In the context of a trace width calculator for printed circuit boards (PCBs), temperature rise refers to the increase in the temperature of a copper trace on the board due to the flow of electrical current.
Understanding Temperature Rise
Based on industry standards and practical experience, temperature rise is specifically defined as:
- How much hotter the trace will get with current flowing in it compared to without.
When current passes through a conductor like a PCB trace, resistance causes power to be dissipated as heat. This heating effect is proportional to the square of the current and the resistance of the trace ($P = I^2R$). The temperature of the trace will rise above the ambient temperature of the surrounding air or board until the heat dissipated is balanced by the heat transferred away (via convection, conduction, and radiation).
Why Temperature Rise Matters
Controlling temperature rise is critical in PCB design for several reasons:
- Component Reliability: Many electronic components have maximum operating temperature limits. Excessive trace heat can raise the temperature of nearby components, potentially shortening their lifespan or causing immediate failure.
- Board Integrity: PCB laminate materials (PWB - Printed Wiring Board material) also have temperature limits. Exceeding these limits can cause delamination (separation of layers), degradation of dielectric properties, or physical damage to the board itself.
- Performance: High temperatures can affect the performance of sensitive analog circuits or impact signal integrity in high-speed designs.
Factors Influencing Trace Temperature Rise
Trace width calculators use empirical formulas or data based on standards (like IPC-2152) to estimate temperature rise. Key factors considered include:
- Current: Higher current leads to significantly more heat generation.
- Trace Width: A wider trace has lower resistance, reducing heat generation for the same current. It also has a larger surface area for heat dissipation.
- Trace Thickness (Copper Weight): A thicker trace also has lower resistance and more mass to absorb heat, reducing temperature rise. Copper weight is typically measured in ounces per square foot (oz/ft²).
- Trace Length: Longer traces have higher total resistance, contributing to more heat.
- Board Layers: Traces on outer layers dissipate heat more easily into the air than inner layers, which rely on conduction through the board material.
- Ambient Temperature: The starting temperature of the environment the board operates in. Temperature rise is added to this ambient temperature to get the final trace temperature.
- Airflow/Cooling: Convection cooling significantly impacts how much heat can be removed. Forced airflow (fans) reduces temperature rise compared to still air.
Deciding Acceptable Temperature Rise
Choosing an appropriate maximum temperature rise is a crucial design decision. As the reference states:
- You have to decide how much temperature rise your board can handle based on the operating environment and the type of PWB material used.
This decision involves:
- Knowing the operating environment: What is the maximum ambient temperature the board will experience? Is there airflow?
- Understanding PWB material limits: Different laminates (e.g., FR-4, High-Tg FR-4, Polyimide) have different maximum operating temperatures (often related to the material's glass transition temperature, Tg, or decomposition temperature, Td).
- Considering component limits: The hottest spot on the board, including trace temperature, must not cause nearby components to exceed their maximum ratings.
Designers typically set a maximum allowable trace temperature rise (e.g., 10°C, 20°C) as an input to the trace width calculator. The calculator then determines the minimum trace width (and potentially thickness/copper weight) required to stay within that limit for a given current.
Example Scenario
Let's say you have a trace carrying 5A and your FR-4 board is operating in a 40°C ambient environment. The FR-4 material might have a maximum recommended operating temperature of around 105°C. You need to ensure the trace temperature doesn't exceed this, with some margin.
You might decide your maximum allowable trace temperature is 90°C. This means your maximum allowable temperature rise is 90°C - 40°C = 50°C. You would input the current (5A) and the desired temperature rise (50°C) into the calculator to find the minimum required trace width and copper weight. Often, a lower temperature rise is targeted (e.g., 20-30°C) for better reliability.
