Now check out Eutectic mixtures ... old-timers may remember soldering with 63-37 tin/lead solder.
The reason? With any other mixture of lead/tin, the liquid solder freezes over a temperature range, often resulting in what very-old-timers called a "cold solder joint". For example, 50-50 tin/lead mixture starts melting at 183C and is fully melted at 214C.
Using Eutectic Solder, the phase transition happens at exacctly 183 C ... the lump is solid at 182C and liquid at 184C.
Geologists take advantage of this: when non-eutectic mixtures of lava freeze (say, a basalt flow in Hawaii or on the moon), different minerals will be found in the rocks. Analyzing the minerals, and assuming equilibrium, you can understand temperatures and pressures in the origination magma.
(ps - yep, new ROHS rules have largely eliminated lead based solder)
Without lead you get tin whiskers. I wonder how the math works out in terms of what’s better for the environment if electronics break a lot more often…
A giant pile of scrap electronics is probably easier to deal with than a very large pile of scrap electronics that's leaching toxic heavy metals into the ground and runoff water.
Most electronics these days seem to get upgraded before tin whiskers can cause problems anyway.
Given the explanation above, this now makes a lot more sense: if the (non-eutectic) solder has cooled gradually, some the higher melting component will freeze first at the coolest location, and things will move around, leaving less of the "low-melting" components in the remaining molten solder. Once it's all solidified, some areas will have more tin and other areas will have more of the other elements. This will induce some weirdness, maybe the tin will want to diffuse to other parts where there is less (is this possible?) or maybe just there will be areas of relatively pure tin, perhaps more susceptible to whisker formation?
On the other hand an evenly-frozen mix of lead and tin (eutectic) wouldn't have these non-uniformities as there would be no driving force for it. One location freezing before the other wouldn't change the composition of the remaining melted solder.
I would also imagine the very quick solidification also prevents larger crystal formations. Crystal growth itself can also lead to separation of the metals and the extra few seconds or more of a half-liquid state solder could represent decades or more worth of crystal growth in ambient temperatures.
The reason? With any other mixture of lead/tin, the liquid solder freezes over a temperature range, often resulting in what very-old-timers called a "cold solder joint". For example, 50-50 tin/lead mixture starts melting at 183C and is fully melted at 214C.
Using Eutectic Solder, the phase transition happens at exacctly 183 C ... the lump is solid at 182C and liquid at 184C.
Geologists take advantage of this: when non-eutectic mixtures of lava freeze (say, a basalt flow in Hawaii or on the moon), different minerals will be found in the rocks. Analyzing the minerals, and assuming equilibrium, you can understand temperatures and pressures in the origination magma.
(ps - yep, new ROHS rules have largely eliminated lead based solder)