Werner and all those still interested,
I cannot accept the way you reply to messages. You reply off line, then publish on line. Let's keep
it on line for the benefit of others needing your wisdom and knowledge. This message is in reply to
your, again and again, off line message as there can be no winners but those needing this
information. Again, I tried to end this, but you have many valid points as well as those not so.
I must declare you accurately reach back, or remember back, as far as do I. I admit the Manko
definition is dated, but there is something you said that is of importance: "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."
My understanding of a surface's definition is an object's area having no depth. Please refresh me
as to how many atomic monolayers, or angstroms, constitute depth. Even Manko, in the "early days,"
indicated a requirement for atomic (not subsurface) level bonding to effect solder joints without
REQUIRING difussion or intermetallic formation. What has changed in that part of the definition?
I certainly agree with you about the overused term "wetting." You clearly shed more light on the
subject while stating it better than could I.
I now believe my initial response (to Afri's question) concering HASL re-processing constitutes a
mutual agreement. You state: "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." This exactly re-states my position and
a main concern together with additional thermal stress or shock diminishing foil/pad and laminate
bond strength – that we also seem to agree upon.
I still would like to focus on what I said in "MY LAST HASL – 1" regarding someone having said you
believe HASL to be representative of most subsequent soldering processes. I, again, totally
disagree as no protected solder termination surface area is available for introduction to the HASL
process – as it is in all other soldering processes. However, during thermal stress testing (using
quality conformance test circuitry having been protectively coated or plated), a much closer
representation is made concerning subsequent solder processing, even though excessive relative to
time and temperature.
You also say: "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.)."
An AMP (INC.) study says: "Reductions in solder joint strength must be avoided due to the stress
the solder joint must withstand. The cyclic forces of expansion and contraction, caused by
coefficient of thermal expansion (CTE) mismatches between the package and the board material, are
mostly absorbed by the lead and the solder joint. Lead compliance, determined by the lead stiffness
or diagonal flexural stiffness, is chosen to enable the lead and joint to resist fatigue,
especially in surface mount devices. Defects which reduce the joint strength will allow fatigue to
destroy the bond. In addition to temperature induced stresses, the solder joint must withstand
vibrational fatigue." I simply question whether the last part of the above study contradicts what
you said before it?
This stuff never ends, but it's too important to end.
And on it goes,
Earl Moon
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