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October 2001

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From:
Ryan Grant <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 10 Oct 2001 10:14:19 -0600
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Hi Earl,

I didn't gather from Werner's comments that thick IMC's are good; rather
that thick IMC's are NOT bad.  This is an important difference because the
pervasive industry "myth" is that thick IMC's are bad.   Like you, I haven't
seen the lower limit defined.

Further, the major failure mechanism contributor is the lead layer.  If this
layer is removed with each rework, then there is no limit to the number of
rework operations provided the pads are solderable.  (of course, ignoring
board and pad damage).  I have validated this through my own experiments
that compared BGA's seeing no rework on up to BGA's seeing six reworks and
then temp cycled.  There was no correlation between joint failures and
rework cycles, but the IMC's did get visibly thicker.

I hope this answers two of your questions.

BTW, if Manko, Rahn, and Wassink do not agree, remember how the scientific
process works.  A hypothesis is formed based on observations made up to that
point.  The hypothesis is then tested and based on the observations of that
test, the hypothesis is either validated or invalidated.

The caveat is that the hypothesis must allow for all previous observations,
and any subsequent observation that invalidates the hypothesis requires that
the hypothesis be changed to reflect the new observation.  The point being,
something that is published will only reflect the current wisdom at the time
of publication; and the scientific process allows for changes in past
wisdom.  Meaning, Manko, Rahn, and Wassink reserve the right to change their
minds from whatever they may have published in the past.


> -----Original Message-----
> From: Earl Moon [SMTP:[log in to unmask]]
> Sent: Tuesday, October 09, 2001 10:34 AM
> To:   [log in to unmask]
> Subject:      [TN] IMC'S - NO LONGER STIRRING THE POT
>
> Thanks folks for all your great input, but I'm still without a clear
> understanding of the IMC subject and may never be.
>
> It is not hard for me to get confused at my advanced age and congenital
> ignorance. I?m just not getting a consistent picture. Could someone paint
> it
> for me ? for us all. First, the following is what I think I know:
>
> 1) I should have said non-surface scientists (me especially) look at a
> surface as what it is apparent as an objects area having no depth.
>
> 2) Surface scientists clearly understand surface as definitely having
> depth
> down to the surface?s beginning (about 1,000 angstroms or so) and
> understand
> that atomic layers (not mono-layers ? very poorly put) comprise this
> ?depth
> to surface.?
>
> 3) Soldering is defined as the joining of two metal surfaces, with a
> solder
> medium (eutectic most commonly composed of tin-lead but changing to
> something lead free soon) requiring both diffusion and intermetallic
> formation (not Manko?s original definition) at temperatures below 800 F.
>
> 4) I know all processes are capable of being mis-managed including HASL. I
> just find it more difficult to manage effectively across so many
> suppliers.
> I'll not go farther with this. Each of us has to make a decision
> concerning
> a process and its effects.
>
> 5) I recognize that rework cycles, be they two or three, have more impact
> on
> circuit integrity as pad bond strength, etc. I still need to know how much
> is too much concerning intermetallic growth and its correlation to solder
> joint reliability.
>
> What follows is what?s confusing to me:
>
> 1) I THINK Werner says thick IMC formations are essential and good for
> reliable solder joints. Is this right?
>
> 2) Aside from board/pad damage, can there be too many soldering operations
> performed
>
> 3) Is there a maximum or minimum IMC thickness to assure reliable solder
> joints
>
> 4) Does Phil agree about IMC thickness
>
> 5) Does Peter Duncan agree about IMC thickness
>
> 6) Do Manko and Rahn agree
>
> 7) Is this what Wassink says and knows and is it converse to all the above
> except Werner
>
> The main reason I?m asking is to gain objective evidence concerning
> eutectic
> lead soldering techniques having been around since before the time of
> Christ. Then, I would like to apply some of this understanding to what
> concerns us all as lead free soldering using who knows what. So, this
> means,
> I?m really no longer interested in stirring the pot, nor am I interested
> in
> becoming an expert. I just want a handle on this stuff as there still
> seems
> to be too much opinion.
>
> I just want to learn more about what I once thought I knew a little
> something about.
>
> Earl Moon
>
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