Paul A. Lauro

Effect of Sn grain orientation on electromigration degradation mechanism in high Sn-based Pb-free solders

Electromigration induced damage strongly depends on Sn-grain orientation in Pb-free solders. Rapid depletion of intermetallic compounds and under bump metallurgy led to significant damages caused by the fast diffusion of Cu and Ni along the c axis of Sn crystals. When the c axis of Sn grain is not aligned with the current direction, electromigration (EM) damage is dominated by Sn self-diffusion, which takes longer to occur. This is a direct proof of the highly anisotropic diffusion behavior in Sn. Due to the presence of twin structures and stable Ag3Sn network, SnAg(Cu) solders are less susceptible to grain orientation effects and showed better EM performance than SnCu solders.

The microstructure of Sn in near-eutectic Sn–Ag–Cu alloy solder joints and its role in thermomechanical fatigue

During the solidification of solder joints composed of near-eutectic Sn-Ag-Cu alloys, the Sn phase grows rapidly with a dendritic growth morphology, characterized by copious branching. Notwithstanding the complicated Sn growth topology, the Sn phase demonstrates single crystallographic orientations over large regions. Typical solder ball grid array joints, 900 μm in diameter, are composed of 1 to perhaps 12 different Sn crystallographic domains (Sn grains). When such solder joints are submitted to cyclic thermomechanical strains, the solder joint fatigue process is characterized by the recrystallization of the Sn phase in the higher deformation regions with the production of a much smaller grain size. Grain boundary sliding and diffusion in these recrystallized regions then leads to extensive grain boundary damage and results in fatigue crack initiation and growth along the recrystallized Sn grain boundaries.

Microstructure and mechanical properties of lead-free solders and solder joints used in microelectronic applications

The replacement of lead (Pb)-bearing solders used in the electronic industry with Pb-free solders will become a reality in the near future. Several promising Pb-free solders have recently been identified, including Sn-0.7Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and Sn-3.5Ag-4.8Bi (in wt.% with slight variations in composition). These are all Sn-rich solders with melting temperatures between 210°C and 227°C, and are recommended for various soldering applications, including surface mount technology (SMT), plated-through-hole (PTH), ball grid array (BGA), flip-chip bumping, and others. Although a considerable amount of information on Pb-free solders has been published in the last few years, the database on these new materials is still at an infant stage compared with that for Pb-containing solders. This paper addresses several aspects of the current fundamental materials understanding associated with Pb-free solders and various issues regarding their imminent use in electronic interconnect applications, including microstructure-processing-property relations, mechanical properties, interfacial reactions, and the thermal-fatigue life and failure mechanisms of Pb-free solder joints.

Kerf-Less Removal of Si, Ge, and III–V Layers by Controlled Spalling to Enable Low-Cost PV Technologies

Kerf-less removal of surface layers of photovoltaic materials including silicon, germanium, and III-Vs is demonstrated by controlled spalling technology. The method is extremely simple, versatile, and applicable to a wide range of substrates. Controlled spalling technology requires a stressor layer, such as Ni, to be deposited on the surface of a brittle material, and the controlled removal of a continuous surface layer could be performed at a predetermined depth by manipulating the thickness and stress of the Ni layer. Because the entire process is at room temperature, this technique can be applied to kerf-free ingot dicing, removal of preformed p-n junctions or epitaxial layers, or even completed devices. We successfully demonstrate kerf-free ingot dicing, as well as the removal of III-V single-junction epitaxial layers from a Ge substrate. Solar cells formed on the spalled and transferred single-junction layers showed similar characteristics to nonspalled (bulk) cells, indicating that the quality of the epitaxial layers is not compromised as a result of spalling.

Interfacial reaction studies on lead (Pb)-free solder alloys

Recently, the research and development activities for replacing Pb-containing solders with Pb-free solders have been intensified due to both competitive market pressures and environmental issues. As a result of these activities, a few promising candidate solder alloys have been identified, mainly, Sn-based alloys. A key issue affecting the integrity and reliability of solder joints is the interfacial reactions between a molten solder and surface finishes in the solder joint structures. In this paper, a fundamental study of the interfacial reactions between several Pb-free candidate solders and surface finishes commonly used in printed-circuit cards is reported. The Pb-free solders investigated include Sn-3.5 Ag, Sn-3.8 Ag-0.7 Cu, and Sn-3.5 Ag-3.0 Bi. The surface finishes investigated include Cu, Au/Ni(P), Au/Pd/Ni(P), and Au/Ni (electroplated). The reaction kinetics of the dissolution of surface finishes and intermetallic compound growth have been measured as a function of reflow temperature and time. The intermetallic compounds formed during reflow reactions have been identified by SEM with energy dispersive x-ray spectroscopy.

High-efficiency thin-film InGaP/InGaAs/Ge tandem solar cells enabled by controlled spalling technology

In this letter, we demonstrate the effectiveness of the controlled spalling technology for producing high-efficiency (28.7%) thin-film InGaP/(In)GaAs/Ge tandem solar cells. The controlled spalling technique was employed to separate the as-grown solar cell structure from the host Ge wafer followed by its transfer to an arbitrary Si support substrate. The structural and electrical properties of the thin-film tandem cells were examined and compared against those on the original bulk Ge substrate. The comparison of the electrical data suggests the equivalency in cell parameters for both the thin-film (spalled) and bulk (non-spalled) cells, confirming that the controlled spalling technology does maintain the integrity of all layers in such an elaborate solar cell structure.

Layer transfer by controlled spalling

In this communication, we present what may be the simplest method yet devised for removing surface layers from brittle substrates. The process is called controlled spalling technology (CST) and works by depositing a tensile stressor layer on the surface of a substrate, introducing a crack near the edge of the substrate, and mechanically guiding the crack as a single fracture front across the surface. The entire process is performed at room-temperature using only common laboratory equipment. We present here, for the first time, the specific process conditions required for controlled spalling of Ge 〈0 0 1〉 substrates using Ni as the stressor layer. We also illustrate the versatility of CST by removing completed CMOS circuits from a Si wafer and demonstrate functionality of the flexible circuits. Raman spectroscopy of spalled circuits with the Ni stressor intact indicates a residual compressive Si strain of 0.0029, in good agreement with the calculated value of 0.0022. Therefore, CST also permits new opportunities for strain engineering of nanoscale devices.

Evaluation of thermal fatigue life and failure mechanisms of Sn-Ag-Cu solder joints with reduced Ag contents

The electronic industry is making substantial progress toward a full transition to Pb-free soldering in the near future. At present, the leading candidate Pb-free solders are near-ternary eutectic Sn-Ag-Cu alloys. The electronic industry has begun to study both the processing behaviors and the thermomechanical fatigue properties of these alloys in detail in order to understand their applicability in context of current electronic card reliability requirements. In recent publications, the solidification behavior of the near-ternary eutectic Sn-Ag-Cu alloys has been reported in terms of the formation of large Ag/sub 3/Sn plates and their effects on mechanical properties of Pb-free solder joints. It was also demonstrated that reducing Ag content in the near-ternary eutectic Sn-Ag-Cu alloys was very effective in controlling the formation of large Ag/sub 3/Sn plates and thereby reducing the reliability risk factor of solder joints. In this study, thermal fatigue behavior of CBGA (ceramic ball grid array) solder joints was investigated in terms of Ag content, cooling rate, and thermal cycling conditions. Extensive failure analysis was conducted with thermal-cycled solder joints to understand the failure mechanisms operating during the accelerated thermal cycling (ATC) tests.

Comparison of electromigration performance for Pb-free solders and surface finishes with Ni UBM

A series of electromigration (EM) experiments were undertaken to evaluate the time to failure performance of solder joints comprised of Sn-Ag and Sn-Cu alloys in combination with three solderable surface finishes, Cu, Ni-Au and Ni-Cu. The opposing pad structure in the solder joints was the same in all experiments and was comprised of a layered structure, simulating Ni based under - bump - metallurgies (UBM) for controlled collapse chip connection (C4). As anticipated the Sn grain size was large with the typical solder joint containing only a few grains. In all experiments, reported here, the electron current exited the pad with the surface finish under evaluation and passed into the solder. Two failure modes were identified. The manifested failure mode depended on the orientation of the c-axis of the larger Sn grains in the solder joint with respect to the applied current direction. When the c-axis is not closely aligned with the current direction, cavitation at solder-IMC interface leads to electrical failure. A more rapid failure mode occurred when the c-axis was closely aligned with the current direction. With this alignment the interfacial IMC structures were swept away by rapid diffusive processes from the pad surface and the pad material was quickly consumed. Interfacial void formation leads to rapid failure in this mode. The Sn-Ag solder appeared to demonstrate greater microstructural stability. But, clearly the best EM performance was seen with the addition of significant levels of Cu to the Sn-Ag alloy. This alloy modification showed the best EM lifetime in combination with a Ni pad structure.

Critical Factors Affecting the Undercooling of Pb-free, Flip-Chip Solder Bumps and In-situ Observation of Solidification Process

The undercooling of flip-chip Pb-free solder bumps was investigated by differential scanning calorimetry (DSC) to understand the effects of solder composition, solder volume, minor alloying elements, presence of an under bump metallurgy (UBM), and cooling rate. The undercooling is defined as the temperature difference between the melting temperature of a solder during heating and the solidification temperature during cooling. Although the undercooling of a bulk sample can be easily measured by DSC, it is difficult to measure the undercooling of flip-chip solder bumps because an individual solder bump is too tiny to be handled and the amount of heat associated with melting or solidification is also too small to be detected. In this study, a large amount of the undercooling (as large as 90degC) was observed with Sn-rich, flip-chip-size solder bumps sitting in a glass mold, while the corresponding undercooling was significantly reduced in the presence of a wettable UBM surface on a Si chip. In addition, the solidification of an array of individual solder bumps in a glass mold was monitored in-situ by video imaging technique during both heating up and cooling down cycles. Random solidification of an array of bumps was demonstrated during cooling, which also spans a wide temperature range of 40-80degC. Comparing the amount of undercooling observed in flip-chip vs. BGA-size bumps or bulk solders, it is evident that the solder volume factor is more dominant than the metallurgical factor such as Sn vs. Pb. It is also demonstrated that a small addition of minor alloying elements such as Co, Zn, Ni and Fe was effective in reducing the amount of undercooling of Pb-free solders.