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This experimental study is devoted to a phenomenological and quantitative investigation of the physics of spreading kinetics of eutectic lead solder systems (Pb-63Sn) over Cu 6 Sn 5 /Cu 3 Sn/Cu (IMC) substrates. The spreading kinetics was studied experimentally by using real-time in situ monitoring of the triple-line movement. Data gathering was facilitated by a hot-stage microscopy system under a controlled atmosphere. Three intermetallic surfaces with different heat treatment conditions were used for this investigation. Dramatically more pronounced spreading of Pb-63Sn on all intermetallic Cu 6 Sn 5 /Cu 3 Sn/Cu substrates compared with Cu substrates was observed, along with a much larger spreading area. Hence the physics/ chemistry of spreading over IMCs significantly differs from the spreading of the same system over a virgin Cu substrate. A power-law initial stage of spreading was confirmed, and the corresponding power-law exponents were determined as a function of substrate topography.

This study is devoted to a phenomenological and quantitative investigation of the physics of spreading of lead-free solder systems (Sn, Sn-0.7Cu, and Sn-3.5Ag) over Cu 6 Sn 5 /Cu 3 Sn/Cu substrates. Wetting kinetics was studied using real-time in situ monitoring of the triple-line kinetics under a controlled atmosphere. Three different intermetallic (IMC) substrates were used for this investigation. It was found that the grain size of the deposited IMC compound coating has a sizable influence on the triple-line movement of the liquid solders considered. More pronounced spreading of solders on IMC substrates was observed for all lead-free solders over an intermetallic with the smallest grain size. Lead-free solder spreading over IMC surfaces exposed to aging at elevated temperatures, hence featuring larger grain size, did not feature a significant improvement of wettability when compared with the behavior of the same solders on virgin copper substrates.

Reactive wetting of solders on Cu and Cu 6 Sn 5 /Cu 3 Sn/Cu substrates was investigated using both (1) the wetting balance, and (2) the hot-stage real time, in situ visualization of the triple-line movement. To understand the phenomenology of the spreading behavior better, comprehensive real-time in situ observations were performed. It was found that the wetting time during the wetting balance tests for both the lead solder (63SnPb) and lead-free solder systems (Sn0.7Cu and Sn3.5Ag) is shorter on Cu substrates than it is on Cu 6 Sn 5 /Cu 3 Sn/Cu substrates. The wetting force was not remarkably different on these two substrates for the same solder system. The hot-stage tests indicate a more pronounced spreading of 63Sn-Pb on Cu 6 Sn 5 /Cu 3 Sn/Cu substrates, along with a much larger spreading area. Spreading of lead-free solders in terms of the triple-line kinetics studied by using the hot-stage visualization shows no significant difference in the spreading evolution either over Cu or over Cu 6 Sn 5 /Cu 3 Sn/Cu substrates.

Wetting kinetics of Sn, eutectic Sn-Ag, eutectic Sn-Cu, and eutectic Pb-Sn was studied using real-time in situ monitoring of the triple-line movement, facilitated by a hot-stage microscopy system under a controlled atmosphere. Significantly different kinetics of lead versus lead-free solders is documented. In case of the eutectic lead solder, four characteristic spreading stages were identified. Spreading of lead-free solders features two stages with a sharp change of the spreading rate at the early stages of rather insignificant spreading. Scanning electron microscopy and energy-dispersive x-ray spectroscopy analysis of the resolidified solder surface within a halo region is discussed.

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In order to make up for the shortage of public-private partnership (PPP) model, more and more sewage treatment PPP projects have adopted the asset-backed securitization (ABS) model. To ensure success of sewage treatment PPPABS projects, risk evaluation, which has remained scarcity and unscientific, is becoming an urgent problem to be solved. Firstly, this paper identifies critical risk factors by literature analysis and expert interview. The final risk system is established from the perspectives of macrorisks, basic asset risks, transaction structure risks, operational risks, and other risks, which include 17 second risk factors. Then, the overall risk evaluation method is proposed based on combination weight method and Dempster–Shafer (D-S) evidence theory. Next, Beijing capital Co. Ltd. sewage treatment PPPABS project as a case is employed to verify the feasibility and effectiveness of the proposed method. Finally, awareness of existing risks, suggestions from law risk, quality risk, underwriting and issue risk, and credit enhancement are provided for sewage treatment PPPABS projects. All above studies are expected to provide helpful references for evaluating overall risk of sewage treatment PPPABS projects.