John is perfectly right, but if you want to research it further, the
technical term of a eutectic alloy if one that goes from solidus to
liquidus without passing through a phase of solid crystals in a liquid.
Generally speaking, it occurs at a minimal temperature. For example, the
Sn63Pb alloy melts at 183°C. If you have, e.g., Sn55Pb, as you heat it
up, it becomes pasty at 183°C because the excess lead will not melt and
will remain solid in the eutectic liquid that has melted. Heat it
further and it will not attain liquidus until about 205°C. The Sn96.5Ag
alloy is eutectic at 221°C (pure tin melts at 232°C). Many binary alloys
have eutectic compositions. I don't believe ternary or higher alloys
have true eutectic compositions, although they may pass from solidus to
liquidus over a very short temperature range of, say, 1°C or less,
making them behave as if they are eutectic, but that may be
hair-splitting. No doubt Werner or others with a better knowledge of
metallurgy than I will jump down my throat. The reason why near-eutectic
compositions are desirable is that even a tiny mechanical shock to the
assembly when it is between liquidus and solidus during cooling will
derange the orderly crystal formation and thus weaken the joint.
Brian
Lum Wee Mei wrote:
> Hello,
>
> What is actually eutectic solder? All along I have this impression that when we talk of eutectic solder, it is refer to Sn63. However, yesterday, I attended a course and was told that Sn96 (4% silver) is also eutectic. Can someone enlightenment me?
>
> Thanks and regards,
> Wee Mei
>
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