Edward
Important: The Intermetallics are solid, when the solder is liquid due
to their high melting temperature. The growth of the intermetallics
depends on the supply of copper and tin. These two elements can be
provided by diffusion in the solid material. Naturally, this is a
process which depends on the temperature as any diffusion. Hence, the
warmer the faster. Makes sense again since Collin states in his book
that the equilibrium of the thickness of the intermetallics is bigger
the higher the temperature. This means, a thicker barrier is needed
until the growth rate of the intermetallics is slowed down so much that
it equals the dissolution in the solder.
Kelly
I don't know whether I understand the expression solid state diffusion
the right way. Lets assume it is right if I take it as diffusion in a
solid material:
It was always one of the most fascinating facts for me, that atoms have
the ability to move trough a solid material. But they rally can. In
crystalline materials this is usually along the grain boundaries which
are a somewhat weird zone where a certain anarchy reigns. You can
imagine it the following way:
Any atom is moving around its theoretical position in space. This
movement is felt as temperature. Due to this movement it is possible
that the atom changes it place. If a lot of the places available are
occupied in one direction the atom will choose to move away from the
crowd in the direction of the lesser concentration. It behaves very
much like human beings on holiday in search of a nice spot on the
beach. Since there are heaps of other atoms having the same idea it is
clear that the atom keeps moving away of its original place. It's
diffunding away. It is also imaginable that an atom moving more
vigorously around its theoretical position will move faster or will
more readily change its place. We all know this, higher temperatures
means faster diffusion.
Well, I don't know whether this kind of explanation is what you
expected. If not, let me know. But I must say, that diffusion is not a
field where I am particularly strong. It can be explained in an
academic manner but this makes my brain boiling.
Best regards
Guenter
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