Page 138 - Six Sigma for electronics design and manufacturing

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Determining the Manufacturing Yield and Test Strategy
will set goals for each type of test in order to plan for test and trou-
bleshooting equipment and train operators for the production phase.
Usually, PCBs are tested on automatic in-circuit testers (ICTs), which
remove some of the assembly or part defects. The PCB test programs
and the effort to develop them depend on these goals. A high yield in
PCB test will reflect a higher turn-on ratio at the product level, saving
the company valuable product test time and resources. Final assembly
of the electronic product is accomplished from these tested PCBs and
other components, such as power supplies and display devices, and
turned on for final test. Yield calculations for PCBs and final product
are similar to the ones discussed in this section.
4.2.3 PCB yield example
A product contains 10 PCBs, and a goal of 95% turn on yield was set
for each PCB at in-circuit test (ICT). The product final test turn-on
yield will be as follows:
DPU (PCB) = 0.05
Product turn-on yield = Y T = e – DPU = e –10 · 0.05 = 0.606 = 61%
A turn-on yield of 61% is disappointing, especially when 95% in-cir-
cuit PCB yield could be difficult to achieve. To achieve a 95% final
product turn-on, what should the PCB ICT test goal be?
Expected product turn-on yield = Y T = e – DPU = e –10 · (DPU)
= 95% or 0.95
10 · DPU = –ln (0.95) = 0.05
DPU (of each PCB test) = 0.005
PCB individual test yield = 1 – DPU = 0.995 = 99.5%
When a final test DPU of 95% is required for a product of 10 PCBs,
the individual PCB ICT yield goals should be set at 99.5%.
It can be seen that the test yield for each component making the fi-
nal product has to increase substantially in order to increase the turn-
on yield of a large electronic product. The manufacturing process has
to increase its quality level in order to match increased product com-
plexity. Several methodologies and tools can be used for each part of
the PCB assembly process. These steps do not necessarily require the
use of more sophisticated inspection methods and equipment, but
simple problem solving techniques such as:
Incoming electronic component quality can be improved with better
supplier certification and supplier process control methods.