The yield of a manufacturing process or of any production line is of paramount importance to manufacturers as it is directly related to corporate profitability. For many manufacturing companies, the only alternative to remain competitive is to improve yield. Manufacturing yield is generally considered to be a measure of manufacturing success, and it is defined as the ratio of the number of usable devices after the completion of a production process to the number of devices at the beginning of the process. Yield also refers to how much salable product can be produced. So, a higher yield means that more items can be produced for the same overhead cost, which permits unit cost and unit sale price to decrease. This may contribute toward making the company competitive and profitable.
Manufacturers are generally preoccupied with eradicating the factors or failure mechanisms, which affect the yield of a product. However, before doing so, the manufacturer must understand how each stage of the manufacturing process affects yield. Since the definition of yield indicates that it is a statistical parameter, the various stages of product manufacture can be represented as probability functions, which are multiplied in order to attain the overall yield. These probability functions include failure probabilities for various defect types. Yield can be effectively controlled if these failure probabilities can be reasonably determined. This further indicates that there is some relationship between yield and reliability as reliability is simply the failure probability subtracted from one. Reliability is also defined as the probability that a device will perform its intended function for a specified time under the stated environmental condition.
There are two types of reliability problems: intrinsic failures, which are inherent to the manufactured device; and extrinsic failures, which are due to manufacturing defects. Extrinsic factors include poor design, improper manufacturing operations, contaminated environments, defective raw materials, poor incoming inspection, or inadequate shipping and handling.
An example of how yield can be affected by reliability problems can be understood by looking at the manufacture of integrated circuits (IC). There is a trend in the microelectronics industry to produce smaller integrated circuit chips that are capable of more sophisticated functions. Fabrication of advanced integrated circuits usually involves 300 to 400 process steps. However, the fabrication of ICs can be simplified into two major stages known as the front-end and the back-end. In the front-end, the components of the integrated circuits are manufactured, whereas in the back-end, metal is added to connect the components of the chip. Each stage consists of several hundred steps, and it is easy to envisage defects being introduced during any step. For instance, the first step in the front-end stage involves the construction plan and circuit diagrams of the chip. Poor plan design may lead to chips that do not function as intended, thereby diminishing yield. Another instance in which defects can be introduced is in the back-end stage where layers of wires are added to connect the various components. Improper connection sequence or poor connections can also lead to defective chips, thereby leading to poor yield.
Using proven technology makes it easier to keep yields higher, but old technology soon becomes expensive to maintain, which ultimately leads to increased overhead and reduced profit margins. At first, it may be difficult to achieve high yields with new technology because large amounts of items may be discarded during the learning phase. Nevertheless, overhead cost should eventually become lower and the benefits of the new technology will be realized as the new technology is grasped. However, there is a great snare that many companies fall into. Particularly, small companies with smaller budgets tend to learn as they go along, which can lead to poor revenues and terrible backlogs that eventually cripple production schedules. It may be worth the cost to hire engineers to optimize the new technology as early as possible.
Manufacturing yield can also be improved by implementing simple corrective actions, which include correcting process defects before they occur. This can be done by thoroughly characterizing all machinery whereby the parameters that affect the performance of the machine are clearly understood and documented. Other corrective measures involve real-time control and monitoring of process variation; alert systems to indicate when a process is moving out of control; sufficient training to enable operators to take immediate corrective action; and advanced statistical analysis to identify the root causes of defects.
Another technique that can be applied to improve manufacturing yield is known as design for manufacturability or design for manufacture (DFM). The essence of this technique is to encourage manufacturers to use simplified designs for their products. Simplified designs through a process of standardization result in fewer parts, which minimizes the opportunity for a defective part or an assembly error. This reduces production cost, improves quality, and contributes to increased yield. Design for manufacturability is built on several product design guidelines, which are summarized in its definition.
In the article by David Anderson on “Design for Manufacturability and Concurrent Engineering,” DFM is defined as the process of proactively designing products to (1) optimize all the manufacturing functions: fabrication, assembly, test, procurement, shipping, delivery, service, and repair, and (2) assure the best cost, quality, reliability, regulatory compliance, safety, time-to-market, and customer satisfaction. An article on design for manufacturability by Kenneth Crow states that research has indicated that decisions made through the design period account for approximately 70 percent of product cost. If this is accurate, it is easy to visualize how poor product design can adversely affect manufacturing yield.
It is therefore imperative that manufacturers strive to attain better and better yield levels for their products. In order to achieve this objective, manufacturers must implement the necessary measures to mitigate or even eliminate the introduction of defects into their products. Failure to do so will lead to poor yields, ultimate loss of competitiveness, and possible extinction of the business.
- M. Anderson, “Design for Manufacturability,” www.design4manufacturability.com (cited March 2009);
- Crow, “Design for Manufacturability,” www.npd-solutions.com (cited March 2009);
- O. Kim and H.-S. Oh, “Reliability Functions Estimated From Commonly Used Yield Models,” Microelectronics Reliability (v.48, 2008);
- Kim and W. Kuo, “Modeling Manufacturing Yield and Reliability,” IEEE Transactions on Semiconductor Manufacturing (v.12/4, 1999);
- Kim and W. Kuo, “Prolog to an Overview of Manufacturing Yield and Reliability Modeling for Semiconductor Products,” Proceedings of the IEEE (v.87/8, 1999).
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