BQR’s software provides several methods for calculating and documenting the components’ actual stress (power, current and voltage) values:
- Semi-automatic: stresses can be easily assigned to components using the BQR E-CAD plug-in on the schematic
- Stress simulation: Components stress is calculated by a unique automated circuit stress simulator (CircuitHawk)
- During stress analysis, design errors and rating issues are detected before layout
- Exact stresses allow for optimal thermal design, saving space and cost
- MTBF calculation using exact stresses provides better (higher) MTBF values
Tj of 137.1oC was calculated for IC U2a based on the absolute maximum power rating (6.12W). Based on this result the mechanical engineer was planning to add a fan to the design. Furthermore, the IC failure rate was calculated to be 6.129 failures per million hours.
By using the actual power dissipation result from the CircuitHawk simulation for the calculation, (see IC U2 in fig. 1), Tj was found to be 95oC, and the natural cooling was sufficient. Additionally, the failure rate decreased to 0.804 failures per million hours.
A difference of 42.1oC was found in Tj, and the failure rate (FR) decreased by a factor of 7.6.
To summarize: The use of actual power dissipation reduces the unnecessary use of costly cooling elements, and results in a much higher MTBF.
MTBF of an actual board was calculated for both actual stress (Parts Stress Method) and 50% stress (parts Count Method).
MTBF calculated with actual stress was found to be higher by a factor of about 1.5.