Tolerance / Worst Case Analysis
DEFINITION
Tolerance / Worst Case Analysis is the method of analyzing a piece of a design using the high and low end of the tolerance spread for each parameter.
SITUATION
Tolerance / Worst Case Analysis is used whenever we need to evaluate critical parameters within a design and the effects of these parameters in worst-case conditions.
OBJECTIVE
If sensitive areas of the design are discovered during the analysis, a Tolerance Analysis can be performed to design in safety margins.
VALUE TO YOUR ORGANIZATION
A Tolerance / Worst Case Analysis has value whenever tolerance issues and tolerance stacking can effect a design, especially in circumstances in which testing will not be able to catch the issues due to low sample size.
RELIABILITY INTEGRATION
An example of Reliability Integration during Tolerance / Worst Case Analysis is as follows:
Tolerance / Worst Case Analysis Used with HALT
If a part of the design has a wide tolerance spread, then issues related to tolerance may not be picked up in HALT due to the low sample size being used. For these cases, then worst-case design practices may need to be employed. Many mechanical assemblies fall into this category. If issues can arise due to tolerance stacking of dimensions, designing for worst case is the best approach.
METHODOLOGY
A tolerance analysis shall provide detailed calculations showing tolerance, temperature, and life effects on the performance of the product.
The critical circuitry shall be analyzed for component variation effects. For example, each circuit specific (voltage, current, etc) for a given node will be analyzed in terms of component tolerance, component temperature effects, and component life effects.
These will include elements such as resistance, capacitance, and inductance. Component parasitics such as e.s.r. of the output capacitors will be included and analyzed as to temperature and life effects.
The circuitry shall be analyzed for circuit design criteria conformance. Maximum variations in input voltage and frequency shall be considered in the analysis.
In addition, the most unfavorable (worst case) combination of realizable conditions to be considered in the parts/circuits tolerance analyses shall be presented in this report. This is typically performed using computer simulation techniques.
CASE STUDIES/OPTIONS
The following case studies and options provide example approaches. We shall tailor our approach to meet your specific situation.
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Mechanical Tolerance Analysis
A Medical Device company was designing a mechanical assembly with tight tolerances. In this situation, we used a Tolerance Analysis rather than HALT because HALT would likely not have been able to find mechanical tolerance issues due to low sample size.
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Electrical Tolerance Analysis
A Military Power Supply manufacturer was required to perform a tolerance analysis on all critical electrical parameters. We linked this analysis to a Spice model and simulated these worst case conditions.
