Moon Gi Cho

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.

Development of low temperature bonding using in-based solders

In-based solders were chosen for the low temperature bonding at lower than 180degC. Three kinds of bonding types on Au/Cu/Ti/SiO2/Si dies, which were Sn/In and Au/In for Type 1, Au/In and Au/Sn for Type 2, and InSn alloy and InSn alloy for Type 3, were studied expecting that the whole In- solder layer is converted to the mixed intermetallic compound (IMC) phases of In-Cu and In-Au IMCs after bonding below 180degC and annealing at 100~120degC. The IMC in the joints were characterized in terms of the micro structure observations and the compositional analysis with Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX), the phase identification with X-ray Diffraction (XRD) and the re-melting temperature with Differential Scanning Calorimetry (DSC). The phase equilibriums of the joints were examined by thermodynamic calculations to understand the re-melting behavior. As a result, complete bonding consisted of only high melting temperature IMCs, Cu11ln9, Cu2In, eta-Cu6Sn5, and Auln2, was successfully made at 120degC followed by annealing at 100degC in Type 3, and at 160degC with annealing for lOhrs or at 180degC without annealing for Type 1, which was confirmed by DSC measurements and explained through thermodynamic calculations.

Enhancement of heterogeneous nucleation of β-Sn phases in Sn-rich solders by adding minor alloying elements with hexagonal closed packed structures

The measured undercooling of pure Sn was about 30 °C due to the difficulty of nucleating a solid β-Sn phase from a liquid phase. To promote the heterogeneous nucleation of β-Sn phases, the addition of impurity elements to the solders was suggested. Among the impurity elements, alloying elements with hexagonal closed packed (hcp) structures, such as Co, Zn, Ti, and Mg, were found effective to enhance heterogeneous nucleation of β-Sn phases in Sn-rich solders. Calculations of the density functional theory indicate that the interfacial energy between β-Sn and Zn was relatively low. Minor alloying elements with hcp crystals are expected to provide more favorable heterogeneous nucleation sites for β-Sn phases.

Effects of under bump metallization and nickel alloying element on the undercooling behavior of Sn-based, Pb-free solders

A significant reduction of the undercooling of Sn-based solder alloys was previously reported when they were reacted with various under bump metallurgies (UBMs). In the present study, new experiments have been designed and carried out to understand the undercooling behavior of various Cuand Ni-doped solders on Ni UBM. Two competing mechanisms were further investigated that includes the formation of intermetallic compounds (IMCs) at solder/UBM interface and the change of solder composition due to the dissolution of Ni UBM into solder. Two types of IMCs, including both Ni3Sn4 and Cu6Sn5, that were formed at the interface were correlated with the undercooling of Sn-0.2Cu and Sn-3.8Ag-0.2Cu solders. In addition, the compositional changes of various Sn-based solders after reactions with Ni UBM were analyzed. Based on the experimental results, it was found that the significant reduction in undercooling is primarily caused by dissolved Ni atoms from Ni UBM and the concurrent formation of Ni3Sn4 IMC in the solder matrix. Finally, the beneficial effect of Ni dissolution is thermodynamic favorable as confirmed by the thermo-calculation and DSC measurements with various Ni-doped solder alloys.

Effect of Ag Addition on the Ripening Growth of

The ripening growth (or lateral growth) of Cu₆Sn₅ grains that form between molten Sn-rich solders and Cu under-bump metallurgies (UBMs) was investigated with Sn-0.5Cu, Sn-1.0Ag-0.5Cu, and Sn-3.0Ag-0.5Cu solders. After reactions with Cu UBMs, the size and morphologies of Cu₆Sn₅ grains were compared with each other through the observation of the top views. The Cu₆Sn₅ intermetallic compounds (IMCs) that form at the interface of Sn-3.0Ag-0.5Cu solders exhibit much smaller grains, though the thickness of the grains is similar to that of the grains that form in Sn-0.5Cu and Sn-1.0Ag-0.5Cu solders. In other words, the ripening growth of Cu₆Sn₅ grains during a reflow is reduced in Sn-3.0Ag-0.5Cu solders. The effects of Ag on the ripening growth of Cu₆Sn₅ grains were examined by comparing the angles of two neighboring Cu₆Sn₅ grains that formed at the interface of each solder, in addition, the interfacial energies between the Cu₆Sn₅ grains and the molten solders are discussed. The effect of Ag on the ripening growth of the Cu₆Sn₅ grains depends on the solder volume: as the volume increases, the ripening growth of the Cu₆Sn₅ IMCs at the interface is reduced more effectively, and small Cu₆Sn₅ grains are produced during the reflow. The tendency of the way smaller Cu₆Sn₅ grains to reduce the formation of Cu₃Sn IMCs during thermal aging is also discussed.

{\rm Cu}_{6}{\rm Sn}_{5}

In microelectronic packaging, the reliability of Pb-free solder joint is critically depending on its interfacial reactions with Under-Bump-Metallurgy (UBM) or pad finishes. Factors such as intermetallic compound (IMC) formation, IMC spalling, UBM dissolution, interfacial void formation, all affects the integrity of solder joints. The failure mechanisms in thermal fatigue or electromigration tests of solder joints are often influenced by the interfacial reactions. When Sn-rich solders are used for Pb-free applications, the interfacial reactions become more aggressive since the reflow temperatures of Sn-rich solders are higher than eutectic Sn-Pb solders and the corresponding solubility of barrier metals in a molten Sn-rich solder is expected to be much higher. Recently, many studies have been conducted to control the reaction or consumption rate of a Cu layer into Sn-rich solders. However, few studies have been reported on the consumption of Ni barrier layers in Sn-rich solders during reflow. In addition, it has been known that the dissolution rate of a Ni layer varies widely depending on its material deposition processes and parameters, such as electrolytic, electroless or sputtered Ni. In microelectronic applications, an electroless Ni (P) layer is commonly used to provide a reaction barrier layer on Cu pads of laminates or modules, while an electrolytic or sputtered Ni layer is used in a UBM structure of a flip-chip metallization. In this study, the consumption rate of different Ni layers (sputtered, electrolytic and electroless plated) are investigated with several Sn-rich solders, such as Sn-Ag and Sn-Cu in terms of reflow conditions. A method of controlling the consumption of Ni barrier layers is also reported by adding minor alloying elements, such as Ni or Cu into Sn-rich solders. The controlling mechanisms of Ni consumption are discussed for different alloying elements. The effects of minor alloying elements on the mechanical and thermal properties of Sn-rich solders are also reported.

Grains at the Interface of Sn-xAg-0.5Cu/Cu During a Reflow

In microelectronic packaging, the reliability of Pb-free solder joint is critically influenced by the interfacial reactions with under bump metallurgies (UBMs), and therefore controlling the intermetallic compound (IMC) formation and growth is recognized as a very important factor for improving the reliability of the solder joints. A silver (Ag) alloying element is broadly used in Pb-free solder alloys since it is beneficially effective on the decrease of melting point, and the improvement of wettability and mechanical properties, such as tensile strength and fatigue. However, the Ag element is often neglected in controlling the IMC growth, because it does not form any IMCs with two common UBMs, such as Cu and Ni(P) UBMs. Actually, in the interfacial reactions between Sn-rich solders and Cu UBMs, Cu<inf>6</inf>Sn<inf>5</inf> IMCs are mainly formed during reflow, and Cu<inf>3</inf>Sn IMCs are additionally formed between Cu<inf>6</inf>Sn<inf>5</inf> IMCs and Cu UBMs during thermal aging. In addition, most of the previous studies have reported that the morphologies and growth of Cu<inf>6</inf>Sn<inf>5</inf> IMCs are controlled by the reflow temperature or time, the cooling rate, the addition of minor alloying elements (e.g. Ni, Co and Zn), and the crystal orientation of Cu UBMs. In this study, the lateral growth (or ripening growth) of Cu<inf>6</inf>Sn<inf>5</inf> grains formed between molten Sn-rich solders and Cu UBMs was investigated with Sn-0.5Cu, Sn-1.0Ag-0.5Cu and Sn-3.0Ag-0.5Cu solders. After reactions with Cu UBMs, the size and morphologies of Cu<inf>6</inf>Sn<inf>5</inf> grains were compared each other through the observation of top views. The Cu<inf>6</inf>Sn<inf>5</inf> IMCs formed at the interface of Sn-3.0Ag-0.5Cu solders exhibited much smaller grains, but their thickness was not much different from those that formed in Sn-0.5Cu and Sn-1.0Ag-0.5Cu solders. In other words, the ripening growth of Cu<inf>6</inf>Sn<inf>5</inf> grains during reflow was reduced in Sn-3.0Ag-0.5Cu solders. To understand the effects of Ag on ripening growth of Cu<inf>6</inf>Sn<inf>5</inf> grains, the angles of two neighboring Cu<inf>6</inf>Sn<inf>5</inf> grains formed at the interface of each solder were compared, and the interfacial energies between Cu<inf>6</inf>Sn<inf>5</inf> grains and molten solders were discussed. In addition, the volume effect of Ag on the ripening growth of Cu<inf>6</inf>Sn<inf>5</inf> grains and the effect of smaller Cu<inf>6</inf>Sn<inf>5</inf> grains on reducing the formation of Cu<inf>3</inf>Sn IMCs during thermal aging were also reported.

Controlling the interfacial reactions in Pb-free interconnections by adding minor alloying elements to Sn-rich solders

To form reliable Pb-free solder joints, minor alloying additions of Ni or Zn to Sn-rich solders have been recommended recently. Several beneficial effects of Ni or Zn minor alloying additions to Pb-free solders were reported to improve solder joint reliability. But the effects of Ni or Zn minor alloying additions on the bulk properties of solders are not systematically evaluated in light of understanding the electromigration or mechanical reliability of solder joints. Therefore, in this study, the minor alloying effects of Ni or Zn on the microstructure and microhardness in terms of Ni or Zn composition and cooling rate are investigated. The amounts of minor alloying elements investigated are in the range of 0.05–0.15 wt% for Ni, and 0.2–0.6wt% for Zn, which cover the reported composition ranges to enhance solder/UBM joint reliability. Three cooling rates are employed during solidification; 0.02 °C/s (furnace-cooling), about 5 °C/s (air-cooling), and 100 °C/s or higher (quenching). The microstructure of Ni or Zn doped solders is evaluated in terms of composition, undercooling during solidification, and cooling rate. The phase diagram analysis is conducted to explain the microstrctural variations. The microstructures of Ni or Zn doped solders are well correlated to their microhardness data.

Effect of Ag on ripening growth of Cu 6 Sn 5 grains formed between molten Sn-xAg-0.5Cu solders and Cu

Laser display technologies have been developed for an excellent expression of natural color, low power consumption and a long lifetime compared to other advanced displays, such as LCD, PDP and other projection type displays. The micro scanner is one of the key devices to make possible the raster scanning type laser projection displays. And the hermetic package of the micro scanner should be required for the protection from the environmental variations so as to keep the driving behavior uniform. Hermetic package can be ensured when the package is sealed hermetically without generating any outgases in the cavity. Thus, the hermetic sealing process was optimized through DOE (Design of Experiment) method using the Sn-In-Ag solder alloys instead of adhesives. And the characterizations of the packages were carried out in terms of hermeticity, shear strength, and interface microstructures. As a result, we’ve got about 2E−9 atm cc/sec He leak rate, which is low enough to pass the standard (MIL-STD-883E). Shear strength was as high as ∼80 MPa. The C-mode SAM images showed the continuous sealing area without any voids. In addition, the interfacial microstructures revealed good adhesion to the both parts, the glass lid and the ceramic package.Copyright