We are actively pursuing through our hired consultants at
[log in to unmask], whisker mechanism studies, testing, and control
methods........They may have some publications available at a cost,
consulting available, and suggestions as to control of
whiskering......................you need to deal with them directly on
that.
We, at ON Semicondiuctor are also are evaluating a Japanese whisker test
referred to as "black box" as in secret test......just to see what it
can do to accelerate whisker growth.
John Lynch
ON Semiconductor
Kay Nimmo wrote:
>
> We have recently added a number of additional recipients to this dedicated
> whisker email group. If you are a new list member please read the notes
> below on how to send messages or leave the list.
>
> Attached is a recent message from Erik de Kluizinaar of Philips to the IPC
> group. It would be good if we can use this specific whisker email forum for
> further discussions.
>
> If you know anyone who may like to join the list please encourage them to
> email [log in to unmask]
>
> We will shortly send some details of whisker tests planned at Soldertec.
>
> regards
>
> Kay Nimmo
>
> Welcome to the Soldertec Tin Whisker email discussion group
> ************************************************************************
>
> This international discussion group can be used to promote exchange of
> whisker data, and, to resolve some of the issues surrounding the use of
> lead-free tin based finishes such as;
>
> 1. How should we test whiskering?
> 2. What is an acceptable level of whiskering?
> 3. Who is responsible for whisker growth at each process stage?
>
> Initially the recipient list will be limited to members of Soldertec and
> other organisations who can make a significant contribution to the
> discussions.
>
> You can leave the email group at any time by submitting a message to
> [log in to unmask] (see below)
>
> You can send a message by using the address [log in to unmask]
>
> It is *IMPORTANT* to remember that by replying directly to the group your
> message will be distributed among all list recipients. Any sensitive or
> confidential messages should be addressed directly to me at [log in to unmask]
> Personal messages should NOT be sent via this list but directly to the
> intended recipient. The forum is for discussion of TECHNICAL issues only -
> any personal statements of an unacceptable or POLITICAL nature are not
> permitted.
>
> +++++ Visit our lead-free.org website +++++
> ITRI Ltd, Kingston Lane, Uxbridge, Middx, UK, UB8 3PJ
> tel: +44 (0)1895 272406 fax: +44 (0)1895 251841
> email: [log in to unmask] www.itri.co.uk and www.lead-free.org
>
> Dear all,
>
> Introduction:
> Lead-free solder alloys and soldering processes are becoming known and
> understood better and better. Lead-free component finishes have not by far
> been given the attention they require. When WEEE comes into place, lead-free
> (and preferably also
> environmentally friendly) solderable component and board finishes have to be
> available. With nickel and noble metals being extremely environmentally
> hostile, the obvious direction is tin-based platings, particularly for
> components. However, there is a
> long way ahead of us and lot to be developed/solved before that is mature
> for the wide range of components applied in electronics.
> Together with some collegues, we at PHILIPS CFT are considering the
> mysterious phenomenon of whiskering sensitivity of tin-based solderable
> finishes on components. Lead-free, tin-based solderable component finishes
> are seen as the environmentally
> friendly alternative for tin-lead. Nickel/noble metal finishes are extremely
> environmentally hostile so, in the framework of lead-free soldering,
> primarily intended to lower the environmental impact, tin-based finishes
> should have a strong preference.
> However, the unexplained phenomenon of tin whisker growth makes many people
> nervous with respect to reliability of electronic products in particular as
> applied in satellites, planes, missiles, pacemakers, etceteras. Although we
> recently have involved
> some experts already , we would like to involve more people in clarifying
> possibilities and limitations of tin-based solderable finishes on components
> by this contribution to the forum. I am sure that there is a lot of
> experience among the prescribers
> and I hope that they are willing to share that for the benifit of the
> environment and the electronic community.
>
> History:
> Two decades and longer ago, there were processes (like bright tin plating
> producing platings with a lot of built-in water, hydrogen and organic
> brightening agents) that produced platings, which gave tin whiskers,
> hair-like single crystalline tin
> crystallites, tens of microns thick and millimeters long. Improved processes
> do not give this any longer, i.e. Philips Semiconductors' DIL and DIP ICs
> are Sn100 plated during more than 20 years. Billions of products have been
> applied and no single
> whisker has been reported during that period. The stresses produced by
> bending of the leads and scratches of the test jigs do not produce whiskers
> either. Further, most leadless ceramic multilayer capacitors are plated with
> Sn100 on a nickel barrier.
> These also are produced by the billions by several suppliers around the
> world. Again, no whisker problem at all has been reported!
> At the time, say 20 years ago, it was reported that small additions of any
> other metal would largely reduce the whiskering sensitivity. Examples were
> Pb, Cu, Ag, Bi. Therefore, galvanic finishes of SnCu0.7, SnBi3 and SnAg3.5
> are considered as the
> lead-free alternatives today. In Japan a lot of companies have made a
> preliminary choice for SnBi3, whereas others are exploring SnCu. However,
> all these processes are inherently difficult, particularly SnAg, still have
> problems such as deposition of
> the added metal on the anodes and have lower deposition rates than Sn100 and
> SnPb. Further development will be needed to arrive at mature processes.
>
> Discussion:
> As the root cause of whisker formation I can only imagine mechanical
> stresses, which squize out the relatively ductile tin. Stresses can
> originate from various sources, residual stress in the base metal and in the
> finish, the latter by built-in water,
> hydrogen and organic brightening agents. Further, the microstructure can
> vary strongly, from super-fine columnar crystallites (old-fashioned bright
> tin) to large flat crystals (modern matt tin). Various processes and
> suppliers produce strongly different
> finishes, regarding these aspects. Just give it a try by heating tin-plated
> samples with some solder flux applied and observing these under a
> microscope. Some will simply melt showing little to no gas escaping, whereas
> others will foam like a Dutch glass
> of beer at the moment of melting. Further, the formation of intermetallic
> layers between tin and plated metal will change the layer and may induce
> stresses too after some time.
> When additional metals are co-deposited with tin, they will be present in
> different ways:
> - Copper and silver will be transformed quickly into small, hard
> intermetallic particles, chemical formulas Cu6Sn5 and Ag3Sn respectively.
> - Bismuth will be deposited as small, hard polygonal crystallites, mainly
> present at the grain boundaries of the tin crystallites.
> - Lead will be present as a reasonably finely dispersed, very soft second
> phase, particle size comparable to a solidification structure of a soldered
> joint. At low concentrations it will probably be in solid solution in the
> tin.
>
> Pb is extremely mobile in tin, and should in principle be able to decrease
> residual stresses by diffusion to or away from stress points.
> Bi, Ag and Cu are not, and these hard particles will block motion of
> imperfections such as dislocations and vacancies. They will add to the
> stresses rather than lowering these.
> Therefore, I personally have the feeling that Pb is the only addition to
> tin-platings that may reduce the whiskering sensitivity because, as far as I
> can see, it is the only metal that is capable to reduce residual stresses
> simply by diffusion at room
> temperature. All other contemplated additives (Cu, Ag, Bi) probably will
> cause higher stresses instead. By the way, also SnPb10 has been reported to
> produce whiskers if tortured strong enough and, some 20 years ago, I found
> myself even dense beards of
> Pb extrusions on SnPb40 finishes on a pre-plated steel plate after deep
> drawing.
> Looking into literature, many aspects have been suggested as possible causes
> for whisker growth. Some even contradicting, such as platings on brass. Some
> authors state that that is the worst case for whiskering, whereas others
> declare brass as the
> obvious cure against whiskers. I think that the situation is as follows. All
> tin-based finishes are intrinsically prone to whiskering. At the time, the
> sensitive platings were almost producing whiskers spontaniously. In that
> case, any effect was able to
> disrupt this unstable situation and to excite to whiskering mechanism. That
> is why so many possible causes were reported. It is like a coin standing on
> its side. Any vibration, air movement etc. can cause it to fall. The fact
> that it falls, however, lies
> in the fact that it was standing on its side in an unstable balance . I
> think of a similar situation for the reported whiskers, where the finishes
> are in an unstable balance and any influence factor will disrupt that
> balance.
> However, with state of the art processes the situation is different. The
> finishes apparently are much more stable and the risks are very small. I am
> afraid that the finishes with Cu, Bi and Ag additives, which were thought to
> cure the whisker problem
> twenty years ago, were made in such better processes then, and that the
> causes have been erroneously mixed up.
>
> Conclusion:
> If all this is true, it is highly probable that modern Sn100 finish have a
> lower whiskering risk than SnCu, SnBi and SnAg. Therefore, next to SnCu and
> SnBi, Sn100 requires a lot of attention in order to assess this, because it
> is easy and fast to
> process, relatively cheap and environmentally friendly.
>
> Urgent problem:
> Unfortunately, we are still stuck with the problem of a good whisker
> sensitivity test. If anyone has a good suggestion there, it would be very
> helpful indeed.
>
> Looking forward for reactions,
> With best regards,
> Erik
> ----------------------------------------------------------------------------
> ------------------------------------------------------
> Erik E. de Kluizenaar
> Philips CFT - Electronic Packaging & Joining (EP&J)
> Building SAQ-p, p/o box 218, 5600 MD Eindhoven - The Netherlands
> Tel/Fax: (+31 40 27) 36679/36815; E:mail [log in to unmask]
> PHILIPS worldwide homepage: http://www.philips.com
> Internal PHILIPS only:
> http://pww.cft.philips.com/cfteurope/electronics/elpajo/index.htm
> ----------------------------------------------------------------------------
> -------------------------------------------------------
>
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