原创 最坏分析由来和内容 (Worst Case Analysis)

Worst Case Analysis：[Worst
Case Extreme Value Analysis (EVA)]

最坏分析其实就是极端值分析

Benefits From WCCA

好处很多拉，其实一句话，就是让你知道怎么样才会出问题。


Assure acceptable operation throughout the entire product life cycle
under the most unfavorable combination of anticipated conditions

Define Critical Components and Spec. Control Drawing (SCD) Limits

Provide Acceptance Test Procedure (ATP) Limits

Define Need for and Range of Select-At-Test (SAT) components

Improve Reliability through Parts Stress and Derating analysis

Identify design concerns which during test, alignment, and use could result in circuit damage or premature degradation.




WCCA helps ensure increased product reliability. We accomplish this through
rigorous mathematical and simulation-based models along with hardware correlation.
Correlated models are then used to determine part stress margins, and EOL/BOL product
operating specifications. A single over-stressed component can cost your company millions
of dollars. A thorough Worst Case Analysis can eliminate this from happening.

Background & Definition


A Worst Case Circuit Analysis is a quantitative assessment of the equipment
performance, accounting for manufacturing, environmental and aging effects.
In addition to a circuit analysis, a WCCA often includes stress and derating
analysis, Failure Modes and Effects
Criticality (FMECA) and Reliability
Prediction (MTBF).



The specific objective is to verify
that the design is robust enough to provide operation which meets the
system performance specification over design life under worst case conditions
and tolerances (initial, aging, radiation, temperature, etc.).

一般我们做的比较多的就是stress和derating，这个最直观也最明显了。


The Stress and Derating Analysis is intended to increase reliability by providing
sufficient margin compared to the allowable stress limits. This reduces overstress
conditions that may induce failure, and reduces the rate of stress-induced parameter
change over life. It determines the maximum applied stress to each component in the
system.

Methodology

• 
  A WCCA follows this general form:
• Generate/Obtain circuit model
• Obtain Correlation to validate model
  
• Determine sensitivity to each component parameter
  
• Determine component tolerances
  
• Calculate the variance of each component parameter as sensitivity times absolute tolerance
  
• Use at least two methods of analysis (eg. hand analysis and SPICE or Saber, SPICE and measured data) to assure the result
  
• Generate a formal report to convey the information produced
  

自己列公式+Pspice+测试数据，选两个对照一下罗

The design is broken down into the appropriate functional sections.
A mathematical model of the circuit is developed and the effects of various
part/system tolerances (see below) are applied. The circuit's EVA and RSS
results are determined for Beginning-of-Life and End-of-Life states.



Two methods of analysis and/or hardware correlation are always used to
confirm results.



These results are used to calculate part stresses and are applied to other
analyses. In order for the WCCA to be useful throughout the product’s life cycle,
it is extremely important that the analysis be documented in a clear and concise
format. This will allow for future updates and review by other than
the original designer. A compliance matrix is generated that clearly
identifies the results and all issues.

Factors Addressed During WCCA


Analog Circuit Analysis

Some of the factors to be considered during analysis include:

Component Tolerances