# Steady-State vs. Transient Heat Transfers in Electronic Circuit Boards

### Key Takeaways

• In steady-state heat transfer, the temperature is constant throughout time.

• In transient heat transfer, the temperature changes with time.

• The heat flux and temperature field images of a circuit board (obtained by conducting steady-state heat transfer analysis) can help designers optimize heat sink geometry and location.

All processes go through a transient and steady-state. These ripples represent the transient state of a pond

My favorite childhood pastime was throwing pebbles into a pond. I was particularly fascinated with watching the ripples fade out and the water become still. Once the pond was calm, it was time for my next throw. In this scenario, the still water represents a steady-state of the pond and the ripples represent a transient state.

If we consider any physical process—for example, oscillations, vibrations, or heat transfer—there is a steady-state and transient state. All processes come to a stable state of rest after going through a transient state. In a steady-state heat transfer, the temperature is constant throughout time, and in a transient heat transfer, the temperature changes with time.

Let’s compare steady-state vs. transient heat transfers further.

## A Basic Overview of Steady-State vs. Transient Heat Transfer

Heat transfer is the transfer of heat energy from one region to another due to temperature differences between the two. The heat flow is always from a higher temperature to a lower temperature. The temperature gradient is the driving force of any heat transfer and the net heat transfer is equal to zero if there is no temperature gradient. The three modes of heat transfer are conduction, convection, and radiation. Whatever the mode of heat transfer, there is a steady-state as well as transient heat transfer.

Let’s compare steady-state heat transfers vs. transient heat transfers.

If a heat transfer is characterized by a constant, specific rate of heat transfer, then it is a steady-state heat transfer. Steady-state heat transfer can be a conduction, convection, or radiation process.

Irrespective of the mode of transfer, in a steady-state heat transfer, the heat flow rate remains constant at any point in time. Steady-state heat transfer can be described in terms of the temperature as the variable; the temperature of the system remains constant over time in steady-state heat transfer after thermal equilibrium is attained.

### Transient Heat Transfer

Transient heat transfer can also be called unsteady-state heat transfer. This type of heat transfer only exists for a short duration of time. In transient heat transfers, the heat energy transferred through the medium is not constant. The heat flow rate keeps changing, and the cause of the varying rates of heat transfer can be either fluctuating temperature differences over the medium or changes in properties of the medium.

In the physical world, heat transfer starts as transient and then reaches a steady-state until thermal equilibrium is reached.

## The Steady-State Temperature’s Impact on Component Failure Rates

In electronic circuits, the steady-state temperature of a component is harmful to the component’s lifespan when it crosses the limit stated in its datasheet. Failure rates of components exponentially increase as steady-state temperatures increase. However, temperatures can be controlled by attaching heat sinks such as power electronic switches, diodes, or LEDs to components. The geometry of the heat sink, aspect ratio of the fins,  pressure, and aerodynamics all influence the heat transfer from the component to the heat sink and from the heat sink to the ambient.

The heat flux and temperature field images of a circuit board—obtained by conducting steady-state heat transfer analysis—can help designers optimize heat sink geometry and location. The need for forced convection or cooling fans and slit openings in the casing of a computer processor or an enclosed electronic circuit can be determined from steady-state heat transfer analysis.

## Transient Heat Transfer Analysis

If a designer would like to determine the temperature profile versus the time of an electronic circuit board, transient heat transfer analysis should be conducted. In cases where the heat generation of a component stops, designers can determine the rate of cooling using transient heat transfer analysis. The rate of reheating is obtained when heat generation restarts in a component.

Comparing steady-state vs. transient heat transfer analysis shows us that both have their own merits and both are essential to improving the thermal performance of electronic circuit boards.