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Applications of Low-temperature Tin-bismuth and Tin-bismuth-silver Lead-free Alloy Solder Pastes
July 30, 2013 |Estimated reading time: 1 minute
Abstract
The electronics industry has mainly adopted the higher melting point Sn3Ag0.5Cu solder alloys for lead-free reflow soldering applications. For applications where temperature sensitive components and boards are used this has created a need to develop low melting point lead-free alloy solder pastes. Tin-bismuth and tin-bismuth-silver containing alloys were used to address the temperature issue with development done on Sn58Bi, Sn57.6Bi0.4Ag, and Sn57Bi1Ag lead-free solder alloy pastes. Investigations included paste printing studies, reflow, and wetting analysis on different substrates and board surface finishes and head-in-pillow paste performance in addition to paste-in-hole reflow tests. Voiding was also investigated on tin-bismuth and tin-bismuth-silver versus Sn3Ag0.5Cu soldered QFN/MLF/BTC components. Mechanical bond strength testing was also done comparing Sn58Bi, Sn37Pb, and Sn3Ag0.5Cu soldered components. The results of the work are reported.
Introduction
Sn3Ag0.5Cu lead-free solders are widely used for lead-free reflow applications. In general, they are compatible with most reflow applications. The drawback is the relatively high melting temperature of this alloy (217°C) which needs a reflow processing temperature between 235°C to 260°C. For assembly of heat-sensitive components and boards, there is a need to look at lower-temperature lead-free solder alloys as an alternative to high-temperature SnAgCu solder as well as a need for a temperature hierarchy for different soldering operations on the board. The main lead-free alloys which could be used for these lower-temperature soldering operations are tin-bismuth (Sn58Bi) and tin-bismuth-silver (Sn57-57.6Bi0.4-1Ag), with a melting temperature of 138°C and a reflow processing temperature of around 180°C. When using low melting temperature tin-bismuth based solder paste, there would also be lower energy usage versus Sn3Ag0.5Cu paste in the reflow oven as indicated in Figure 1.Read the full article here.Editor's Note: This article originally appeared in the July 2013 issue of SMT Magazine.