Leadframe-based packages are generally molded over bare copper leadframes.
An exception is when a Ni-Pd-Au pre-plated leadframe is used.  Then the
transfer molding of the epoxy mold compound is done over the AuPdNi-Cu
surface.

Even the 'bare' copper leadframes often have metallo-organic adhesion
promoting coating to improve MSL performance.

After molding the 'bare' copper leadframe-based packages are plated with
Sn-Pb.  Sn-Cu intermetallics begin forming right away, even at room temp.
If the Sn-Pb is stripped by successive dilution by dipping into a series of
molten Sn or SAC pots, obviously the package is exposed to thermal
excursions for which it was not designed and reliability/MSL performance
standards can be expected to be degraded.  If the Sn-Pb is removed
chemically, there is a risk of oxidizing the Sn-Cu intermetallic layer
previously formed on the leads.  This can cause serious solderability
degradation.

Unless the leaded package has a fairly robust leadframe and leadframe shape
(like a PLCC) you can expect some lead deformation that can significantly
reduce SMT yield.  Using multiple Sn or SAC solder pots to clean the leads
can also result is solder bridging between leads for fine pitch devices.

You can make money at just about anything these days, but converting leaded
packages from Sn-Pb to a finish with < 1000 ppm Pb seems very painful,
expensive, and risky to me.


Best regards,
Leo

Leo M. Higgins III, Ph.D.
Vice President Technical Support Operations
ASAT, Inc.
3755 Capital of Texas Highway, Suite 100
Austin, Texas     78704

ph     512-383-4593
fx      512-383-1590
[log in to unmask]
www.asat.com


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-----Original Message-----
From: Leadfree [mailto:[log in to unmask]]On Behalf Of to Paul Taylor
Sent: Friday, November 04, 2005 2:29 AM
To: [log in to unmask]
Subject: Re: Making IC's RoHS Compliant


Brian

I don't claim to be an packaging expert, perhaps there is one on this
forum who can answer about the pro's and cons of re-tinning and whether is
actually feasible.  If it was as simple as re-tinning, why would there be
such a push on last-time-buy components as exceptions as they could all be
converted.

regards
Paul




Brian Ellis <[log in to unmask]>
04/11/2005 08:15

To
"(Leadfree Electronics Assembly Forum)" <[log in to unmask]>,
[log in to unmask]
cc

Subject
Re: [LF] Making IC's RoHS Compliant






I don't think you are right. The tinning is done after the packet is
moulded, in my experience. It would be very hazardous to mould a
thermosetting polymer over a tin-lead alloy, which can easily creep,
destroying the hermeticity.

Brian

to Paul Taylor wrote:
> Doug
>
> Stripping and re-tinning the leads does the outside of the IC, but
unless
> you aim to split the IC's down the middle!! then there will be lead
> trapped in the leadframe under the epoxy.  This would constitute a fail
in
> RoHS.
> I suggest you either put pressure on you IC manufacturer to supply in a
> RoHS compliant leadframe or re-design.  Better get your skates on, there
> is not much time left.....(re-design, re-qualify etc)
> Also I agree with Doug Rohm, it would have to survive the higher
> temperatures
> Better get that CAD package out....
>
> regards
> Paul
>
>
> Paul R Taylor
> Senior Engineer, RoHS Compliancy Group, IED
> Pitney Bowes Ltd
> The Pinnacles, Elizabeth Way, Harlow, Essex, CM19 5BD
> tel: +44 1279 449 744      fax +44 1279 449 594
> email: [log in to unmask]         website: www.pb.com
>
>
>
> "Douglas, Scott" <[log in to unmask]>
> Sent by: Leadfree <[log in to unmask]>
> 27/10/2005 20:38
> Please respond to
> "(Leadfree Electronics Assembly Forum)" <[log in to unmask]>; Please
respond
> to
> "Douglas, Scott" <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> [LF] Making IC's RoHS Compliant
>
>
>
>
>
>
> Dear Group,
>
> We are looking for a way to avoid re-designing a number of PWB to
replace
> non-RoHS compliant IC's with other compliant (not drop-in replacement)
> parts. In a discussion today with our component engineer, we wondered if
> we could take our stock of non-RoHS compliant  IC's, send them to some
lab
> someplace, have them (dip, acid etch, whatever) remove the tin/lead
solder
> plating the leads and then have the leads re-plated with lead-free
solder.
> This assumes a plastic or ceramic package with standard metal leads,
maybe
> through hole, maybe surface mount. Could we do this and then claim to be
> RoHS compliant? Anything else we should think about in this?
>
> Scott Douglas, NCE
> Compliance Engineer
> Harman Specialty Group
> 3 Oak Park Drive
> Bedford, MA 01730
> Phone: (781) 280-0406
> Email: [log in to unmask]
> Web: www.harmanspecialtygroup.com
>
>
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