Page 159 - Six Sigma for electronics design and manufacturing

Six Sigma for Electronics Design and Manufacturing
128
bad test effectiveness, and good test effectiveness. They are measured
as percentage values:
1. Test coverage (%): the test coverage for a given fault. Coverage of 0
for a defect category means that this defect is not tested.
2. Bad test effectiveness (%): the percentage of bad components that
fail a test. Thus, a tester with 100% bad test effectiveness will fail
all bad items, whereas one with 0% bad test effectiveness will pass
all bad items.
3. Good test effectiveness (%): the percentage of good parts that pass
a test. Thus, a tester with 100% good test effectiveness will pass all
good items, whereas one with 0% good test effectiveness will fail all
good items.
4.4.4 Factors affecting test operation parameters
Factors that affect test effectiveness can be divided into three broad
categories: technology, management decisions, and design for test
(DFT) efforts. They are listed in Table 4.9 and further explained in
the next section. A factor-based model could be created in order to
make PCB design decisions during the development stage. The model
could help the design team investigate the effect that different design
choices would have on the test effectiveness.
4.4.5 Test coverage
Test coverage (also called defect coverage) is a measure of the ability
of a tester to detect defects. It is the percentage of those defects that

Table 4.9 Factors that affect test effectiveness
Category Examples
Circuits tested Microwave circuits (require shielding)
Digital versus analog versus mixed
Manufacturing Through-hole versus SMT
Test pad size
Pitch size
Nodal access
Fixture design and fit
Management decisions Time and resources for test and fixture development
Time planned for in-line test
Design for test (DFT) Design review for DFT
Use of built-in self-test (BIST)
Unit under test memory space dedicated to test