What Is the Eutectic Point? | Soldering

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Now as you work with soldering, you may learn of a term called the eutectic point. Eutectic solder is a formulation that has a particular percentage of tin and lead. In this case 63 percent tin, 37 percent lead.

At this percentage ratio, solder has a unique property where it has a very low melting point, and if you choose any other formulation with a little bit more tin or a little bit less tin, the melting point is higher. When you have non-eutectic solder, there exists a range of temperatures where the solder can exist as a liquid and a solid simultaneously; and that gives it a sort of pasty or slushy characteristic that you can use to mold or shape the solder as it's cooling and forming.

This can be an advantage in some cases such as with plumbing, where you may have a wiped joint, but in the case of electronics, often is the case you want the solder joint to cool very, very quickly and become solid all at once; in which case you're more apt to use eutectic solder.

Each particular alloy formulation has its own eutectic percentage ratio. In the case of tin lead, it's the 63-37 percent ratio. Different formulation of solder have different proportions for that eutectic point.

When purchasing eutectic solder, you can look at the label and see whether it says the word eutectic or not. If you see the word eutectic on it, that's a good indication that you have a eutectic material and that it will have this low melting point and a very rapid transition from rapid to solid. Eutectic solder is often easier to work with when working with electronics.