Systematic investigation of Sn-Ag and Sn-Cu modified by minor alloying element of titanium

Abstract

Reliability issues of Pb-free solder joints used in microelectronic interconnects, such as BGA, flip-chip, or even 3D-IC, are becoming more critical recently when high performance electronic systems demand more rigorous reliability requirements. The selection of new solder materials has been an important topic in order to enhance the reliability of Pb-free solder joints. In the past few years, numerous studies have been conducted on the beneficial effects of minor alloying elements, including Co, Cu, Fe, Ni, Zn and others, into Sn-rich Pb-free solders [1-6]. Only recently have Liu et al. [7] first reported the beneficial effect of Ti in SAC (Sn-Ag-Cu) solder for BGA applications with superior drop-impact performance. Getting such a superior drop-impact performance usually compromises high temperature mechanical properties such as creep resistance. But they found SAC-Ti having enhanced drop-impact performance without sacrificing a good creep property. Improving simultaneously these two mechanical properties was regarded as an excellent achievement of SAC-Ti. Moreover, considerable suppression of the undercooling and retardation of interfacial IMCs were also reported owing to the addition of Ti to SAC. Nevertheless, V. Vuorinen and his coworkers [8] lately reported the result of a thermal aging experiment on SnAg solder modified with Ti and asserted that the minor addition of Ti cannot change the activities of components nor influence the stability of the IMC layers. This study did not support the beneficial effects of minor alloying addition of Ti in SAC of the previous report [7]. Hence, it is essential to clarify the effect of Ti-addition on Sn-rich solders by further investigation. The objectives of our present work are (a) to understand the intrinsic consequence of Ti-addition on Sn-Ag and Sn-Cu solders, (b) to observe whether Ti-addition can influence the interfacial reactions between solders and under-bump metallurgies (UBMs), and (c) to reveal the contribution of Ti-addition on any other reliability performance, such as electro-migration (EM) resistance. In this study, two Ti-added Sn-Ag and Sn-Cu solders were commercially prepared; Sn-1.0Ag-0.2Ti (wt.%) and Sn-0.7Cu-0.2Ti (wt.%) in the form of solder ingots. They were cut into small pieces and evaluated for their metallurgical properties and interfacial reactions with Cu and Ni UBMs. Pure Sn-1.0Ag (wt.%) and Sn-0.7Cu (wt.%) were also prepared in the form of solder balls as control samples. DSC (differential scanning calorimetry) analysis was used to study the melting/solidification behavior of Ti-added solders. It was confirmed that a small amount of Ti addition can effectively reduce the undercooling to a few degrees, while a large undercooling is persistent in Sn-Ag or Sn-Cu solders without Ti addition. It is worth noting that a small undercooling can lessen a possibility of non-uniform solidification among many neighboring solder joints during reflow, which is beneficial for joint integrity and reliability as well. Some solder samples were also produced in the form of small solder cylinders by the injection-molded-solder (IMS) process [9], which is known for the precursor of IBM's wafer bumping technology, C4NP. Some of the solder cylinders are then exposed at 200 °C for high temperature aging experiment. For Ti-added solders, Ti2Sn3 networks can stabilize the morphology of β-Sn by restraining their grain growth, which implies they may be good candidates for resisting EM failure and maintaining the high-temperature strength of solders. Microhardness of high temperature aged samples are measured to show the trend of hardness change as a function of aging time. The interfacial reactions between different solders and different UBM show interesting results about the effect of Ti addition on the formation of interfacial IMCs. For as-reflowed samples (240 °C, 1 min), significant retardation of interfacial Cu6Sn5 is observed when both the Sn-Ag-Ti and Sn-Cu-Ti solders reacted with Cu UBM. However, Ti addition seems not to be so effective for hindering the formation of Cu3Sn. In the case of Sn-Cu-Ti solders reacted with Ni, the interfacial IMC, (Cu, Ni)6Sn5, is also found to be impeded comparing to the case without Ti addition. When Sn-Ag-Ti solders undergo a reflow process on Ni, Ni3Sn4 forms at the interface as a layer of equilibrium IMC and Ti addition unexpectedly accelerate the formation of Ni3Sn4. The Ti concentration effect and the solid-state growth kinetic of different IMCs will also be discussed in this study. The effects of Ti-addition on electro-migration performance of Sn-Ag and Sn-Cu solder joints will be evaluated with a model joint of Cu wire samples providing a uniform current density during EM stressing in the future.