1. The Relationship Between Component Failure and Temperature
A component's maximum allowable temperature is a hard limit. Once this limit is exceeded, the failure rate can increase sharply, and the component must not be operated beyond it. Even within the specified temperature range, however, component failure rate remains strongly related to temperature: as temperature rises, the failure rate generally increases.
This raises an important question: Is there a safe temperature below which the failure rate is no longer closely related to temperature-
Both theory and practical experience indicate that, in most cases, no such temperature point exists. Component failure rate remains temperature-dependent across the operating range. Above a certain temperature, however, the failure rate increases much more rapidly, creating a distinct knee point in the curve.
Derating means operating a component or product at a stress level appropriately below its specified rated value. The purpose is to reduce the basic failure rate and improve operating reliability. In the 1950s, Japanese researchers found that reducing temperature by 10 °C could reduce the failure rate of components by more than half. Practical experience has also shown that appropriately derating certain component parameters can significantly improve component reliability.
Because electronic-product reliability is particularly sensitive to electrical stress and thermal stress, derating design and thermal design are especially important in electronic equipment.
A power supply designed for a flow meter initially did not use derating. Based on the calculated power dissipation under normal ambient conditions of 20 °C to 25 °C, the adjustment transistor was expected to dissipate 0.8 W, so a transistor with a rated power of 1 W was selected. The product then experienced frequent failures during debugging and field operation.
The analysis showed that the transistor's 1 W power rating applied at an ambient temperature of 25 °C. In actual operation, the surrounding temperature reached 60 °C, and the transistor's actual maximum power dissipation had effectively reached 1 W under those conditions. A reliability engineer recommended replacing it with a transistor having the same electrical parameters but a 2 W power rating. The resulting derating factor was approximately S = 0.5, and the product failures were quickly resolved.
2. Temperature Cycling Is One of the Most Severe Environmental Stresses
Screening tests, also called removal tests, are performed to eliminate products with early-life failures. For electronic equipment, temperature cycling is one of the most effective screening methods. A statistical comparison gives an effectiveness ratio of approximately 3.5:1 for temperature cycling versus vibration.
This comparison indicates that temperature cycling is highly effective at exposing latent defects. Repeated changes in temperature cause materials and structures with different thermal expansion characteristics to expand and contract. When mechanical stress is superimposed on thermal stress, some failures that are otherwise difficult to reproduce can be triggered.
The effectiveness ratio above is a statistical conclusion. Engineering experience also shows that the combination of mechanical and thermal stress can expose intermittent faults that may not appear during ordinary functional testing.
3. Temperature Drift in Component Characteristics Can Cause System Failures
Temperature-related failures can generally be divided into two categories:
- Recoverable soft failures
- Irrecoverable hard failures
A soft failure usually occurs when the temperature tolerance of a component or circuit is insufficient. The failure can disappear after the temperature returns to an appropriate range, making this an important cause of difficult-to-reproduce board-level faults. Although excessive temperature can trigger soft failures, simply lowering the temperature is not always enough to eliminate them. The specific failure mechanism must first be identified so that targeted corrective actions can be implemented.
Temperature-induced soft failures are often caused by drift in component parameters. A transistor's voltage or current characteristics may change with temperature until the circuit no longer meets its operating margin.
Case Study: Parameter Drift Prevents a Transistor from Turning On
- During a HALT test, the test temperature was reduced to -15 °C, at which point the board's serial port consistently hung. The initial suspicion was a clock or power-supply problem.
- The board-management MCU was used to check the power-voltage and clock-status results. The voltage-monitoring results connected to the MCU were normal, but the board's operating clock was detected as missing.
- The missing-clock condition was then analyzed in detail.
After the CPU operating-power module reached the correct output voltage, it generated an enable signal that turned on the clock circuit. However, the CPU operating voltage was not connected to the MCU monitoring circuit because this voltage varied from 0 V to 1.2 V and was not routed to the MCU for monitoring. The clock circuit could operate normally only after receiving the enable signal.
The control logic enabled the CPU operating-power module after detecting that power supplies A and B were operating normally. The status signals for power supplies A and B were passed through two MOSFETs and then sent to a CPLD for detection.
Theoretical analysis showed that the transistor input voltage was 1.05 V. A resistor-divider network reduced this voltage to approximately 0.7 V to turn on the transistor. At room temperature, the required turn-on voltage was between 0.58 V and 0.7 V, so a 0.7 V drive level appeared sufficient.
At low temperature, however, the transistor characteristics drifted and the required turn-on voltage became higher than 0.7 V. The voltage produced by the divider was therefore no longer sufficient to turn on the transistor.
The low-temperature parameter drift was verified experimentally. A board with the resistor-divider network still installed was operated normally at room temperature. An oscilloscope monitored the /1V05_detect pin, where the /1V05_detect signal was initially low. The oscilloscope was set to rising-edge triggering, and liquid nitrogen was then sprayed onto the transistor. Within a few seconds, the /1V05_detect signal changed from low to high, confirming that the transistor parameters had shifted at low temperature and that the VBE turn-on threshold had increased.
4. Temperature Can Physically Damage Components
Hard failures are irrecoverable, but their causes can be widely distributed. During semiconductor manufacturing, small impurities and defects that cannot be completely avoided may remain inside a device. While the device operates, these microscopic defects can gradually grow and spread. Once they affect the device's external function, the device fails.
Temperature commonly accelerates the growth and propagation of these defects, often in combination with electrical stress. The maximum temperature capability of a component remains a hard limit: exceeding it can cause a sharp increase in failure rate and is not acceptable in normal operation. Even below the limit, failure rate remains temperature-dependent and generally increases as temperature rises.
Excessive temperature during soldering or operation can also cause BGA solder balls to crack.
5. Measuring, Monitoring, and Protecting Against Excessive Temperature
Some processors include internal temperature sensors that measure the core temperature. Current processors commonly use digital thermal sensors (DTS) for this purpose.
In multicore processors, Intel integrates multiple DTS units to monitor temperatures in different regions. The temperature data for each region can be read through model-specific registers (MSRs).
The digital thermal sensor is valid only when the processor is in the C0, or normal operating, state. Overtemperature detection is also based on the digital thermal sensor, with the result represented by a bit in an MSR.
The temperature-sensor reading provides the information used to determine whether the board or processor enters the TM1 or TM2 thermal-management state. Temperature sensors can also be added at critical locations on the board to improve thermal monitoring and help characterize the thermal environment.
When an overtemperature condition is detected, the system can automatically issue an alarm, reduce the operating frequency, or restart the system. These protective actions help prevent a temporary thermal excursion from developing into permanent component damage.
