Thermal characterization of an epoxy-based underfill material for flip chip packaging

doi:10.1016/S0040-6031(00)00357-9

Thermochimica Acta

Volumes 357–358, 14 August 2000, Pages 1-8

https://doi.org/10.1016/S0040-6031(00)00357-9 Get rights and content

Abstract

Epoxy-based underfill encapsulant materials are used in advanced microelectronic packaging to reduce thermal stresses on the solder joints and interconnections in integrated circuits. Therefore, their thermal properties directly affect package performance and reliability. In this study, the thermal properties of an epoxy-based underfill material developed for Intel’s flip chip packaging were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermomechanical analysis (TMA), dynamic mechanical thermal analysis (DMTA) and DMA techniques. Experimental results showed that this epoxy can be cured rapidly with low cure weight loss. Near room temperature, the cured epoxy has a low coefficient of thermal expansion (CTE) and a moderate storage modulus, resulting in a low stress index. The glass transition temperature and thermomechanical properties as a function of epoxy curing conditions will be discussed.

Introduction

In traditional integrated circuit (IC) packaging, the silicon chip is wire-bonded to a lead frame and sealed by a ceramic or plastic shell. Continuous improvement of microprocessor performance involves the increase in electronic packing density, device speed and size reduction. This has created high demands for smaller, cheaper, and lighter assemblies for high volume production. The driving forces for advanced packaging technologies are size reduction and device speed improvement. In advanced packaging and interconnection assemblies, the package size and the number of interconnects in the package are greatly reduced by using surface mount technology. Among the advanced packaging techniques, flip chip packaging is receiving increasing attention [1], [2], [3], [4].

Flip chip packaging technology is an interconnect technology which was first invented by IBM over 30 years ago as a packaging solution for high performance computers [5]. In this technology, the active area of the silicon chip surface is mounted facing toward the substrate by a variety of interconnect materials and methods [4]. Compared with traditional face-up wire bonding and tape automated bonding (TAB), the flip chip process provides higher packing density, shorter interconnection length, better electrical performance, improved reliability, and better manufacturability. Over the past 10 years, great efforts have been devoted to the research and development of flip chip technology. One of the most successful flip chip packaging processes is the controlled collapsible chip connection (C4) [1], as shown in Fig. 1.

A major challenge for the flip chip packaging technique originates from thermal stresses caused by the mismatch of coefficient of thermal expansion (CTE) between the silicon chip and the organic substrate [6], as demonstrated in Table 1. In particular, this mismatch produces thermal stresses on solder joints which results in fatigue and crack growth during temperature cycles [7]. This is a major reliability concern because cracks increase thermal and electrical resistance which lead to component failure. To solve this problem, a silica filled epoxy resin with low CTE is used to fill the gap between the silicon chip and the substrate (Fig. 1). This epoxy-based material, called underfill encapsulant, has a CTE which closely matches that of the solder alloy. Therefore, this encapsulation provides mechanical reinforcement and reduces thermal stresses on the solder joints. Consequently, the thermal properties of the underfill material have a direct impact on the reliability of the solder joints. For this reason, the development and characterization of epoxy-based packaging materials are of great interest to the microelectronics industry [8].

In this paper, we studied the thermal properties of an epoxy-based underfill encapsulant which was developed for Intel’s next generation flip chip (C4) packaging. The cure behavior, weight loss profile, filler size distribution and morphology, thermal expansion and dynamic mechanical properties were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), thermomechanical analysis (TMA) and dynamic mechanical thermal analysis (DMTA). These experimental results were used to improve the manufacturing process.

Section snippets

Experimental

The C4 underfill encapsulant was developed by Intel and its material supplier. The encapsulant was stored in a −40°C freezer before the experiments. At room temperature, this uncured epoxy-based material is a black viscous liquid. The chemistry of this epoxy will not be discussed here because it contains proprietary information.

DSC study of curing reaction

It is very important to understand the curing reaction of an epoxy-based underfill material, because its physical and mechanical properties depend on the extent of cure. DSC is one of the most widely used thermal analysis techniques to characterize the epoxy curing reactions [9], [10]. Fig. 2 shows non-isothermal DSC scans at various heating rates. Notice that the exothermic reaction peak is very sharp, indicating that this underfill epoxy can be cured rapidly near 150°C. This is highly

Weight loss profile and filler content

Fig. 7(a) shows a TGA curve during a typical curing process. During this process, the temperature of the uncured underfill epoxy was raised from 25 to 120°C at 3°C/min, then kept isothermally at 120°C for 30 min before being raised to 150°C at 1.3°C/min. The total isothermal time at 150°C was 166 min. From Fig. 7(a), it is clear that the weight loss was completed during the 30 min isothermal at 120°C, and the total weight loss during cure was only 0.18%. The low cure volatile of this epoxy yields

Filler morphology

The morphology and size distribution of these silica fillers were analyzed using SEM. Fig. 8(a) is a low-magnification SEM micrograph showing the filler distribution. From this SEM micrograph, it is clear that all fillers are spherical in shape, and the largest filler particles are approximately 25 μm in diameter with no segregation observed, i.e., small and large fillers are randomly distributed across the sample. This leads to isotropic properties for the material. Fig. 8(b) is a

Thermal expansion behavior

Samples used for TMA studies were prepared using cure profiles labeled as A and B, as discussed before. As suggested by isothermal DSC experimental results, both cure profiles should yield nearly fully cured materials. In practice, it has been shown that cure profile B may reduce void formation in the cured materials. This is because that cure profile B has an additional isothermal step at 80°C, this will initiate a slower reaction rate, which allows cure volatiles to escape instead of being

Dynamic mechanical properties

It is important to understand the dynamic mechanical properties of underfill materials, because they strongly affect thermal stresses and silicon die warpage in the packaging. The dynamic mechanical properties of cured C4 epoxy were measured by DMTA. In addition, dynamic mechanical analysis is more sensitive in detecting T_g [11]. Fig. 11 shows the storage modulus E′, loss modulus E″ and the loss factor tan δ for underfill epoxy. Again, samples prepared using cure profiles A and B have nearly the

Conclusions

An epoxy-based underfill encapsulant material developed for Intel’s next C4 flip chip packaging has been characterized using thermal analysis techniques. DSC measurements showed that this material can be cured rapidly at the processing temperature of 145°C with no post-cure required. Isothermal DSC results showed that the apparent activation energy for cure is 89.1 kJ/mol, which results in a good thermal stability at room and storage temperatures. TGA weight loss analysis revealed that this

Acknowledgements

Y. He wishes to thank Dr. Yuejin Guo for useful discussion and comments on this study.

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