Every solder alloy has both a solidus and a liquidus temperature. The solidus temperature is the point below which the alloy is solid (not melted). The liquidus temperature is the point, above which the alloy is completely liquid (melted). The solidus and liquidus temperatures can be the same for an alloy (eutectic alloy composition) or they can be different temperatures (off-eutectic/non-eutectic alloy composition). For the off-eutectic/non-eutectic alloy composition, there is a plastic or pasty range between the solidus and liquidus temperatures; this is where part of the alloy has melted while another part has not. Once an off-eutectic/non-eutectic alloy has reached the liquidus temperature, then the alloy will be completely liquid.
Two examples of eutectic and non-eutectic alloys are Indalloy 290 (97In3Ag) and Indalloy 3 (90In10Ag). The 97In3Ag alloy melts at 143°C, while 90In10Ag melts from 143°C to 237°C. 90In10Ag has a very large plastic range, but can still be used for a multitude of lower-temperature applications. Think about it like concrete! The alloy essentially is a slurry when it is raised to the temperature of the plastic range (between 143°C and 237°C). When concrete cures it works just fine. The same goes for this alloy. The grain structure will be different than if you raised the temperature of the alloy above 237°C.
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