An optical characterization technique for hygroscopic expansion of polymers and plastic packages

Abstract

In this paper, we have described a new simultaneous 3-D optical system for characterizing electronic packages. This optical system is able to capture high resolution, online whole field data set. We are able to use this optical system to determine the coefficient of hygroscopic expansion (CHE) in moulding compound by correlating the weight gain and hygro-strain information obtained. In this study, we have used this optical system to evaluate the hygroscopic expansion of a fully cured moulding compound, XJA-9. We have also studied the effect of moisture diffusion on the deformation of plastic IC package. It was found that moisture diffusion could reverse the warpage of the plastic package. The study showed that moisture may cause severe changes to the internal stress field of plastic packages that would usually occur unnoticed or underestimated.

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.