Using Peak Current Flow to Optimize PCBA Layout

Key Takeaways

• Why knowing the peak current flow is important.

• How to determine peak current flow for different signal types.

• How to use the peak current flow to  optimize the board layout.

EKG signal pattern

Wellness is on everyone’s mind these days. What actually contributes to a state of wellness; however, depends to a great extent on whom you ask. For example, some say that wellness consists of your states of physical, emotional, social, intellectual and spiritual health. Others may add occupational and environmental health to this list. There are even those who assert that wellness is not a state at all. Instead, it is the pursuit of a fulfilling and healthy life. Based on these interpretations, it may be wise to ponder your response carefully before responding to the question, “How you are doing.”

One commonality to all definitions of wellness is the importance of physical health as a determining factor.  And the most often used indicator of physical health is the condition of the heart. A quick test for general heart health is the blood pressure test, which yields the maximum pressure exerted by the heart (systolic) and the amount of pressure in your veins in between heart beats (diastolic). An even more comprehensive assessment is obtained through an electrocardiogram (ECG or EKG). From the EKG signal pattern, abnormalities; such as irregular heart rate and/or rhythm can be detected, as well as indications of previous or pending heart attack, lack of blood flow or oxygen and even heart enlargement. 

Just as electrical signals in the body can be used to detect abnormalities, so they can be used to identify contingencies on circuit boards. And the most important indicator on PCBAs is the amount of current flow. One measurement, current density, should always be used to help determine adequate trace and via copper weights for functionality. However, signals can fluctuate, especially from external sources. Therefore, it is critical that you know and design for peak current flow to optimize your board layout for reliability. Let’s take a look at why knowing this current parameter is so important, how to determine it and lastly, how to utilize it for the best PCBA designs.

Why Knowing Current Maximums is Critical 

Most sources on PCBAs are referred to by their voltage or as a power supply. Yet, the driving parameter for all electronics is current. And not having sufficient current can present a number of problems, as listed below.

Results of Undercurrent

 Switch failure 

Transistor-based switching is dependent upon signal level. If current is too low, switching may not occur which will negatively impact circuit operation.

 Misinterpreted data

Digital devices interpret signals based on a threshold level. If this level is not surpassed, highs may be interpreted as lows and vice versa, which may cause malfunctions in processors and other components.  

 Erroneous operation

Undercurrent signals passed to other boards or devices may result in erroneous operation or no function at all. 

Undercurrent can significantly affect functionality and board operation; however, overcurrents can have more severe results, as listed below.  

Results of Overcurrent 

 Shorts

Too much current on traces can result in short circuits between traces and/or traces and components.

 Fire

If the overcurrent is high enough or persists, components may explode or the board may be set on fire and threaten other boards or devices nearby, as well as personnel. 

As shown above, undercurrents can be problematic, but overcurrents can be dangerous. Fortunately, there are methods to determine the peak current flow, as discussed below.

How to Determine the Board Max Currents 

Voltage and current signals alike, can usually be defined by their waveform shape, as shown in the figure below.

Relationship between peak and RMS signals

The signal relationships above; although referred to in terms of voltages, apply to current waveforms as well. Most AC signals can be classified as one of the above periodic types, either directly, or after a transformation; such as Fourier, Laplace, etc.  As shown, from this form the peak value can easily be obtained using the following equation.

    I(peak) = I (rms) x Crest factor                                (1)

    where the Crest factor is dependent upon the waveform type 

        and I(rms) is the average or measured value typically obtained from a multimeter. 

For components, current-voltage (I-V) characteristics from which peak current flows can be obtained are typically available from the manufacturer’s datasheet. For transmission paths, I-V characteristics or curves can be determined by first calculating the impedances of the traces or vias that connect the components. Knowing the peak current flows can now be used to generate the best PCBA board layout, as discussed below.

Board Layout Based on Peak Current Flows

In order to determine the best layout and routing for your board, there are a number of factors that must be considered. These include, the type of signals propagating on and through the board, locations for component footprints, how many and what type(s) of vias to use, type of materials and number of layers for the stackup. Additionally, there are environmental concerns; such as temperature variations, vibrational and flexibility issues, etc. However, failing to consider the peak, current flows may be the most important consideration as it can lead to malfunction or even component and board damage. 

To avoid these contingencies, the following steps should be taken.

Peak Current Flow Design Steps

• Determine the maximum current input and output for all components (datasheets)

• Determine the maximum current flow for all traces and vias (impedance calculations)

• Include protection and/or security circuitry to limit current peaks (fuses, circuit breakers)

• Choose copper weights and trace widths based on an approved standard

The recommended standard for sizing traces is IPC-2152 Standard for Determining Current-Carrying Capacity in Printed Board Design. A sample of available data from this publication is shown below.

IPC current carrying capacity curves

In order to utilize standards recommendations; such as shown above, you first need to know the peak current flows for your design, which are best determined using circuit optimization and simulation. 

With Cadence’s industry leading PCB Design and Analysis software, the best circuit analysis program, PSpice, is at your disposal. The advanced Analog/Mixed-Signal Simulation capabilities, as well as graphical flexibility makes analyzing peak current flows for your board design straightforward and easy to perform.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.