to be published ASME 6th Design for Manufacturing Conference, September 2001
Daniel Becker and Peter Sandborn
CALCE Electronic Products and Systems Consortium
University of Maryland
College Park, MD 20742
Yielded cost is defined as cost divided by yield and can be used as a metric for representing an effective cost per good (non-defective) assembly for a manufacturing process. Although yielded cost is not a new concept, it has no consistent definition in engineering literature, and several different formulations and interpretations exist in the context of manufacturing and assembly.
In manufacturing, yield is the probability that an assembly is non-defective. To find the effective cost per good assembly that is invested in the manufacturing or assembly process, cost is accumulated and divided by yield. Yield information can either be found from historical data on the manufacturer or can be interpolated with the process capability ratio, Cp, and a normal distribution table.
This paper reviews and correlates existing yielded cost formulations and presents a new method that enables consistent measurement of process flows. This new method views step yielded cost as the change in process yielded cost when the step is removed from a process. This approach is preferred because it provides an effective cost per good assembly that incorporates upstream and downstream information and is independent of step order between steps that scrap defective product (i.e., test steps).
Conventional wisdom dictates that the best way to improve a process is to increase the yield of the lowest yield step. The new approach developed in this paper produces an auxiliary cost that can be used to determine the best method of improving processes that, for complex processes, does not always correspond to improving the lowest yield step. Simple and complex assembly process examples are presented to demonstrate the new yielded cost formulation and an efficiency ratio computed using yielded cost is proposed as a measure upon which to base process changes. The new approach is applied to a microwave module (MWM) manufacturing and assembly process example.
Complete article is available to CALCE Consortium Members