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Six Sigma for Electronics Design and Manufacturing
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low test coverage, under 90% yield of good PCB’s into the next test
cycle, will result in the need for a functional test of the PCB.
3. Functional test (FT). This type of test is used to eliminate design-
based as well as assembly defects. The latter occur when the set of
components being assembled meet their individual specifications
and are assembled correctly, but the assembled PCB does not meet
its systems specifications. Functional testers use the PCBs in a
similar fashion to their intended use in the product. In its simplest
form, FT is called a box test, which consists of testing the PCBs in
a fully functional product.
4. The customer of PCB test is the next level of production. It usually
is the product assembly process, where the PCBs are combined
with mechanical parts to form the product. The product might un-
dergo additional testing such as burn-in, product test, and system
test.
Product test occurs after the product has been assembled with
PCBs and other mechanical and input/output modules. Burn-in oc-
curs when the product is subjected to environmental stress condi-
tions, and then tested to see if there were any “infant mortality”
failures. System test occurs when the product is combined with
other products with maximum-length cabling to form a system con-
figuration similar to those in customer sites.
The different types of PCB tests have different expectation of de-
fects removal. Test engineers usually communicate quality through
using the PCB yields from each of the test methods mentioned above,
whereas the assembly community communicates through DPU or
DPMO. The management sets enterprise goals at certain Cpk levels
or six sigma. Using the examples in this chapter, it was shown that
quality communications could be just as effective using any of these
common methods outlined above.
The PCB test strategy is formulated based on the lowest-cost alter-
native for removing defects, given the current quality level of the de-
sign and manufacturing process. To achieve a successful strategy, the
costs of each type of test as well as visual inspection should be known,
including nonrecurring costs such as test development and program-
ming, fixture design, and troubleshooting. A test strategy could be de-
veloped to provide the proper balance between investing in improving
the PCB assembly process capability versus performing test and trou-
bleshooting to remove defects generated in design and assembly. This
can be accomplished given the availability of alternative test method
costs, and having the defects and their sources quantified using meth-
ods outlined in this chapter.