應用西北角法建構晶圓廠瓶頸機台管理模式

Abstract

在晶圓代工廠建廠之成本中，機台設備所佔的固定成本比例最高，而機台折舊攤提也是反應在產品成品的固定成本上。產品成本中的固定成本佔晶圓的單位生產成本比例又會依機台使用年限不同而有差異，一般而言，新廠的機台成本可高達晶圓廠總成本的70% 至 90%。因此如何提高機台的利用率，是每座晶圓代工廠的重要課題。

工廠的工業工程管理人員(Industrial Engineer, 以下簡稱 IE) 最重要的責任之一就是幫工廠規劃最佳產能。為能進行長期的產能規劃，或是能以最短的時間獲得某種產品組合下的每個機台的最大負載率，並找出此產品組合的瓶頸機台，IE通常會選擇以靜態產能規劃方式來規劃產品組合，但靜態產能規劃通常不考慮機限，為了提高機台產能，方法之ㄧ就是解除機限。企業界的產能規劃問題，通常不是一般的線性規劃方法就可以解決，而是需要以更有效率的演算法來規劃，這種特殊形式的線性規劃就是一個運輸問題。運輸問題是線性規劃中的一種，運輸問題求起始解的方法之一是西北角法，此方法只求供需平衡之可行方案，並未考慮運輸成本。此方法是自平衡的運輸表格左上角開始，將各供給量成梯形狀依各需要量逐步向右下角分配。此概念與本研究欲將產品先分配給機台限制最多的產品概念雷同，先將限制最多的機台盡力填滿產能後，再開始分配給機台限制次多的機台，一直到機台分配完為止，最後會再提供機限展開報表，供相關人員參考。因此本研究之主要目的即是應用西北角法建構一套管理模式以彌補靜態產能不考慮機限的缺失，使產能規劃更能達到最佳效益。本研究最後以一個晶圓代工廠實例來說明本研究方法之有效性。

Generally speaking, cost of equipments could claim 70%~90% of capital expenses of a new plant. The fixed overhead of equipments for different wafer size foundry accounts for the largest proportion of the cost. The depreciation of equipment will be assigned to the fixed overhead. The percentage of fixed overhead that accounts for wafer unit cost varies from equipment to equipment due to depreciation. Thus, it is very essential for all wafer foundries to increase the utility of equipments.

The most important responsibility of an industrial engineer (IE) is to design the most effective capacity planning for a plant. Static capacity planning (SCP) is the most common method for doing long-term capacity plan. The equipment loading can be evaluated very quickly through SCP under some kind of product combinations. However, equipment limitations are usually not included in SCP simulation model. Therefore, to resolve this problem, one better approach is to dissolve equipment limitations. Objective of this study is to develop a new management model to improve the main drawback of SCP using northwest-corner method. Finally, a real case from a Taiwan wafer foundry is utilized to demonstrate the effectiveness of the proposed method.