>What are the development issues that are crucial for formulating flux
 >medium for Pb-free solders? Would a direct translation of flux
 >mediums of Pb-rich to Pb-free solders be advantageous or detrimental,
 >and why?
 >
 >Regards
 >Dr. Tennyson Nguty, Electronics Manufacturing Research Group,
  >################################################################

Tennyson and all

Good question.  Herewith some observations from the reflow soldering trials
within the IDEALS project.

1)  Lead-free joints (mainly SnAg3.8Cu0.7) exhibited more voids than in SnPb
when the same paste medium was used for both.  Void content in the range 5-10%
of the projected wetted area was common for SnAgCu, depending on component
type.  If not between the land and component, the voids were trapped against
the land and we formed the impression that they did not agglomerate and float
out as readily as in SnPb.  We put this down to the higher surface tension and
lower density respectively of the lead-free alloys.

2) There was an effect of board finish, such that better wetting finishes (e.g.
Ni/Au) showed fewer voids than less well wetted finishes (e.g. OSP-Cu).

3)  The void content was a strong function of the 'style' of paste, e.g.
higher solids pastes yielded fewer voids.

4)  The void content was independent of reflow profile as best as we could
determine.  This included a range of preheat temperature (160C to 175C) and
peak temperatures (225C to 255C) and conventional and linear profiles.

5)  Nitrogen soldering slightly reduced void levels and improved the appearance
of the residue.  It also made the residue easier to remove, although we were
looking at no-cleans, so cleaning was not a major issue.

6)  The lead-free alloys did not spread over copper surfaces - copper halos
were almost universal.

We concluded that, as a first pass, mediums developed for SnPb were acceptable
for lead-free.  The higher void content of the joints prompted much discussion
as to what was acceptable - without us reaching a firm conclusion.  We saw no
evidence that voids up to, say, 10% wetted area were compromising reliability
but this level would exceed the typical 5% criterion where a limiting void
content is specified.  Equally, I have to say that the evidence might be there,
but was not visible in the noise: much more systematic work needs to be done in
this area.  The reduced spreading of lead-free solders on copper is also an
issue, e.g. for intrusive reflow (which we did NOT look at in IDEALS) and
inspection standards.  A rethink on activator systems and OSPs might have a
role to play here.

Not really an answer to your question, but, I hope, a prompt for a fuller
discussion on an area that I personally think is at least as important as the
fine details of alloy composition.

Regards and Season's Greetings

James
+++++++++
    From: James H Vincent
          Caswell Technology
          Caswell, Towcester NN12 8EQ, UK

     Tel: +44 1327 356321 (direct), G-Net: +942 6321
          +44 1327 350581 (switchboard)
     Fax: +44 1327 356775
  E-mail: [log in to unmask]
     WWW: http://www.gmmt.co.uk

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