S. M. L. Nai

Low temperature glass-to-glass wafer bonding

In this paper, results of successful anodic bonding between glass wafers at low temperature are reported. Prior to bonding, a special technique was used, i.e., an amorphous and hydrogen free silicon film was deposited on one of the glass wafers using a sputtering technique. The effects of bonding temperature and voltage were investigated. The bonding temperature and the voltage applied ranged from 200/spl deg/C to 300/spl deg/C and 200 V to 1000 V, respectively. As the bonding temperature and bonding voltage increased, both the unbonded area and the size of voids decreased. Scanning electron microscope (SEM) observations show that the two glass wafers are tightly bonded. The bond strength is higher than 10 MPa for all the bonding conditions. Furthermore, the bond strength increases with increasing bonding temperature and voltage. The study indicates that high temperature and voltage cause more Na/sup +/ ions to neutralize at the negative electrode, which leads to higher charge density inside the glass wafer. Furthermore, the transition period to the equilibrium state also becomes shorter. It is concluded that the anodic bonding mechanisms involve both oxidation of silicon film and the hydrogen bonding between hydroxyl groups.

Using carbon nanotubes to enhance creep performance of lead free solder

Abstract In the present study, the influence of multiwalled carbon nanotubes (CNTs) on the creep performance of 95·8Sn–3·5Ag–0·7Cu lead free solder is investigated. Composite solders containing varying weight percentages of CNTs were synthesised. Solder joints were fabricated and subjected to a series of tests under testing temperatures ranging from 25 to 125°C and applied stresses ranging from 4 to 18 MPa. Creep results revealed that solders containing CNTs exhibited significantly improved creep resistance and also creep time to failure increased. Stress exponents of composite solders were higher than that of monolithic solder. In general, stress exponent was found to increase with increasing applied stress and with decreasing testing temperature. The range of activation energies of composite solder with 0·01 wt-%CNT addition was found to be comparable to that of monolithic Sn–Ag–Cu. However, as the amount of CNT increased to 0·04 wt-%, the range of activation energies increased accordingly.

A modified constitutive model for creep of Sn?3.5Ag?0.7Cu solder joints

In this study, the constitutive behaviour for creep performance of 95.8Sn–3.5Ag–0.7Cu lead-free solder joints was investigated. It was observed that the stress exponent (n) can be well defined into two stress regimes: low stress and high stress. A new, improved constitutive model, which considered back stress, was proposed to describe the creep behaviour of SnAgCu solder joints. In this model, the back stress, which is a function of the applied shear stress in the low stress regime (LSR) and a function of the particle size, volume fraction and coarsening of IMC particles in the high stress regime (HSR), was introduced to construct the relationship between the creep strain rate and the shear stress. The creep mechanism in these two stress regimes was studied in detail. In the LSR, dislocations passed through the matrix by climbing over the intermetallic particles, while in the HSR, the dislocations were glide-controlled. According to the different creep mechanisms in both the stress regimes, the back stress was calculated, respectively, and then incorporated into the Arrhenius power-law creep model. It was demonstrated that the predicted strain rate–shear stress behaviour employing the modified creep constitutive model which considered back stress, was in good agreement with the experimental results.

Development of Lead-Free Solder Composites Containing Nanosized Hybrid (ZrO2 + 8 mol.% Y2O3) Particulates

In this study, lead-free composite solders were successfully synthesized, with varying amount of nanosized ZrO2 + 8 mol.% Y2O3 particulates incorporated into 95.8 Sn – 3.5 Ag – 0.7 Cu solder. These composite materials were fabricated using the powder metallurgy technique involving blending, compaction, sintering and extrusion. The extruded materials were then characterized in terms of their physical properties, microstructural development, thermal and mechanical properties. Experimental results revealed that with the addition of increasing amount of reinforcements, the density values of the composite solders decreased, but there was no influence on the melting point of the composite solders. Thermomechanical analysis of the solder nanocomposites showed that the use of reinforcements lowered the average coefficient of thermal expansion of the solder material. Moreover, the results of mechanical property characterizations revealed that the addition of reinforcements aids in improving the overall strength of the nanocomposite solder. An attempt is made in the present study to correlate the variation in volume percentages of the hybrid reinforcements with the properties of the resultant nanocomposite materials.

DEVELOPMENT OF NOVEL LEAD-FREE SOLDER COMPOSITES USING CARBON NANOTUBE REINFORCEMENTS

In this study, Sn-Ag-Cu based nanocomposites with carbon nanotubes (CNTs) as reinforcements were successfully synthesized via the powder metallurgy technique. Lead-free solder powders were firstly blended together with varying weight percentages of CNTs. The materials were then compacted, sintered and finally extruded at room temperature. The extruded materials were characterized for their microstructural, thermal and mechanical properties. The porosity of the nanocomposites was observed to increase with increasing weight percentages of CNTs, accordingly the density of the nanocomposites was reduced. Thermomechanical analysis of the solder nanocomposites showed that the use of CNTs as reinforcements decreased the average coefficient of thermal expansion of the solder matrix. Furthermore, the results of mechanical properties characterization revealed that the addition of CNTs aids in enhancing the microhardness and the overall strength of the nanocomposite solder. An attempt is made in the present study to correlate the variation in weight percentages of the carbon nanotubes with the properties of the resultant nanocomposite materials.

Silicon-to-silicon wafer bonding with gold as intermediate layer

In this study, eutectic bonding process between two 4-inch, p-type silicon wafers has been successfully achieved. A gold pattern of varying height is firstly deposited on one silicon wafer prior to bonding. To further understand the eutectic bonding process, the effects of bonding temperature and gold height are investigated. When studying the effect of bonding temperature, the gold height is kept constant at 1.00 /spl mu/m while the bonding temperature is varied from 375 to 475/spl deg/C. It is found that bonding temperature of 400/spl deg/C yielded the most satisfactory results, in terms of bonding efficiency, bond strength and interfacial integrity. Moreover, when studying the effect of gold height, the bonding temperature is kept at the optimized temperature of 400/spl deg/C, with gold height varying from 0.20 to 1.40 /spl mu/m. It is found that gold height of 1.00 /spl mu/m yielded the best results.

INDENTATION SIZE EFFECT ON THE CREEP BEHAVIOR OF A SnAgCu SOLDER

In the present study, nanoindentation studies of the 95.8Sn-3.5Ag-0.7Cu lead-free solder were conducted over a range of maximum loads from 20 mN to 100 mN, under a constant ramp rate of 0.05 s-1. The indentation scale dependence of creep behavior was investigated. The results revealed that the creep rate, creep strain rate and indentation stress are all dependent on the indentation depth. As the maximum load increased, an increasing trend in the creep rate was observed, while a decreasing trend in creep strain rate and indentation stress were observed. On the contrary, for the case of stress exponent value, no trend was observed and the values were found to range from 6.16 to 7.38. Furthermore, the experimental results also showed that the creep mechanism of the lead-free solder is dominated by dislocation climb.

Advanced high density interconnect materials and techniques

The trend in micro/nanosystems is to be lighter, smaller and cheaper. At the same time there is a prodigious push for increasing functionalities. Such demands can only be fulfilled by progressively higher density integrated devices and circuits. With 2D IC reaching its physical limitation soon, 3D IC has attracted tremendous attentions and interests worldwide. For either 2D or 3D IC, the interconnection and packaging will be one of the major challenges for the development and commercialization of micro/nanosystems. Flip chip at chip level and wafer-level have the advantages of having the lowest possible inductance per lead, highest frequency response speed as well as the lowest cross talk and simultaneous switching noise. Challenges arise when interconnection method of flip chip is gradually growing into the mainstream in the integration and packaging industry where the issue of size becomes increasingly critical for interconnection and pitches. Therefore, the development of new interconnection materials and techniques is necessary to meet the ever-stringent requirements of mechanical, thermal and electrical properties of interconnection when the interconnection dimension and pitch size are reduced to very fine scales. Furthermore, such advanced interconnection materials and techniques can also be used to stack 3D IC. In this paper, the development of novel lead-free solder nanocomposites, room to low temperature Cu-Cu and Au-Au bonding, and carbon nanotube interconnection techniques will be reported. The developed micro/nanointerconnection techniques can be easily adopted by the industry to realize high density and multifunctional integration and packaging.

NANOMECHANICAL PROPERTIES OF A Sn-Ag-Cu SOLDER REINFORCED WITH Ni-COATED CARBON NANOTUBES

In the present study, 0.05 wt.% of Ni-coated multi-walled carbon nanotubes (Ni-CNTs) were successfully incorporated into the 95.8Sn–3.5Ag–0.7Cu solder using the powder metallurgy technique, to synthesize a new lead-free composite solder. Its mechanical property (in terms of hardness) was investigated at room temperature using the nanoindentation method. The results revealed that the nanoindentation hardness increased by 14.3% with the incorporation of 0.05 wt.% of Ni-coated CNTs. This observation is in good agreement with the microhardness test results. Moreover, the addition of Ni-CNTs improved the creep resistance of the composite solder. The test results established that nanotechnology coupled with composite technology in electronics solders can result in the enhancement of mechanical properties. These advanced interconnect materials will thus benefit the microelectronics assembly and packaging industry.

Enhancing the performance of Sn-Ag-Cu solder with the addition of titanium diboride particulates

In this study, varying volume percentage of titanium diboride particulates were successfully incorporated into Sn-Ag-Cu solder, to synthesize lead-free composite solders. The composite solders were synthesized via the powder metallurgy route of: blending, compaction, sintering and extrusion. The extruded materials were then characterized for their microstructural development, thermal and mechanical properties. With increasing volume percentage of titanium diboride particulates, the composite solders experienced a corresponding decrease in density values but porosity levels were observed to increase. Microstructural characterization using scanning electron microscopy revealed uniform distribution of reinforcement particulates and a fairly good interfacial integrity between the particulates and the solder matrix. Thermomechanical analysis showed that presence of titanium diboride particulates as reinforcements, decreased the average coefficient of thermal expansion of the solder composites. A change in the mechanical properties was also observed with the presence of increasing particulates. An attempt is made, to correlate the increasing presence of titanium diboride particulates with the microstructural development, as well as the mechanical and thermal properties of solder matrix.Copyright