Yin_Hsain Koh
All these quesions on intermetallics.
Wow, I hope you are only 20 years old and can spend the next 40 years getting
all of your answers about intermetallics.

I have been studying them for 40 years and have metallurgical degree and
could only answer a few of your questions.

First, intermelallics are compounds of metals that form with specific atomic
ratios of these metals in specific crystallographic forms, as opposed to an
alloy that is a mixture of several metals.  Tin is a great one for forming
intermetallics and does with great ease when it is in the molten state or
when it is in solid state and in contact with another metal.  It forms
multiple intermetallics with copper, nickel, gold, palladium, bismuth, silver
and many others that are not of great interest to the PCB industry.  Lead
also forms some with palladium.  If you look at the phase diagrams books,
they usually have the additional data such as the crystallographic size and
shape and atomic ratios such as Cu6Sn5 and Cu3Sn.

If you have and accurate phase diagram and a X-ray diffraction unit (not
EDAX) you can determine which intermetallics you are looking at by measuring
their size and crystal arrangment and then looking up to see which one you
have.  If you are smart with your metallographic techniques and have a SEM,
or a good optical, you can often see the different layers of intermetallics
in the solder joint extending from the non-fusible metallic surfaces.

All of the intermalllics have different strengths, fracture resistance or
ductility and you usually cannot change that. This will contribute to the
capability of you solder joint to withstand stresses.  Additionally, as the
temperature increase, greater thicknesses of the intermetallics form and
different intermetallics may form.

There are many good studies that have been made on specific intermetallics
such as tin/gold, but I do not know of any one book that covers the range you
are looking for.  International Tin Research Institute located in the UK,
London I think, has published many articles on the tin componds.  Many of the
soldering books have vague information about intermetallics, mainly they
leave you with the feeling if you form intermetallics it must be a bad solder
joint.  In reality, you must form intermetallics to have a tin-solder alloy
type solder joint.  Many authors who write wildly about intermetallics do not
know if "intermetallic" is spelled with one "L" or two "LL's".

I will not say more, but it is a very complex subject and I wish you luck.

Phil Hinton