[tinwhiskers] Re: Response to numerous postings
- From: "Mark Vaughan" <mark@xxxxxxxxxx>
- To: <tinwhiskers@xxxxxxxxxxxxx>
- Date: Tue, 18 Mar 2008 12:28:04 -0000
Thanks for your reply
Regarding micro fractures as tin cools / crystallizes, there is a project at
one of the British Universities, I don?t remember which one, Surrey springs
to mind but I may be wrong.
I know of this since I met the professor conducting the research about a
year ago, I discussed my experiences as below, and he confirmed this was
exactly what their research was showing.
We have also experienced PCB?s manually soldered in lead free that passed
full test initially, but failed to be capacitive only a week or so later.
For a while we extended burnin tests from our usual 3 days, to two weeks to
find the bad joints.
We found if we conformal coated the boards with a simple lacquer the problem
never occurred, our simple theory was that there were fractures within the
joint that would oxidise to cause it to fail and the lacquer stopped this.
We now use a different alloy that doesn?t seem to suffer from the problem.
I know someone else that may know the professors name, I believe he?s a
member of a local ham radio club, can?t be that many professors.
Regs Mark
Dr. Mark Vaughan Ph'D., B.Eng. M0VAU
Managing Director
Vaughan Industries Ltd., reg in UK no 2561068
Water Care Technology Ltd, reg in UK no 4129351
Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH UK.
Phone/Fax 44 (0) 1872 561288
RSGB DRM111 (Cornwall)
_____
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of
gordondavy@xxxxxxxxxxx
Sent: 18 March 2008 00:55
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Response to numerous postings
This is a response to an amazing number of postings on March 13 and 14.
Apparently there has been a pent-up demand for the new tin whiskers forum. I
list below some comments from a number of those postings, and my response. I
will start with the general comment that it helps to keep in mind that the
real goal is not to prevent tin whiskers but to prevent short circuits
caused by tin whiskers.
I await someone who can do the math on how statistically significantly
conformal coatings "mitigate" tin whiskers. (Bob Landman)
Bob, you?re going to have to wait longer than you can afford to get a
statistically significant answer to your mitigation question. First, you?re
going to have to live with the fact that conformal coating will not entirely
prevent short circuits caused by tin whiskers. But that really doesn?t
matter. The requirement is not that all such short circuits be prevented,
but that their frequency of occurrence is acceptably low ? no higher than
the frequency of occurrence of other kinds of circuit failure. I believe,
for reasons elaborated on below, that this is a reasonable expectation.
If a whisker can grow from one pin on an IC package, then certainly, it can
also grow from adjacent pins as well and then don't we have the perfect
opportunity for shorts? (Bob Landman)
Two whiskers can grow through conformal coating and meet. The presence of an
electrical potential between the whiskers increases the probability of their
meeting, but as Henning has already commented, there?s still a question of
whether they will actually meet, and if they do, whether (because of their
oxide coatings) they will cause a short circuit. Keeping in mind my comment
above, I believe that the probability of two whiskers penetrating a
conformal coating and meeting is low enough for most high-rel applications.
??any trace of silicon on the surface?? Raymond Bennett
Silicone on a surface will prevent the proper adhesion of a conformal
coating. However, the primary concern is how this lack of adhesion prevents
the coating from performing its primary function, which is to adhere to the
surface so as to prevent the opportunity for a continuous layer of liquid
water from forming. I don?t think anyone knows how much a poorly adhering
conformal coating affects the probability of penetration of a whisker from
below, but in preventing penetration from above (i.e., from the outside) it
will probably work as well as a well-adhered coating (see Joe Kane?s point
5). For typical voltages (<50 V) the thickness required to prevent a short
circuit is probably on the order of tens of nanometers.
?I seem to remember somewhere that the tips of the whiskers are so thin,
that they can bore right through a conformal coating...no matter how well
applied.? Phil Shewmaker
There?s been no such evidence that I know of. Certainly a thin whisker is
sharper than a thick one, but it also buckles more easily.
??there has not been adequate research into the use of ternary or quaternary
lead-free solder alloys for whisker mitigation?? John Wayt
The research on what elements reduce the tendency of tin to form whiskers
was done fifty years ago. The only element that has a well-established
ability is lead. Bismuth may have a small reducing effect. That?s the
complete list.
?The tin crystallizes and grows?? Steve Smith
Tin is crystalline when it is electroplated, and it is crystalline when it
freezes. There?s no way to get amorphous tin.
?Remember that we don't have an intact layer of tin oxide on the surface.
The thermal cycling continually shatters it, just as colliding sheets of ice
on the ocean break and reform creating a jagged ice-sheet.? Steve Smith
The time required for an oxide coating to start forming on a freshly exposed
layer of metallic tin is measured in microseconds. The layer quickly grows
to a few nanometers and then effectively stops growing.
?Moisture has been shown to be a major driver of whiskering, so any idea
that any conformal coating can prevent whiskering I would hold up in the air
between thumb and forefinger and at arms length, and drop into a
high-humidity environment for a few thousand hours, and then come back and
gingerly poke it with a long stick.? Steve Smith
Moisture does seem to accelerate whisker growth, but it is not necessary.
The greater concern for moisture should be what it does to the properties of
conformal coatings. They all absorb moisture to some extent and therefore
become softer. That still might not make any difference on how easy it is
for a whisker to penetrate from above.
?As far as I am aware the promotion of use of conformal coatings to aid RoHS
issues were not to help tin whiskers, but stop oxidisation of crystalline
tin joints. Using a conformal coating stops the oxidation process and
improves short term reliability.? Mark Vaughn
Conformal coatings are applied to prevent dendritic growth between adjacent
metalizations at different electrical potentials. It does not stop
oxidation, nor does it need to.
?Tin loves to form a crystal structure, plumbers tell the difference between
tin and solder bar, by bending the bar and listening to it, the crystalline
structure of tin will crackle.? Mark Vaughn
Pure tin makes a noise called ?tin cry? when it is bent. That has no
relevance to anything being discussed in this forum.
?With Pb joints the eutectic point, that is the potentially crystalline
phase during cooling lasts for about a 2 degree temperature change, with a
Tin based lead free solder that is around 39 to 42 degrees.? Mark Vaughn
I have no idea what Mark is intending to say.
?This makes it very difficult to ensure a manually soldered joint is a good
one, especially when the joint looks so damn awful in lead free.? Mark
Vaughn
Manually soldered solder connections have remained satisfactory for many
decades. If there?s enough of it and the wetting to the surfaces to be
connected is satisfactory, their appearance (i.e., texture and luster) is
unimportant.
?So there is a high probability that manual joints with contain crystalline
fishures that over time usually a few days to weeks will oxidise and form a
bad/dry joint.? Mark Vaughn
Manually soldered connections do not contain fissures, and their oxidation
is virtually complete by the time the solder has cooled to room temperature.
?I expect it's more related to the electrical current flowing and voltage
charges across the tin lattice structure. Some crystalline structures are
not complete and are said to have acute voids, in which nucleation sites
become seeded, brought on by electrical charge to form crystals that due to
differences in angles of the crystal and void become ejected, sounds like a
whisker to me.? Mark Vaughn
The relationship between potential or current and whisker growth is tenuous
at best. The tin plating does not ordinarily contain ?acute voids? (whatever
that term might be intended to convey), and while nucleation of whiskers is
still not well understood, since metals are electrical conductors,
electrical charges are unlikely to have anything to do with it.
?Sadly this is not my field, trying to remember some college stuff 20 years
old, and I don't remember the differences with between pure and solvent
crystalline grow, so I may be talking bunkum, but I don't think I'm far
off.? Mark Vaughn
Solvent crystalline growth is irrelevant to tin plating.
?I was also wondering if anyone has evaluated the effects of Hydrogen
Sulfide on these connections.? Jacob Brodsky
I don?t think that hydrogen sulfide has a measured effect on the growth of
tin whiskers (tin sulfide is not a very stable compound), but it has a major
effect on the growth of silver whiskers, as discussed at length on the NASA
whisker web site.
?Are there conformal coatings that we could put in a purchase specification
that might help us here? Is it something we could do ourselves after
purchase?? Jacob Brodsky
As discussed above and below, conformal coating is the only practical
mitigation technique available to the OEM other than solder dipping or (in
the case of chip components) replating.
?<While the solder alloys are not pure tin, the soldering process does not
wet the entire surface of the parts that have become pure tin plated.>
Wetting the entire surface is rare.? Henning Leidecker
Small terminations are completely coated with a layer of solder and hence if
the solder contains lead they have very little risk of whisker formation.
Larger terminations are at risk of whisker growth from any tin-plated area
not reached by the solder. Even areas under but near the edge of the solder
can grow whiskers since the solder froze before it could completely dissolve
the tin.
?If the solder has lead in it, and is applied as a hot liquid, then the lead
alloys nicely into the tin and no long whiskers grow. But evaporating or
electroplating some lead onto the pure tin is not so effective at getting
the lead into the pure tin.? Henning Leidecker
Neither evaporating nor electroplating lead onto tin is a commercially
available process. There is a proprietary process for replacing existing tin
plating with electroplated tin-lead, and that is only practical for chip
components. That process does not leave any lead-free tin.
?If the tin surface has already grown some whiskers, and then a leaded alloy
is flowed on, the whiskers can sometimes remain: they melt at 232C (and the
tough oxide on their surface can maintain them as needles encased in a
shell, at up to 260C and sometimes a bit more), while eutectic lead-tin
solder melts at 183C and might be applied at less than 250C.? Henning
Leidecker
While it is true that because of its oxide sheath (thin, but very tough!) a
tin whisker can survive heating well above the melting temperature of tin,
because of its very tiny cross section it will dissolve instantly upon
contact with molten solder.
?Whiskers were able to push through THIN areas of our coating.? ?Warning:
Conformal coating processes often suffer from incomplete coating. For
example, shadowing effects during spray application, thinning of the coating
from surface tension and other forces during curing can leave you with ?less
coverage? than the process documentation tells you that you have. Good idea
to INSPECT and verify YOUR coating processes ability to coat all areas of
interest to you.? Jay Brusse
Any conformally coated assembly will have thin areas (anywhere there is a
small radius of curvature as well as any shadowed area). Therefore it is
only practical to assume that some whiskers will grow on a conformally
coated assembly. I see no point of inspecting for thickness since there
remains no practical way to make the coating thick in the
small-radius-of-curvature regions. See my comments above about how much of a
concern thin coating is.
??large grained 100% matte tin plating on copper-lead frames resists
whiskering even when exposed to compressive forces and held at elevated
temperatures in humid environments. Bright tin, however, had whisker growth
reaching 700 µm even at ambient temperatures. Extensive cyclic compression
tests with select matte tin chemistries, considered a worst-case evaluation,
yielded whisker growth of less than 20 µm.? ?A nickel barrier under tin
helps prevent the formation of compressive stresses caused by the growth of
CuSn intermetallics and hence reduces the one driving force for tin whisker
growth. If a nickel barrier layer is not possible, tin plating is annealed
after plating for 1 hour at 150°C to promote controlled intermetallic
compound formation and reduced tin whisker risks.? Patrick Lavery, quoted by
Bob Landman
The problem any OEM has with the type of tin plating being received is that
it can?t be inspected to find out if it is ?good? (matte, large grain) or
not so ?good? (bright, fine grain). Any assembly will contain components
from dozens if not hundreds of component manufacturers, and even one
manufacturer can have unnoticed changes in the tin plating process from day
to day. Similarly, there is no practical way to determine whether a nickel
underplate is present or whether the plating had been reflowed within one
hour. (What would you do if you knew?)
?Of these finishes, ENIG has been shown to be the most robust for lead free
soldering.? Patrick Lavery, quoted by Bob Landman
ENIG won?t grow tin whiskers, but it can have failures due to black pad and
brittle fracture. Consider OSP or immersion silver.
?A thick conformal coating (silicone-based polymer with 90% to 92% filled
ceramic particles) used in power brick products as part of a thermal
management strategy?adds resistance to whisker growth.? Patrick Lavery,
quoted by Bob Landman
The power brick approach seems to me to be a very effective technique to
prevent short circuits due to tin whiskers, provided the coating doesn?t
have voids that span the space between tin-plated surfaces at different
electrical potentials.
Gordon Davy
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