اسکن از تداخل نور سفید در کنترل کیفیت نوار تک نقطه اتصال خودکار

We report on applying a scanning white light interferometry (SWLI) for quality control of aluminum lead single-point tape automated bonding (spTAB). A spTAB process was used to connect Al leads on a thin polyimide flex to Al bond pads on a flexible Al-polyimide cable. In the experiment three different bonding process parameters, i.e. bond force, ultrasonic power, and ultrasonic treatment time were varied in order to maximize the pull force. A custom built scanning white-light interferometer was used to measure the bond height in order to correlate this parameter with the tensile bond force. This force was obtained in a destructive way by a consecutive pull test. All bonds with a height within (7.22 ± 1.80 μm), possessed a tensile strength exceeding 85 mN. This was verified by a separate validation measurement where the pull force of bonds complying with the height requirement was recorded. Based on the 3D observations the conditions for an acceptable bond quality were revisited and refined.

Optical techniques provide non-destructive means for quality assurance in microelectronics production. Typically, 2D imaging of devices during production is carried out [1] and [2]. For instance the geometry of interconnects, e.g. ball bonds, has been used as a signature of acceptable quality [1]. Scanning white light interferometry (SWLI) is a fast and accurate 3D inspection method [3]. This technique could prove useful in cases where the interconnections are optically accessible after bonding, as with single-point tape automated bonding (spTAB), should the bond geometry provide information about the interconnection quality. Flexible interconnections and the TAB technique have typically been used in applications requiring dynamical features like inkjet printers and in flat structures such as memory cards and displays [4] and [5]. Standard materials in flexes for TABing use polyimide as base material and copper as conductor. However, extremely light structures can be realized using thin Al instead of Cu. In addition, being softer than Cu, Al allows a delicate bonding process with low bonding force and ultrasonic power, even at room temperature [6]. Therefore, the Al-technology is an attractive technology for miniaturized components with critical thermo-mechanical features. Moreover, Al–Al bonds are known to be reliable and additional advantage accrues from the fact that the flexible base material can absorb part of the coefficient of thermal expansion (CTE)-based deformations caused by temperature changes in the hard microchip [6]. The bonding process is purely ultrasonic and provides lead-free interconnections. The aim of this work is to verify the applicability of SWLI for rapid quality control of spTAB samples. The (bumpless) spTAB was used to assemble samples for optical 3D investigation. Pull tests were used to establish a correlation between the optically measured bond height and the bond strength measured by a pull test.

It has been shown that using designed experiment and statistical tools it is possible to optimize the bonding parameters for a spTAB process using a relatively small number of bonds. Bonds performed using these optimized parameters were non-destructively studied by a scanning white-light interferometer. The results showed that by retaining bonds that exceed a certain height, it was possible to non-destructively ensure a minimum strength for these bonds. Particularly, for the studied application, bond thicknesses of (7.22 ± 1.80 μm) ensured bond strengths exceeding 85 mN – well beyond the limits set by the MIL-STD 883F standard. Furthermore, quantitative information obtained with the SWLI studies can be used to reformulate the criteria for an acceptable spTAB interconnection. In the light of these results we would like to conclude that the SWLI has shown its potential as a quality assurance tool within the framework of spTAB interconnections.