- The Joule heating effect can be described as the power loss in the form of heat energy in a conductor, PCB trace, or interconnect in an IC.
- Once the thermal limits of the PCB and IC are reached due to Joule heating, the electrical current constraints are imposed on the device or board.
- Joule heating effects in ICs increase the DC voltage drop or DC IR drop in the package.
The miniaturization trend has aggravated heating issues in printed circuit boards (PCBs) and integrated chips (ICs).
The miniaturization trend has aggravated heating issues in printed circuit boards (PCBs) and integrated chips (ICs). The high-density packaging adopted in PCB and IC fabrication significantly affects their performance. The Joule heating effect challenges the electrical and thermal performance of circuit boards and ICs. The reliability of a PCB drops under the influence of the Joule heating effect. Designers must take the Joule heating effect into account when designing circuit boards and ICs.
Thermal Design Challenges in PCBs
PCB thermal design poses great challenges as designs become more complex. Assumptions should not be made regarding the temperature distribution on the board during the design phase. Circuit board reliability and integrity are affected by an increase in temperature due to self-heating or other phenomena that demand design accuracy. It is necessary to consider the number of layers, the thermal conductivity of the PCB, component power dissipation, and temperature distribution in power delivery networks (PDN) to achieve better reliability and integrity in circuit boards.
Joule Heating Effect
The Joule heating effect can be described as power loss in the form of heat energy in a conductor, PCB trace, or interconnect in an IC. When a conductor is heated due to the Joule heating effect, there is an increase in the temperature, electrical resistivity, and current density. In effect, it becomes a cumulative process where self-heating increases other quantities such as current density and temperature. Subsequently, a further increase in the Joule heating effect is experienced.
The Joule heating effect is also called ohmic heating, self-heating, or resistive heating. It is the resistance of the material that generates power loss in the form of heat, hence the name ohmic or resistive heating. The mathematical expression for the Joule heating effect is:
H = I2Rt
H is the heat energy produced in Joules and I is the current in Amperes, which flows through the conductor. R is the electrical resistance in ohms and tis the time in seconds for which the current flows through the conductor.
Power Delivery Networks
The power delivery networks (PDNs) in printed circuit boards and ICs are adversely impacted by the Joule heating effect. As the current density of the PDN increases, the heat dissipation also increases. PDNs get heated and spread this heat to other parts of the board and IC. Once the thermal limit of the PCB and IC is reached due to Joule heating, the electrical current constraints are imposed on the device or board.
Joule heating in PCB PDNs can influence electrical performance by imposing current constraints. When the PDN lines reach the glass transition temperature of the dielectric substrate under the effect of Joule heating, it affects the structural integrity of the board. The structure of the PCB or IC package can degenerate under such circumstances.
As IC fabrication technology is downscaled, two changes occur in devices:
- Power supply voltage is reduced
- Power density is increased
These changes critically influence the thermal performance of the IC. The electrical resistance of the interconnects in the IC is temperature-dependent. The electron flow increases the temperature and ultimately leads to the Joule heating effect.
The Joule heating effect in ICs increases the DC voltage drop or DC IR drop in the package. It affects the characteristics of the interconnects, power delivery networks, and internal components as well. The overall performance characteristics of the IC are altered under the influence of the Joule heating effect, and an increase in electromigration-induced problems is observed.
Considering the electrical, thermal, and mechanical reliability of PCBs and ICs, it is important to take into account the Joule heating effect during the design phase. Designing the heat management system in the board and package according to practical thermal challenges helps improve the overall performance.
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