Fab cycle time improvement through inventory control : a wafer starts approach

Thesis (M.B.A.)--Massachusetts Institute of Technology, Sloan School of Management; and, (S.M.)--Massachusetts Institute of Technology, Engineering Systems Division; in conjunction with the Leaders for Manufacturing Program at MIT, 2007.Includes bibliographical references (p. 59-60).Intel's Colorado Springs wafer fabrication facility, known internally as F23, has undertaken several initiatives to reduce cycle time including High Precision Maintenance (HPM), content reduction through the application of Manufacturing Excellence (mX), effective utilization of production equipment, and aggressive inventory control. Each of these efforts has contributed to the marked improvement F23 achieved throughout 2006. F23's cycle time efficiency, the ratio of raw process cycle time to actual fab cycle time, improved from 12% (worst amongst Intel facilities) to greater than 35% (best amongst Intel sites), and overall cycle time was reduced by more than 61% in 2006. Inventory control was found to have a major impact on factory cycle time and performance. F23 controls its factory work-in-process, WIP, inventory through the F23 Wafer Starts Protocol. F23 utilizes Little's Law (Cycle Time = Inventory / Output) to identify target WIP inventory levels required to achieve particular cycle time goals. The target inventory is then achieved by modulating wafer starts. To do this, the Wafer Starts Protocol monitors the inventory of the overall fab and the constraint operations and suggests the amount of wafers to start for each shift.(cont.) Maintaining the target inventory level drives the overall factory cycle time towards the cycle time goal. Using the starts protocol, F23 has reduced its inventory by 44% while ramping factory output. During the implementation of this wafer starts protocol, F23 began tracking a new inventory metric to determine factory performance. Critical WIP ratio was introduced to evaluate the factory's inventory relative to the theoretical minimum inventory based upon a given factory output level and raw process time. F23 also found that this metric provides an effective comparison of inventory level between fabs. The Fab23 Wafer Starts Protocol is one of the ways in which F23 has applied Manufacturing Science tactics and principles to drive cycle time improvements. F23 has found that inventory control can have significant impacts on factory cycle time. This is one of the reasons why F23 was able to achieve dramatic cycle time improvement.by Matthew J. Ward.S.M.M.B.A

Development and Simulation Assessment of Semiconductor Production System Enhancements for Fast Cycle Times

Long cycle times in semiconductor manufacturing represent an increasing challenge for the industry and lead to a growing need of break-through approaches to reduce it. Small lot sizes and the conversion of batch processes to mini-batch or single-wafer processes are widely regarded as a promising means for a step-wise cycle time reduction. Our analysis with discrete-event simulation and queueing theory shows that small lot size and the replacement of batch tools with mini-batch or single wafer tools are beneficial but lot size reduction lacks persuasive effectiveness if reduced by more than half. Because the results are not completely convincing, we develop a new semiconductor tool type that further reduces cycle time by lot streaming leveraging the lot size reduction efforts. We show that this combined approach can lead to a cycle time reduction of more than 80%

Introducing pull methodologies in a semiconductor fab

Thesis (M.B.A.)--Massachusetts Institute of Technology, Sloan School of Management; and, (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering; in conjunction with the Leaders for Manufacturing Program at MIT, 2005.Includes bibliographical references (p. 85-86).Semiconductor manufacturing is a highly complex and re-entrant process. In a fabrication facility, hundreds of decisions are made during a production shift regarding how shared tool capacity will be prioritized. These decisions contribute to how balanced or unbalanced the manufacturing line will be. Characteristics of an unbalanced line are large WIP bubbles, long queue times, and expediting. A balanced line has less WIP bubbles, shorter queues, and WIP is positioned at all points throughout the line to be in position to meet the demand forecasted. This thesis focuses on work performed at Intel's Fab 23 to improve the process for assigning production priorities through the introduction of pull methodologies. Existing processes and tools were studied, and an improved methodology and decision-support tool was developed to aid operations managers in driving towards a balanced line. An experiment was also designed and executed in production to test the methods and tools developed. Target cycle time data was used along with throughput goals to construct a target inventory profile throughout the line. Actual inventory was then compared to the ideal "balanced" profile to determine where WIP deficits and surpluses existed.(cont.) Using this information, the operations managers had objective metrics that could be used in determining which operations should receive priority. Significant externalities inhibited performance during the experiment, preventing measurable improvements in line balance and cycle time from being realized. However, these externalities were known prior to experimentation, and a decision was made to learn from the experiment.The tool proved helpful in promoting consistency across shifts in how the factory was run. There were many anecdotal examples of the decision-support tool driving more intelligent priority decisions than operations managers would have made without the tool.by Jason Walker Connally.S.M.M.B.A

Systematic review and discussion of production control systems that emerged between 1999 and 2018

There exists a large literature on well-known Production Control Systems (PCS) such as Kanban, Constant Work-In-Process (CONWIP), Material Requirements Planning (MRP) and Paired Cell Overlapping Loops of Cards with Authorisation (POLCA). However, there are also many new systems that have emerged in the last 20 years. These systems are less well known, and no systematic review exists. Through a systematic literature review, 13 PCS’s were identified, such as COBACABANA, REDUTEX, BK-CONWIP and B-CONWIP. For each system, we describe its characteristics, how they work and in what productive environments it proves useful. A cross-comparison of the 13 systems was then conducted using seven variables: primary control variable (WIP or throughput), degree of centralisation, material flow (productive environment), whether it was introduced as card-based, types of authorizations, number of articles published and type of articles published (theoretical or empirical). Most new PCS’s (7 out of 13) show characteristics that are similar to Kanban, CONWIP and POLCA: they are decentralised, card-based and use WIP as primary control variable. This may be a result of the strong influence of Lean Manufacturing paradigms Meanwhile, there is a general lack of empirical studies, being 11 PCS’s developed from mathematical simulations

On the meaning of ConWIP cards:an assessment by simulation

The simplicity of Constant Work-In-Process (ConWIP) makes it one of the most widely adopted card-based production control solutions. Its simplicity, however, also limits the opportunities that are available to improve the concept. There are arguably only two major search directions: (i) to alter the meaning of cards away from controlling jobs; and (ii) to adopt alternative, more sophisticated backlog sequencing rules. In this study, we outline a simple, practical load-based ConWIP system that changes the meaning of cards. Rather than controlling the number of jobs, cards are associated with a certain amount of workload. Simulation results demonstrate the positive performance impact of limiting the total shop load. The Workload Control literature advocates the use of a corrected load measure as it better represents the direct load queuing at a station; but this worsens performance when compared to a shop load measure in the context of ConWIP

Card-based systems: systematic literature review of new systems and proposal of a list of soft factors for systems implementation

Although Production Control Systems are considered a mature topic in literature, up to now, the focus of literature has been on optimizing the operating parameters of each system, assigning little importance to factors related to human influence (soft factors). Implementation, however, remains a complex problem. This paper aims to reduce this gap by proposing, through a combination of research methods, a list of soft factors critical to success in implementing card-based systems, which which are the most studied and implemented and which share as a characteristic a strong human influence on their functioning. However, to achieve this goal, it was initially necessary to identify which systems are based on cards. Although for systems such as Kanban and CONWIP there is a large literature available, systems developed after POLCA proposal in 1998 were little explored. Therefore, this dissertation initially performs a systematic literature review identifying 13 systems developed between 1999 and 2018, such as COBACABANA, DDMRP, Redutex, B-CONWIP, BK-CONWIP, among others. Briefly, it presents how each system works, its characteristics, current research stage and environments in which it has been proved to be useful, aiming to increase researches about them. The systems are also compared in relation to six variables defined in the literature, identifying that many of the new systems are card-based. The soft factors proposed for the implementation of card-based systems are based on the analysis of problems cited in the literature as well as difficulties identified through a longitudinal case study. This list was validated by experts as well as a group of employees from the focus company that participated in the implementation of kanban. This dissertation also proposes a soft factor house for the implementation of card-based systems, in which the factors are classified as exclusive to this theme or classic management factors, as well as in relation to the organizational level in which it operates (organization, implementation group, or individual). This house aims to assist managers in implementing card-based systems, increasing success rates in this process. In addition, by reviewing recent production control systems, this dissertation aims to increase the repertoire of managers on existing systems, enabling the implementation of more appropriate options for the productive environment in which they are located.Não recebi financiamentoEmbora os Sistemas de Coordenação de Ordens sejam considerado um tópico maduro na literatura, o foco da literatura até o momento recaiu especialmente sobre a otimização dos parâmetros de funcionamento de cada sistema, atribuindo pouca importância aos fatores relacionados a influência humana (soft factors). A implementação, contudo, continua a ser um problema complexo. Esse trabalho visa reduzir essa lacuna, propondo, através de uma combinação de métodos de pesquisa, uma lista de fatores soft críticos para o sucesso na implementação de sistemas baseado em cartão, os quais são os mais estudados e implementados e que compartilham como característica uma forte influência humana em seu funcionamento. Contudo, para realização desse objetivo, inicialmente foi necessário identificar quais são os sistemas baseados em cartão. Embora para sistemas como o Kanban e o CONWIP exista uma grande literatura disponível, sistemas desenvolvidos após a proposta do POLCA em 1998 foram pouco explorados. Assim, essa dissertação inicialmente realiza uma revisão sistemática de literatura identificando 13 sistemas desenvolvidos entre 1999 e 2018, tais como COBACABANA, DDMRP, Redutex, B-CONWIP, BK-CONWIP, dentre outros. Brevemente, é apresentado o funcionamento, características, estágio atual de pesquisa e ambientes propícios para cada sistema, visando aumentar as pesquisas sobre eles. Os sistemas são também comparados em relação a seis variáveis, identificando-se que muitos dos novos sistemas são baseados em cartão. Os fatores soft propostos para a implementação de sistemas baseado em cartão se baseiam na análise de problemas citados na literatura bem como de dificuldades identificadas através de um estudo de caso longitudinal. Essa lista foi validada por especialistas assim como por um grupo de colaboradores da empresa foco que participou da implementação do kanban. Nessa dissertação é proposta também uma casa de fatores soft para a implementação de sistemas baseados em cartão, nas quais os fatores são classificados como exclusivos dessa temática ou fatores clássicos de administração, bem como em relação ao nível organizacional em que atua. (organização, grupo de implementação ou indivíduo). Essa casa tem como objetivo auxiliar os gerentes na implementação de sistemas baseados em cartão, aumentando as taxas de sucesso nesse processo. Além disso, através da revisão de sistemas de coordenação de ordens recentes, essa dissertação visa aumentar o repertório dos gerentes sobre os sistemas existentes, possibilitando a implementação de opções mais adequadas para o ambiente produtivo em que se encontram

On the backlog-sequencing decision for extending the applicability of ConWIP to high-variety contexts:an assessment by simulation

Constant Work-in-Process (ConWIP) is a card-based control system that was developed for simple flow shops – a lack of load-balancing capabilities hinders its application to more complex shops. In contrast, load balancing is an integral part of Workload Control, a production planning and control concept developed for high-variety environments. One means of load balancing evident in the Workload Control literature is through the use of a capacity slack-based backlog-sequencing rule. This study therefore investigates the potential of the backlog-sequencing decision to improve load balancing in the context of ConWIP, thereby making it suitable for more complex, high-variety environments. Using simulation, we demonstrate that: (i) the choice of backlog-sequencing rule significantly impacts throughput times and tardiness-related performance measures; and (ii) capacity slack-based sequencing rules achieve significant performance improvements over ‘classical’ ConWIP backlog-sequencing rules. These results significantly extend the applicability of ConWIP. Results from the Workload Control literature however do not directly translate across to ConWIP. The simplified release procedure of ConWIP makes backlog-sequencing based on planned release dates dysfunctional. This negatively impacts the performance of modified capacity slack-based sequencing rules that were recently shown to be the best choice for Workload Control

Constant Flow Management - Investigating manufacturing flow variability

This project investigates the manufacturing flow variability in order to stabilize the factory process flow. Nowadays, in manufacturing production lines and particularly in modern front end semiconductor lines, processes and equipments are very complex. Any disturbance of the process creates variability in the line, and causes substantial losses in productivity for manufacturing corporations. These disturbances are unpredictable, difficult to control and result in long recovery times. Variability occurring in a production system disturbs the whole processing flow and results in long product cycle times. Hence, a range of sources of variability was determined from the literature and analyzed. This lead with the cooperation of factory managers to the development of four main objectives: (1) Determine a proper metric to measure the variability in the production system. (2) Determine the effect of batching and tool availability on the process flow. (3) Understand the interaction between operations. (4) Develop a release strategy in order to stabilize the production flow. First, from the observation of real production data, a difference metric was developed and operations creating or removing variability were identified. The propagation of variability can be followed using a correlation coefficient. Nevertheless, the data were not detailed enough to explain the origin of the variability. Consequently, several simulation models were created to investigate variability. The simulations‘ results show that the release strategy should be adjusted as a function of batch, tool availability and constraint parameters, in order to stabilize the flow of items in the line and control cycle time and cycle time variability. The notion of critical availability is introduced and defined. Improvement of the line performance is obtained through a tighter control of the availability of high capacity operations. This lead to the development of a new hybrid push pull release strategy, named CONFLOW, to regulate the flow of items reaching the constraint operation. CONFLOW was tested under many simulating conditions (batching, parallel processing, and different line length). Compared to a push system, CONFLOW release strategy results, into significant improvement (up to 80%) in cycle time, cycle time standard deviation and WIP level at the cost of 13% reduction in throughput. CONFLOW performances were compared to common TOC strategies (SA and DBR). The results are encouraging. In the specific conditions considered, CONFLOW performances are similar to SA and slightly better than DBR

Designing a robust production system for erratic demand environments.

Production systems must have the right type of material in the right quantities when required for production. They must minimize the work in progress while ensuring no stock-outstock-out occurs. While these twin opposing goals are achievable when demand is stable, they are difficult to realize under an erratic demand pattern. This dissertation aims to develop a production system that can meet erratic demands with minimal costs or errors. After a detailed introduction to the problem considered, we review the relevant literature. We then conduct a numerical analysis of current production systems, identify their deficiencies, and then present our solution to address these deficiencies via the ARK (Automated Replenishment System) technique. This technique is applied to a real-world problem at Methode Engineering ©. We conclude by detailing the scientific benefit of our technique and proposing ideas for future research

Bridging the Gap: Refining a Workload Control Concept for Practical Implementation.

Workload Control (WLC) is a Production Planning and Control (PPC) concept designed for complex manufacturing environments such as the job shop, a configuration commonly found in Make-To-Order (MTO) companies. The Lancaster University Management School (LUMS) has been a major contributor to the popularisation of WLC since the early nineteen eighties. This thesis contributes to the further development of a WLC methodology developed at Lancaster, hereafter referred to as the LUMS approach. The majority of contemporary WLC research is simulation based; as a result, there is a lack of understanding regarding practical elements of the concept. This has created a void between theoretical aspects of WLC and the practice of MTO production. This thesis is case study based and considers two research questions important to improving the feasibility of applying WLC in practice and to the future development of a body of empirical evidence. The first question focuses on bridging the gap between the theory and practice of WLC by refining the LUMS approach. The second centres on identifying the implementation issues surrounding WLC and exploring how these can be addressed in a case study setting. A three stage process to refine the LUMS approach was undertaken. Stage One revisits the literature, identifying areas in which the LUMS approach can be restructured. Stage Two focuses on developing a Decision Support System (DSS) based on the LUMS approach, while considering the needs of present day MTO companies. This resulted in changes to key assumptions underpinning the methodology. Through the partial implementation of the DSS in a case study setting, Stage Three contributes towards both research questions. This produced inductive refinements to the concept, while issues involved in preparing the company for implementation were also explored, leading to the development of an implementation strategy and framework for WLC