BRAZING & SOLDERING TODAY 56 FEBRUARY 2014 TECHNOLOGY NEWS Surface Tension and Density of Liquid Solders, Brazing Alloys in the In-Sn-Zn System The surface tension and density of liquid In-Sn-Zn alloys with 5, 10, 14, 20, 35, 50, 75 at.-% zinc content, and various Sn:In ratios, were measured at the Institute of Metallurgy and Materials Science, Krakow, Poland (Ref. 1). Surface tension measurements were also performed in a wide temperature range of 500–1150 K using the method of maximum pressure in gas bubbles. Density decreases linearly from 500 to 1150 K for all alloy compositions. Linear dependence of surface tension on temperature was also observed with a negative gradient dσ/dT. Generally, there was observed an increase of surface tension with increasing zinc content. Only two exceptions were found — alloys containing 5 at.-% of Zn at Sn:In = 1 and Sn:In = 24. These alloys, therefore, seem to be most suitable for practical applications in brazing or soldering. However, in comparison with the Sn-Pb eutectic solder, surface tension values of In-Sn-Zn alloys are higher, about 60 mN/m. A difference in composition of the surface phase and bulk alloy was also found at 673 and 973 K. The surface of liquid alloys is enriched with tin for all In-Sn-Zn alloys. It is enriched with indium for alloys containing >10 at.-% of zinc. Predicting Thermal Conductivity of Lead-Free Solders, Intermetallics Lead-free solders of the Sn-Ag-Cu system are replacing Sn-Pb solders in electronics. A new method of calculating thermal conductivity for tin-based solders was developed and experimentally tested at Northwestern University, Evanston, Ill. This method is based upon the fact that heat and electrical transport both involve free electrons. Therefore, the Wiedemann-Franz-Lorenz (WFL) relation is presented as a possible solution to predict thermal conductivity with known electrical conductivity (Ref. 2). Generally, analysis of experimental data showed that the WFL relation is obeyed in both solder alloys and intermetallic compounds, especially those matching close to the relation for tin, with a positive deviation from the theoretical Lorenz number. Thus, from available electrical conductivity data, thermal conductivity of solders and intermetallics can be obtained based on an appropriate WFL relation, and vice versa. Several sets of thermal conductivity data are presented in this paper, particularly thermal conductivity of widely used solders such as Sn-3.5Ag-0.7Cu, Sn- 37Pb, and intermetallics Cu6Sn5 and Cu3Sn. By applying the WFL relation, the number experiments required to determine material properties for different solder/intermetallic interconnects For info, go to www.aws.org/ad-index
Welding Journal | February 2014
To see the actual publication please follow the link above