Determining Units of Defects
The basic definition of a defect is one that is based on the Poisson distribution. The defect rate, or defects per unit (DPU), is calculated based on defects, opportunities, and units. Defects are any deviation of the product functions that causes customer dissatisfaction or nonconformance to specifications. Units are the number of parts, subassemblies, assemblies, or systems that are inspected or tested. Opportunities 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:
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 operations, each with its own DPU.
• Each manufacturing operation or material part represents a distinct transformation of product or material.
• In order for the next level of part aggregation (assembly or fabrication) to achieve six quality without test, the individual DPUs have to be much greater in quality than the aggregation output.
• There is a need to translate the DPU of each operation into a DPXJ for the next level.
• For product design and manufacturing process engineers, it is much more useful to communicate and plan manufacturing tests using process and product yields as opposed to DPUs for the higher levels of product,
• There is a need to manage the conversion of DPUs into yields.
To address these issues, particular industries have developed the concept of defects per million opportunities (DPMO). These are standards that define the total defect opportunities per particular product or assembly. They use specific methods to combine the DPUs of parts and manufacturing operations, to arrive at the total number of opportunities. Opportunities can be defined in terms such as:
• Opportunities are characteristics or features of the product or the manufacturing process.
• Opportunities must be measurable and have a standard or specification with which they can be compared.
• Opportunities must be appraised. If a product has features that are not appraised, they should not be counted as opportunities.
• Opportunities are assumed to be independent.
• There cannot be more defects in a unit than opportunities.
• The opportunity count for a product is constant until the design or the manufacturing process changes.
An example of a DPMO methodology is the Institute of Printed Circuits (IPC) Standard 7912 for calculations of DPMO for PCB assembly, which will be discussed later in Section 4.3.
