What is the Current Carrying Capacity of Microvia PCBs?

13 October 2023
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Microvias are used to form connections between HDI layers with an aspect ratio of 1:1. They establish conducting pathways between two or more layers. Microvias are created with the help of ultraviolet excimer laser drilling through several millimeters on HDI PCBs. Their diameter is less than 150μm.

They are used to reduce parasitic capacitance in HDI PCBs, which could result in EMI and crosstalk.

Microvias mostly consist of the following.

Pads: These are used to connect the starting and end points of microvias

Antipads: This is a clearance hole between the conductive copper layer and the no-connect metal layer on which vias are not connected to prevent shorting.

Conductive barrels: Barrels are connective tubes that fill the drilled holes and pads connecting each end of the barrel to the plane, trace, or component.

So Much Current Can a Microvia PCB Carry?

The main purpose of microvias is to improve the electrical characteristics of HDI PCBs and aid in the miniaturization of applications. They build a much higher route density by reducing the layer count and eliminating the need for through-hole vias. Microvias also boost the processing power of a circuit board.

The current carrying capacity of a microvia PCB is calculated with the help of the following formula:

I = (K)(A𝜷2)(𝝙T𝜷1)

Where I denotes the current in amperes,

𝛽1 and 𝛽2 have the values of 0.44 and 0.75 respectively
K is a correction factor that is equal to 0.048 in external conductors and 0.024 in internal conductors
A is the cross-sectional area in mils
𝝙T represents the temperature change compared to the ambient temperature denoted in °C

In other words, the current capacity of a microvias PCB depends on the temperature change and the cross-sectional area of the trace.

Factors Affecting the Current Carrying Capacity of a Microvia PCB

There are a few factors that all designers should keep track of to measure the current carrying capacity of microvia PCBs. Let’s take a look at some of the critical ones below.

Temperature Rise vs. Ambient Air Temperature

Ampacity, also known as current carrying capacity, is the maximum current a microvia can carry without exceeding its temperature rating. Since microvias PCBs conduct current, they will, as a consequence of Ohm’s law, generate heat that will increase the temperature of the components and surroundings. It is vital to keep this temperature at a minimum otherwise it could lead to microvia failure.

Furthermore, the temperature of a material changes its resistivity. As a rule, resistivity changes by an appreciable amount for every 1℃ change. Despite being flame resistant, FR-4 materials do not have the thermal properties to tolerate excessively high temperatures. Some designers recommend using thermal lamination in this situation.

The table below takes a look at the resistivity of various metals at 20℃.

Material	Usage	Resistivity at 20℃
Aluminum	Wires, alloys, and traces	2.65✕10^-8
Gold	Gold plating to prevent corrosion	2.44✕10^-8
Copper	Wires, alloys, and traces	1.68✕10^-8
Silver	Silver plating to prevent corrosion and improve resistivity	1.59✕10^-8

Trace Cross-Sectional Area

The cross-sectional area of the trace is directly proportional to current carrying capacity. In other words, increasing the surface area allows for improved heat dissipation, which translates to a higher current carrying capacity. It is recommended to increase trace width to achieve the required current rating.

We can also increase trace thickness (as opposed to width) to achieve the required current rating. However, this may not be possible in most cases because of manufacturing limitations. The cross-sectional area of the trace acts as a heat sink. As the temperature rises, the surface area of the trace can allow current to pass through more effectively compared to a thinner trace. But it may not always be possible to increase trace width because of limited spacing (especially in HDI PCBs). In this case, designers may increase the thickness of the trace by applying solder over the top of the trace.

Other factors one might need to consider include the following.

Capacitance
Impedance
Step response
DC resistance
Power dissipation measured in dBm
Resonant frequency
Inductance
Thermal resistance
Voltage drop

Importance of Measuring Current in Microvias

The size of microvias will determine the overall current rating because they resist the smooth flow of electrons. We know that the resistance of a conductor has an inverse relationship with conductivity. This explains why small microvias are important in high-density interconnect boards. Their smaller resistivity allows manufacturers to pack more functionality into smaller devices.

Microvias allow designers to take advantage of novel PCB design methods to improve the functionality of smaller electronics and innovated the HDI design. Devices like smartphones and wearables are made possible because of the high current carrying capacity of microvias.

The Importance of the IPC-2152 Standard

The IPC-2152 standard makes it possible to understand a microvia PCB’s current carrying capacity based on factors such as thickness board material, via placement, copper weight, trace cross-sectional area, and thermal conductivity.

The IPC-2152 standard also takes into account the distance between copper planes. This eliminates the need to consult imaging software to derive accurate values from charts which is time-consuming.

However, designers should use an online calculator to save time and calculate the correct values. It is possible to directly calculate the current capacity and trace area of microvia PCBs for various temperature changes. The best part is that all this data from IPC-2152 is accessible to the general public.

Wrapping Up

Current calculations are an integral component of HDI PCB design. These calculations will be even more important with next-generation PCBs with more layers and components. Microvias are an absolute necessity in circuit designs because they allow designers to create shooter paths without sacrificing signal integrity or performance.

However, it is important to keep track of current-carrying capacity to avoid the components from overheating.

If you need more information on microvias or need help with your project, talk to the experts at Hemeixin PCB today.