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394 Chapter Eleven
require deeper study for all causes to the lowest level using, possibly,
fault-tree analysis (FTA). “Significant” elements are important for the
design itself. “Control” elements are regulated by the government for
any public concern. A control plan is needed to mitigate the risks for
the significant and critical elements. The team needs to develop proac-
tive design recommendations. This information is carried to the
PFMEA after causes have been generated in product projects. Process
mappings are analogous to the block diagram in transactional DFSS
projects.
5. Potential causes. Generally, these are the set of noise factors and
the deficiencies designed in because of the violation of design axioms
and best practices (e.g., inadequate assumptions). The study of the
effect of noise factors helps the DFSS team identify the mechanism of
failure. The analysis conducted by the DFSS team with the help of the
block diagram allows identification of the interaction and coupling of
their scoped project with the environment, with the customer, and
within the DPs themselves. For each potential failure mode identified
in column 2 of Fig. 11.2, the DFSS team needs to enter a cause in this
column. See Sec. 11.3 for cause-and-effect tools linking FMEA columns 3
and 5. There are two basic causes: (1) the design is manufactured and
assembled within specifications, (2) the design may include a deficiency
or vulnerability that may cause unacceptable variation (misbuilds,
errors, etc.), or (3) both.
6. Occurrence. Occurrence is the assessed cumulative subjective rat-
ing of the physical entities (parts/components or subsystems) as fail-
ures that could occur over the intended life of the design; in other
words, it is the likelihood of the event “the cause occurs.” FMEA usu-
ally assumes that if the cause occurs, so does the failure mode. On the
basis of this assumption, occurrence is the likelihood of the failure
mode also. Occurrence is rated on a scale of 1 (almost never) to 10
(almost certain), based on failure likelihood or probability, usually
given in parts per million (ppm) defective. In addition to this subjective
rating, a regression correlation model can be used. The occurrence
rating is a ranking scale and does not reflect the actual likelihood. The
actual likelihood or probability is based on the failure rate extracted
from historical service or warranty data with the same parts or surro-
gate. See Table 11.2 for examples. In DFEMA, design controls help pre-
vent or reduce the causes of failure modes, and the occurrence column
will be revised accordingly.
7. Current controls. The objective of design controls is to identify
and detect the design deficiencies and vulnerabilities as early as pos-
sible. Design controls are usually applied for first-level failures. A wide
spectrum of controls is available, such as lab tests, project and design
reviews, and design modeling (e.g., simulation, CAE). In the case of an