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December 1998

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 3 Dec 1998 13:18:53 EST
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text/plain
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Hi Earl,
You quote a 'definition' of the soldering process straight out of Manko's
book, which represents 1962 thinking and understanding (on the macro-level,
you may actually see on IMC layers, but they are there on the micro-level). If
you consult more recent works (Klein-Wassink, Lea, Frear, etc.), you will find
that in soldering, solubility  of the metals involved is important and that
some intermetallic compound formation (it may be only a couple of atomic
monolayers thick) is required to form a metallurgical bond.
It is unfortunate, that the term 'wetting' has taken on the definition of
forming a metallurgical bond, because I can wet (spread/cover with solder) a
metal surface with liquid solder without forming a metallurgical bond by means
such as a gold flash on nickel but inadequate temperatures and time for the
process of metallic solution formation between nickel and tin (this, of course
would be not possible with a metal more readily soluble in tin, e.g. copper).
Intermetallic compounds are typically not involved in the failure  of solder
joints (Morris, Frear, etc.), but the visible lack of an IMC layer which often
is an indication of a failure to 'wet', is involved in 'solder joint failures'
(in quotation marks because no real solder joint had been formed) showing
interfacial separations (no indication of solder on the metal surface).
Intermetallic compounds are essentially all brittle, but most are brittle and
much stronger than solder. The important exceptions are gold/tin and
silver/tin IMCs, which are both brittle and weak—thus, the observed problems
with them in sufficient concentrations.
IMC layers grow with time and temperature following an Arrhenius relationship
with activation energies ranging from about 0.5 to 1.8 eV. As they grow, they
'consume' tin from the adjacent solder region and form a lead-rich region
ahead of them, which has lower strength than the nominal solder composition.
Thus, solder joint failures frequently show fractures close to, but not in the
IMC layers (but, frequently there are other reasons for this fracture
location, as well). For a thin HASL surface prior to the formation of solder
joints, the growth of the IMC layer can deteriorate the solderability of that
surface; re-processing these boards will cause a degradation of quality and
reliability.
Also solder joints, unless they first have been subjected to thermal cyclic
fatigue, do not fail as the result of mechanical shock and/or vibration. Many
studies have shown this; other parts of the assembly will invariably fail
first. The very property of solder that makes solder joint fatigue such an
important issue—the readiness to creep because of the use temperatures close
to the material's melting temperatures, makes solder joints much less
susceptible to mechanical shock and/or vibration as well as non-uniform
geometries creating stress concentrations than would be the more common
structural metals (steel, copper, aluminum, etc.).

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]

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