L. Zavalij

Effect of sample size on the solidification temperature and microstructure of SnAgCu near eutectic alloys

The degree of undercooling of Sn in near eutectic, SnAgCu solder balls upon cooling at a rate of 1 °C/s from the melt was examined and found to increase linearly with inverse nominal sample diameter (for balls of radius between 100 and 1000 μm). The mean undercooling for SnAgCu solder balls in a flip chip assembly was 62 °C. The microstructures of these different samples were examined by means of scanning electron microscopy. The Sn dendrite arm width was observed to monotonically increase with ball diameter, indicating a possible dependence of the mechanical response of such solder balls upon size.

Solder metallization interdiffusion in microelectronic interconnects

We investigated the growth of intermetallic compounds in Cu/Ni/Au/PbSn solder joints. The substrates that we investigated had been Au plated by one of two different techniques. The Au finish thicknesses ranged from 0.25 to 2.6 /spl mu/m. After solder renew, structural examinations using optical and electron microscopy of cross-sectioned solder joints revealed the growth of Ni/sub 3/Sn/sub 4/ at the solder/Ni interface after reflow. Solder joints with thicker layers of Au annealed in Ar gas at a temperature of 150/spl deg/C for up to 450 h, displayed an appreciable growth of Au/sub 0.5/Ni/sub 0.5/Sn/sub 4/ at the Ni/sub 3/Sn/sub 4//solder interface. Previous investigators correlated growth of a Au-Sn alloy with the degradation of the mechanical properties of the solder joint. The determination of the stoichiometry of the Au/sub 0.5/Ni/sub 0.5/Sn/sub 4/ phase provides some understanding of why this phase grew at the Ni/sub 3/Sn/sub 4//solder interface, as Sn, Au and Ni are all readily available at this interface. The growth of this ternary alloy is also consistent with trends observed in the kinetics of formation of solder alloys.

Microstructure and Damage Evolution in Sn-Ag-Cu Solder Joints

The implementation of no-Pb soldering is progressing rapidly, often without immediately obvious problems. However, our current understanding of pertinent materials issues is far from sufficient to prevent surprises, and potentially serious miscalculations. The fundamentally different nature of no-Pb from SnPb solder joints has serious consequences for the design and interpretation of accelerated tests. The present work addresses the behavior of ball grid array (BGA), wafer level chip scale package (WL-CSP) and flip chip assemblies with SnAgCu solder joints in long term thermal cycling. Components were assembled onto either Cu or Ni pads on high-Tg printed circuit boards and cycled between 0 o C and 100 o C. Samples were removed at various stages for cross sectioning and microstructural characterization at various stages. Careful examination of the formation of intermetallic compounds at the pad surfaces, the size and orientation of the Sn grains, the morphology and number of precipitates, and the growth of cracks, revealed the sensitivity of the damage evolution to the solder microstructure. The consequences of the observed pattern of damage evolution are discussed.

Aspects of the structural evolution of lead-free solder joints

Studies of the formation of intermetallic compounds at some lead-free solder/metallization interfaces are briefly reviewed in this article. SnAgCu/Ni and SnAgCu/Cu interfaces are examined in particular. It has been found that (Cu,Ni)6Sn5 forms at SnAgCu/Ni interfaces until copper is depleted from the solder matrix. This article also contrasts the formation of (Au,Ni)Sn4 and related compounds in PbSn/Ni solder joints and lead-free solder joints.

Crystal structure of Au1-xNixSn4 intermetallic alloys

Abstract Ni coated with a thin layer of Au is commonly used as a metallization in electronics packaging. Such Ni/Au metallizations are alloyed with Sn at temperatures above the Pb–Sn solder liquidus. Thermal treatment of these joints in the solid state, for instance at 150°C, has been shown to result in the formation of Au 1− x Ni x Sn 4 compounds at the solder interface. These compounds are found to be deleterious to joint integrity. In this work, synthesis, microstructural analysis and crystal structure determination of Au 1− x Ni x Sn 4 alloys were performed. The alloys were examined by powder and single crystal X-ray diffraction, metallography, electron microprobe analysis, and thermal analysis. It was found that Ni atoms substitute for Au atoms in the AuSn 4 structure giving compounds with general composition Au 1− x Ni x Sn 4 (with x ≤0.5). The structure of the title compounds is considered isotypical with the AuSn 4 phase (the PdSn 4 structure type), with a symmetry increment from Aba2 to Ccca . Crystal structure refinement from powder data using the Rietveld method was performed for two alloys with x =0.25 and 0.50 yielding Au 0.73 Ni 0.27 Sn 4 (sp. group Ccca , a =6.448 A, b =11.606 A, c =6.441 A) and Au 0.51 Ni 0.49 Sn 4 (sp. group Ccca , a =6.424 A, b =11.522 A, c =6.384 A). A single crystal experiment was performed for compound Au 0.91 Ni 0.09 Sn 4 (sp. group Ccca , a =6.523 A, b =11.676 A, c =6.485 A).

The kinetics of formation of ternary intermetallic alloys in Pb-Sn and Cu-Ag-Sn Pb-free electronic joints

A simple model of the formation of Au/sub 0.1/Ni/sub 0.1/Sn/sub 0.8/ in Pb-Sn solder/Ni interconnects is examined by numerical simulation. Previous experimental observation has shown that after reflow the interface consists of the Ni/sub 3/Sn/sub 4/ alloy between Pb-Sn solder and Ni, with Au distributed through the PbSn solder ball. Au/sub 0.1/Ni/sub 0.1/Sn/sub 0.8/ was observed to form at the Pb-Sn solder/Ni/sub 3/Sn/sub 4/ interface during annealing at 150/spl deg/C in a number of studies. The numerical simulation was used to calculate the maximum flux of Au to the interface, and with the assumption that this Au was immediately incorporated in to Au/sub 0.1/Ni/sub 0.1/Sn/sub 0.8/ a maximum rate of formation of Au/sub 0.1/Ni/sub 0.1/Sn/sub 0.8/ was calculated. This rate was found to be similar to measured rates of formation of Au/sub 0.1/Ni/sub 0.1/Sn/sub 0.8/ from two different studies. The formation of(CuNi)6SnS in Sn-Ag-Cu/Ni solder interconnects was discussed within the context of these observations.

Calorimetric investigation of the formation of metastable silicides in Au/a-Si thin film multilayers

Metastable phase formations were studied in Au/a-Si thin film multilayers using differential scanning calorimetry and x-ray diffraction. Two nonequilibrium crystalline silicides were found to form below 500 K. Each phase formed by a different mechanism, and the competing growth of the two phases over the temperature range of 375 to 500 K, was found to depend greatly on thickness and grain size in the Au layers. At higher temperatures (500–600 K), these metastable phases decomposed and the a-Si crystallized by metal-induced crystallization to yield a phase mixture of Au and x-Si. The enthalpy of crystallization of a-Si was measured from the calorimetry data to be −12.1±1.2 kJ/mol.

Growth of Cu-Ni-Sn Alloys in Pb Free CuSnAg Solder/Au-Ni Metallization Reactions

Diffusion and phase formation were monitored in solder joints consisting of flip chips with Ni(V) under-bump metallizations bumped with Ag 3.8 Cu 1.85 Sn94.35 (atomic percentage) solder reflowed on printed circuit boards with Cu/Ni/Au metallizations. A ternary alloy, (CuNi) 6 Sn 5 , was observed to form during reflow at solder/Ni interfaces in these Ag 3.8 Cu 1.85 Sn94.35/Au/Ni solder joints. After extended thermal aging at 150 o C, a second ternary compound, (CuNi) 3 Sn 4 forms at the Ni/(CuNi) 6 Sn 5 interface. The growth of these alloys depletes the solder of essentially all Cu, changing its mechanical properties and melting temperature.