Cost of yield

Abstract

Every semiconductor manufacturer carefully manages fixed and variable costs to achieve profitability. Usually this is well defined in terms of facilities, equipment, labor, and materials, and for the high volume single product producer, the story is complete. Allocating a factory's capacity across several or many products quickly complicates the picture, and the engineering staff is forced to prioritize issues across product subsets. Cost of yield (COY) is a measure of the additional dollars debited (credited) to production expenses when the aggregate productivity falls below (exceeds) the business plan. It provides a way to understand the economic impact of yield variation from the plan weighted by the costs and volumes of each product. COY has been calculated at IBM Microelectronics since 1995 when the division moved to a "Standard Cost" accounting system, and it continues to be an important business management tool. The management staff interprets COY data to validate resource allocations and insure the deployment of the correct activities that cover the products with the largest COY variances. In fact, COY can be subdivided by a number of dimensions, and the implications of these analysis reaches beyond the realm of accounting. A manufacturer develops the cost targets during its planning cycle by projecting productivities (and yield). Measuring COY validates and audits the planning process, and large COY variances trigger profit motivation questions. Moreover, the manufacturer can make an informed decision as to when to scrap degraded hardware. This paper discusses the mechanics of the COY calculation and illustrates the concepts with analysis examples. Suggested integration into and investigation by the manufacturing engineering community are shown. Gaining a true understanding of how specific decisions affect yield in terms of actual dollars forces the design community to be accountable for conscience yield-limiting decisions, and provides the right benefit measure of some design-for-manufacturing initiatives. Finally, the senior business decision makers can assemble an enterprise-level COY perspective to guide future investment and actions.