Soldering and brazing are two techniques of joining metals where the base metal, in the case of electronics which is copper, the base metal doesn't actually melt and it's natural to ask how does the solder get inside of the copper if the copper doesn't melt?
Now, copper is a metal that's not made of a pure single crystal but is formed of lots and lots of small micro crystals. We call it a polycrystalline material. And in between these crystals are little spaces where the tin and lead from solder can diffuse into cracks and crevices, microscopic crevices and useful to form the bond. So the tin lead doesn't really get into the crystal structure itself of copper but gets in between the crystals. And the same is true for brazing as well.
So if you were to look at the material under a microscope you'd see lots and lots of little grains of salt, if you will. And if you were to look at a microscopic image of soldered copper you'd see that the solder flows in between the crystals and leaving little islands of copper in between.
Much of the soldering takes place at the microscopic scale. Things like these oxides that disrupt solder are little particles that get in between the solder and the copper and prevent this diffusion of lead and tin into the crevices of copper. So there's a lot going on at the microscopic scale. It's very important to the features that we're interested in at much larger scales.
And you can look at a piece of soldered material. If you were to cut it you'd see a layer of tin lead and a very sharp line where the copper begins. But look closely at that layer and you'll see the that the tin and lead are actually intermingling with the copper and so there's a boundary layer where the tin and lead have flowed into the copper a short distance but you'll see that there's actually a zone where the two metals exist but the three metals exist all together.