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
David Gowlett <[log in to unmask]> on 10/25/99 01:00:00 PM
Please respond to [log in to unmask]
To: [log in to unmask]
cc: (bcc: Yun Zhang/ECSDOMAIN)
Subject: Re: [LF] ATP Lead-Free Solder Project
Dear Christopher,
This project by Lucent Technologies raises a few questions that I would like
discussed on the leadfree forum:
Pure Tin
It is claimed in a news release from Lucent Technologies on April 28, 1999
that Murray Hill has developed a new electroplating process for pure tin
that inhibits whisker growth. From the technical work I have read about
whiskers the cause is attributed to internal stress in the tin deposit. This
is normally overcome by reflowing the deposit, however, in the case of
forming components after plating the resultant internal stress can lead to
whisker formation.
I have also read an alarming report from the Department of Health, Education
and Welfare issued on March 14, 1986 dealing with failure of pacemakers due
to tin whiskers.
Has the risk of whiskering of pure tin been overcome?
I assume from Lucent's new project that pure tin is not acceptable.
Viable alternatives to tin-lead electroplating solutions.
Lucent talk about developing viable processes for electroplating tin-silver
and tin-bismuth solder as a drop-in replacement for tin-lead solder.
As far as I am aware tin-silver will most likely involve cyanide chemistry
which is hardly environmentally friendly. The alloy fails the EPA test
designed to simulate waste disposal and groundwater contact. The price of
silver is significantly higher than lead.
Tin-bismuth, there are two concerns over the use of tin-bismuth, the first
is the availability of bismuth, the second is the reaction of lead with
bismuth which forms a brittle intermetallic. It is going to be very
difficult to ensure that all components are leadfree.
The only obvious alternative is tin-copper, which although it has a high
melting point it is lower than pure tin. The advantages are that the
chemistry should still be based on methane sulphonic acid and therefore
there should be little plant modification required. The cost are reasonably
comparable to tin-lead solder. The down side is that this is most alloy
composition vary with the applied current density, it therefore may be
impossible to control the process.
My final comment is that the European Commissions draft proposal allows for
the use of lead as an alloying element in steel, aluminium and copper. In
the case of copper there can be 4% lead by weight, why is this not also
applied to solder?
The electroplating of a 95/5 Sn/Pb solder would be by far the simplest route
to lead reduction. The original reason for leadfree solder was to eliminate
lead from tin cans, this was suggested by the Organisation for Economic
Co-operation and Development back in 1990, they did not consider solder used
in electronics a risk as the quantity of lead used was not significant.
I look forward to receiving comments on the above points.
Best regards,
David Gowlett
Technical Director
Palmer Plating Ltd
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]]On Behalf Of Christopher
Jorgensen
Sent: 14 October 1999 18:10
To: [log in to unmask]
Subject: [LF] ATP Lead-Free Solder Project
Lucent's Electroplating and Chemicals Services (EC&S) is sponsoring a
project to
develop commercially viable electrochemical processes to enable the use of
tin-silver
and tin-bismuth as drop-in replacements for lead-based solders. The project
is being
done with the support of the Advanced Technology Program (ATP) of the
National
Institute for Standards and Technology (NIST).
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