When you are choosing a low temperature solder alloy, there are some key properties that you should to consider. The first, and most important, is the melting temperature. This is commonly referred to as the solidus and liquidus temperatures. So understandably a couple of times now I have been asked about the liquidus temperature of Durafuse® LT.
Now here is where new technology starts making things tricky. If you have read my last few posts, you probably know Durafuse® LT is a low-temperature mixed alloy solder paste with great drop shock reliability. But with two different alloys in Durafuse™ LT, we have two different liquidus temperatures - one lower and one higher than the the peak reflow temperature. During reflow we never go past the liquidus temperature of the higher melting alloy - in one sense there is no "time above liquidus".
But understanding the impact of "time above liquidus" is not wasted because just like Kim explained, solder NEEDS some version of liquidus in order to form a bond with the substrate and component. In Durafuse® LT, we also get fusion - where the liquid low-melting alloy bonds not only with the substrate and component, but also with the high-melting alloy. So in a way... liquidus is even more important to the Durafuse® LT alloy system, even though we never really reach it.
Fun fact: I was looking up the melting point of peanut butter while trying to come up with an example of two things melting together with different melting points (peanut butter and chocolate chips - yum)... but it was WAY too different. Peanut oil is liquid well below room temperature and apparently they keep the semi-solid texture using emulsifiers - so it's almost liquid to start with? I like the visual, but Durafuse® LT definitely starts as solid powders and ends as one solid, continuous metal joint. I'll let you know when I find a better simile!