Understanding PCB operating temperature
A guide to controlling PCB temperature
Our aim is to help you understand the role of operating temperatures in PCBs, and how you can ensure the long life of your design. We’ll cover:
First, PCB temperature is measured in degrees Celsius, which is used worldwide. To give you an idea of how degrees Celsius translates to fahrenheit, familiarize yourself with this formula, with T being temperature:
T(°F) = T(°C) × 1.8 + 32
Electronic applications depend on printed circuit boards, which makes reliability, performance and above all, safety, critical. This means engineers must understand how temperature affects the integrity of their PCB assembly in the design stage, before PCB production starts.
PCB design involves components such as holes, traces, through holes, layers and solder masks. Each of these areas are impacted by heat. Standard PCBs – or any PCB – can suffer adverse reactions if the temperature is too high. The PCB material involved determines the PCB max temperature, or PCB temperature limit.
But what is the average temperature for PCBs? What matters is the material used in your circuit board, which means every PCB’s temperature will differ.
The most common substrate in PCBs is FR-4, a flame-retardant glass epoxy laminate. It can withstand temperatures up to 90°C to 110°C.
A high-temperature PCB is typically defined as one with a Tg (glass transition temperature) greater than 170°C, although other people consider the starting point as 150°C. Depending on the high-temperature PCB supplier, this will, of course, vary.
Monitoring temperature is critical to determining the hot-power areas and power-hungry ICs. This will help you adjust or correct any issues early. But before you check your PCB temperature range, you need to think about thermal expansion.
If your PCB has more than one layer, you should ensure that the coefficient of thermal expansion (CTE) aligns.
CTE measures how much materials expand in high temperatures. Ideally, the dielectric layers should share a similar CTE to your copper layers. If not aligned, each layer will expand at vastly different rates, which leads to serious problems.
Measuring PCB temperature
Before you determine your PCB maximum operating temperature:
- Determine the PCB’s primary heat source (usually the microcontroller or microprocessor).
- Check that the PCB temperature sensor is in place.
- Find the ground (GND) pins, which you should have attached to the substrate of the heat source. It’s through these GND pins that around 60%, 65% of the heat generated moves to the temperature sensor. Because the GND pins connect to the substrate, they have the least thermal resistance of the components between the sensor and heat source.
How to measure PCB temperature:
- To create effective thermal equilibrium, place a ground plane between your temperature sensors and the heat source.
- Connect the GND pins of each temperature sensor to the heat source’s ground plane.
- Ensure the temperature sensors and the heat source are as close as possible to each other.
Glass transition temperature (Tg) determines at what temperature the PCB’s base material becomes mechanically unstable. PCB glass transition temperature is lower than PCB melting point.
PCBs all have different glass temperature values based on their substrates. If the PCB’s operating temperature exceeds its Tg value, the PCB will change its state. PCBs with a higher glass transition temperature possess a better stability at higher temperatures, as the substrate offers good heat resistance, and chemical and mechanical stability.
Your Tg value should be at least 20°C to 25°C higher than the expected operating temperature of the device you’re designing for. So if your PCB’s Tg value is 130°C, then the device’s working temperature should be lower than 110°C.
PCB copper traces
Copper traces generate heat due to I2R (the formula for electrical efficiency) power dissipation in the trace itself. Cooling occurs due to thermal conductivity into the substrate material, convection and radiation. When the rates of heating and cooling are equal, the result is a stable PCB temperature.
The current capacity of your PCB’s copper trace is determined by several factors, two of which are the trace width and maximum temperature rise required.
Trace width is a function of several variables. This includes the resistance and current through it, along with the allowed temperature rise.
Typically, a temperature rise of 10°C is permitted above the ambient temperature of 25°C. If your board material and PCB design allow, a 20°C temperature rise is even permitted.
What happens if the temperature rise is too high?
Let’s say your temperature rise is 50°C or even 100°C. A copper trace on an FR 4 board can handle that, but it’s going to impact the ambient temperature of components nearby, affecting the integrity of your board and resulting in power loss.
The causes of too high temperatures in printed circuit boards are many. The most common reasons are:
1. Malfunction causes heat dissipation
If a component on your circuit boards malfunction, heat dissipation occurs. The component can no longer generate high power. To compensate, nearby components have to work harder to produce more power. Generating more power creates more heat.
2. Through-hole interference
Through-holes have copper plating in the hole barrels, which make electrical connections between layers. It’s the through-holes and heat sink components that supply power. Problems occur if the heat sink is not soldered correctly or if another component interferes with the through-hole. This causes other components to work harder, generating more heat and reaching maximum temperature and possibly beyond.
3. Lead-free solder
As a hazardous material, lead raises concerns about health and the environment. More design engineers worldwide are complying with EU restrictions of hazardous substances (RoHS), which limits the use of lead solder in electronic applications. However, lead-free replacements tend to have a high-melting point. This translates into PCBs requiring high heat to operate, which can lead to component stress.
4. The distance of your surface mount
Your surface mount device (SMD) enables a smooth flow of current via the through-hole and heat sink. For this to happen, the through-hole components and SMDs should be the correct distance to each other. Otherwise, the current travels farther, and the extra travel time gives the receiving components time to cool. To compensate, other components on your circuit board have to generate more power, with the risk of overheating.
5. External thermal issues
During the design phase, don’t overlook the environment that the circuit board and its components will be exposed to. Failure to make this consideration can put thermal stresses on boards in extreme low- and high-temperature environments. Your components, such as resistors, capacitors and semiconductors change with both lower and higher temperatures.
6. High-frequency circuits
Higher temperatures are inevitable when boards use high-frequency circuits. These are common in mobile, microwave, radio frequency and high-speed design applications, such as smartphones. High-frequency circuits require power that generates more heat.
Considerations for effective PCB design must include controlling heat to ensure high performance. If the environment will include extreme temperatures, your design needs to take that into consideration. To maintain the appropriate PCB temperature, follow these tips.
1. Use heat sinks
Thermal conductivity is key. This is why heat sinks are usually made of aluminum or copper – both of these metals offer excellent thermal conductivity and low thermal resistance.
Heat sinks are usually attached to the components on the board, such as power transistors. The role of heat sinks is to absorb and dissipate heat into the air. The heat generated naturally moves from the components to your heat sinks.
Fastens heat sinks to PCBs. The integrated spring provides continuous pressure to the heat sink until the thermal adhesive cures. Heat-stabilized nylon 6/6 & steel chrome plated.
2. The importance of fans
Most PCBs use multiple fans. They disperse heat out of the case while drawing in cool air to prevent overheating. You’ll need fan guards to prevent debris from falling in and causing obstruction, which in turn causes overheating.
Simple to install with snap-in application for PCB cooling fans. Made of ABS. Operating temperature range: 0°C – 60°C.
Silicone mountings offer simpler, faster installation than conventional screw mounts. High elongation rubber provides shock and vibration protection.
3. Choosing the right materials and components
Heat-resistant materials are fundamental to good PCB design. PCBs that rely on thick copper plates can withstand high temperatures and enhance the life of the application. Compared to standard PCBs, their features include handling higher levels of currents, resisting high temperatures for longer, while providing stronger connection points.
The most common of PCB materials is FR 4, a glass epoxy that tolerates high temperatures. You can also look at high-frequency materials such as PCB laminates that provide excellent dielectric properties at microwave frequencies. The laminate you choose should withstand absorbing moisture.
Thermal interface materials are another excellent choice, as they improve heat transfer between surfaces. You can learn more in How to ensure a smooth PCB assembly and the Ultimate guide to PCB hardware.
Your considerations also need to include high-quality components to ensure reliable performance. Features you need to pay attention to is their ability to withstand the temperatures you need them to.
Low resistance to heat transfer while providing electrical insulation between power devices and metal parts. Silicone rubber and fiberglass. Operating temperature range: -60°C – 150°C
4. Adequate Spacing
Each component on your boards need enough space between them to prevent interaction. That is, one component might interact with another component, which can create heat. Maintaining distance between them will prevent this.
Use for a high-temperature PCB. Installed by hand – no tools required. Ceramic. Operating temperature range: -40°C – 1700°C
Easily mounted on boards by hand. Arrowhead fastener on one end is held firmly in place with tension wings. The other end is a teardrop fastener that fits into a small hole, leaving very little protrusion. Nylon 6/6. Operating temperature range: -40°C – 85°C
You can learn more about spacers in our A guide to PCB spacers.
We can talk in terms of fahrenheit for this.
Turn on your soldering iron. Set the temperature above your solder’s melting point. If you’re using lead-based solder, start at 600°F- 650°F (316°- 343°C).
For lead-free solder, start at 650°- 700°F (343°- 371°C).
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