Dear Mr. Zhang,

Thank you for your reply to my e-mail, is till have a couple of points that
I would appreciate comment on.

1. Pure tin whiskers
There is still surely a risk that if tin deposited by your process is formed
after plating without any stress relief, then whiskers may form. I have
found on the Internet at
http://www.satellitetoday.com/viaonline/backissues/1998/1098dollars.htm that
the failure of four satellites has been attributed to tin whisker growth.
The potential for costly litigation makes pure tin a difficult route to
take. I believe whilst pure tin is often on cheaper components, the problems
associated with tin whiskers is reduced as the pure tin will alloy with lead
during the soldering process. There is also a lot of historical reluctance
to the use of pure tin. The requirement standard from Telcordia (formerly
Bellcore), document No. SR-3151, states that "no matter what process is
used, any electroplated tin finish may grow whiskers". The problem is not
helped by the lack of a definitive test for whisker growth, in my twenty
years of electroplating I have yet to see a whisker in real life.

2. Tin-silver
The issue with the cost is not to do with the amount of silver in the
electroplating solution. If we take a cost of the raw metals ($/t) as tin
5590 Lead 488 and silver 159,646, then reducing the lead content from 60/40
to 90/10 will increase raw material costs by 43%, however the eutectic
Sn3.5Ag increase cost by 309%.

3. Lead reduction
Because the European WEEE directive allows lead to be present in certain
metals, it is possible that they will accept the same for tin. Reducing the
amount of lead in solder is by far the cheapest solution to the problem as
no one can offer a simple drop in alternative.

I wish you all the best with the development work you are undertaking, I
intend to investigate further the tin-copper.

Best regards,
David Gowlett

-----Original Message-----
From: Leadfree [mailto:[log in to unmask]]On Behalf Of Yun Zhang
Sent: 02 November 1999 21:54
To: [log in to unmask]
Subject: Re: [LF] ATP Lead-Free Solder Project


Dear Mr. Gowlett,

Lucent electroplating chemicals and services (EC&S)?s position is to
introduce
pure tin as one of the alternatives for lead-free applications.  Our
understanding of the whisker issue, development of SnTech pure tin
processes,
and the many advantages of pure tin supports this position.  Electroplated
pure
tin coating could be used as a board finish for high temperature reflow
operations and as a component finish.  Today's electronic industry requires
multi-step, multi-temperature soldering, electroplated tin alloy coatings
with
lower melting temperatures will be required to meet the industry needs.
That'
is why we are developing tin-bismuth and tin-silver.  This does not suggest
that
pure tin is not acceptable.

The SnTech pure tin process, owing to its large well-polygonized grains and
low
organic content, appears to resist whisker formation.  Detailed results have
been presented at IPCWorks'99.

While it is true that it will be very difficult to develop commercial viable
tin-silver and tin-bismuth electroplating processes, it does not mean it can
not
be done.  The traditional approach, i.e., the "cocktail-mix" approach has
inherent problems.  Because of the likely interactions of multiple
components in
these lead-free systems, the process window and control will be undoubtedly
more
challenging. It is essential that we investigate electrodeposition with an
entirely different approach, namely, at the molecular level.  This is
precisely
what Lucent Electroplating Chemicals and Services (EC&S) proposes to do in
aforementioned lead-free solder project.   Though the chemistry composition
for
tin-silver system is yet to be determined, cyanide will not be used.

The cost of silver is not going to be an issue with an electroplating
solution,
since very little silver metal is in the plating bath.  On the other hand,
replacement of lead with small amounts of silver will reduce cost for lead
waste
disposal (both in liquid and solid forms) and monitoring and reporting of
lead
levels at manufacturing facilities.  The same argument could be made for
bismuth.

It is true that there is a compatibility issue of bismuth containing solders
with the lead containing solders because of the low melting phase of
tin-bismuth-lead.  This suggests that the bismuth containing solder be
implemented at a later date when all components are lead-free or utilized in
Pb-free systems.  However the case may be, we need to be ready with a lower
melting temperature solder.

It is important to bear in mind that in order to replace tin/lead solder,
the
electronic industry needs more than one lead-free solder. Various
compositions
of tin/lead solders are used presently (e.g., 100 Sn, 97/3, 95/5, 85/15,
60/40,
5/95 SnPb) to accommodate a range of melting temperatures and mechanical
properties requirements for various applications.  Therefore, different
tin-based solders with different melting temperatures and materials
properties
will be needed.

We do not foresee any appreciable plant modifications for these lead-free
solder
plating processes.

(90-95)Sn(5-10)Pb is one of the simplest ways to reduce lead, however, it is
not
lead-free.  When the industry converts to lead-free in the near future, we
must
be prepared!  Pure tin plating will be one of the alternatives.

I hope this is helpful.

Best regard,

Yun Zhang, Ph.D
Lucent Technologies EC&S
236 Richmond Valley Road
Staten Island, NY 10309

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