Page 134 - Six Sigma for electronics design and manufacturing

Determining the Manufacturing Yield and Test Strategy
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portunities are the characteristics that are inspected or tested. DPU
is traditionally based on the opportunities of defects provided in one
unit.
Defects can be attributes of units, as defined by a time or region.
Units can be incoming materials, individual designs, transistors in an
IC, repetitive manufacturing processes such as welds in a joint, etc.
They can be individual units in a product, such as printed circuit
boards (PCBs), or a single product. Defects represent the total defects
found on that unit, expressed as a number called defects per unit
(DPU). Since six sigma quality implies a very low DPU of 3.4 parts in
a million operations, this definition has been converted to units of
parts per million (PPM) in order to make it easier to communicate six
sigma quality requirements. The following are the equations used to
describe these units and their relationships:
number of defects found anywhere
DPU = (4.1)
number of units processed
DPU (PPM) = DPU (fractional) · 1,000,000 (4.2)
DPU (PPM) is the normalization of the DPU by a factor of 1,000,000
in order to facilitate equating a lower number with lower defects and
driving it down to zero. Sometimes it is shortened to just PPM.
The definition of units is sometimes confusing. A unit could be a
single transistor on an IC chip containing a million transistors. A unit
could also be the IC itself, or it could the PCB containing many ICs, or
the product containing many PCBs. In addition, the manufacturing
steps needed to produce the transistors up to the final product have
their own defect rate. Clearly, a uniform approach to these situations
needs to be taken.
A historical approach to this dilemma has been to declare that six
sigma or Cpk targets have to be achieved in “everything that we do.”
That means every material part or manufacturing operation has a six
sigma goal. The collective aggregation of six sigma parts or operations
will also have to be equal to six sigma. This approach would logically
lead to the following strategy:
Divide the manufacturing process into the smallest defined opera-
tions, each with its own DPU.
Each manufacturing operation or material part represents a dis-
tinct transformation of product or material.
In order for the next level of part aggregation (assembly or fabri-
cation) to achieve six sigma quality without test, the individual
DPUs have to be much greater in quality than the aggregation
output.