Wetting, solder temperature, effect

The adhesion of specimens subjected to environmental exposure tests was evaluated prior to and subsequent to the contact. Wet thermal shock testing consisted of five cycles each for IS minutes in boiling water and 2 minutes in ice water. The maximum transfer time between the two baths was 30 seconds. Thermal and temperature/humidity exposures were performed in controlled atmosphere chambers for 200 hours. Simulated solder tests comprised immersing the test. specimen in a silicone oil followed by solder flotation. The solder temperature varied from 232 to 288 C and the contact time was S or 10 seconds. In some cases, two flotations were performed on the same sample. The effect of a heat treatment at 135 C prior to solder testing was examined the heat treatment time varied from 0 to 16 hours. ... 

A detailed study on the wetting characteristics of eutectic Bi-Sn solder has been reported in terms of impurity effects, fluxes, base metals, and soldering temperature [18]. It was determined that eutectic Bi n solder is far less tolerant of impurities than eutectic Plr n [19]. In particular, the presence of impurity elements which form intermetallic compounds with Bi-Sn solder, such as Cu, Ni, Fe, and Pd, is especially critical, while Sb and Pb appear to be beneficial in terms of promoting wetting characteristics. Table 1 compares the solderabihty of several low-melting-point solders on various surface metallizations [20]. Only the Au/Ni-plated metallization is considered acceptable for Bi n solder, while the Cu, Ni, and Au/Ni metallizations are all acceptable for eutectic Sn Pb solder when a rosin-base flux is used. Another study confirmed that Bi Sn solder does not wet Cu-base metallizations as well as eutectic Sn-Pb solder does when a rosin-base flux is used [21]. However, if the Cu surface is pre-tirmed, then Bi-Sn wetting is acceptable even with a rosin-base flux [22]. Based on the wettabihty studies, eutectic Bi-Sn solder can only be considered a viable candidate if a suitable flux system is developed which allows it to be utilized for metallizations other than a Au/Ni overplate. 

Reflow The solder paste temperature exceeds the liquidus point and reflows, wetting both the component leads and the board pads. Surface tension effects occur, minimizing wetted volume. 

Strictly from an assembly process point of view, the mixing of Pb-free and traditional Sn-Pb solder can be beneficial. The Sn-Pb solder can improve the wetting and spreading performance of the Pb-free solder by two phenomena. First, Pb contamination lowers the molten solder surface tension of the solder. Second, the Pb contamination reduces the melting temperature of the Pb-free alloy. However, concerns are raised by the mixing of Sn-Pb and Pb-free solders and its effect on the long-term reliability of interconnections under thermal-mechanical fatigue environments. 

Takemoto, T. Miyazaki, M. Effect of excess temperature above liquidus of lead-free solders on wetting time in a wetting balance test. Mater. Trans. JIM 2001, 42 (5), 745-750. 

FIG. 42 Wave soldering results for Sn-Ag-Bi-Cu alloy that show the effect of increasing the wave temperature or reducing the conveyor speed to improve wetting. As the wave temperature is increased from 245 to 270°C, the percent coverage increases. In addition, as the wave speed is reduced from 1.0 to 0.75 m/min, the percent coverage is increased while the bath temperature is kept constant at 250"C. (Courtesy of Panasonic.)... 

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