of the non-conductive type. As its barrel always has a layer of copper, a via always retains electrical continuity, regardless of whether the filling is conductive or not.

In highly dense PCBs, especially those with fine-pitch components such as BGA, fabricators fill PCB vias with epoxy and planarize them to make them flat. Flash plating over them makes them perfectly flat and suitable for mounting BGAs.

Other applications may need a Faraday shield on one side of a chip, which could double as a heat sink as well. Stitching the underside of the chip with vias is a standard practice, while filling them up with a conductive epoxy fill, helps in the heat conduction.

When concerned with EMI, you may use multiple vias in the region of a ground strap, filling them up to provide a conductive wall. An impedance-controlled structure may also benefit from closely spaced and filled vias on either side.

Special Vias

Although you will find warnings about placing vias within pads as these can siphon off solder paste while soldering, leaving the joint devoid of solder, you may not have much choice when designing with very closely pitched BGA packages. The space available around the pads may not be adequate for a dog bone, and the only option may be a via in the SMT pad, or partially in it.

You can get over the solder siphoning by having the via epoxy filled, flattened, and plated to encapsulate it. The other side of the PCB via may not be important enough and you can wall it off with a mask.

Sometimes, to attain very high routing densities, it may be necessary for the designer to use landless vias. The trace directly enters the hole without a PCB pad. As the vias do not have PCB pads, the designer can pack in more traces in between adjacent vias.

Stacked Vias and Laser drilling

Even after using blind and buried vias, you may still not have enough room for proper routing. In such circumstances, you may consider using laser drilled micro vias and or stacked vias. The two major benefits of laser drilling are extremely fine holes (sub 0.004”), and excellent registration. Both are obvious benefits for very dense parts.

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Fig.5: Laser-Drilled Micro Via

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Fig.6: Staggered and Stacked Micro Vias

Laser drilling does not pass through the layers, unlike that in mechanical drilling. It vaporizes the top copper layer, burns through the substrate dielectric layer beneath it, and stops when it touches the bottom copper layer. This accounts for the v-shaped pit as against a straight hole a mechanical drill-bit creates.

For stacked vias, designers place laser drilled vias directly on top of each other. However, designers typically use stacked vias only when board real estate is at a premium.

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Vias and Signal Integrity

Just as people do, electrical signals too find it much easier to take a direct route when traveling from point A to point B. A via in the signal path forces the signal to take a detour, and the signal integrity suffers as a result.

For high-speed signals, there is also the challenge of via stubs. For instance, a via taking a trace from L1 to L3, may leave a stub down to L16. A high-speed signal will typically traverse all the way from L1 to L16 before reflecting to L3. This will attenuate the signal, as the effective electrical stub length will be almost double its mechanical length. Designers of thick boards take care of the problem by removing the unnecessary part of the barrel by back drilling. HDI boards do not face this problem, as they use laser drilled micro vias and stack them up to the desired layer.

Conclusion

The above types of via in PCB can increase density and bring down the cost in volume production. Laser drilled PCB vias can increase multilayer density and reduce layer count, without reducing the trace width or trace spacing.

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