In response to Peter Roth's question in the whiskering discussion thread, we did
some digging on whiskers and my colleague David Ormerod came up with the
following comments which we felt might be useful to share with the group.
_________________
Alan Rae
Director of Technology
Cookson Electronics Inc.
225 Foxborough Blvd. Suite 150
Foxborough MA 02035
USA
[log in to unmask]
Phone: (508) 541-5843
Fax: (508) 541-5871
"We have a fair amount of information on tin whisker which we collated for
the FST product project. Here are a few comments off the bat:
Essentially, whiskering of Tin coatings appears to be driven by an onset
of a series of environmental conditions, although in some cases this
appears to be almost a random phenomenon, many different tests have
been devised to study, simulate and precipitate this condition.
Whisker can range in size from a few angstroms thickness to much
thicker, with length up to several millimetres.
The critical precursors which increase the propensity towards
whiskering, include: the stress in the coating; the purity; the
thickness range; and to a degree, the crystal structure.
Highly stressed deposits of 2 - 10 microns of pure tin are probably the
most susceptible and, hence electrolytically plated deposits can be a
particular problem. This stress effect can also be demonstrated by
scratching tin deposits and watching these areas preferentially whisker.
Immersion deposits which are thinner (say 0.5 - 1.5 microns) and which
have a more polygonized structure are much less prone. This is the basis
of the FST technology.
Adding a small amount of noble or other metal can retard of eliminate
the effect. A small % of Bismuth or Lead is very effective at achieving
this.
Test conditions include: The DIN 41640 test - a 56 day exposure at
55oC/20% RH and also a long term storage conditioning test at 88 - 93%
RH 35 - 45C can stimulate the formation.
Some people would argue that storage at 50C (worst temp for whisker
growth) and 50% +/-10% RH (humidity) is the best medium term test
environment.
Brass substrates are worse than copper as the zinc apparently creates
nucleation sites for whisker growth, and even tin-lead may not be immune
to this effect when plated on brass. However, there is no guarantee
that this will happen (back to the random question).
There is an arguable link between metallic dendritic growth testing ( a
water drop across conductors under electrical bias -which generates
dendrites which ultimately cause a short) and whiskering propensity.
With respect to the questions below I would suggest that the "mechanical
burr" may generate more critical level of surface stress on the coating,
which may be caused by the local variation of current density of the
plating process. CD may be much higher at this point. Regarding what can be
done to address this ? - Additives can be used to regulate deposit stress
and also co-deposit inhibiting materials in electrolytic baths or,
alternatively a modified immersion tin like FST can be used to control the
thickness and crystal structure.
Hope this helps.
Regards, David
David. H .Ormerod,
Enthone - Polyclad Technologies
144 Harvey Road
Londonderry, NH 03053
Tel: (603) 645 0021 X203
Cell Phone: (603) 759 3865
Pager: 1 800 467 3700 ; PIN# 597 9351
Fax: (603) 645 4901
Email : [log in to unmask]"
"Roth Peter (RtW1/TEF1)" <[log in to unmask]> on 04/17/2000 08:25:18
AM
To: [log in to unmask]
cc:
Subject: Re: [LF] DTI update report: Tin Whiskers
Dear David, Forum members,
The Tin-whiskers issue is, in my opinion, the most important unanswered
question for component suppliers.
Experience in semiconductor assembly shows different types and causes of
whisker:
(a) A copper burr from the cutting of the 'Dam-bar' between adjacent pins
becomes 'plated up'. This sharp mechanical burr grows during the
tin-plating process to form a thick whisker.
The electro-plating people can now successfully regulate the process to
all but eliminate this problem.
(b) The 'classical' whisker you can see in the textbooks: This is very thin
indeed, maybe only atoms thick.
It grows during the lifetime of the product.
I believe this has little if any relation to the type (a) above.
My questions:
(1) Can the plating process also affect or prevent the classical whiskers
in
(b) above ?
(2) What conditions are needed to promote or accelerate the growth of the
classical whiskers ?
What temperature / time / humidity / voltage etc. or other conditions
should we use in a qualification- or life-test to confirm or
eliminate
the existence of a problem with a given product or process ?
Your answer would be greatly appreciated.
Thanks and Best Regards, Peter Roth
RH-14665
* Robert Bosch GmbH, Dept: RtW1/TEF1, 72703 Reutlingen, Germany.
* Tel: +49-7121-35-1962 * Fax: +49-7121-35-1498
* mailto:[log in to unmask]
> -----Original Message-----
> From: David Gowlett [SMTP:[log in to unmask]]
> Sent: Saturday, April 15, 2000 12:29 PM
> To: [log in to unmask]
> Subject: Re: [LF] DTI update report
>
> Hello Mike,
>
> Thanks for the reply; I still believe that the myths and legends of tin
> whiskers need to be dealt with.
>
> Many technical papers still claim that whiskers are a serious problem. I
> have read an alarming report from the Department of Health, Education and
> Welfare issued on March 14, 1986 dealing with failure of heart pacemakers
> due to tin whiskers this problem caused a Food and Drug Administration
> Class
> 1 recall.
>
> http://www.satellitetoday.com/viaonline/backissues/1998/1098dollars.htm
>
> The above web site contains details on the failure of satellites which
was
> attributed to tin whiskers
>
> I have read a lot of technical paper about the elimination of tin
> whiskers.
> They mostly claim that the risk of whiskers can be reduced by reducing
the
> organic content of the tin and by not subjecting the plated deposit to
> stress.
> No one has ever stated that whiskers can be eliminated!
> If the plated component is subjected to any forming after plating the
> resulting stress can increase the risk of whiskering, unless this stress
> is
> removed by either annealing or reflowing the tin. The vast majority of
> components and wires that we plate are subjected to subsequent forming
and
> in a lot of cases cannot be reflowed.
>
> The main problem that exists is that there is no standard test for
> whiskers
> on tin although I understand that Ms. Kay Nimmo from ITRI is
co-ordinating
> a
> test program, aiming at coming up with a standard method.
>
> I will not recommend pure tin as a replacement for tin/lead until we can
> at
> least test the risk. If any of our customers ask for pure tin I will
> supply
> it but only after making them aware of the risk.
>
> We need a decision on the risk of whiskers from a Government level.
>
> Best regards,
> David Gowlett
> Technical Director
> Palmer Plating Ltd
> Cirencester
> England
>
> -----Original Message-----
> From: Leadfree [mailto:[log in to unmask]]On Behalf Of Mike Schatzberg
> Sent: 15 April 2000 01:27
> To: [log in to unmask]
> Subject: Re: [LF] DTI update report
>
>
> Hello David and Everyone.....
>
> It is indeed true that the American side of the leaded component industry
> has been slow to react.....at least as far as proving or disproving some
> of
> the myths and legends about whiskering, or dendritic growth from
unalloyed
> tin plating, over copper substrates for example.....
>
> We were only recently challenged here to produce the next generation of
> "lead-free" lead wires, for the capacitor industry, and have only shipped
> out our first prototype samples for evaluation recently.....Component
> level
> testing has now begun.....
>
> Here at Torpedo Specialty Wire, Inc., we are following the direction of
> Lucent Technologies, regarding the use of "pure tin", to avoid the
> whiskering problem.....Our plating technology solution involves assuring
> that extremely low levels of organic content are contained within the
> plated
> coating.....Our processes are also aimed at assuring very short wetting
> times and lower wetting forces, to approximate the lower wetting angles
> associated with tin/lead plating systems....and be compatible with higher
> wave soldering temperatures.....
>
> We are working in concert with one of our universities here in North
> Carolina, to detail our findings regarding this electro-plated solution
to
> a
> "drop-in" replacement for tin/lead alloy plated lead wire.....This
> solution
> is anticipated to be fully compatible with any of the newer solders
> predicted to be used in wave soldering and reflow applications......
>
> We have begun the journey now, and it will surely transform our company
> over
> the next several years.....although we are approaching these changes
> cautiously, we are excited to be participating in such a major industrial
> revolution.....
>
> Mike Schatzberg
> General Manager
> Torpedo Specialty Wire, Inc.
> Rocky Mount, NC
> 252-977-9345
> [log in to unmask]
> www.torpedowire.com
>
>
>
>
>
> ----- Original Message -----
> From: "David Gowlett" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Thursday, April 13, 2000 12:50 PM
> Subject: [LF] DTI update report
>
>
> > Dear all,
> >
> > II have just loaded from www.lead-free.org the new DTI Report UPDATE
> 2000
> > "Analysis of the current status of lead-free soldering - one year on".
> >
> > The section on "Component Terminators" irritates me enough to write
this
> > message.
> >
> > The report claims that "In general, component companies have been
fairly
> > slow to react to the impending change to lead-free soldering, possibly
> due
> > to a low level of customer demand".
> >
> > Alternatives to Sn/Pb are given they are as follows:
> >
> > Pd/Ni
> > The DTI now admit that this finish does not provide the solderability
> > expected from conventional soldering technologies unless the soldering
> > temperature was in excess of 250 deg C.
> > Ignoring the problems that may be caused by this elevated temperature
> there
> > is still the simple fact that palladium is currently twice the price of
> > gold!
> >
> > Sn/Bi, Sn/Ag & Sn/Cu
> > For these alternatives the DTI state "Each has advantages and
> disadvantages
> > in processing and performance and a 'favourite' system has yet to be
> > established". The main disadvantage from the point of electroplating
any
> one
> > of these finishes is that they do not currently exist as a commercial
> > process!
> >
> > Sn
> > This is the only viable alternative to Sn/Pb that currently is
available
> and
> > does not involve excessive sums of money. The DTI report states
> "Concerns
> > over tin whiskering are being addressed by NPL, ITRI and others, though
> work
> > has yet to be fully completed". Kay Nimo of ITRI in November 1999
> requested
> > information on tin whiskers from this forum. I spoke to Kay in February
> and
> > was informed that my reply was the only one received!
> >
> >
> > Conclusion
> > The component companies cannot be held liable for the fact there are no
> > acceptable alternatives to Sn/Pb. The industry as a whole has to decide
> if
> > pure Sn is acceptable (i.e. whiskers are not a problem) or will have to
> pay
> > the increased cost for precious metals.
> >
> > My personnel opinion is that we are all wasting our time and I believe
> this
> > is highlighted by the reduction of mail on this subject.
> >
> >
> > Sorry about the length of this message, I look forward to receiving
> > comments.
> >
> > Best regards,
> > David Gowlett
> > Technical Director
> > Palmer Plating Ltd
> > Cirencester
> > England
> >
> >
> > This message has been checked for all known viruses by Star Internet
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