Manufacturing is a multistep process, with each step generating its own variability, and therefore contributing to the overall defect rate. In a large multistep operation, individual process quality has to be very high in order for the overall yield to be reasonably acceptable. Otherwise, the probability of producing one good part is very low. In the case of PCB or IC fabrication, with 30-50 steps each, there are usually several in-process inspections or tests to cull out the intermediate defects, so that good parts can be produced when all production steps are completed. This chapter will examine methods to allocate for and plan these tests based on the expected quality of production.
It is important to measure quality in terms of the total number of defects found anywhere in the manufacturing process, and prior to any test or inspection. This will reduce confusion when setting quality targets or benchmarking similar operations in different plants. In addition, it will result in a true measure of quality that is not masked by the test or inspection costs.
Units of these quality measures are expressed in terms of first time yield (FTY) and defects per unit (DPU), expressed in parts per million (PPM). Recently the term defects per million opportunities (DPMO) has been used to reduce confusion on how to calculate defect rates in a complex multistep process such as PCB fabrication and assembly. Re- pairs are not considered as part of the definition of first time yield (FTY).
The issues of calculating FTY have become important in light of the increase in subcontracting the manufacturing of high-technology electronic products. Project teams and their leaders need accurate estimates of new product yields to plan and budget for test and troubleshooting equipment and personnel. In addition, management needs to benchmark potential suppliers in terms of their manufacturing quality. The results have been beneficial in several categories, and will be further highlighted in this chapter:
• By rolling up the yields of its various product components and manufacturing operations, the total product yield can be estimated. Project teams are thus able to manage carefully where additional resources are needed in terms of improving particular designs or manufacturing capabilities. By using these yield estimates, the new product team can also increase the accuracy of the new product cost estimates.
• Design for manufacturer (DFM) principles，as championed by manufacturing engineers, can be emphasized to the design team in order to increase the FTY of new products, since a direct relationship can be made between the two concepts,
• FTY yield calculations can influence the focus of quality improvement teams.
• Yield calculation can clarify the best test strategy for reducing the overall test and troubleshooting costs.
In this chapter, the issues of yield and test strategy will be examined in a hierarchy of steps:
1. Determining units of defects
2. Determining manufacturing yield on a single operation or a part
with multiple similar operations
3. Determining design or manufacturing yield of multiple parts with multiple manufacturing operations or design specifications
4. Determining overall product testing strategy