Ultimately, all defects have to be removed by testing the individual assemblies that make up the product, and then finally testing the product. Test engineers are concerned about the yield of the product, in order to budget and plan for test and troubleshooting equipment and operators. The six sigma quality defect rate and yield calculations are excellent tools to help in the planning of electronic product test strategy.
It is common knowledge in the test industry that the cost of inspecting for and removing defects can be as high as 30% of the overall manufacturing cost. In addition, the earlier a defect is caught and re- moved in the manufacturing cycle, the cheaper it is in terms of equipment cost and operator skills. The best alternative to expensive test equipment and skilled operators is achieving six sigma quality and the resultant assembly yield goals.
As shown in the examples in this chapter, the quality of the individual elements of an assembly can be linked to its total quality performance. In Example 4.2.3,10 PCB assemblies, each with 95% test yield, can result in the next level of assembly (final product made up of the 10 PCBs) having a yield of only 61%. If a higher yield for the next step in the assembly is desired, then the yield of the individual components have to be improved further.
In Example 4.3.2, it was shown that increasing the number of components or steps in the assembly have a similar effect on reducing the yield. The yield for an assembly of 90% based on 100 components or steps quickly drops to 59% yield with 500 components, and then to 35% yield with 1000 components.
This combined effect of setting the yield goal and the number of the underlying steps in the assembly operations have led test engineers to examine the test strategy based on the ability of various test equipment to remove certain level of defects.