Hi Daan,

I am currently in the process of writing a paper on Material Control for
Lead-Free and here is some relevant information that I found on the subject
of alloy compatibility :

Backward Compatibility (i.e. using Pb-Free components in Sn/Pb process):
BGAs with Pb-Free solder balls are not recommended for Sn/Pb assembly using
temperatures below 220C because the solder joints are poorly formed if the
balls do not melt. As a result 2nd level reliability may be adversely
affected. There is also a potential for increased Sn-whisker growth if the
finish does not melt.

Forward Compatibility (i.e. using Sn/Pb components in Pb-free process): Less
of an issue, some reports of increased voiding in PBGA solder ball joints
due to flux trapping. Another issue that was already pointed out is the
resulting Lead-Contamination that may affect the solder joint structure and
decrease its reliability.

As a result of the above issues, some component vendors, including Intel do
not recommend using their components in forward or backward compatible
assemblies.

References :

- Pb-free IC Component Issues & IPC/JEDEC Specification Update - Rick Shook,
Agere Systems
http://www.turi.org/messages/Rick_Shook-Dec3-03.pdf

- Lead Contamination in Lead-Free Electronics Assembly - Karl Seelig and
David Suraski, AIM
http://www.aimsolder.com/techarticles/Lead%20Contamination%20in%20Lead-Free%
20Electronics%20Assembly.pdf

- Lead-Free Program Overview - Intel Web Site
http://www.intel.com/design/flcomp/packdata/wccp/download/chpt8.pdf

I hope this information is helpful. Feel free to use it for your web site if
you wish.

Francois Monette
Cogiscan Inc.
[log in to unmask]
www.cogiscan.com



Date:    Mon, 7 Jun 2004 15:38:34 +0200
From:    Daan Terstegge <[log in to unmask]>
Subject: Re: Mixed assembly of ROHS & Non ROHS components

Hi Blair,

Out of the many articles that I have read about this subject not a
single one clearly states that leaded parts in leadfree solder is a big
"NO".  The researchers seem to limit their conclusions to statements
like "combination A fails after 3000 cycles, combination B after 2930
cycles" or "combination A is stronger than combination B". I welcome all
papers that support other opinions, and I'm willing to add them to my
leadfree webpage for everyone's benefit.
To my opinion the issue of thermal detoriation of components that were
not designed for the higher leadfree soldering temperatures is a much
more viable concern.

Daan Terstegge
Thales Communications
Unclassified mail
Personal Website: http://www.smtinfo.net



>>> [log in to unmask] 06/04/04 03:25pm >>>
I have no actual experience with this, however, from what I have
learned in following this list and the LeadFree list is that if you use
components with Pb (lead) based finishes on the leads in a lead-free
process, the reliability of the solder joints is questionable.

If you use lead-free finishes in a tin-lead soldering process effect is
minimal.

Others on this list can explain why, and you may be able to find that
information in a search of the archives.

Bottom line is if you are using a lead-free soldering process, it is
highly recommended that all components used are also lead-free. If you
still have a tin-lead process, you can mix.

Blair Hogg
QA Manager
GAI-Tronics Corp.

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