How does Sherlock calculate the overall CCA score?

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How does Sherlock calculate the overall CCA score?

Post by natalieh » 16 May 2017, 10:49

Sherlock’s Scoring Scheme

Have you ever thought about how Sherlock computed your overall CCA score? If you're curious, keep reading!

How is the overall CCA score determined?
When assessing the overall CCA score, the scoring process is completed by addressing each type of failure. Failures can be sorted into two basic categories which are distinct by the certainty or eventuality of a failure.

The first set of issues is certain to cause failure after some period of time (deterministic). These types of issues are typically wearout mechanisms (solder joint fatigue, semiconductor wearout, etc.). Essentially, damage from these mechanisms will evolve and eventually induce failure.

The second set of issues is not certain to cause failure (probabilistic). They simply increase the likelihood of failure. The uncertainty is often due to a lack of additional information on materials, environment, etc., to be able to make an accurate prediction. Examples of probabalistic failures include CAF, flex cracking, etc.

The overall CCA score is determined by using a weighted average, where 50% of the overall CCA score will be from deterministic scoring and 50% of the overall CCA score will be from probabilistic scoring.

Overall CCA Score = [0.5 x Deterministic Score] + {0.5 x Probabilistic Score]

However, if any of the probabilistic or deterministic scores are 0, then the associated type is scored as a 0. Also, if there are any module scores of 0, then the associated type (deterministic or probabilistic) is scored as a 0. A CCA score of 10 is assigned if all modules are completed with a score of 10. For example, if one runs only the Solder Fatigue Analysis, even if it scores a 10, the overall CCA score will not be a 10 because not all modules were implemented.

What do these overall CCA scores mean?
A score of 10 is in accordance with industry best practices.
A score of 7 to 10 is designated green and indicates a preferred design.
A score of 5 is in accordance with minimum acceptable practice within the electronics
A score of 4 to 6 is designated yellow and indicates a marginal design.
A score of 0 strongly suggests a high likelihood of failure (> 90%) during the desired
A score of 0 to 3 is designated red and indicates a high-risk design.

How is the solder fatigue module score calculated?
Determining this score involves comparing the ratio between the life cycle goal and the failure probability.
If the ratio is less than 1.0 the score is 0.
If the ratio is less than 1.4, the score is Log10(1.4)*((ratio-1.0)/0.4).
If the ratio is less than 10, the score is Log10(ratio).
Otherwise the score is 10.0.

As always, if you have any questions, please feel free to reach out to me directly!
Natalie Hernandez, PhD
Product Manager
DfR Solutions

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