An optical characterization technique for hygroscopic expansion of polymers and plastic packages
Introduction
Polymers are widely used in electronics packaging because of their relatively low cost, lightweight, and suitability for mass production. However, current plastic packaging materials tend to absorb moisture from surrounding environment with time. The absorbed moisture often results in component corrosion [1], [2], deterioration in mechanical property of packaging material [3], [4], and reduction in the adhesion strength at constituent interfaces [5], [6]. It has also been shown that hygroscopic swelling in polymeric materials give rise to increased stress in the component due to swelling mismatch and leading to delamination between moulding compounds and IC chips [6], [7]. The evolution of tensile stress caused by hygroscopic expansion of underfill is one of the reasons for the failure in under-bump metallization (UBM) in flip-chip [8], [9]. Wong et al. [9] have developed a technique to measure the coefficients of hygroscopic expansion (CHE) but the method has some severe technical limitations, namely: long testing time may be needed for low desorption temperature and more importantly the system is not suitable for analysing whole package due to its non full-field measurement mode.
Hence the solution to these limitations is to develop a full-field and on-line optical characterization system that has sub-micron resolution or better. In fact, Moiré interferometry [10] has been successfully used to characterize the hygroscopic expansion of moulding compound and plastic packages [11], [12]. In this paper, we present a novel 3-D optical testing system and apply this 3-D characterization technique to investigate the hygroscopic expansion of moulding compound and its large effect on the deformation of plastic packages. In fact, preliminary characterization results were presented by the authors in 2001 [13]. Advantage of 3-D characterization over conventional 2-D one is that more information can be achieved without cross sectioning the specimen.
Section snippets
Principle of the 3-D optical testing system
The 3-D optical testing system consists of a two conventional interferometer systems, namely a Moiré and a Twyman/Green. The Moiré interferometer provides information for the in-plane deformation and the Twyman–Green interferometer, the out-of-plane deformation measurements. Schematic diagram of Moiré interferometry system is shown in Fig. 1(a). In this configuration, a high frequency cross-line diffraction grating is replicated on the surface of the specimen and it deforms together with the
Coefficient of hygroscopic expansion (CHE) measurement
In this study, CHE is defined as the ratio of the ultimate hygro-strain over the percentage total weight gain in the sample saturated with moisture. Post-cured moulding compound (XJA-9) from ST Microelectronics is used in this study.
Summary and discussion
Based on the above results, the following conclusions were drawn:
- (1)
The simultaneous 3-D optical testing system is an effective tool for characterizing the hygroscopic expansion and determination of CHE. Advantages of this optical technique include high resolution, comprehensive information, full field and in situ testing mode. Although in this study, the hygroscopic expansion of polymer material and its effect on plastic packages was characterized in room ambient, the system could be used for
Acknowledgements
The authors would like to express their gratitude to Mr. W.D. van Driel, from Philips semiconductor, for his invaluable suggestions and comments. We also thank Mr. Tee Tongyan, from ST Microelectronics, for providing the moulding compound used in this investigation.
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2012, Thermochimica ActaCitation Excerpt :These techniques include measuring the moisture-induced curvature or bending of a bilayer strip (such as cured epoxy-laminate on a sheet of Cu) [29,30] or using the Archimedes’ principle to monitor the change in sample volume [31]. Others have used a non-contacting projector microscope [32] or a novel 3D testing system consisting of a Moiré and a Twyman/Green interferometer systems [33] to directly monitor the swelling strain. Park et al. have used digital image correlation (DIC) to determine the temperature dependence of the coefficient of hygroscopic swelling (CHS) of several molding compounds between 20 and 180 °C [34], while the same technique has been used to measure hygroscopic swelling of thin polymer films [35].
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