Effect of the Soldering Atmosphere on the Wettability Between Sn4.0Ag0.5Cu (in wt.%) Lead-Free Solder Paste and Various
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JMEPEG https://doi.org/10.1007/s11665-018-3419-2
Effect of the Soldering Atmosphere on the Wettability Between Sn4.0Ag0.5Cu (in wt.%) Lead-Free Solder Paste and Various Substrates D. Soares, H. Leita˜o, C.S. Lau, J.C. Teixeira, L. Ribas, R. Alves, S. Teixeira, M.F. Cerqueira, and F. Macedo (Submitted October 24, 2017; in revised form March 27, 2018) The ‘‘degree of wetting,’’ which is related to the contact angle (h) between the molten solder and the substrate, is a useful parameter on the solderability process control. The contact angle, however, is strongly dependent on the type of substrate surface finish and used atmosphere (inert or non-inert). Furthermore, the surface tension, being an important parameter on the solderability process and performance, can also be achieved if the contact angle is known. In this study, the SAC405 [Sn4.0Ag0.5Cu (in wt.%)] solder paste contact angle was measured, by the ‘‘sessile drop’’ method, as a function of the temperature, surface pad finish and used atmosphere. The results are discussed, and the contact angles obtained for the different conditions are compared and discussed. Then, the surface tension (experimental) was obtained from the measured contact angle and compared with the obtained by using computation models (theoretical). The experiments performed in high vacuum conditions, i.e., low oxygen content, over a temperature range, allowed the evaluation and understanding of the surface oxides layers role on the solder wettability. The present study shows that in the soldering process, even in an inert atmosphere, usually used in industry, occurs the formation of superficial oxides, over the liquid solder and/or at the pad surfaces, that strongly affects the solder paste wettability, specially with Sn and OSP (organic solderability preservative) finishing. Differences in contact angle of ‡ 10 were determined between the two types of used atmospheres. The experimental surface tension and theoretical surface tension obtained, for the NiAu substrate type, present good correlation. The lower contact angle values were obtained for the NiAu and OSP finish types, independently of the atmosphere type. Keywords
atmosphere, Pb-free solders, surface finishing, wettability
1. Introduction The soldering process consists of the formation of a joint between the molten solder and the metal surface (from now on designated as substrate). Thus, the ability of the molten solder to flow or spread on the substrate is important for the formation of a proper metallic bond (Ref 1-4). To achieve a reliable solder joint to sustain the performance and overall product reliability, the study of the wetting behavior is a crucial step in the characterization of the solder alloys (Ref 2, 5-10), especially for the lead-free solders. This article is an invited submission to JMEP selected from presentations at the Symposium ‘‘Interface Design and Modelling, Wetting and High-Temperature Capillarity,’’ belonging to the topic ‘‘Processing’’ at the European Congress and Exhibition on Advanced Materials and Processe
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