Robert
I am very interested to know on immersion silver. Can I have a copy of the
published paper referred to in your notes please. I am also interested to know
about hole tolerances used for backplanes, eg. standard 2mm eye-of-the-needle
connectors. Have you adopted the same dimensional size/tolerances as tin-lead?
From your experience do the incidents referred to cause for concern?
Regards
Mehrdad Kalantary
"Furrow, Robert Gordon (Bob)" <[log in to unmask]> on 28/08/2001 15:31:14
Please respond to "(Leadfree Electronics Assembly Forum)" <[log in to unmask]>;
Please respond to "Furrow, Robert Gordon (Bob)" <[log in to unmask]>
To: [log in to unmask]
cc: (bcc: Mehrdad Kalantary/MAIN/MC1)
Subject: [LF] RE: [LF] Lead Free Board Finishes
Thomas,
At Lucent, we have also struggled with the optimal choice of a surface
finish. I will try to give you my thinking as to each coating.
Electroless Nickel Immersion Gold (ENIG) can be an excellent choice, but I
feel its success is very dependent on the board vendor. Controlling the ENIG
process is the most difficult of the surface choices about which you
inquired. If the nickel deposition is not well controlled, it can lead to
problems. If the bath is run in low volumes, I feel that control can be very
difficult. Also, the phenomena of "Black Pad" has caused us great concern.
It is a very difficult failure to detect because you can easily pass ICT
with these fractured joints, and then get into a an intermittent problem
once in the field. Also, our experience on wetting to nickel to form
nickel/tin intermetallics is only a fraction of that which we have attained
over the years for wetting to copper and forming copper/tin intermetallics.
Soldermask attack can also be an issue. Finally, during our evaluation of
ENIG we found that it could not hold up to temperature/humidity for even 24
hours. Although, this is not real production conditions, it indicated that
the coating wasn't as robust as the Immersion Silver (ImAg) tested in
parallel.
Immersion Tin (ImSn) is also felt to be suspect. With the thin coatings
expected on the board, we are concerned with shelf life. We do use ImSn for
press-fit backplanes, but do not recommend it for SMT or TH applications. We
completed assembly testing with ImSn a few years ago that indicated there
was a higher defect rate at assembly than with other finishes. However, I
want to stress that this was one formulation and that improvements may have
been made. However, there is still the issue of intermetallic formation
consuming the tin, reducing shelf life and making multiple thermal cycles
problematic. Also, the chemistry involved is more of a health hazard than
those used with OSP's or immersion silver.
We have used OSP's for almost two decades now and in general have had good
success. With proper care in handling, and usage of nitrogen blanketing to
reduce oxygen levels during thermal excursions, we have been able to
accomplish good soldering on multiple thermal cycles. The reason we no
longer prefer OSP is not because of the coating itself, but due to isolated
busts that have occurred. We have seen defects caused by very thin layers of
soldermask residue on the pads and also due to incomplete removal of tin
strip residues after etching. Also, with via in pad product we have
experienced where the inside of the small holes are not completely dried and
thereby break down the surface of the pad and reduce solderability. These
busts can't be detected until after expensive components have been attached
to the board. These are low level type situations, but when they occur they
cost a lot of money because we end up junking that product. I know you could
scrape the pad and repair the individual defect, but you can't determine if
there are other marginal areas on the board that meet visual requirements
yet may be marginal and be a reliability concern.
Immersion Silver (ImAg) is now the surface of choice for Lucent. We
underwent extensive testing during 1997 and introduced the finish into
production in 1998. A paper detailing our testing was presented at IPC
summit on Surface Finishes and PWB Solderability in September 1999. We have
since produced a large quantity of circuit packs with this finish and are
very satisfied with our results. Although have been a couple of isolated
incidents, the problems encountered with this coating are less than any
other with which we have experience.
If you would like further information, please contact me.
Thanks,
Robert Furrow
Printed Wiring Board Engineer
Strategic Supply Global Account Manager
Supply Chain Networks
Lucent Technologies
978-960-3224 [log in to unmask]
-----Original Message-----
From: Thomi [mailto:[log in to unmask]]
Sent: Wednesday, August 22, 2001 5:06 PM
To: [log in to unmask]
Subject: Re: [LF] Lead Free Board Finishes
Dear all,
we are engaged in different activities regarding lead free soldering R&D. We
are also concerned with what will become a major standard in circuit board
finishes for leadfree. We are going to start experimenting with wave
soldering and will begin with two solder surfaces:
* One is electroless Ni - immersion Au, which we see as the current
wide spread fine pitch answer to the problems with uneven HAL solder
surfaces, and which happens to be a no lead constitution.
* The other is chemical Sn, which we see as a flat simple composition
less costly alternative to Ni-Au.
But what about OSP (organic solderability preserve on bare copper)? We see a
problem of decreasing wettability after multiple solder steps in an
electronics manufacturing process.
And what about chemical Ag (with "built-in" organic inhibitor, or organic
preservation layer)? Is there a tendency to test and use it more frequently,
or is it a dead borne child?
Any comments are appreciated, and at the end of this year we will be able to
communicate some test results.
Thomas Ahrens, D-Boostedt
-----Ursprüngliche Nachricht-----
Von: Vadali Mahadev < [log in to unmask] <mailto:[log in to unmask]> >
An: [log in to unmask] <mailto:[log in to unmask]> < [log in to unmask]
<mailto:[log in to unmask]> >
Datum: Freitag, 17. August 2001 20:08
Betreff: [LF] Lead Free Board Finishes
Hello,
I agree with all the people voting against splitting the group. The reasons
for switching to lead free is just important a discussion topic as the
technical details. In most instances they are dependent on each other.
Also, I am looking for information on changes that are necessary in
designing the boards to accommodate the lead free packages. Are there any
guidelines/information on what works good for the Sn-Ag-Cu and Matte Sn
based solders? for BGAs? for QFPs?
Thanks
Vadali Mahadev
PS: As always all information presented here are only mine and in no way
connected to my employer.
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