Page 221 - Six Sigma for electronics design and manufacturing

Six Sigma for Electronics Design and Manufacturing
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termine how many times the innerlayer processing rate is applied,
and the material cost of copper-clad dielectric and prepreg material
for adhesion of the layers. Since the drill cost rate is often expressed
per hole, this cost will depend on the stack height, which is in turn
based on the PCB thickness, hole diameter, and the required accuracy
of the location of the drilled hole. Too tight a hole tolerance, which af-
fects the solder mask operation, will result in a lower drilled stack,
and therefore higher drilling costs but less misregistration defects.
Most fabricators process full panels (such as the 18 × 24 size), so the
more images or PCBs that can be designed to fit on a single panel, the
smaller the PCB cost.
The cost of a fabricated PCB should provide a conversion process
from PCB design features to manufacturing cost. An important part
of this process is the designer’s understanding of the manufacturing
process, capabilities, and constraints. For example, the dry film solder
mask is the most expensive, yet offers the best quality in terms of sol-
dering defects in PCB assembly. The inverse is true of the screened
solder mask.
To calculate the effect of different design alternatives, a manufac-
turing engineer must provide information about each cost parameter
in the fabrication process that would influence the final cost. The cost
(C i ) for each PCB parameter is:
(6.5)
C i = P i · F i
Where P i is the PCB specific number applied for each cost parameter
and F i is the respective cost factor for the fabricator.
The cost factor, F i , is derived from the fabricator cost rates based on
their actual material, labor overhead, and support costs. For example,
the relative cost factor F ilp of inner layer processing is calculated from
the following formula:
(6.6)
F ilp = (C ll + C lo + C ls )/N l
Where C ll and C lo are the inner layer imaging and etching direct labor
and overhead expended from the last financial period reported, respec-
tively, and C ls is the department support and maintenance costs such
as percentage of utilities, maintenance, and general management costs
incurred to support the inner layer department. N l is the number of
layers consumed by the fabricator during that period. Obviously, this
cost factor system necessitates alternative accounting procedures by
which costs are accumulated based on the cost factor structure.
Cost factor systems provide a standard measure of the contribution
of individual design features to overall PCB manufacturing cost. They
allow manufacturing engineers to compare multiple design alterna-
tives in order to select the optimum design.