Fay Hua

Electromigration in flip chip solder joints having a thick Cu column bump and a shallow solder interconnect

In advanced electronic products, current crowding induced electromigration failure is one of the serious problems in fine pitch flip chip solder joints. To explore a strong resistance against current crowding induced electromigration failure, a very thick Cu column bump combined with a shallow solder interconnect at 100μm pitch for flip chip applications has been studied in this paper. Results revealed that these interconnects do not fail after 720h of current stressing at 100°C with a current density of 1×104A∕cm2 based on the area of interface between Cu column bump and solder. The reduction of current crowding in the solder region by using thick Cu column bumps increased the reliability against electromigration induced failure. The current distribution in a flip chip joint of a Cu column bump combined with a shallow solder has been confirmed by simulation. However, Kirkendall void formation was found to be much serious and enhanced by electromigration at the Cu∕Cu3Sn interface due to the large Cu∕Sn ra...

The creep properties of lead-free solder joints

This paper describes the creep behavior of three tin-rich solders that have become candidates for use in lead-free solder joints: Sn-3.5Ag, Sn-3Ag-0.5Cu, and Sn-0.7Cu. The three solders show the same general behavior when tested in thin joints between copper and Ni/Au metallized pads at temperatures between 60‡C and 130°C. Their steady-state creep rates are separated into two regimes with different stress exponents. The low-stress exponents range from ∼3–6, while the high-stress exponents are anomalously high (7–12). Strikingly, the high-stress exponent has a strong temperature dependence near room temperature, increasing significantly as the temperature drops from 95°C to 60°C. The anomalous creep behavior of the solders appears to be due to the dominant tin constituent. Research on creep in bulk samples of pure tin suggests that the anomalous temperature dependence of the stress exponent may show a change in the dominant mechanism of creep. Whatever its source, it has the consequence that conventional constitutive relations for steady-state creep must be used with caution in treating tin-rich solder joints, and qualification tests that are intended to verify performance should be carefully designed.

Pb-free solder challenges in electronic packaging and assembly

Soldcrs are used in inan?. areas of clectronic packaging and assemblies. They have been uscd as electrical and mccllanical interconnects. such as fliu cluu buinuing and .I att;iclunent, ball grid amy (BGAs). surface mount technology (SMT). and soldering surface finishes, such as lcadfnme coating and print circuit board metal surface finish for several decades. As future electronics bccoine smaller. faster and cheapcr. lherc are scveral critical soldering technology gaps. Fundanrental understanding of Pb-frce soldcrs. Industry has ovcr 50 years of expcriencc with SnPb solders in electronics packagcs. Empirical rules for SnPb solders are not autornatically applicable to Pb-free solders. In addition, smaller. faster and cheaper electronics require increasing elcctrical power. The solder joints undergo mcchanical. electrical cirrrcnt as acll as thermal stresses in thc application. Tlrc responds of Pb-frec solders to the stresses are different from SnPb solders. Fundamental understanding of Pb-frec solders is kcy task for solder material researchers for next decadc.

Thermal properties of oxide free nano non noble metal for low temperature interconnect technology

Tin (Sn) nanoparticles with various sizes were synthesized from a chemical reduction method. Their morphological and thermal characterizations were studied. The high resolution transmission electron microscopy (HRTEM) study showed that significantly low level of oxides was formed. The thermal characterization by differential scanning calorimetry (DSC) exhibited the size dependency of the melting points

Thermal Properties of Tin/Silver Alloy Nanoparticles for Low Temperature Lead-free Interconnect Technology

Tin/silver alloy nanoparticles with various sizes were synthesized by the chemical reduction method and their thermal properties were studied by differential scanning calorimetry. Both the particle size dependent melting temperature and latent heat of fusion have been observed. The melting point can be achieved as low as 194degC when the diameter of the nanoparticles is around 10 nm. The 64 nm (average diameter) SnAg alloy nanoparticle pastes showed good wetting properties on the cleaned copper foil surface and the intermetallic compounds formed. These low melting point SnAg alloy nanoparticles could be used for low temperature lead-free interconnect applications.

iNEMI Pb-free alloy characterization project report: Thermal fatigue results for low and no-Ag alloys

Significant innovations in Pb-free solder alloy formulations are being driven by volume manufacturing and field experiences. As a result, the industry has seen an increase in the number of Pb-free solder alloy choices beyond the common near-eutectic Sn-Ag-Cu (SAC) alloys first established as replacements for Sn-37Pb. The increasing number of Pb-free alloys provides opportunities to address shortcomings of near-eutectic SAC, such as poor mechanical shock performance, but also introduces a variety of technical and logistical risks. Since 2008, the Pb-Free Alloy Characterization Program sponsored by the International Electronics Manufacturing Initiative (iNEMI) has been working to fill the gap in knowledge associated with thermal fatigue resistance of these new solder alloys. Results from the extensive experimental program are now becoming available and are being published through a series of publications (see References). This paper provides a summary of the overall iNEMIs program goals, the experimental structure, and the results and analysis of thermal cycling for low silver alloys, containing 1 wt.% or less Ag. Results indicated that there is a correlation between the characteristic life of short dwell thermal cycles and Ag content. Increase in the Ag content increased the characteristic life. Another important finding is that all low-and no-Ag alloys performed better than Sn-37Pb under the test conditions. Finally, as the stress levels increase during thermal cycling, the performance differences between the Pb-free alloys diminish, and their performance appears to be approaching that of Sn-37Pb.

Densification in Electronics Packaging and Assembly

Summary form only given. Demand for smaller, faster, and low cost electronics has led the electronics industry move to very fine interconnects on both Ball Grid Array (BGA) surface mount assembly, flip chip packaging level. The trend of densification challenges board assembly material, processes and reliability. HDI PCB materials will become mainstream for electronics to address the increasing I/O density. The electrical, mechanical, thermal and chemical interaction impacts to solder joint reliability needs to be addressed. Low stress flip chip assembly technology will be needed for next generation flip chip technology.

Tin/silver alloy nanoparticles for low temperature lead-free interconnect applications

A chemical reduction method was used to synthesize tin/silver alloy nanoparticles with various sizes and their thermal properties were studied by differential scanning calorimetry. Both the particle size dependent melting temperature and latent heat of fusion have been observed. The melting point can be achieved as low as 194 °C when the diameter of the nanoparticles is around 10 nm. The 64 nm (average diameter) SnAg alloy nanoparticle pastes showed good wetting properties on the cleaned copper foil surface and the intermetallic compounds formed. These low melting point SnAg alloy nanoparticles could be used for low temperature lead-free interconnect applications.

Electromigration study on Cu-Sn interconnections

This research concerns the influence of electron current on the diffusion of Sn and Cu in simple Cu-Sn and Cu-Sn-Cu diffusion couples. The diffusion couples are designed to permit in situ studies of the progress of diffusion. Initial tests were done in 60/spl deg/C air with a current density of 1 /spl times/ 10/sup 4/A/cm/sup 2/. The results showed Cu movement into Sn in the direction of the electron current. With accompanying grain boundary sliding of the Sn grains. There is also evidence for Sn flow in the direction opposite the electron current.