Temperature Cycling consists of multiple cycles of changing temperature extremes. The termperature extremes, the ramp rates, and the dwell periods are variables used in calculating the test's screening strength. This calculation is based on MIL-HDBK-344A Section 5.4 Procedure C and includes the following test methodology:
Determing thermal cycling ranges
- Investigage Bill of Materials (BOM)
- Reduce part list to components affected by thermal cycling
- Search component data sheets for thermal ranges
- Develop matrix of parts vs. thermal ranges
- Assess matrix for the most restrictive parts. If the UUT is going to be powered, do thermal survey to determine assembly hot spots
- Determine min and max thermal level. Thigh = most restrictive high temperature, Tlow = most restrictive low temperature
- Determine thermal range
- Evaluate proposed test which is a function of: chamber air velocity, dwell times, ramp rates, UUT hardware mass
- Determine stabilization and soak periods.
- The UUT must be instrumented internally to help determine the thermal dwell times.
- Thermal stabilization must be achieved at each temperature dwell extreme
Determine Screening Strength
- The screening strength is a function of the thermal rate of change, the temperature range, and the number of thermal cycles performed
- To carry out an effective thermal cycling, do the following: maximize the rate of change and the temperature range, and use the number of cycles to optimize the process. The number of cycles is used to achieve a target screening strenth probability. **Note: Screening Strength is the percent of defects that could be found by that screening process.
- The rate of change and the temperature range are determined from the thermal matrix and from experimental test data
- The number is cycles is caluclated from the screening strength formula (TR-82-87) test report.
- At card level, the screening strength is 70-80%
- At system level, the maximum screening strength is approximately 50%. This is due to the slow rate of change of cards inside the system.
Result: The constant rate of change provides the expansion and contraction necessary to sufficiently stress the product. The screening strength provides a measure of defect precipitation and detection efficiency.