Integrated process capability analysis with an application in backlight module

Backlight application specializes in supplying light, with notable examples including liquid crystal display (LCD), hand-phone LCD, and PDA LCD. The integrated process capability and integrated process yield for cold cathode fluorescent lamp backlight are unknown. Process capability analysis is a highly effective means of assessing the process ability of backlight that meets specifications. A larger process capability index (PCI) implies a higher process yield, and lower expected process loss. Chen et al. [Int. J. Product. Res. 39 (2001) 4077], applied indices Cpu, Cpl, and Cpk to evaluate the integrated process capability for a multi-process product with smaller-the-better, larger-the-better, and nominal-the-best specifications, respectively. However, Cpk suffers from the weakness of being unable to reflect the specific process yield. This study selects index Cps to replace Cpk. Meanwhile, an integrated PCI for the entire backlight module is proposed, and the relationship between the PCI and process yield is described. A multi-process capability analysis chart, which reasonably accurately indicates the status of process capability for the backlight module, is designed for practical applications. 2002 Elsevier Science Ltd. All rights reserved. 1. Product description––backlight module Initially, backlight module was used primarily in advertising light boxes. However, because of the vigorous marketing of liquid crystal display (LCD) manufacturing and customer demand for night version and full color LCD, backlight module is closely associated with optical products. Generally, backlight module comprises a piece of light-guide plate, which guides a spontaneous light source from a light emitting diode or light tube to create a lager and more uniform surface illuminant. Backlight module is generally simply termed backlight. The application of backlight specializes in supplying the light required by liquid crystal. LCD, Hand-phone LCD, and PDA LCD are examples of backlight applications. The advantages of backlight are uniform light distribution and high brightness. Backlight module provides the light source for TFT monitors. TFT monitors are growing in size, and the present production size ranges from 13 to 18 inches. Generally, the brightness equalization becomes increasingly difficult to control as TFT monitors increase in size. The backlight modules of LCD are categorized into LED, EL, and cold cathode fluorescent lamp (CCFL). Forhouse Corporation is located in central Taiwan and specializes in CCFL production. The bright white light provided by CCFL offers clear and even illumination over a large viewing area. The features of CCFL backlight include high brightness, long lifetime, low power consumption, and white color. A thin structure with even illumination emits light from a tube-like light source over a large area. CCFL inverters output a high pressure AC current. A CCFL comprises mold frame, lamp, film, ALcase and light guide panel (LGP). Fig. 1 illustrates the structure of the backlight module. The key quality variables of the backlight module currently produced by Forhouse Corporation include: Microelectronics Reliability 42 (2002) 2009–2014 www.elsevier.com/locate/microrel * Corresponding author. Tel.: +886-4-23924505; fax: +886-423934620. E-mail address: [email protected] (M.L. Huang). 0026-2714/02/$ see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0026-2714 (02 )00126-9 (1) length, (2) width, (3) thickness, (4) brightness, and (5) equalization. Table 1 describes the specifications for the above quality characteristics. Currently, the X–R control chart and process capability indices Cp, and Cpk are using to assess each quality characteristic. Meanwhile, the integrated process capability and integrated process yield for the CCFL backlight are unknown. 2. Process capability evaluation for a backlight module Process capability indices are widely used to assess whether product qualities meet specifications in semiconductor and IC assembly industries. Numerous statisticians and quality engineers, such as Kane [9], Chan et al. [5], Choi and Owen [1], Boyles [6], Pearn et al. [11], Kotz and Johnson [10], Boyles [7], Singhal [8], V€annman [15], Chen [2] and Spiring [14], have investigated process capability indices with the aim of proposing more effective methods of assessing process potential and performance. The key quality characteristics of a backlight module include (1) length, width and thickness, which are nominal-the-best specifications, and (2) brightness and equalization, which are larger-the-better specifications. Customers expect all of the quality characteristics of a backlight module to meet or exceed expected levels. Chen et al. [3] has noted that the integrated process capability for multi-process products cannot be evaluated by either univariate process capability indices or multivariate process capability indices. Furthermore, in assessing the integrated process capability for a multiprocess product, Chen et al. [3] has applied indices Cpu, Cpl, and Cpk to assess the process capability for a multiprocess product with smaller-the-better, larger-thebetter, and nominal-the-best specifications, respectively. Since a backlight is designed to have two larger-thebetter specifications and three nominal-the-best specifications, the integrated process capability index (PCI) for a backlight module becomes