[tinwhiskers] Re: Conformal Coating ? When Reliability Goes Astray
- From: "Mark Vaughan" <mark@xxxxxxxxxx>
- To: <tinwhiskers@xxxxxxxxxxxxx>
- Date: Fri, 14 Mar 2008 11:09:22 -0000
As far as I am aware the promotion of use of comformal coatings to aid RoHs
issues were not to help tin whiskers, but stop oxidisation of crystalline
tin joints.
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.
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. 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. 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. Using a conformal
coating stops the oxidation process and improves short term reliability.
As to tin whiskers, well I suppose if something is in the way like a
conformal coat then perhaps that might make it harder for the whisker to
penetrate, some two pack laquers are very very hard. But to make any further
comment requires more knowledge of how tin whiskers truly form, there's a
lot of conjecture, but it seems no one is 100% sure of the mechanism. Some
say it is an environmental reaction so a conformal coating might help, but
tin whiskers are problems in transistors of the 1960's and they are totally
sealed in a can. (actually give them a good whack with a hammer and it will
often break the whiskers to allow the transistor to function again, that?s
what the restorers do), 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.
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.
Regs for now
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)
-----Original Message-----
From: tinwhiskers-bounce@xxxxxxxxxxxxx
[mailto:tinwhiskers-bounce@xxxxxxxxxxxxx] On Behalf Of Bob Landman
Sent: 13 March 2008 20:38
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Conformal Coating ? When Reliability Goes Astray
We are told that conformal coatings are a successful tin whiskers mitigation
strategy. Correct me if I'm wrong, but I don't believe it's been
demonstrated to any significant degree that any conformal coating on the
market today will "prevent" tin whiskers from punching through the coating.
I note that it's popular to use the word "mitigate" and that's a word that
is not as strong a word as "avoid" or "prevent". I await someone who can do
the math on how statistically significantly conformal coatings "mitigate"
tin whiskers.
A dictionary states that the word means "To moderate (a quality or
condition) in force or intensity; alleviate."
By how much? The word itself gives us no clue.
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?
I just read the article below on reliability of conformal coatings that I
thought worth sharing if we are going to have to count on such a coating to
save our lives.
-Bob Landman/H&L Instruments,LLC
SMT
Conformal Coating - When Reliability Goes Astray
By Umut Tosun, Ms.ChE., ZESTRON America
Proper functioning of electronic assemblies under the most stringent
conditions can be guaranteed only when conformal coatings perfectly adhere
to board surfaces. The usual requirement for such proper adhesion is
attained through the highest cleanliness level on the assembly surface.
Cleanliness level assessments can be performed by implementing quick,
innovative, and economical analytical cleanliness procedures.
In automotive-, military-, and aerospace-related industries, electronic
assembly requirements have increased steadily over the years. Increasing
package density brings higher standards for assembly cleanliness during the
manufacturing and assembly process. The use of assemblies under harsh
climatic conditions, such as temperature fluctuations and moisture exposure,
increases the risk for malfunctions. Thus, failure mechanisms such as
leakage current and electrochemical migration are initiated through
environmental influences (Figures 1A and B).
With the introduction of lead-free solder pastes, increased amounts of rosin
and activator content also must be taken into consideration. The latter has
proved responsible for an increase in corrosion-related malfunctions, and
reductions in the reliability and life of electronic assemblies.
Coating as a Reliable Protective Measure
Protecting electronic assemblies with conformal coatings is an important,
necessary measure to ensure reliability of electronic products. As conformal
coating is usually the last step in the manufacturing process, application
failures may have a drastic effect on production costs and lead to
unavoidable field failures. To guarantee optimum adhesion of the protective
coating and prevent subsequent crack formation or delamination, it is of
utmost importance to ensure the highest cleanliness level for assemblies
prior to coating.
Minimum Surface Cleanliness
The minimum cleanliness requirement prior to conformal coating application
is specified in the J-STD 001 D standard. Accordingly, the following methods
are required for proper qualification:
. Visual inspection with 20 or 40× magnification (according to IPC
A610D);
. Measurement of flux residues (257.95 µg/inch2 for Class-3
assemblies);
. Measurement of ionic contamination (10.06 µg/inch2 eq. NaCl);
. Evidence of other organic impurities;
. SIR measurement during or after climatic storage.
Visual inspection can be performed with a microscope. No visual impurities
on the assembly should be observed during this scrutiny.
The amount of resin on assemblies plays a significant role as it directly
influences conformal coating adhesion. Resin residues can lead to
insufficient adhesion and result in coating delamination. Acknowledging the
threshold for Class-3 assemblies with a set limit of 257.95 µg/inch2, one
has to be aware that this value is equivalent to the amount of resin that
can be found around a single soldered joint. However, the amount of resin
left by lead-free solder pastes has increased enormously due to their
changed composition. Over the years, resin residues have been detected by
means of extensive and lengthy procedures such as high-pressure liquid
chromatography (HPLC). They now can be detected easily through a quick
chemical test, such as the resin test. This method ensures that residual
resin is identified and removed effectively.
In assessing the surface's cleanliness according to the J-STD 001D standard,
the ion equivalent represents an important test method as well. A high ionic
equivalent value indicates the existence of a large amount of hygroscopic
impurities. Over time, the found impurities might lead to a coating
delamination, ultimately resulting in failure (Figure 2).
Other organic impurities, such as flux residues, influence coating quality
and trigger failure mechanisms underneath the conformal layer. In accordance
with J-STD 001D standard, the presence of organic impurities either can be
tested with infrared spectroscopy or detected with a discoloration method
such as the flux test. By means of a color reaction, organic acids used as
activators in fluxes specifically are revealed by the flux test on the
assembly. This innovative and non-destructive test method naturally provides
a visual distribution of the critical residues on the board surface.
Surface insulation resistance (SIR) measurement is done to demonstrate the
surface insulation resistance, as a high degree of insulation ensures that
electrical signals on the assembly are not distorted. Flux residues and
conductive impurities may cause leakage current bridges, thereby leading to
malfunctions. While performing SIR measurements, operators store a comb
structure in a climatic exposure test cabinet and the surface resistances
between the individual comb structures are measured.
All previously mentioned methods ensure detection of the various impurities
that could disrupt the functionality and viability of a conformal coating.
Nevertheless, the integration of a cleaning process usually is required to
be able to maintain all of the production thresholds set by the J-STD 001-D
standard. The cleaning process should not only remove impurities, but also
guarantee the proper adhesion of the conformal coating to minimize long-term
risk of crack formation and delamination.
Conclusion
Associating an appropriate cleaning process with innovative surface
cleanliness tests will yield an economical and cost-effective solution. A
proper coating process will result, and process and operational reliability
of coated assemblies will increase. The Society for Corrosion Protection
endorses this statement in its latest "Use and Processing of Conformal
Coatings for Electronics Assemblies" guideline. Authored by a consortium of
coating manufacturers, this guideline provides a selection of reliable and
economical process solutions. When using optimized cleaning and
qualification test methods in production steps preceding the coating
process, the adhesion of coatings is ensured, thus preventing field failures
of the coated assemblies. With the arising board complexity, partnerships
between manufacturing process engineers, cleaning process suppliers, and
equipment manufacturers become increasingly important.
Umut Tosun, M.S. Chem. Eng., accredited cleaning expert and application
technology manager, Zestron, may be contacted at u.tosun@xxxxxxxxxxxxxx
<http://smt.pennnet.com/Articles/mailto:u.tosun@xxxxxxxxxxxxxx> .
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