From the solder paste supplier side, I would have to agree with James.  AIM
has thus far had good success using our "latest and greatest" flux
chemistries (which were originally intended for Sn/Pb) with lead-free
alloys.  Printing characteristics were the same, of course, and reflow
should more than adequate results.

It's important to keep in mind, though, that solder paste manufacturers are
constantly updating and developing their flux chemistries.  That being said,
as the industry moves more and more towards lead-free alloys, you should see
chemistries that work (reflow) better and better with the lead-free, higher
melting point alloys.

Regards,

David
AIM
----- Original Message -----
From: "James Vincent" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, December 14, 1999 6:38 AM
Subject: Re: [LF] Leadfree paste formulation and printing


> >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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To subscribe/unsubscribe, send a message to [log in to unmask]
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################################################################
IPCWorks -October 25-28 featuring an International Summit on Lead-Free Electronic
Assemblies.
Please visit IPC's Center for Lead-Free Electronics Assembly
(http://www.leadfree.org ) for additional information.
For technical support contact Gayatri Sardeshpande [log in to unmask] or 847-790-5365.
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