DPMO Methods for Standardizing Defect Measurements
In the previous example, the yield calculations involved two types opportunities—components and solder joints or terminations. This is similar to the problem presented in Figure 4,1, where the IC wastlj5 component and the bonding was used for the terminations. A common problem in electronics manufacturing quality has been to decide which of the two choices, components or terminations, should be the basis for defect opportunities when calculating the yield of assemblies.
An additional problem is defining the cause for termination failures. If the IC in Figure 4.1 was not placed properly in the frame some of the terminations could become defective, even if the bonding process was completed successfully. If one IC chip was misplaced in the assembly step of the process, it could lead to 256 defects in the bonding process. This would falsely penalize the bonding process, even if it was functioning properly. Obviously, a set of rules need to be applied in order to clarify the quality of the assembly operation and to benchmark it with similar operations in the supply chain.
The defects per million opportunities (DPMO) concept was developed for the PCB assembly operation to tackle the problems outlined above. Developed as IPC Standards 7912 and 9261, they set the rules for counting opportunities and defects. They define £i mix of defects and opportunities for components and assembly operations consisting or placements and terminations. Table 4.3 shows a basic grouping of defects and opportunities for PCB assemblies. A number of defects and a number of opportunities are defined for each operation. The defects for each operation could be influenced by prior operations. For example, a misaligned component in the placement operation might cause many termination defects, as discussed earlier. In this methodology, it would be counted as one placement defect and zero lion defects for the PCB. The number of opportunities for components include all of the components plus the fabricated (raw) PCB. The number of termination is the actual number of solder joints on the PCB, Some the definitions are as follows:
The DPMO for each operation is equivalent to DPU (PPM) defined earlier in this chapter. The DPMO index is a useful tool for calculating the actual yield of the PCB, since it is based on the total number of defects divided by the total number of opportunities. It is usually dominated by the termination count. The DPMO index is the basis for DPMO charts, discussed in the next section.
The overall manufacturing index (OMI) is an attempt to equalize the weight of all three basic operations in PCB assembly. The yield of each operation is calculated using the power expansion formula 4.8, then the yields are multiplied together to form a multiplier yield for the assembly line. A multiplier defect rate for the assembly line is derived from the one-multiplier yield, and then multiplied by 1 million to obtain the OMI index.
The OMI index represents an overall theoretical defect rate in which each operation is given equal weight, based on the its own calculated yield. The OMI index is independent of the number of opportunities of each operation, and therefore can be used to compare the quality of alternate PCB assembly lines.
