[log in to unmask] wrote:
> 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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Dear TechNetters!
For two years now I am waiting each day to open my mail and to
participate to all the interesting debates on our meeting place.
Earl has touched now a subject I was not daring to touch since some
time.
Though I am very thankful to all the friends who answered to my
questions
off line, I would be interested in answers other people got off line to
their questions, if there wasn't a special request or cause.
Please, let this forum be an open one, and don't let me fear that in
future,
we will find here only questions, and when searching the archives, we
will only wonder what the answer might be.
Love to all
Gaby
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